PII: S1618-3169(05)00100-X

From Sensory to Long-Term Memory

Evidence from Auditory Memory Reactivation Studies

Istva

n Winkler

1,2

and Nelson Cowan

Institute for Psychology, Hungarian Academy of Sciences, Hungary,

Cognitive Brain Research Unit, Department of Psychology, University of Helsinki, Finland,

Department of Psychological Sciences, University of Missouri, USA

Abstract. Everyday experience tells us that some types of auditory sensory information are retained for long periods of

time. For example, we are able to recognize friends by their voice alone or identify the source of familiar noises even years

after we last heard the sounds. It is thus somewhat surprising that the results of most studies of auditory sensory memory

show that acoustic details, such as the pitch of a tone, fade from memory in ca. 10Ð15 s. One should, therefore, ask (1) what

types of acoustic information can be retained for a longer term, (2) what circumstances allow or help the formation of

durable memory records for acoustic details, and (3) how such memory records can be accessed. The present review discusses

the results of experiments that used a model of auditory recognition, the auditory memory reactivation paradigm. Results

obtained with this paradigm suggest that the brain stores features of individual sounds embedded within representations of

acoustic regularities that have been detected for the sound patterns and sequences in which the sounds appeared. Thus, sounds

closely linked with their auditory context are more likely to be remembered. The representations of acoustic regularities are

automatically activated by matching sounds, enabling object recognition.

Keywords: memory, auditory sensory memory, long-term sensory memory, reactivation, event-related brain potentials, mis-

match negativity (MMN)

1. Introduction

Traditionally, the processing of sensory and categori-

cal information have been distinguished from each

other on the basis of performance differences found

in some experimental procedures and by evidence

showing anatomical separation of sensory and cate-

gorical processing in the human brain (see, however,

Näätänen, Tervaniemi, Sussman, Paavilainen, & Win-

kler, 2001, for recent evidence concerning the “intelli-

gent” functions of auditory sensory-specific areas in

the human brain). Previous research in experimental

psychology identified four important features charac-

terizing sensory memory traces, distinguishing them

from categorical memory representations (Broadbent,

1958; Cowan, 1984, 1988; for a full discussion of the

supporting and contradictory evidence, see Section

3.2.): 1) the formation of sensory memory traces does

not depend on attention; 2) the information stored in

sensory memory traces is modality-specific; and 3)

has a resolution, which is finer than the conventional

meaningful categories; but 4) it is lost within a short

period of time. The goal of the current review is to

reexamine these distinguishing features of sensory

” 2005 Hogrefe & Huber Publishers Experimental Psychology 2005; Vol. 52(1):3Ð20

DOI: 10.1027/1618-3169.52.1.3

memory traces in the light of recent evidence obtained

with electrophysiological and behavioral methods,

mostly using memory-reactivation procedures.

1.1 Properties of Sensory Memory:

Behavioral Studies

Classical multi-store models of memory postulated

separate stores for the retention of sensory informa-

tion (e.g., Atkinson & Shiffrin, 1968). The sensory

memory stores (a separate one for each modality)

were assumed to serve as temporary buffers from

which information could be accessed for a short time,

after which they were lost due to decay or to interfer-

ence from more recent stimuli. The information se-

lected from the sensory buffers was categorized, or

transformed into a common internal code, allowing

modality-independent operations. Only categorized

information was assumed to be stored in more durable

stores.

These features of the multi-store models corre-

spond well with the majority of the results of sensory

memory research. For example, it has been found in

4 I. Winkler & N. Cowan: From Sensory to Long-Term Memory

many studies that subjects can only tell the difference

between two closely similar sounds if the sounds to

be compared are presented within ca. 10 s (taken to

suggest decay) Cowan, 1984). Also, presenting irrele-

vant sounds between the to-be-compared ones deterio-

rates performance, the more so, the closer the sim-

ilarity between the to-be-compared and the interven-

ing sounds (taken to suggest interference) (Cowan,

1984; Deutsch, 1975; Massaro, 1970). When the

sounds to be compared are separated by long silent

intervals, subjects can only discriminate them if they

substantially differ from each other. The degradation

of sensory resolution is compatible with the idea that

once the trace of the first sound is eliminated from

the sensory buffer, subjects can only rely on those

memory stores that have a longer retention interval.

These stores, however, only contain categorized infor-

mation representing stimulus features with a cruder

resolution than the sensory buffer. Importantly, it ap-

pears that the retention interval of the sensory buffer

cannot be extended much by top-down control (e.g.,

Keller, Cowan, & Saults, 1995), though there is a

small effect of that nature. Three related findings in

the area of speech perception leading toward the no-

tion of separated sensory and categorical memory

stores are (1) the category boundary effect, or more

successful discrimination between two phonemes fall-

ing across a category boundary than between two allo-

phones (instances of a single category) of comparable

physical separation, (2) the delay effect, or poorer

comparison of two allophones as a function of the

temporal separation between these allophones in the

range of a few seconds, and (3) the vowel advantage,

or much more rapid forgetting of allophonic detail for

the acoustically-complex stop consonants than for the

acoustically-simpler vowels (Fujisaki & Kawashima,

1971; Pisoni, 1973). Acoustic theory has emphasized

this distinction between unstable sensory information

and more stable categorical information (e.g., Dur-

lach & Braida, 1969).

However, there also exist results indicating that not

all sensory information is lost within a few seconds.

Craik and Kirsner (1974) reviewed studies showing

that people often remember voice information for

longer than the period of 30 s or so that has been the

presumed duration of auditory sensory memory, and

they carried out their own interesting experiments re-

inforcing that point. In their studies, spoken target

words were presented 4 s apart in two different voices

(male and female) in random order. The test for each

word consisted of another version of the word (the

probe word) that had to be recognized. Foils that had

not been presented as targets also appeared as negative

probes, and the question was whether the probe had

Experimental Psychology 2005; Vol. 52(1):3Ð20 ” 2005 Hogrefe & Huber Publishers

appeared before in the experiment. The distance be-

tween the target and the corresponding probe word was

a lag of 1, 2, 4, 8, 16, or 32 words. At a lag of 1, the

target and probe occurred with no intervening word; at

a lag of 2, they occurred with one intervening word; and

so on. It was found in several of the experiments that

spoken probe words were recognized faster and more

accurately when the voice remained the same from the

target word to the probe word. This occurred even at the

long delays, indicating that some memory of the voice

persisted for over 2 min in the presence of intervening

words. It also was found that subjects maintained an

ability to recall (at better-than-chance accuracy) the

voice in which a particular word had been presented.

For example, in one experiment, in which the probe

words were visually presented, the proportion of recall

of the voice conditional upon correct recognition of the

word was, at the six lags, 1.00, 0.98, 0.87, 0.75, 0.76,

and 0.73, respectively. Thus, at lags of 8 words (32 s)

and above, recall of the voice reached an asymptote of

much-better-than-chance accuracy.

In a very different type of procedure demonstrating

long-term storage of memory for sound, Crowder

(1989) presented a pure tone followed by a note played

by a musical instrument. Comparison of the pitches of

the two sounds was speeded when participants had ad-

vanced knowledge of which instrument was to be used

for the second sound. Presumably, long-term memory

was used to generate a mental image of the frequency

of the pure tone as played in the timbre of the instru-

ment that was used for the second sound.

As a consequence, modern memory models sug-

gest that sensory stimulus codes are processed along

with categorical stimulus representations by special-

ized subsystems or activation processes (e.g., see

Cowan, 1988, 1995, 1999). However, although these

models assume that some sensory details are retained

for longer periods of time, they do not explain the

apparent contradiction between the classical findings

of limitations in accessing sensory information and

longer-term retention of these data. Our approach to

this issue was based on everyday experience. One can

recognize concrete objects (not just object categories)

by sensory details even long after the object was last

encountered. For example, we are often able to recog-

nize our friends’ voices over the telephone or shouting

from another room, even though there are no known

categorical properties distinguishing one voice from

another. A model of the recognition situation may re-

veal important information about what type of sensory

information is retained in the brain as well as about

the circumstances that help in forming such memory

representations. The auditory memory reactivation

paradigm was designed to test these questions. By ba-

5I. Winkler & N. Cowan: From Sensory to Long-Term Memory

sing the test of “recognition” on an event-related brain

potential (ERP) that can be measured independently

of the subject’s task, we avoided confounding factors

stemming from the task and strategy of the subject,

thus providing an unbiased measure of what sensory

information is stored in the brain for longer time

periods.

1.2 The Mismatch Negativity

Event-Related Potential

Following a brief introduction to the ERP component

used to test the recognition of sounds, the reactivation

paradigm will be described in detail. We will show

that with this paradigm, we are able to tap auditory

sensory information that is resistant to decay and in-

terference and discuss what processes and memory

structures may underlie the reactivation phenomenon.

Evidence will be provided showing that reactivated

sound information can also be accessed in active

discrimination tasks. In the discussion, we turn to the

question of what kind of sound information is retained

for longer periods of time. Finally, we relate our find-

ings to two current models of working memory.

The ERP component involved in testing memory

reactivation has been termed the mismatch negativity

(MMN). MMN is elicited whenever a sound violates

some regular aspect of the preceding sound sequence

(for recent reviews, see Näätänen & Winkler, 1999;

Picton, Alain, Otten, & Ritter, 2000). The elicitation

of MMN requires the presence of some representation

of the violated acoustic regularity. The auditory sen-

sory information encoded in these regularity represen-

tations corresponds to that appearing in perception.

MMN is elicited whether or not the subject’s task is

related to the test sounds. In fact, in the majority of

MMN studies, subjects were engaged in some activity

involving visual stimuli (e.g., they read a book,

watched a movie, or performed some reaction task

with visual stimuli) and were instructed to disregard

the sounds presented to them (the “passive” condi-

tion). Thus MMN can be used to obtain a task-inde-

pendent index of the retention of auditory information

in the brain.

The MMN component typically peaks between 100

and 200ms from the onset of the regularity violation

with negative polarity over the fronto-central scalp and

positive over scalp locations above the opposite side of

the Sylvian fissure. This is because the main cortical

generators of MMN lie within or in the vicinity of the

supratemporal plane with additional contribution from

sources located in the frontal cortex (Halgren et al.,

1995; Opitz, Rinne, Mecklinger, von Cramon, & Schrö-

” 2005 Hogrefe & Huber Publishers Experimental Psychology 2005; Vol. 52(1):3Ð20

ger, 2002). The simplest and most commonly used

paradigm for obtaining MMN is the auditory oddball

sequence. When a repeating sound (termed the “stan-

dard” sound) is occasionally exchanged for a different

sound (“deviant” sound) MMN is elicited. However,

MMN is also elicited by violations of far more com-

plex auditory regularities (for reviews, see Näätänen et

al., 2001; Winkler, 2003). The MMN wave can be delin-

eated from other concurrent ERP components by

subtracting from the ERP response elicited by the devi-

ant stimulus the ERP elicited by some control sound.

For a good assessment of the MMN response, the con-

trol sound should share as many features as possible

with the deviant sound, but it should not itself elicit an

MMN (i.e., it should be a stimulus that conforms to the

regularities of the sound sequence in which it appears).

The current explanation of MMN elicitation sug-

gests that incoming sounds are compared with extrap-

olations (sensory inferences) calculated from the rep-

resentation of regularities detected in the preceding

sound sequence. Sounds that mismatch these extrapo-

lations activate the MMN-generating process (Win-

kler, Karmos, & Näätänen, 1996b). It is important to

note that the presence of a sensory memory record of

a sound in the brain is not a sufficient prerequisite of

MMN elicitation; MMN is only elicited once some

auditory regularity has been detected and a subsequent

sound violates this regularity (Cowan, Winkler,

Teder, & Näätänen, 1993; Sussman, Sheridan, Kreu-

zer, & Winkler, 2003a; Winkler, Schröger, & Cowan,

2001). Thus, stimulus change per se (e.g., two dif-

ferent sounds presented successively at the beginning

of a sound sequence or within an ever-changing se-

quence of sounds) does not result in MMN elicitation

(Cowan et al., 1993; Horva

th, Czigler, Sussman, &

Winkler, 2001; Winkler, 1996).

The MMN-generating process appears to be inde-

pendent of top-down control (Rinne, Antila, & Wink-

ler, 2001; Sussman, Winkler, & Wang, 2003b). It has

been shown that in most cases, the MMN results ob-

tained in the “passive” situation match those that can

be obtained in the same paradigm with attention di-

rected away from the sounds in a controlled manner

(Sussman et al., 2003b; Winkler et al., 2003). How-

ever, it should be noted that some of the processes

underlying the detection of auditory regularities can

be modulated by top-down control and the outcome

of these processes is also reflected in the elicitation

and/or amplitude of the MMN response (Sussman,

Winkler, Huotilainen, Ritter, & Näätänen, 2002; Suss-

man et al., 2003b). The possible functions of the

MMN-generating process are to initiate further pro-

cessing of the deviant sounds (Nääatänen, 1990) and/

or to update the regularity representations that did not

6 I. Winkler & N. Cowan: From Sensory to Long-Term Memory

Figure 1. Schematic illustration of an MMN reactivation paradigm. Top panel: The test condition. Standard trains consist of sounds

or sound patterns, which conform to some regularity. Elements of these trains are termed “standards”, marked with “Std” on the

figure. Standard trains are followed by the retention interval and the reactivation train. The first element of the reactivation train is

termed the “reminder” (marked “Rem”). It conforms to the regularities of the standard train (i.e., it is also a “standard”). Reactivation

is tested by the second element of the reactivation train, which violates some regularity of the standard train. Thus it is termed

“deviant”, marked as “Dev”. The reactivation train ends with a further standard or standards (their role is to provide a homogeneous

context for the deviant). Bottom panel: One possible control condition. Standard trains are exchanged for trains in which the sounds

or sound patterns vary in the regular feature of the standard trains (e.g., if the regularity used in the test condition is the constancy

of tone frequency, then frequency is randomly varied in the control condition). The trains substituted for the standard trains are

termed random-sound trains and their elements marked with “Rnd”. Reactivation trains are exchanged for comparison trains that

start with the same sounds as the reactivation trains (the reminder and a deviant, termed comparison tone and marked as “Cmp”).

The rest of the train is made up of random sounds, again varying in the critical regularity of the test condition.

correctly predict the deviant (Winkler & Czigler,

1998; Winkler et al., 1996b).

2. Auditory Memory Reactivation

2.1. The Reactivation Paradigm

For testing reactivation with the MMN measure, at

least two trains of sounds are needed (Figure 1, top

panel). The first train sets up a regularity (tone repeti-

tion in Figure 1). It is termed the “standard train.” The

standard train is followed by the retention interval.

The second train (“reactivation train”) starts with a

sound (or sound pattern) that conforms to the regulari-

ties of the standard train. This sound is termed the

The requirement of establishing a regularity in the standard train provides a good possibility for delineating the MMN

component from other overlapping ERP responses. Figure 1 (bottom panel) shows the optimal control sequence. Standard trains are

exchanged for trains that do not show the same regularity (termed “random-sound trains”). For example, if the deviant is set up to

violate the common frequency of the standard-train tones, then a train of tones randomly varying in frequency can be used in the

control condition. The “reminder” and the “deviant” are unchanged (compared with the test condition) but are again followed by

Experimental Psychology 2005; Vol. 52(1):3Ð20 ” 2005 Hogrefe & Huber Publishers

“reminder.” The second sound of the reactivation train

then violates the regularity of the standard train. This

sound is termed the deviant or position-2 deviant in

paradigms testing MMN elicitation also in later posi-

tions of the trains. In some paradigms, each train

served both functions: the first two sounds of the train

tested reactivation with respect to the preceding train,

whereas later sounds in the train set up the standard

for the next train. If the (position-2) deviant sound

elicits the MMN component, one can conclude that

the regularity of the standard train was represented in

the brain and this representation was available to the

MMN-generating process. This is because, as was

mentioned in the previous section, stimulus change

alone does not activate the MMN-generating process.

MMN can only be elicited if the deviant sound vio-

lates some detected regularity.

Figure 2 (top left

7I. Winkler & N. Cowan: From Sensory to Long-Term Memory

Figure 2. ERP difference waves (deviant minus same-position standard-tone responses) obtained by Cowan et al. (1993) at the

frontal (Fz) electrode location in position 2 (reactivation, first row) and in position 1 (deactivation test, second row) of the tone

trains. In the constant standard condition (left column), standard and deviant tones were fixed within the stimulus blocks. In the

roving standard condition (right column), the frequency of both standard and deviant tones changed from train to train. Tone onset

is at the crossing of the x and y axes. Calibration is marked at the lower right corner. Tick-marks at the bottom are spaced 100ms

apart. The MMN response, which appears only in position 2 of the constant standard condition (upper left corner), is a negative

wave peaking between 100and 200ms post-stimulus.

panel; adapted from Cowan et al., 1993) shows the

MMN response elicited by a position-2 frequency-de-

viant tone.

However, the elicitation of MMN by a position-2 de-

viant, in and of itself, does not prove that reactivation

occurred. One should also check whether the represen-

tation of the regularity of the standard train was still

available to the MMN-generating process following the

retention interval. That is, it should be tested whether a

deviant presented in the first position of a stimulus train

elicits the MMN. Figure 2 (bottom panels) shows that

in Cowan et al.’s (1993) study no MMN was elicited by

position-1 deviant tones. It has been repeatedly found

that when trains are separated by a silent interval of 11 s

or more, no MMN is elicited by deviant sounds pre-

sented at the beginning of a train (Cowan et al., 1993;

Gaeta, Friedman, Ritter, & Cheng, 2001; Winkler et al.,

2001). Therefore, when the retention interval is longer

than 11 s, MMN elicited by position-2 deviants tells

that the representation of the regularity of the standard

train, which was not available to the MMN-generating

process at the beginning of the reactivation train be-

random sounds. This setup ensures that the control-condition deviant will not elicit MMN while the rest of its ERP components

match those elicited in the test condition. The first reactivation experiments used for comparison the response elicited by a standard

sound that was presented in the same position as the deviant. More recent studies used the control described above.

” 2005 Hogrefe & Huber Publishers Experimental Psychology 2005; Vol. 52(1):3Ð20

came available by the time the second sound of the reac-

tivation train was processed. Cowan et al. (1993)

termed this phenomenon “memory reactivation” as

they regarded it to be similar to the reactivation phe-

nomena described by Rovee-Collier and Hayne (1987).

Note, however, that the findings of Rovee-Collier and

Hayne refer to learned actions rather than sensory infor-

mation and that the timescale of their effect is much

longer (several days) than that ever tested with the cur-

rent reactivation paradigm.

The reminder is a critical element of the reactiva-

tion paradigm. Cowan et al. (1993) found that when

the first tone of a train substantially differed from the

tone repeated in the preceding train, it did not set up

a subsequent “deviant” for MMN elicitation. In their

“roving-standard” condition, Cowan et al. presented

short trains of tones in which all but possibly one tone

had the same (standard) frequency. This standard fre-

quency, however, changed from train to train. In ca.

17 % of the trains, a tone whose frequency differed

from the current as well as from the previous standard

frequency (deviant) appeared in the second position

8 I. Winkler & N. Cowan: From Sensory to Long-Term Memory

of the train. The first tone of the train had the fre-

quency of the new standard, which was different from

that of the previous train. Position-2 deviants did not

elicit MMN in this situation (Figure 2, top right

panel). One could still argue that the roving-standard

condition of Cowan et al. did not set up a stable regu-

larity, because the standard frequency changed from

train to train. However, Ritter and his colleagues (Rit-

ter, Sussman, Molholm, & Foxe, 2002) have shown

that reactivation occurs even if the standard frequency

changes from train to train, when the reminder

matches the standard of the preceding train. Winkler

et al. (2002) narrowed the frequency range within

which the reminder is effective (i.e., reactivation oc-

curs). These authors found no MMN elicited by devi-

ants following a reminder that differed only by 3 %

from the standard frequency. Winkler et al.’s result

suggests that the representation against which the re-

minder is checked encodes features of the standard

sounds with a resolution characteristic of sensory

memory traces. Thus reactivation can indicate the

existence of finely resolved auditory information in

the human brain.

2.2. Characteristics of the Memory

Involved in Reactivation

The first question is whether the characteristics the

memory traces involved in reactivation are the same

as those describing the classical notion of auditory

sensory memory. Auditory sensory memory is subject

both to decay (estimates ranging from 10 to 20 s) and

to interference by similar sounds (Cowan, 1984). The

first reactivation study (Cowan et al., 1993) tested re-

tention intervals just beyond the most commonly ac-

cepted value for the duration of auditory sensory

memory (11Ð15 s). It is thus possible that a weak resi-

due of the auditory sensory memory trace of the re-

peated standard tone could have been reinforced by

the reminder. However, in a more recent study (Wink-

ler et al., 2002), the retention interval was set to 30 s.

The standard was again a repeating tone and the devi-

ant tone differed from it in frequency. All subjects

showed reactivation in this situation, even though only

one of them performed above chance level in discrimi-

nating the standard and deviant tones when the tones

were separated by a 30-s silent interval.

Interference from similar sounds has also been

tested (Winkler, Cowan, Cse

pe, Czigler, & Näätänen,

1996). Trains were composed of 12 tones. At least 5

of the first 6 tones were identical, whereas tones 7

through 12 (termed intervening tones) varied ran-

domly in frequency. Deviants appearing within the

Experimental Psychology 2005; Vol. 52(1):3Ð20 ” 2005 Hogrefe & Huber Publishers

first 6 tones also differed from the standard tones in

frequency. Trains were separated by 5.9 s of silence.

No MMN was elicited by the random-frequency in-

tervening tones presented in the last 3 positions of the

train (from position 10Ð12, i.e., the 4thÐ6th interven-

ing tones). Because these tones deviated from the

standard in the same way as the deviant did, the fact

that no MMN was elicited by them at the end of the

trains demonstrated that the frequency-regularity rep-

resentation was no longer available to the MMN-gen-

erating process after the 1st through 3rd intervening

tones, even before the onset of the silent interval that

separated the trains. Therefore, if the sensory informa-

tion involved in reactivation was vulnerable to inter-

ference from similar sounds, no MMN should be ex-

pected to be elicited by position-2 deviants (i.e., the

deviants testing reactivation following the silent in-

terval separating the trains). However, MMN was elic-

ited by position-2 deviants in this situation. This result

suggests that the memory involved in reactivation is

not subject to the type of sensory interference that

characterizes short-term auditory sensory memory.

The experiments reviewed above set up tone repeti-

tion as the standard. In most natural situations, however,

sound sequences include substantial amount of vari-

ance and the regularities have to be extracted from the

ever-changing input. One possible way to model such

variability is to set random changes in some sound

features while fixing the level of others. Ritter,

Gomes, Cowan, Sussman, and Vaughan (1998)

showed that constant tone-intensity was reactivated

when tone-frequency was varied throughout the stimu-

lus trains (including the frequency of the reminder

tone) and vice versa, constant tone-frequency was re-

activated when tone-intensity was varied. These re-

sults suggest that the memory representation involved

in reactivation encodes constancies extracted from the

variable input.

One could, however, argue that feature constancies

may be detected directly by neurons sensitive to nar-

row ranges of auditory features (see, e.g., Ritter, Dea-

con, Gomes, Javitt, & Vaughan, 1995). Furthermore,

the type of sensory information whose recognition we

intended to model with the reactivation paradigm is

based on secondary, rather than primary sound fea-

tures. For example, the timbre of a human voice that

allows one to recognize a friend by his/her voice alone

is characterized by the ratio between energy emitted

in certain frequency ranges, rather than by the abso-

lute pitch of the voice. A simple model of a secondary

auditory feature has been constructed by Saarinen,

Paavilainen, Schröger, Tervaniemi, and Näätänen

(1992), who presented tone pairs, 90% of which were

ascending in frequency (i.e., the frequency of the se-

9I. Winkler & N. Cowan: From Sensory to Long-Term Memory

cond tone of the pair was higher than that of the first

tone). The absolute frequencies of the tones varied

randomly throughout the sequences. Infrequent de-

scending-pitched tone pairs and tone repetitions elic-

ited the MMN response (for a study controlling all

aspects of pitch ascension, see Paavilainen, Jaramillo,

Näätänen, & Winkler, 1999).

Korzyukov, Winkler, Gumenyuk, Alho, and Näätä-

nen (2003) investigated whether this pitch-ascension

regularity can be reactivated similarly to regularities

based on primary sounds features. Seven tone-pairs

of ascending pitch varying randomly in their absolute

frequency levels were presented in the standard trains.

The retention interval was followed by a train starting

with one ascending-pitch (reminder) and a descend-

ing-pitch (deviant) tone pair. (Control sequences were

composed according to the scheme shown on the bot-

tom panel of Figure 1: pitch direction of the tone-

pairs varied randomly in the random-sound trains.) A

significant difference was observed between the re-

sponses elicited by the deviant and the corresponding

comparison tone-pair in the MMN latency range. The

scalp distribution of this potential difference matched

that of the MMN component elicited by descending-

pitch tone pairs presented infrequently amongst fre-

quent ascending-pitch tone pairs. This result demon-

strated that regularities based on secondary sound fea-

tures can be reactivated. Thus we can conclude that

the reactivation paradigm can indeed model real-life

situations in which recognition occurs on the basis of

acoustic subtleties.

2.3. Interpretation of the Reactivation

Phenomenon

Cowan et al. (1993) offered two alternative explana-

tions of their findings of “memory reactivation.” One

hypothesis suggests that during the retention interval,

the sensory memory traces involved in detecting

acoustic deviance enter a dormant state (i.e., a state in

which they cannot be directly accessed). The reminder

activates the corresponding dormant memory trace(s),

bringing them back to immediate memory (and thus

allowing the detection of the following deviants). The

alternative explanation assumes that, although the

memory traces required for deviance detection are

present and accessible, they are not consulted by the

MMN-generating process because the retention in-

terval causes a context change. That is, the sounds

presented after the relatively long retention interval

are not initially considered to be a continuation of the

preceding sound sequence, but rather the start of a

new sound group, whose regularities are yet to be de-

” 2005 Hogrefe & Huber Publishers Experimental Psychology 2005; Vol. 52(1):3Ð20

termined. In this explanation, the function of the re-

minder is to provide a link to the previous train,

reinstating its context as current and the related regu-

larities as relevant to the processing of the new

sounds.

The lack of MMN elicitation by the first sound of

the trains following the retention interval should thus

be taken either as a sign of the dormancy of the mem-

ory traces or as a sign that a change of context have

taken place. Based on the results of a behavioral test

and the analysis of the MMN studies known at the

time, Ritter and his colleagues (2002) argued that, in

just 10Ð15 seconds, auditory sensory memory traces

do not decay beyond usefulness. Evidence supporting

Ritter et al.’s conclusion has been obtained by Winkler

and his colleagues (2001; see also Gaeta et al., 2001),

who found that deviant tones may not elicit MMN at

the beginning of a short train when the trains are sepa-

rated by a silent interval of just 7 s duration. The trains

consisted of 4 tones of uniform stimulus duration (equi-

probably 100 or 300 ms) and were delivered equiproba-

bly with a stimulus onset asynchrony (SOA; onset-to-

onset interval) of 0.5 or 7 s. The silent interval sepa-

rating successive trains was always 7 s. The ERP re-

sponse to the first tone of those trains in which tone

duration differed from the preceding train was com-

pared with that elicited when tone duration matched

with the preceding train. The change in tone duration

elicited MMN in ca. half of the subjects when the

within-train SOA of the preceding train was 0.5 s. In

contrast, the same deviants following the same silent

interval (the between-train interval was always 7 s)

elicited MMN in all subjects when the within-train

SOA of the preceding train was 7 s (i.e., equal to the

inter-train interval). This result cannot be explained

on the basis of sensory memory alone, because, at the

time when the deviant tone was delivered, the sensory

memory trace of the standard tone must have been

stronger when the preceding train were delivered with

the short SOA than when it was delivered with the

long SOA. This is because more standard tones were

delivered with short than the long SOA within the last

ca. 10 s preceding the deviant tone. Therefore, we

must assume than the sensory memory trace of the

standard tone was present and accessible in both con-

ditions at the time when the deviant tone was deliv-

ered. Thus, this result demonstrates that the lack of

MMN cannot be taken as proving that no sensory

memory trace can be accessed by the MMN-generat-

ing process. Winkler et al. (and also Gaeta et al.) inter-

preted their result in terms of the context change hy-

pothesis. On the basis of these results, Ritter et al.

(2002) favored the context reactivation (or reinstate-

ment) explanation of the MMN reactivation phenome-

non.

10 I. Winkler & N. Cowan: From Sensory to Long-Term Memory

However, since then, Winkler et al. (2002) showed

reactivation after a much longer (30 s) retention in-

terval and that reactivation occurred also in those sub-

jects who could not discriminate the standard and de-

viant tones when these were separated by 30 s of si-

lence. The latter result is all the more important since

Keller et al. (1995) found a small effect of tone re-

hearsal on tone comparisons performance for over-10-

s retention intervals. This may explain why Ritter et

al. (2002) as well as our own test (see the next section)

found significant residual memory after 11Ð15 s in

active tone comparison tasks. However, the finding of

reactivation in subjects who could not perform the

comparable discrimination task suggests that reactiva-

tion may involve both of the processes brought up by

Cowan et al. (1993): The reminder connects the new

sounds to a previous context (reinstatement) as well

as reactivating the possibly dormant memory repre-

sentations that describe the regular characteristics of

this context.

2.4. Reactivation in an Active

Tone-Comparison Experiment

The comparisons between results obtained with MMN

and in behavioral studies (discussed in the previous

section) make it imperative to test reactivation in an

active paradigm that is analogous to the ones tested in

the passive situation. Although the reactivation para-

digm was intended to model situations in which re-

cognition occurs without voluntary effort, one would

assume that reactivation should also occur in situa-

tions in which the subject actively tries to maintain

some sensory information. This was tested in two ex-

periments in which subjects were required to judge

whether a test tone was the same or different com-

pared with a previously presented standard tone. The

two experiments differed only in whether the standard

tone was presented only once (1-Standard Experi-

ment; 21 participants, 9 male, 16Ð23 years of age,

19.4 years mean age) or six times in a row with 0.75 s

SOA (6-Standard Experiment; 25 participants, 13

male, 18Ð31 years of age, 22.4 years mean age).

2.4.1. Stimuli and Procedure

Eighteen sets of tones and a burst of white noise, band-

pass filtered between 300 and 1500 Hz, were generated.

The duration of all sounds was 250ms (including 5ms

rise and 5ms fall times), their intensity 70 dB (SPL).

Each tone set consisted of 3 tones separated in fre-

Experimental Psychology 2005; Vol. 52(1):3Ð20 ” 2005 Hogrefe & Huber Publishers

quency by proportionally equal steps of 2.5, 3, 3.5, 4,

or 5 % (see later). (For later reference, the middle tone

of the sets will be denoted as “A” as it was always the

first to be presented; the lower-pitched one will be

denoted as “B,” the higher as “C,” and the aperiodic

noise as “N”). The frequency of the middle (A) tone

of the lowest set was 400 Hz; the middle tones of

neighboring sets were separated by 6 % in frequency.

Each trial presented tones from only one set. Tone sets

appeared in the trials in a randomized order and with

equal probability. Trials started with the standard

tone(s), which was always the middle tone of a tone

set (“A”), followed by a silent retention interval,

which was, with equal probability, 11, 12, 13, 14, or

15 s. The sound following the retention interval was

selected with equal (25 %) probability from the three

tones of the set and the noise-burst. These second

tones were considered to be reminders (A valid; B, C,

and N invalid). Following the reminder by an SOA of

0.75 s, the test tone was delivered. The test tone was

equiprobably chosen from the three tones (A, B, and

C) of the set selected for the trial. Thus, 12 types of

trials were presented (the order of the sounds being

standard, reminder, test): AAA, AAB, AAC, ABA,

ABB, ABC, ACA, ACB, ACC, ANA, ANB, and

ANC. Each trial type occurred 18 times, each time

based on a different tone set (216 trials, overall). Sub-

jects were instructed to press a button if they thought

that the test tone was identical to the standard tone(s)

and a different button if they thought that the two

tones were different. The reminder (the sound preced-

ing the test tone) was introduced to them as a warning

signal preparing them for the delivery of the test tone.

They were informed that the warning sound carried

no information about the task and so they should not

rely on it in their judgment. The instructions empha-

sized the requirement of correct responses and placed

no time pressure on giving the response. Subjects

were also motivated by a scheme of performance-de-

pendent bonus payments to do the task as best they

could. Subjects started each trial when they felt ready

for it.

Prior to starting the main test, we established the

frequency difference at which subjects could reliably

(⬎ 90 %) discriminate two tones of slightly different

frequencies. Subjects were presented with pairs of

tones separated by a 2-s SOA. The two tones of the

pair were either identical or slightly different from

each other in frequency. In separate stimulus blocks

(10 pairs per block), the frequency difference was

either 2.5, 3.0, 3.5, 4.0, or 5.0 %. Absolute tone fre-

quencies varied across the 18 preselected frequency

sets (see above). Testing started with the highest (5 %)

frequency separation. After each successful stimulus

11I. Winkler & N. Cowan: From Sensory to Long-Term Memory

block (a block in which at least 9 of the 10 responses

were correct), frequency separation was decreased.

Frequency separation was increased after two succes-

sive blocks in which performance did not reach the

criterion level. Testing was finished when a frequency

separation was found at which the subject performed

at the desired level, but had to turn back twice from

the one-step lower frequency-separation level. On

average, subjects of the 1-standard experiment needed

4.1 %, whereas subjects of the 6-standards experiment

needed 3.8 % frequency separation for reliable dis-

crimination performance. Frequency separation be-

tween the B and A and A and C tones was set up in

the main experiment to equal the level established in

the preliminary testing. Subjects then received train-

ing in the task of the main experiment. Two blocks of

18 trials each were presented to them with the reten-

tion interval set to 3.5 s and feedback given after each

response. All other parameters were identical to the

corresponding ones in the main experiment.

2.4.2. Model for Analyzing the Results

Performance was analyzed with the help of a mathe-

matical model

of performance that assumed additiv-

ity between the following effects:

(1) Memory (“M”) Ð Performance based on the sen-

sory memory trace present at the time the warning

(reminder) tone was presented.

(2) Reactivation (“R”) Ð Only present on AAX type

of trials (X can be either A, B, or C); increases

performance.

(3) Interference from the warning tone (“I”) Ð

Decreases performance by degrading the residual

memory trace. Interference was assumed to be

zero for noise (ANX) trials. This assumption may

not be accurate. As a consequence, “M” will be

slightly underestimated.

(4) Strategy to answer according to the relationship

between the warning and the test tone (“S”) Ð

Using this strategy boosts performance on some

types of trials, such as ABC, but degrades perfor-

The formal model of performance is as follows. P(AXY) stands for the performance in the AXY trials (X can be A, B, C, or

N and Y can be A, B, or C); for other abbreviations, see the text. Equations are numbered from (1) to (7).

(1) P(ANA)=M+Bn

(2) [P(ANB) + P(ANC)]/2 = M - Bn

(3) P(AAA)=M+R+S+Ba- I

(4) [P(AAB) + P(AAC)]/2=M+R+S- Ba - I

(5) [P(ABA) + P(ACA)]/2 = M - S + Bbc - I

(6) [P(ABB) + P(ACC)]/2 = M - S - Bbc - I

(7) [P(ABC) + P(ACB)]/2=M+S- Bbc - I

” 2005 Hogrefe & Huber Publishers Experimental Psychology 2005; Vol. 52(1):3Ð20

mance on others, such as ABB. The frequency

separation between the warning and the test tones

was larger in the ABC and ACB trials than in any

of the other trials. This may have increased the

likelihood in these types of trials that answers were

given on the basis of the relationship of the warn-

ing and the test tones. By assuming that “S” was

equal in all trial types, we may thus underestimate

“S” for the ABC and ACB type of trials and, as a

consequence, underestimate the size of the reacti-

vation (“R”) effect.

(5) Bias to answer “equal” over answering “different”

(“B”) Ð Bias boosts performance on some types

of trials, such as ABA, but degrades performance

on others, such as ABB. This tendency was as-

sumed to depend on the relationship between the

standard tone and the warning sound. Therefore

three separate “B” values were considered, one for

the AAX trials, another for the ABX and ACX

trials, and the third for the ANX trials: Bias vari-

ables were named Ba, Bbc, and Bn, respectively.

The number of variables (effects) in the model

equaled the number of independent measurements.

Therefore, the effect values (the amount by which

each effect contributed to performance) could be un-

ambiguously determined, separately for the 1- and

6-Standard experiments.

2.4.3. Results and Discussion

Figure 3 presents the results of both active reactivation

experiments (hit percentages according to trial cate-

gories on the top panel and the calculated model val-

ues on the bottom panel). Although the memory factor

was responsible for the largest segment of perfor-

mance, reactivation proved to be significant in both

experiments (p ⬍ 0.05 in the 1-Standard Experiment

and p ⬍ 0.01 in the 6-Standard Experiment; one-

group Student’s t tests). The strategy and interference

factors were found to be significant only in the 6-

Standard Experiment (p ⬍ 0.01 and p ⬍ 0.05, respec-

tively). No significant differences were found between

the two experiments for any of the model factors (dif-

12 I. Winkler & N. Cowan: From Sensory to Long-Term Memory

ANA

ANY

AAA

AAY

AYA

AYY

AYZ

1-Standard

6-Standard

0,00

10,00

20,00

30,00

40,00

50,00

60,00

70,00

80,00

90,00

Hit %

Trial types

Tone Comparison

1-Standard

6-Standard

Memory

Reactivation

Interference

Strategy

1-Standard

6-Standard

0,00

10,00

20,00

30,00

40,00

50,00

60,00

70,00

Hit %

Model Values

1-Standard

6-Standard

Figure 3. Active reactivation experiments. Top panel: The percentage of correct responses (hits), separately for the 1-Standard (front

row) and 6-Standard (back row) experiments and sorted by trial categories. The 12 trial types (see the text) were analyzed in 7

categories created according to the different effects included in the model. ANY presents the average performance in the ANB and

ANC trials, AAY the AAB and AAC trials, AYA the ABA and ACA trials, AYY the ABB and ACC trials, and AYZ the ABC and

ACB trials. Bottom panel: Contribution of the modeled effects (in hit percentage) to the performance in the two experiments. Note

that the interference factor decreases performance (all other factors, when their value is positive, increase performance).

Experimental Psychology 2005; Vol. 52(1):3Ð20 ” 2005 Hogrefe & Huber Publishers

13I. Winkler & N. Cowan: From Sensory to Long-Term Memory

ference in the strategy factor was marginally signifi-

cant: F(1, 44) = 3.80, p = 0.0575; one-way between-

group ANOVA). In the 6-Standard but not in the 1-

Standard Experiment, reactivation was positively

correlated with memory (Spearman rank correlation

0.44, p ⬍ 0.05), whereas strategy was negatively

correlated with memory (-0.43, p ⬍ 0.05). These re-

sults suggest that reactivation occurs also when sub-

jects actively maintain auditory sensory information,

although its effect is relatively small as long as the

residual memory is still sufficiently strong (i.e., in the

present situation, over 60 % of the performance can

be attributed to residual memory, whereas only ca.

10 % to reactivation).

3. General Discussion:

A Reevaluation of Sensory Memory

3.1. What Kind of Memory Representations

Are Reactivated?

MMN elicitation is based on a memory representation

describing auditory regularities, not just auditory sen-

sory memory traces. This has been shown by the re-

sults of those studies that tested violations of various

nonrepetitive regularities (for a review, see Näätänen

et al., 2001). However, these memory representations

are not independent of the concrete sound feature

levels even in the most “abstract” cases. For example,

Paavilainen, Simola, Jaramillo, Näätänen, and Win-

kler (2001) presented subjects with tones varying in

frequency and intensity. Most tones conformed to a

rule, which was, in separate stimulus blocks, either

“the higher the frequency the higher the intensity” or

“the higher the frequency the lower the intensity”. In-

frequent tones violating the standard rule (high-fre-

quency soft and low-frequency loud tones or high-fre-

quency loud and low-frequency soft tones, depending

on the rule) elicited MMN. This result suggests that

the memory representation involved in MMN genera-

tion encoded the abstract feature-conjunction regular-

ity. One aspect of the results, however, suggested that

the representation of the regularity was not fully inde-

pendent of the actual levels of the relevant auditory

features. The MMN amplitude was somewhat higher

in response to deviants that fell farther from the center

of the feature distribution of the regular sounds.

(However, no regular tone elicited the MMN, not even

the ones with extreme levels in both features.) The

MMN-amplitude differences suggest that the standard

tones of medium frequency and medium intensity

were regarded as a prototype within the abstract regu-

larity.

” 2005 Hogrefe & Huber Publishers Experimental Psychology 2005; Vol. 52(1):3Ð20

The picture emerging from the MMN literature is

that the representations of the stimulation must in-

clude not only regularities, but also certain feature val-

ues that serve as reference points or anchors. An an-

chor point would allow an abstract rule or regularity

to be translated into specific feature values. That way,

the same regularity could be associated with different

anchors in different stimulus situations, allowing for

the efficient storage of information and the possibility

to maintain alternative descriptions of the same se-

quence of sounds. Simultaneous representation of

multiple redundant regularities describing the same

sound sequence has been demonstrated for MMN gen-

eration (Horva

th et al., 2001). Horva

th and his col-

leagues presented a regular sequence of tones alternat-

ing in pitch (ABABAB . . ., where A and B are two

tones differing only in pitch). The presence of repre-

sentations for different redundant regularities was

tested by presenting deviants that violated one pos-

sible description (rule) while conforming to a different

description of pitch alternation. It was found that

memory representations of at least one “local” rule (A

tone is always followed by B and vice verse) and one

“global” rule (every second tone is A and every other

is B) are simultaneously maintained and incoming

sounds are checked against them in parallel. Further

results suggested that possibly also representations of

more general versions of alternation are kept active at

the same time (rules, such as Higher toneÐLower

toneÐHigher toneÐLower tone . . . and alternation

with variable interstimulus intervals). If, as was shown

by Horva

th et al. (2001), the auditory system main-

tains multiple redundant representations even for sim-

ple acoustic regularities, concise abstract regularity

descriptions actualized with a minimal amount of sen-

sory data offer an economical form of information

storage.

The hypothesized structure of regularity represen-

tations suggests that we can reactivate sensory infor-

mation that was encountered within a context with dis-

tinctive regular characteristics. A stimulus that con-

forms these regularities serves as a reminder, activa-

ting the regularity representations and thus bringing

the corresponding context to immediate memory. Re-

activation of the characteristic features of an object

could serve as the basis of its recognition. Features of

the reactivation process are in perfect correspondence

with everyday experience of object recognition. We

can easily recognize objects appearing in their usual

context. The more experience we have with the given

object and the more distinctive its sensory features the

more likely that we can recognize it.

The characteristics of the memory representations

as shown by the reviewed reactivation experiments,

14 I. Winkler & N. Cowan: From Sensory to Long-Term Memory

i.e., resistance to decay and interference, encoding of

higher-order regularities extracted from acoustical

variance, are compatible with the assumed characteris-

tics of long-term memory records. Thus it seems pos-

sible that in the auditory modality, the regularity rep-

resentations indexed by the MMN component form

the link between immediate sensory memory and

long-term storage of sensory information.

3.2. What Distinguishes Auditory Sensory

Memory from Categorical Memory?

The MMN and new behavioral results discussed above

help to establish several points about memory for

acoustic stimuli. First, the regularities of the acoustic

pattern, including grouping and organization of acous-

tic information, are important for memory. Second,

aspects of these regularities can be stored in long-term

memory and reactivated later. Given these results, sev-

eral issues arise with respect to behavioral research on

memory and the models that have been based on it.

First, does it still make sense to think of this informa-

tion as especially “sensory,” as opposed to categorical,

in nature? Second, what types of overall models of

processing can represent all of the results?

In the following we shall re-examine the evidence

supporting the distinction of sensory memory traces

form categorical memory storage for each of the four

critical qualities mentioned in the introduction:

(1) sensory memory does not depend on attention,

(2) it is modality-specific, (3) it includes information

that is finer than any meaningful set of categories, and

(4) it is short-lived. How do these qualities apply to

the retention of acoustic regularities?

3.2.1. Attention-Independence

A traditional objection to the attention-independence

of sensory processing comes from findings showing

attentional modulation of neural signals associated

with early afferent processing. Results showing that

mid-latency ERP components, such as the auditory

P50 wave are attenuated when attention is directed

away from the stimuli have been interpreted in terms

of “sensory gating” (e.g., Guterman, Josiassen, & Ba-

shore, 1992). It has been suggested that the afferent

flow of sensory information is under control from the

prefrontal cortex, a structure linked with the voluntary

direction of behavior (Knight, Staines, Swick, &

Chao, 1999). The notion of sensory gating may be

compatible with the attentional spotlight theory. Sup-

Experimental Psychology 2005; Vol. 52(1):3Ð20 ” 2005 Hogrefe & Huber Publishers

porting results for the auditory modality have been

obtained in experiments investigating the detection of

auditory targets as a function of the target’s distance

from a precued (expected) location. A graded

decrease of accuracy and slower reaction times have

been found with increased distance between the target

and the precued location (Arbogast & Kidd, 2000;

Mondor & Zatorre, 1995; Rorden & Driver, 2001).

Comparable amplitude gradients have been found

both for some of the early, exogenous (N1) as well

as for later, endogenous (MMN, P3) ERP responses

(Arnott & Alain, 2002; Teder-Salejärvi, Hillyard,

Roder, & Neville, 1999).

Effects in sound organization provide another line

of argument against the attention-independence of au-

ditory sensory memory. Bregman (1990) suggested

that several heuristic processes analyze the auditory

input in parallel. These processes provide alternative

solutions to breaking down the complex acoustic input

into coherent sequences of sound. When two strong

alternatives of sound organization emerge from the

initial analysis (ambiguous auditory scenes), one can

voluntarily choose between them. Since the auditory

information stored about sounds depend on how they

are organized (Dowling, 1973), ambiguous auditory

scenes allow voluntary modulation of auditory sen-

sory memory. In a similar vein, Sussman et al. (2002)

found that informing subjects about the large-scale

structure of a sound sequence determined what sounds

are detected as deviants in a sound sequence. Sussman

et al. presented a tone sequence composed of a repeat-

ing tone pattern (AAAABAAAAB . . .; where B was

higher in pitch than A). Tones were presented at an

intermediate (700ms) SOA, because previous studies

showed that at shorter SOAs (100ms), the sequence is

unambiguously represented as a repeating tone pattern

(Sussman, Ritter, & Vaughan, 1998), whereas at

longer SOAs the large-scale regularity is not automati-

cally detected (Scherg, Vajsar, & Picton, 1989). Of the

A tones, 2.5 % were exchanged for a lower-pitched (C)

tone. When subjects were instructed to press a re-

sponse button for the rare low-pitched C tones MMN

was elicited by the B tones, which were relatively rare

(appeared in 20 % of the time) compared with the A

tones. In contrast, when subjects were informed about

the regularly repeating tone pattern and were in-

structed to press the response button whenever the

pattern was broken (that is, to the same C tones), the

B tones did not elicit MMN. The lack of MMN by the

B tones indicates that the tone sequence was repre-

sented in terms of the repeating tone pattern. In this

case, the B tone is part of the repeating pattern and,

therefore, it does not violate a regularity. Thus it ap-

pears that, at least in certain cases, auditory memory

15I. Winkler & N. Cowan: From Sensory to Long-Term Memory

can be influenced by top-down processes. Moreover,

some authors suggest that even the unambiguous cases

of sound organization require attention (e.g., Carlyon,

Cusack, Foxton, & Robertson, 2001), although this is-

sue is still controversial (see, e.g., Winkler et al.,

2003).

Taken together, this literature does call into ques-

tion whether complex acoustic patterns are held in a

way that is independent of attention; but it also calls

into question the more fundamental assumption that

sensory memory is held in a manner that is independ-

ent of attention. That never has been rigorously exam-

ined, and the aforementioned study by Keller et al.

(1995) did find that attention had an effect on memory

for tone frequencies necessary for comparisons within

a musical category.

3.2.2. Modality-Specificity

Regarding the second criterion of sensory memory, its

modality-specificity, here the results seem unequivo-

cal. Frankish (1985) and Cowan, Saults, Elliott, and

Moreno (2002) compared memory for grouped and

ungrouped lists of spoken or printed verbal items. The

result in both studies was that grouping has a much

larger, beneficial effect in the auditory modality. In

fact, Cowan et al. found that, with visual pre-

sentations, grouping could be accomplished just as

well on trials in which the stimuli themselves were

presented at a steady pace as on trials in which the

stimuli were temporally grouped into 3 clearly distinct

sets of 3 digits. Both presentation schedules produced

serial recall results for the 9-digit lists that were

slightly scalloped in a manner, suggesting that the 9

digits had been mentally grouped accordingly. (The

stimulus grouping on some trials apparently induced

a similar mental grouping on the remaining trials with

visual presentation.) In contrast, with auditory pre-

sentation of digits, the outcome for grouped lists was

far superior to the outcome for lists of digits presented

at a steady pace. It is as if the acoustic modality, un-

like vision, inherently carries timing information that

cannot be ignored within the mental representation.

In accordance with this notion, Näätänen & Winkler

(1999) suggested that the time-line of acoustic stimu-

lation serves as the core for integrating the outcome

of the various auditory feature analyzers into a unitary

representation of the auditory stimulus: “We suggest

that the critical step between the stage of fragmentary

stimulus information (maintained in the feature traces)

and the emergence of the auditory stimulus represen-

tation is the synthesis of the static stimulus features

with the temporal envelope of the stimulus event. An

” 2005 Hogrefe & Huber Publishers Experimental Psychology 2005; Vol. 52(1):3Ð20

auditory stimulus cannot be fully described by static

features alone. Therefore, in the formation of an audi-

tory stimulus representation, the extracted features

must be aligned with the passage of time.” (Näätä-

nen & Winkler, 1999, p. 848).

3.2.3. Resolution

The third traditional hallmark of sensory information

is that it is more fine-grained than categorical infor-

mation. The question is whether this fine-grained in-

formation is present within the same memory repre-

sentation that stores extended acoustic patterns. Evi-

dence to that effect is that the ability to detect a differ-

ence between two tones differing less than a musical

note category is affected by the pattern of prior tone

presentations. Cowan, Saults, and Nugent (1997) var-

ied not only the time between tones to be compared,

but also the time between trials. It was found that

when trials were further apart, performance improved.

One way to explain that result is that, when trials are

too close together, the first tone in the current pair

may be grouped in memory with tones from the pre-

ceding trial. So, it appears that there is no such thing

as a simple, pure acoustic memory for a single tone

separate from the pattern of tones that has formed.

3.2.4. Duration

The fourth typical characteristic of sensory memory

was its short life. However, we already have discussed

evidence that there is memory for acoustic properties

that lasts a long time, such as information about a

friend’s voice. There is abundant evidence for long-

term auditory memory that allows a recall advantage

over visual presentation, even after an intervening dis-

tracting task, provided that the items to be recalled

also are separated by other items. Research on long-

term modality effects recently was reviewed by Gardi-

ner and Cowan (2003). Also, Cowan, Saults, and Nu-

gent (2001) reanalyzed the evidence on tone compari-

sons by Cowan et al. (1997) and found that, under

certain circumstances in which the separation between

trials was the largest, it was unclear if there was any

loss of information at all as a function of the time

between tones to be compared. In other words, the

“decay” of auditory sensory information across sev-

eral seconds may actually be a matter of the tone be-

coming more and more confusable with tones from

previous trials, as opposed to an actual loss of sensory

persistence as has typically been assumed. However,

this dramatic conclusion awaits further testing.

16 I. Winkler & N. Cowan: From Sensory to Long-Term Memory

Although these findings suggest that sensory infor-

mation lasts in memory rather than decaying rapidly,

there are some apparent contradictory findings. Some

studies have shown that the final serial position of

acoustically presented lists carries information that is

especially vivid and may be represented differently

from items presented in any other position of the list.

There has been some debate on this point (cf. Ba-

lota & Engle, 1981; Bloom & Watkins, 1999). How-

ever, Cowan, Nugent, Elliott, and Saults (2000) found

a clear distinction of the final serial position in a de-

velopmental study of memory for digit lists that were

ignored during their presentation while a silent game

involving rhyming pictures was played. Occasionally,

the rhyming game was interrupted and the computer

keyboard was to be used to recall the most recent spo-

ken list. The delay between the end of this last list and

the recall cue was 1, 5, or 10 s. For most serial posi-

tions, children of two ages and adults all forgot the

items at the same rate across the variable retention

interval. For the final serial position, though, the result

was strikingly different than for any other serial posi-

tion. Young children (in second grade) forgot this final

item much more rapidly than subjects in the older

groups.

There is another finding suggesting that there may

be a special form or function of memory that is spe-

cific to the final item (although see Bloom & Watkins,

1999 for evidence that it is not quite that specific). It

has been obtained in studies of the suffix effect, in

which a final list item that is not to be recalled (e.g.,

the word “go”) interferes with memory for items at

the end of the list (Crowder & Morton, 1969; Morton,

Crowder, & Prussin, 1971). Balota and Ducek (1986)

presented a suffix immediately after a spoken list or

after a 20-second delay and found that there was a

suffix effect in either case, but that the similarity be-

tween the voice of the speaker of the list and suffix

mattered only if the suffix was presented immediately.

This again suggests that there is some specific acous-

tic information that is very fragile and some broader

pattern of information that is more durable. It is un-

clear if the specific acoustic information disappears

as a function of absolute time or, more in keeping

with other evidence we have presented, disappears as

a function of the shifting context; the specific acoustic

information apparently would lose its contextual rele-

vance especially quickly over time.

3.3. The “Regularity-Record Plus Anchor”

Hypothesis

The theoretical need, then, is to tie together findings

suggesting that acoustic information is retained for a

Experimental Psychology 2005; Vol. 52(1):3Ð20 ” 2005 Hogrefe & Huber Publishers

long time and the findings suggesting that the final

item may be of special importance. Information on

long-term retention includes, for example, the mis-

match negativity results showing that patterns are

taken into account and that rapid decay does not de-

scribe performance well (e.g., Winkler et al., 2001),

and behavioral results on long-term modality effects

(e.g., Gardiner & Cowan, 2003) and sensory memory

stability (Cowan et al., 2001). Information on the spe-

cial importance of the most recent item includes the

role of this item as an anchor, or as a reminding or

reactivating stimulus, in mismatch negativity pro-

cedures (Cowan et al., 1993; Korzyukov et al., 2003;

Winkler et al., 1996) and, behaviorally, the develop-

mental difference in memory decay for the list-final

item only (Cowan et al., 2000) and the tendency of

modality and suffix effects to be largest at the end of

the list.

One way to combine these factors theoretically is

with the notion that acoustic memory is a record of

regularities, and that the last item serves as an anchor

for this record. As an analogy, every member of an

orchestra has a mental record of the music that is to

be played, but still needs a common pitch to anchor

the memory and tune the instrument before the musi-

cal piece is played. To carry the analogy further, the

melody can be held in long-term memory but the an-

choring note must be played on the spot for it to be

of any use (except for musicians with the absolute

pitch ability). If an intruder came into the room during

tuning and played a deviant note, the process would

be corrupted and the correct anchor probably would

have to be repeated. That is analogous to the use of a

reactivating reminder in the mismatch negativity pro-

cedure and to the corrupting effect of a suffix in list-

recall procedures.

Characteristics of the suffix effect are compatible

with the above-suggested structure of auditory mem-

ory (i.e., regularity plus anchor). Most studies of the

suffix effect show that it can occur even when the

suffix remains the same from trial to trial. However,

the situation is different when list items are separated

by distracting tasks, making them temporally very dis-

tinct and producing long-term modality effects. Under

that situation, a suffix effect eliminating the auditory

modality advantage occurs, but only if a different suf-

fix is used on every trial (Glenberg, 1984). In the

short term, the most recently presented acoustic item

may have a special vividness that is susceptible to in-

terference from any other sound. In the long term,

though, the most recent item may carry acoustic infor-

mation that reminds the subject of the rest of the

acoustically presented list. When that item was a suf-

fix that was not to be recalled, it may detract from the

17I. Winkler & N. Cowan: From Sensory to Long-Term Memory

power of the list-final item to serve as a unique anchor

for the list.

3.4. Considering some Current

Memory Models

Last, we can ask the question of what established

models can or cannot handle these results. The fore-

most model of temporary memory, currently, is the

working-memory model of Baddeley (1986). That

model postulates the existence of two buffer stores, a

phonological buffer and a visuospatial buffer. Re-

cently, a short-term episodic buffer also has been pos-

tulated (Baddeley, 2000). However, in our view, none

of these buffers make sufficient accommodation for

specifically sensory information. The phonological

buffer cannot be used to explain the auditory modality

superiority effect. To explain it, it seems necessary to

postulate the existence of sensory memory stores in

addition to the more abstract, phonological, and spa-

tial representations that are highlighted in Baddeley’s

model.

In the similar but alternative model of Cowan

(1988, 1995, 1999), sensory memory and categorical

memory both are considered to reflect activated sub-

sets of the long-term memory system (though new

links are formed between items that are attended con-

currently, and these new links then become part of the

long-term memory system). Results showing fast

task-independent effects of information stored in

long-term memory records on auditory change detec-

tion are fully compatible with Cowan’s model. For ex-

ample, phonetic category boundaries and prototypes

have been shown to affect the MMN response elicited

in ignored sequences of speech stimuli (Aaltonen,

Eerola, Hellstrom, Usipaikka, & Lang, 1997;

Näätänen et al., 1997; Winkler et al., 1999). In Cow-

an’s model, the modality-specific effects are handled

by assuming that there will be interference between

the activated representation of a recently presented

item and a new stimulus when the two share similar

features. Cowan’s (1988, 1995, 1999) model does not

specifically explain why the auditory modality superi-

ority effect occurs; nor does it specify the nature of

memory for regularities in the auditory memory sys-

tem. Yet, it seems more open to the possibility of sepa-

rate acoustic and phonological information sources

than does Baddeley’s (1986, 2000) model.

Given the way in which the expectations of the

earlier models of processing have failed, it is perhaps

not surprising to learn that some researchers have pro-

moted models in which there is no distinction at all

between shorter-term and longer-term memory (e.g.,

” 2005 Hogrefe & Huber Publishers Experimental Psychology 2005; Vol. 52(1):3Ð20

Nairne, 2002). Such models are well suited to accom-

modate the finding that memory for acoustic regulari-

ties is long lasting, and to accommodate similarities

between the results of ostensibly short-term and long-

term memory phenomena. However, such models do

not yet appear to have a ready explanation for differ-

ences that are observed between shorter-term and

longer-term memory phenomena. One issue here is

what we mean by the term “reactivation.” It carries

with it the assumption that there is such a thing as

activation. If there is no distinct form of memory in

the short term, then there is no such thing as tempo-

rary memory activation. Instead, what we term “acti-

vated” would actually be “relevant to the present

context.” In that case, reactivation effects would have

to be more accurately portrayed as reminder effects.

Ultimately, new modeling efforts will be needed to

explain precisely how it is that regularities can be

stored in acoustic memory, and to determine whether

some minimal amount of attention is needed to assist

in the formation of the memory for regularities that

is stored and later can be reactivated by a reminder

stimulus.

Acknowledgement

Due to space constraints this article has been pub-

lished outside of the special issue “Working Memory

and Cognition” (Issue 4, 2004) for which it was origi-

nally accepted.

This research was supported by the Hungarian Na-

tional Research fund grant OTKA T034112 and U.S.

National Institutes of Health grant HD-21338. We

thank Lı

via Pato

for conducting the behavioral reacti-

vation experiments.

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20 I. Winkler & N. Cowan: From Sensory to Long-Term Memory

Received October 15, 2003

Revision received December 5, 2003

Accepted December 15, 2003

Adress for correspondence

Istva

n Winkler

Institute for Psychology

Hungarian Academy of Sciences

P.O. Box 398

H-1394 Budapest

Hungary

Tel. +36 1 354 2296

Fax +36 1 354 2416

E-mail winkler@cogpsyphy.hu

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