Event-related potentials to repetition and change of auditory stimuli

306 Eleetroencephalography and clinical Neurophysiology, 83 (1992) 306-321 © 1992 Elsevier Scientific Publishers Ireland, Ltd. 0013-4649/92/$05.00

EEG 91196

E v e n t - r e l a t e d p o t e n t i a l s to r e p e t i t i o n a n d c h a n g e o f a u d i t o r y s t i m u l i *

Walter Ritter a, Petri Paavilainen b, Juha Lavikainen b, Kalevi Reinikainen b, Kimmo Alho b, Mikko Sams c and Risto N~i~ifiinen b

Department of Neuroscience, Albert Einstein College of Medicine and Department of Psychology, Lehman College, City Unit,ersity of New York, Bronx, NY (U.S.A.), b Department of Psychology, Unicersity of Helsinki, Helsinki (Finland),

and '" Low Temperature Laboratory, Helsinki Unit:ersity of Technology, Espoo (Finland)

(Accepted for publication: 23 June 1992)

Summary The major intent of this study was to compare the role of st imulus repetition and change in the elicitation of the MMN, an ERP component specific to st imulus change, and N2b, usually partially overlapping the MMN when stimuli are attended. Event-related potentials were recorded in one set of conditions where subjects ignored the stimuli and read a book, and in another set of conditions where subjects counted stimuli designated as targets. Stimuli were delivered in 4 ways, the common feature between all these conditions being the occurrence of infrequent events at a probability of 0.20: (1) an oddball paradigm with 1 deviant, (2) an oddball paradigm with 2 deviants, each with a probability of 0.10, (3) a regular alternation of tones of 2 pitches where either of the 2 tones infrequently repeated (P = 0.20), and (4) a random presentation of tones of 5 different pitches, where any of the 5 tones infrequently repeated (P = 0.20). In the count conditions, the infrequent events were designated as targets. It was found that the M M N was elicited by stimulus change and not st imulus repetition in the ignore and count conditions, whereas the N2b was elicited by both st imulus changes and repetitions in the count conditions. It was also possible, in the count conditions, to disentangle the part of the late positive complex which is related to st imulus deviation and the part which is related to st imulus significance (target).

Key words: Auditory evoked potentials; Mismatch negativity; N2b; P3a; P3b

The N2 wave of the event-related potential (ERP) elicited by auditory stimuli has been divided into 2 components, the mismatch negativity (MMN) and N2b (N~i~it~inen et al. 1978, 1982). The shorter latency component, the MMN, is usually obtained by presenting two tones that differ along some dimension in an oddball paradigm, with one of the tones occurring with high probability (the standard) and the other occurring with low probability (the deviant). The MMN can be obtained both when subjects attend to the tones to perform a task and when subjects ignore the stimuli while reading a book (N~i~ifiinen et al. 1982; Sams et al. 1983). The MMN is elicited by the deviant and delin- eated by subtracting the ERPs elicited by the standard from the ERPs elicited by the deviant. However, when local sequences of stimuli within a run are examined, the standard as well as the deviant elicit the MMN, depending on the immediately preceding sequence of stimuli (Sams et al. 1983). When the immediately pre-

Correspondence to: Dr. Walter Ritter, Dept. of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461 (U.S.A.).

* This work was supported in part by U.S. Public Health Service Grant NS30029, the Academy of Finland, and the V~iin6 Tanner Foundat ion (Helsinki, Finland).

ceding stimuli are different than the current stimulus, the MMN is elicited, whereas when the immediately preceding stimuli are the same as the current stimulus, the MMN is not elicited. The reason why the deviant is associated with a larger MMN than the standard, when the ERPs are averaged across an entire run, is because the deviant stimuli are usually preceded by different stimuli (i.e., the standards) whereas the standards are usually preceded by other standards (i.e., identical stimuli). In general, then, the MMN is obtained when the eliciting stimulus is different than the preceding stimuli. The MMN can also be obtained using two equiprobable stimuli (Sams et al. 1983); as just reported for the oddball paradigm, both stimuli elicit the MMN, depending on the immediately preceding sequence of stimuli. Thus, whatever the overall probability of the stimuli, the MMN is elicited when a stimulus is different from the immediately preceding stimuli.

The MMN appears to be based on a memory that is accessed very early in the processing of stimuli. It has been hypothesized that a neuronal model of the physical features of repetitive stimuli is generated (N~i~it~inen 1985). Subsequent stimuli that are different than the model elicit the MMN, whereas stimuli that match the modei do not. Put in other terms, the MMN is the outcome of a mismatch operation carried out by a

ERPs TO REPETITION AND Ct lANGE OF AUDITORY STIMULI 307

comparator mechanism. Since the MMN can usually be elicited whether subjects attend the stimuli or not, it can be thought of as registering stimulus change. Whereas the MMN is associated with stimulus change, it is not associated with violations of expectation concerning which stimulus will be delivered next. Scherg et al. (1989) have shown that the MMN is elicited by a low probability stimulus in a completely predictable sequence (4 tones in a row of one pitch were regularly alternated with one tone of another pitch), and that the amplitude and latency of this MMN was very similar to that of the MMN obtained for the same tones in an unpredictable sequence.

Nordby et al. (1988), however, reported that infrequent repetitions in a sequence of regularly alternating tones of two pitches el ici ted an MMN. The results were not interpreted as indicating that the MMN is associated with match operations, but rather that the conception of the neuronal model described above requires broadening, such that it encodes the temporal ordering of pitch. In this view, the MMN can be elicited by mismatch operations based on a deviation in the temporal sequence of stimuli. One of the intents of the present study was to attempt to replicate Nordby et al. (1988).

The longer latency component of the N2 wave, N2b, is also usually obtained by presenting two stimuli in an oddball paradigm. By contrast with the MMN, N2b ordinarily is only elicited when the stimuli are attended. Another way in which these two components differ is their scalp distribution. The MMN is generally largest at Fz and inverts in polarity at the mastoid and often also in posterior recordings when the nose is used as reference (Alho et al. 1986; Novak et al. 1990; Paavilainen et al. 199l). The N2b, on the other hand, is usually largest at Cz and does not invert in polarity across the scalp (Novak et al. 1990; Sams et al. 1990). However, the MMN and N2b are alike in several ways. Both components are inversely related in amplitude to stimulus probability and longer in latency as the difference between the stimuli is reduced (N~i~t~inen and Gaillard 1983). Perhaps the most important way in which the two components are alike, for the purposes of the present study, is that they behave in a similar manner when local probabilities are examined. Both the MMN and N2b are obtained when stimulus change Occurs .

In contrast to the MMN, N2b has been hypothesized to be related to expectation (Ritter et al. 1984). As with P3b (P300), N2b is generally only elicited when stimuli are presented in an unpredictable manner. But in virtually all studies to date, N2b has been elicited by stimulus sequences similar to those used to elicit the MMN. Thus, it could be that N2b is elicited only when the eliciting stimulus is different than the preceding stimuli and, therefore, is also associated with a mis-

match operation. In this view, the MMN and N2b could be considered to reflect registration and detec- tion, respectively, of stimulus change.

Most studies of N2b have used the active oddball paradigm. In this arrangement, stimulus change and low probability are confounded. Consequently, it is not possible to determine to what degree the enhanced N2b associated with the deviant stimulus is due to stimulus change or to a violation of expectation.

P3b has also been largely studied using the active oddball paradigm and behaves like the MMN and N2b when local stimulus sequences are examined (Squires et al. 1975). However, P3b is clearly related to expectation rather than only stimulus change. Friedman et al. (1978) presented the numbers 02 to 19 on a visual display in a random manner and, also randomly, on 20% of the trials any of the numbers could repeat. The subject's task was to extend the wrist whenever any of the numbers repeated. The P3b associated with the targets was significantly larger than for the non-targets. Since the targets were stimulus repetitions the P3b cannot be associated only with stimulus changes.

In order to unconfound probability and stimulus change in an N2b study, Breton et al. (1988) altered the oddball arrangement by regularly alternating two visual stimuli and infrequently repeating either stimulus. Subjects were instructed to respond to the infrequent repetitions. Hence, instead of the target being an infrequent change, as in the usual oddball task, targets were stimuli that infrequently did n o t change from one trial to the next. It was found that N2b was elicited by the infrequent repetitions, but it tended to be smaller than when the same two stimuli were presented in the customary oddball arrangement.

The latter data suggest that N2b can be elicited by stimulus matches in addition to stimulus mismatches. However, there is more than one interpretation of the results. On the one hand, N2b could indeed have been associated with a match operation. On this view, subjects could have used the physical characteristics of each stimulus as it occurred to form a template (i.e., representation) against which the next stimulus would be compared, and when the next stimulus matched the template an N2b was elicited. But it is possible that the subjects used the template based on the stimulus that preceded the one presented on the current trial in anticipation of the next stimulus. This then would have resulted in a mismatch for the infrequent repetition, which in turn would have elicited N2b. In other words, N2b could have been associated with a mismatch operation.

It is not known which strategy the subjects used in Breton et al. (1988). That subjects can voluntarily choose which stimulus a template for N2b is based on has been suggested by Sams et al. (1983). For example, in an oddball arrangement in which a target occurs

308 w. RITI'ER ET AL.

infrequently, subjects will form a template of the frequent, non-target stimulus because its frequent repetition will help support the maintenance of an adequate template. In this case, the targets will be identified via a mismatch with the template and elicit N2b. Hence, the deviant will have a larger N2b than the standard. On the other hand, if targets and non-targets are equiprobable, then each can equally well support a template. In a study by Sams et al. (1983), which used two equiprobable stimuli, N2b was larger for the non- targets than the targets. They therefore suggested that the target was selected by the subjects for the template, thus leading to a larger N2b for the non-targets. Im- plicit in this interpretation is the notion that N2b is elicited when a stimulus does not match the template.

To test whether an N2b can also be elicited by a stimulus match when it is a rare event, a condition in which tones of 5 equiprobable pitches were randomly presented was included in the present study. In this condition, on a random 20% of the trials any of the 5 pitches could repeat. Targets were designated as the infrequent repetition of any pitch. The notion was that if these targets elicited N2b, then N2b can be associated with a match operation since the only plausible strategy subjects could have used to identify targets was to search for a match between the pitch of the current stimulus and that of the preceding stimulus.

Thus, the main purpose of the present study was to compare the role of stimulus repetition and change in the elicitation of the MMN and N2b. As already ex- plained, in one stimulus arrangement , two tones differ- ing in pitch were presented, a standard tone with a probabity of 80% and a deviant with a probability of 20%. Both ignore and attend (count deviants) conditions were used. On the basis of previous studies (e.g., N~i~it~inen et al. 1982), it was expected that in the ignore conditions only the MMN would be elicited, whereas both the MMN and N2b would be elicited in the attend conditions.

In another stimulus arrangement, the probability and magnitude of deviation was equivalent to the previous condition, but two different, equiprobable deviant tones were used instead of one. The purpose of this ar rangement was to study the effect of repetition versus change among deviant stimuli on the mismatch process (indexed by the MMN) elicited by the deviant tones. It is possible that the mismatch process elicited by the deviant stimuli in association with the trace of the standard stimulus is weakened by the concurrent match process associated with the deviant stimulus' own trace. In this case, if the deviant-stimulus trace is weaker (when there are two different deviant stimuli), the match process between the deviant stimulus and its trace would be weaker than when the deviant-stimulus trace is stronger (when there is one deviant stimulus). Thus, if there is a match process between the deviant

stimulus and its trace which reduces the strength of the parallel mismatch process between this deviant stimulus and the trace of the standard stimulus, one would expect a larger MMN when there are two deviants compared with only one deviant. This match process elicited by deviants might be one reason why an in- crease in the probability of deviant stimuli at tenuates the amplitude of the MMN (see Winkler et al. 1990, 1992).

Hence, the main idea of the present study was to compare the results obtained for the oddball stimulus arrangements where the infrequent event was a change from the previous stimulus with those from stimulus arrangements in which the infrequent event was a stimulus repetition. In one arrangement, the repetition occurred against a background of a regular change in stimuli, and in the other arrangement the repetition occurred against a background of irregular change in stimuli.

Methods

Subjects Ten subjects between the ages of 20 and 35 years of

age, consisting of laboratory personnel and paid uni- versity students, participated in the experiment. Three of the subjects were laboratory personnel and the remaining were students.

Experimental procedure Subjects sat in a comfortable chair in a sound-

damped, electrostatically shielded room with their eyes open. Tones produced by 60 msec bursts of sine wave (including 10 msec rise and fall times) were presented binaurally through earphones with an intensity of about 80 dB SPL. The stimulus blocks consisted of 500 tones presented at a constant rate of 1 every 710 msec. There were 4 different kinds of stimulus arrangements. For each arrangement, in one condition the subjects counted designated targets and in another condition they ignored the stimuli and read material of their own choosing.

Oddball (l deviant). Tones of 2 pitches were presented. A standard tone of 1000 Hz occurred on 80% of the trials and was randomly interspersed by a deviant tone of 1050 Hz on 20% of the trials. The deviant tone was designated the target in the count condition.

Oddball (2 deviants). Tones of 3 pitches were presented. A standard tone of 1000 Hz occurred on 80% of the trials, randomly interspersed on 10% of the trials by a deviant tone of 1050 Hz and 10% of the time by a deviant tone of 952 Hz. Thus, the overall probability and relative magnitude of stimulus deviation in the two conditions were equal, the only difference being the number of deviants employed. In the count condi-

ERPs TO REPETITION AND CHANGE OF AUDITORY STIMULI 3(19

tion, the subjects counted the total number of deviant stimuli.

Alternation. Two tones, one of 1000 Hz and the other of 1050 Hz, regularly alternated on 80% of the trials, randomly interspersed by a repetition of either of the tones on 20% of the trials. The repetitions were designated targets in the count condition.

Fit:e Tones. Tones of 5 pitches were presented (1075, 1025, 1000, 975, and 930 Hz); the mean relative magnitude of deviation (high/ low) being the same (5%) as in the other conditions. On 80% of the trials, the tones changed pitch equiprobably from that of the current trial to a pitch of one of the other 4 tones on the next trial. Randomly, on 20% of the trials, the pitch repeated from one trial to the next, repetitions occurring with equal probablity for all 5 tones. In the count condition, the repetitions were designated targets.

During every other block in the first half of the experiment, the subjects ignored the stimuli and during the remaining blocks they silently counted the tones designated as targets. The sequence of the stimulus arrangements was identical to the order of the descrip- tion above. After completing these 8 blocks, the subjects were presented with another 8 blocks, in the reverse order, beginning with a counting task and alternating between count and ignore conditions. After each count condition, the subjects reported the number of targets counted in that condition. There were between 91 and 102 targets in the various conditions.

The electrical activity (0.1-100 Hz, - 3 dB points) was recorded along the midline at Fpz, Fz, Cz, Pz and Oz. Additional lateral electrodes were placed at the left (L3) and right (R3) mastoids, along with electrodes one-third (LI and R1) and two-thirds (L2 and R2) of the distance from Fz to the left and right mastoids. The horizontal E O G was recorded with an electrode placed at the outer canthus of the right eye. All recordings were referenced to the nose. The E E G analysis period of 600 msec (sampling frequency 200 Hz) included a pre-stimulus interval of 50 msec to determine the baseline for the ERP amplitude measures. The computer automatically rejected E O G and E E G changes exceed- ing 150 /zV. After averaging, frequencies higher than 30 Hz were digitally filtered out from the ERPs.

Amplitude and latency measurement and statistical analysis

For each subject, ERPs from the two runs of each condition were combined. In order to establish the presence of the MMN and N2b, amplitude measures at Fz, Cz, Oz, El, R1, L3 and R3 were obtained in difference wave forms. For Oddball (1 deviant), the difference wave forms were obtained by subtracting the ERPs associated with the standard tone from the ERPs associated with the deviant tone. In Oddball (2 de-

viants), two difference wave forms were obtained by subtracting the ERPs associated with the standard tone from the ERPs associated with each of the deviant tones. For Alternation, the ERPs elicited by the alternating tones were subtracted from the combined ERPs elicited by the repetitions of each pitch. In Five Tones, the ERPs elicited by the frequent (80%) pitch changes were subtracted from the combined ERPs elicited by the repetitions of all 5 pitches.

In each ignore condition, the MMN was measured as the average amplitude during 90-190 msec from stimulus onset and t tests used to determine whether the mean value across subjects was significantly different than zero.

In the count conditions, the average amplitude of the N2b wave was measured in two latency windows: 90-150 msec (presumably mainly due to the MMN) and 180-230 msec (presumably due to N2b). For Five Tones, where the N2 wave had a longer latency, the latency windows were 180-240 and 240-300 msec. For each latency window of each count condition, a t test was used to determine whether the mean value across subjects was significantly different than zero.

In the conditions where both the MMN and N2b were determined to be present in the same condition by the preceding analysis, 2-way ANOVAs with repeated measures were used to compare their scalp distributions. The factors were component and electrode. Similar analyses compared the topography of N2b obtained in the following count conditions: Odd- ball (1 deviant) versus Five Tones, and Oddball (2 deviants) versus Five Tones. The factors were condition and electrode. In each case, two kinds of compar- isons were made: one based on the 5 midline sites and the other on the tilted coronal line (L3, L2, L1, Fz, R1, R2, R3). The amplitude values were scaled in the manner recommended by McCarthy and Wood (1985) and the Greenhouse-Geisser correction employed.

In the count conditions, N2b latency was measured as the peak negativity at Cz in the difference wave forms in the range of 100-500 msec.

Results

Performance The average percentage of counting errors for the 4

count conditions were: 3.6 for Oddball (1 deviant), 4.8 for Oddball (2 deviants), 11.6 for Alternation and 24.1 for Five Tones. Most of the errors in the latter two conditions consisted of reporting too few deviant stimuli. There was no clear trend for errors in either direction for the two oddball conditions. The performance mirrored the subjects' reports of the relative difficulty of the count conditions: both oddball conditions were relatively easy, the Five Tones condition was

310 W. RITTER ET AL.

very difficult, with the Alternation condition some- where in between.

ERPs of the ignore conditions Oddball (1 deviant). The upper rows of Figs. 1 and 2

present the grand mean ERPs elicited by the standard (thin lines) and the deviant (thick lines) tones in Odd- ball (1 deviant) for the midline and coronal lines, respectively. The ERPs associated with the deviant stimulus began to separate from the ERPs associated with the standard stimulus around 80 msec, becoming more negative until about 220 msec. The difference between the ERPs is illustrated in Fig. 3 (see the solid lines) which presents the subtraction of the standard ERPs from the deviant ERPs for the midline and

coronal lines, respectively. The MMN can be seen in the difference wave forms as a negative wave between roughly 80 and 220 msec, which peaks around 140 msec. It was largest at Fz, Cz, L1 and R1, and inverted in polarity at the mastoids (L3 and R3) and Oz. The polarity inversion of the MMN has been observed before (Alho et al. 1986; Scherg et al. 1989; Novak et al. 1990) and, in conjunction with magnetoencephalo- graphic recordings (Hari et al. 1984) and dipole-source analysis (Scherg et al. 1989), has been inferred to be caused by a source located in the vicinity of auditory cortex along the supratemporal plane of the temporal lobe.

Table I presents the results of the t tests employed to establish the presence of the MMN as a voltage

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STAHDARD DEUIAHT Fig. 1. Grand mean ERPs recorded along the midline (and horizontal EOG) while reading (upper panel) and while counting (lower panel) during the 4 kinds of stimulus sequences. OB 1 = Oddball (1 deviant); OB 2 = Oddball (2 deviants); ALT = Alternation; 5 T = Five Tones. The thin wave forms were associated with the standards and the thick wave forms with the deviants. In OB 2, the thick wave forms were associated with

the 1050 Hz deviant and the dotted wave forms with the 952 Hz deviant. S indicates stimulus onset.

ERPs TO REPETITION AND CHANGE OF AUDITORY STIMULI 311

significantly greater than zero in the difference wave forms at Fz, Cz, Oz, L3, L1, R1 and R3. The amplitude of the MMN was significantly greater than zero at Fz, Cz, L1 and R1, where it was negative in polarity, and at the mastoids (L3 and R3), where it was positive in polarity.

Oddball (2 det,iants). The ERPs and associated difference wave forms obtained in Oddball (2 deviants) essentially replicated the results of the Oddball (1 deviant) condition (see the second row of Figs. 1 and 2; for the difference wave forms, see the solid lines in Fig. 3). There was an MMN for both deviant stimuli that peaked around 140 msec and was largest at Fz, Cz, L1 and R1, and inverted in polarity at the mastoids. Table 1 shows that the amplitude of the MMN was significantly different from zero at Fz, Cz, L1, R1, L3 and R3 for the 1050 Hz deviant, and at all 7 sites analyzed for the 952 Hz deviant (being positive at Oz for both

deviants as well as at the mastoids). The MMN in the Oddball (2 deviants) condition was larger than that in the Oddball (1 deviant) condition, but the difference was not statistically significant.

Alternation. The Alternation condition exhibited very small differences between the ERPs elicited by the deviant and standand tones (see the third row of Figs. 1 and 2; for the difference wave forms, see the solid lines in Fig. 3). Along the midline, there was a slightly greater negativity for the deviant than the standard ERP, which was largest at Fz and peaked at about 170 msec in the difference wave forms presented in the third row of Fig. 3. Table I shows, however, that there was no significant negativity in the latency region of the MMN at Fz, Cz, Oz, L1 or R1. There was, however, a significant and marginally significant positivity at R3 and L3, respectively.

Fit,e Tones. The Five Tones condition was similar

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STFtbtI}RRI} I}EU IRbtT Fig. 2. Grand mean ERPs recorded along the coronal line. Conditions, wave forms and abbreviations as in Fig. 1,


to the Alternation condition in that both exhibited virtually none of the differences between the ERPs elicited by the standard and deviant tones described for the two oddball conditions (see the 4th row of Figs. 1 and 2; for the difference wave forms see the solid lines in Fig. 3). In Table l, there was no significant activity at any of the recording sites analyzed in the latency region of the MMN.

ERPs of the count conditions Oddball (1 deuiant). The fifth rows of Figs. 1 and 2

present the grand mean ERPs elicited by the standard and deviant stimuli at the midline and coronal recording sites for Oddball (deviant 1), respectively. As in the

reading condition for this stimulus arrangement, the ERPs elicited by the deviant tones began to become more negative than the ERPs elicited by the standards around 80 msec (the MMN), However, when the subjects counted the deviants, the MMN associated with the deviants was followed by a second negative wave (N2b) that peaked at approximately 210 msec. The N2b, in turn, was followed by a prominent positive wave that was largest at Pz. The positive wave peaked at Fz around 320 msec (P3a) and about 350 msec at Pz (P3b). Overlapping with the P3 complex and extending to the termination of the recording epoch was a slow positive wave that was largest at Pz and Oz, and a slow negative wave at Fpz, Fz, Cz, L1 and R1.

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RER.O I 1"16 .............. C O U b t T ] I'1@ Fig. 3. Upper panel: grand mean difference wave forms obtained along the midline (and horizontal EOG) by subtracting the ERPs elicited by the s tandards from the ERPs elicited by the deviants. In OB 2, only the wave forms derived from the subtraction which used the ERPs elicited by the 1050 Hz deviant are shown for the sake of clarity. The 952 Hz deviants elicited very similar potentials (see Figs. 1 and 2). Lower panel: grand mean difference wave forms obtained along the coronal line. Conditions and abbreviations as in Fig. 1. The solid lines are from the reading

condition and the dotted lines are from the counting condition.

E R P s T O R E P E T I T I O N A N D C H A N G E O F A U D I T O R Y S T I M U L I 313

The wave forms obtained by subtraction (Fig. 3, dotted lines) depict the differences between the ERPs elicited by the standard and deviant stimuli. The MMN was largest at Fz, L1 and R1 and can be seen to invert in polarity at the mastoids and Oz. The N2b was largest at Fz, Cz, L1 and R2 and, by contrast to the MMN, spread along the midline to Pz and Oz and along the tilted-coronal line to L2 and R2, and was small or absent at the mastoids. Table II presents the results of t tests used to establish the presence of the MMN and N2b at selected recording sites. The amplitude of the MMN was marginally significant at Fz and significant at R1, R2, k3, R3 and Oz (the latter 3 sites all positive in polarity). The amplitude of N2b was significantly different from zero at Fz, Cz, L1 and R1, and marginally significant at Oz.

The scalp distributions of the MMN and N2b along the midline and the tilted-coronal line obtained in the reading and count conditions are presented in the upper left panels of Figs. 4 and 5, respectively. Figs. 4 and 5 show that the MMNs obtained during the read and count conditions were very similar in distribution. The amplitude measures of the N2b were probably contaminated by a partial overlap with the MMN. Note that at Fz in the difference wave form for the Oddball (1 deviant) reading condition, for example, the MMN does not return to the baseline until about 220 msec (Fig. 3). If the MMN lasted about as long in the Oddball (1 deviant) count condition, then it would have extended into the latency region used to measure N2b (180-230 msec). This was unavoidable. In the latter condition, N2b peaked at about 200 msec at Fz. The likely overlap can be visualized by comparing the solid and dotted lines for the Fz recording in Fig. 3. Never- theless, when the amplitude data were scaled, 2-way ANOVAs employed to compare the scalp distributions of the MMN and N2b yielded a significant interaction between the factors of component and recording site for the midline sites (F (4/36) = 10.8, P < 0.01), and a marginally significant interaction for the tilted-coronal line (F (6 /54) = 2.7, P < 0.101. If anything, whatever overlap occurred between the MMN and N2b would have made it harder to obtain a significant interaction.

In order to bet ter delineate the N2b, Fig. 6 presents the difference wave forms obtained in the counting conditions at Fz, Cz and Pz (the thin lines) superim- posed on the difference wave forms obtained by subtracting from the thin lines the difference wave forms obtained in the reading condition (the thick lines). In other words, the difference wave forms obtained during the reading condition were subtracted from the difference wave forms obtained during the counting condition. The rationale behind this subtraction was the supposition that the MMN is equal in the two conditions and would therefore cancel, leaving a "pu re" N2b. Support for this supposition was obtained in

M E G studies (Kaukoranta et al. 1989; Lounasmaa et al. 1989), which showed that the MMN was equivalent in magnitude for a read and count condition when the same stimuli are used. As can be seen, the "pure" N2b obtained in Oddball (1 deviant) (the thick lines) is more posterior in its topography than for the thin lines, which include some overlap of the MMN with N2b. Were an A N O V A on scaled data done comparing the "pu re" N2b with the MMN obtained in the reading condition, an even more significant interaction between component and electrode would most likely have been obtained than that reported in the preceding paragraph.

Oddball (2 deciants). The ERPs and associated difference wave forms of this condition essentially replicated the results of the count Oddball (1 deviant)

T A B L E 1

R e a d i n g . M M N m e a s u r e d f r o m the d i f f e r ence wave fo rms (dev ian t -

s t a n d a r d ) as a m e a n a m p l i t u d e (izV) d u r i n g 90-1911 msec. S t a n d a r d

e r r o r s o f the m e a n s a re given in p a r e n t h e s e s .

E l e c t r o d e A m p l i t u d e t (9) P

O B 1 Fz - 1.56 (().38) 4.05 **

Cz - 1.10 (0.34) 3.24 *

O z 0.47 10.211 2.18 #

L3 1.27 (0.28) 4.57 * *

L1 - 1.62 (11.51/) 3.25 **

R1 - 1.43 (0.40) 3.60 **

R3 1.21 (/).25) 4.78 **

Fz -2 .1/1 (0.43) 4.64 **

Cz - 1.79 (1/.53) 3.40 **

O z 0.58 (0.50) 1.16 ns

(1050 Hz) L3 /).96 (0.29) 3.34 **

L1 - 1.95 (il.47) 4.14 **

RI - 2 . 1 5 (1/.551 3.89 **

R3 1.13 (0.37) 3.03 *

O B 2 Fz - 2 . 1 2 (0.49) 4.35 **

Cz - 1.60 (0.44) 3.62 **

O z 1.11 (0.42) 2.63 *

(952 Hz) L3 1.69 (11.411 4.09 **

LI - 2 . 0 5 (1t.43) 4.79 **

R I - 2.49 (I).47) 5.25 **

R3 1.79 (0.34) 5.21 **

A L T Fz 0.01 (0.36) 0.03 ns

Cz 1/,28 (0.36) 0.77 ns

Oz 0,62 (0.39) 1.59 ns

L3 0.76 (0.39) 1.92 #

LI - 0 , 1 4 (1/.38) 0.37 ns

R1 - 0 . 2 0 10.351 0.57 ns

R3 0.87 (0.27) 3.19 *

5 T Fz 0.46 (0.36) 1.26 ns

Cz 0.34 (0.33) 1.02 ns

O z - 0 . 0 1 (0.26) 11.05 ns

L3 0.01 (0.33) 0.03 ns

L1 0.17 10.38/ 0.45 ns

R I 0.40 (0.26) 1.54 ns

R3 - / ) . 2 8 (0.36) I).79 ns

# P < 0 . 1 0 ; * P < 0 . 0 5 ; * * P < 0 . 0 1 .

314 W. R I T T E R E T AL.

condition. The bottom panels of Figs. 1 and 2, and the dotted lines in Fig. 3, present the pertinent data. There was an M M N for both deviant stimuli that inverted in polarity at the mastoids from about 80 to 200 msec. N2b peaked around 200 msec and was followed by a prominent positive wave that was largest at Pz. The positive wave peaked at Fz at about 320 msec (P3a) and around 340 msec at Pz (P3b). The period from 400 msec to the end of the recording epoch was character- ized by a slow positive wave over the posterior region and a slow negative wave over the frontal region of the scalp.

The results of t tests to establish the presence of the MMN and N2b are presented in Table lI. In the latency region of the MMN, the difference wave forms of both deviant tones were associated with mean nega-

tivity across subjects at Fz, Cz, L1 and R1, and mean positivity at R3, all significantly different from zero. The positive activity at L3 for the 952 Hz deviant tone was also significantly different from zero. There was mean positive activity for both deviants at Oz, but only the activity for the 952 Hz deviant approached significance. For N2b, the difference wave forms of both deviants were associated with mean negativity across subjects that was significantly different from zero at Fz, L1 and R1. For the 952 Hz deviant, significant negativity was also obtained at Cz. Otherwise, the N2b in the difference wave forms was negative at all recording sites for both deviants (see Fig. 3), with the exception of a small, non-significant mean positivity across subjects at R3 for the 952 Hz deviant.

The scalp distributions of the MMN and N2b are

T A B L E 11

Counting. M M N and N2b measured from the difference wave forms (deviant - standard) as a mean amplitude (IzV) during the time windows given in the table. Standard errors of the means are given in parentheses .

El 9 0 - 1 5 0 msec ( M M N ) 181)-230 msec (N2b)

Ampl, t (9) P Ampl . t (9) P

OB 1 Fz - 1.15

Cz - 0.48

Oz 0.73

L3 0.94

LI - 1.58

R 1 - 1.49

R3 1.24

Fz - 1.72

Cz - 1.22

Oz 0.32

(105(1 t t z ) L3 0.49

El - 2 . 1 0

RI - 2 . 1 8

R3 1.04

OB 2 Fz - 1.85

Cz - 1.21

Oz /).41

(952 Hz) L3 1.21

El - 2 . 5 9

RI - 2 . 1 5

R3 1.20

A L T Fz - 11.42

Cz - 0.04

Oz 0.04

L3 0.24

LI - 0 . 2 4

R1 - 0 . 6 1

R3 0.16

5 T

(0.52) 2.211 # - 3 . 1 5 (0.63) 5.1tl **

0.48) 0.99 ns - 2 . 8 2 (0.80) 3.54 **

0.31) 2.30 * - 0 . 7 1 10.311 2.25

0.27) 2.51 * - 0 . 3 9 (0.30) 1.39 ns

0.63) 2.51 * - 3.86 (0.87) 4.43 **

0.56) 2.65 * - 3 . 5 9 (0 .7 l ) 5.115 **

0.25) 5.(10 ** 0.12 (0.23) [I.53 ns

0.36) 4.81 ** - 2 . 6 4 (0.61) 4.34 *

(0.38) 3.22 * - 1.97 (1.091 1.81 ns

(0.38) 0.83 ns - 0.75 ([).57) 1.32 ns

(0.57) 0.85 ns - 1.31 (0.63) 2.08

(0.62) 3.39 ** - 3 . 1 9 11.05) 3.03 *

(0.52) 4.22 ** - 3.25 (0.84) 3.87 **

(0.38) 2.77 * - 0.29 (0.52) 1/.55 ns

(0.31 ) 5.29 * * - 2.94 (0.54) 5.41 * *

(0.37) 3.22 * - 2.54 (0.77) 3.28 **

(0.71) 0.57 # - I ) . 97 (0.51) 1.91 #

(0.44) 2.78 * - 0.65 (0.77) /).84 ns

(0.56) 4.59 ** - 4 . 0 5 (0.75) 5.42 **

(0.25) 8.58 ** - 3 . 7 1 (0.72) 5.13 **

(0.46) 2.62 * 0.12 (0.58) 0.2l ns

(0.30) 1.41 ns - 1 . 5 1 10.51) 2.98 *

(0.38) 0.12 ns - 1.43 (0.611 2.36 *

(0.33) 0.14 ns - 0.74 (0.49) 1.50 ns

(0.38) 0.62 ns - [ I .81 (0.91) t).89 ns

(0.39) 0.63 ns - 2.11 (0.65) 3.23 *

(0.31) 1.95 # - 1.92 (0.57) 3.34 **

(0.26) 0.59 ns - 0.63 (0.79) 0.79 ns

180-240 msec ( M M N ) 240-300 msec (N2b)

Fz - 0 . 4 6 (0.37) 1.23 ns - 0 . 8 2 (0.36) 0.24 *

Cz - 0 . 2 0 (0.39) 1/.52 ns - 0 . 1 3 (0.52) 0.24 ns Oz 0.78 (0.32) 2.41 * I).64 (0.55) 1.17 ns

L3 0.48 (0.23) 2.10 # 0.11 10.51) 1/.22 ns

L1 - 0 . 8 3 (0.42) 1.96 # - I ) .93 (0.57) 1.63 ns

RI - 0 . 9 0 (0.48) 1.89 # - 1.05 (0.42) 2.51 *

R3 0.57 (0.47) 1.21 ns 0.33 (0.651 0.51 ns

# P < 0 . 1 0 ; * P < 0 . 0 5 : ** P < 0 . 0 1 .


-5

-4

-3

-2

-1

0

1

2

Oddball (1 Deviant)

Amplitude [pV]

Oddball

Amplitude [~V] -5

-4

-3

-2

-1

0

1

2

(2 Deviants)

, . - °

×

I I - - t I - - I - + - - - - I I - - - + - - - - - - - - + - -

Fpz Fz Cz Pz Oz Fpz Fz Cz Pz Oz

Electrode Electrode

Latency Window [ms]: Latency Window [ms]:

" 90-150 (C) " 180-230 (C) " 90-150 (C) " 180-230 (C)

. . . . 90-150 (R) ..... 90-150 (R)

Alternation Five Tones

-5

-4

-3

-2

-1

!t

Amplitude [pV] -5

-4

-3

-2

-1

0

1

2

Amplitude [IJV]

I I I - - I I - - - - 4 - I I - - I

Fpz Fz Cz Pz Oz Fpz Fz Cz Pz

Electrode Electrode

- _ _ q _ _

Oz


90-150 (C) • 180-230 (C) ---*-- 180-240 (C) " 240-300 (C)

" 90-150 (R) ..... 180-240 (R) Fig. 4. Scalp distribution (in /zVs) along the midline of the negative activity obtained during the reading and counting conditions in difference wave forms obtained by subtracting the ERPs elicited by the standards from the ERPs elicited by the deviants. The thin lines represent the activity in the counting condition (C) in an early latency window (MMN) and the thick lines represent the activity in a late latency window (N2b), indicated beneath each panel. The dotted lines represent the early activity in reading (R). Twice as many lines occur in the upper right panel

because there were two deviants in that condition.

316 W. R I T T E R ET AL.

depicted in the upper right panels of Figs. 4 and 5 for the midline and tilted-coronal line recording sites, respectively. As mentioned above for the count Oddball (1 deviant) condition, the measurement of the N2b

amplitude was probably contaminated by overlap with the MMN. For the ANOVAs that were used to compare the topographies of the MMN and N2b, there were no significant interactions between component

-5

-4

-3

-2

-1

0

1

2

Oddball (1 Deviant)

Amplitude [jaV]

I ~ - - - I - - - - - - + t I ~

L3 L2 L1 Fz R1 R2 R3

Electrode

-5

Oddball (2 Deviants)

Amplitude [luV]

-4

-3

-2

-1

0

1

2 '4-

- t I ~ - + - ~ - - - - - + ~

L3 L2 L1 Fz RI R2 R3

Electrode

Latency Window [ms]:

- " 90-150(C) " 180-230(C)

........ 90-150(R)

Latency Window [ms]:

90-150(C) • 180-230 (C)

90-150(R)


-5

-4

-3

-2

-1

0

1

2

Amplitude [pV]

. . . . . . . - - " " ' . . . o

I I I I I ~ - - - - - k - - -

L 3 L2 LI Fz R1 R2 R3

Electrode

-5

-4

-3

-2

-1

0

1

2

Amplitude [tuV]

................................... "%} - - I t I - ~ - I I

L3 L2 L1 Fz R1 R2 R3

Electrode


" 90-150 (C) " 180-230 (C) - ' 180-240 (C) " 240-300 (C)

" " 90-150(R) " " 180-240(R) Fig. 5. Scalp distribution of the negative activity obtained along the coronal line, as in Fig. 4.

ERPs TO REPETITION AND C H A N G E OF A U D I T O R Y STIMULI 317

Fz Cz Pz

oBl_@ OB2

ALT i r , ,

5 T V

deviant-standard (counting) [deviant-standard (counting)]- [deviant-standard (reading)]

Fig. 6. Grand mean difference wave forms obtained at Fz, Cz and Pz. The thin wave forms were derived by subtracting the ERPs elicited by the s tandards from the ERPs elicited by the deviants in the counting conditions. The thick wave forms were derived by subtracting the difference wave forms obtained during reading (ERPs elicited by s tandards subtracted from ERPs elicited by deviants) from the

thin wave forms.

and electrode for the midline recordings. However, for the coronal line, there was a significant interaction for the 1050 Hz deviant ( F (6 /54) = 4.4, P < 0.03), and an interaction for the 952 Hz deviant that just missed reaching significance ( F (6 /54) = 3.2, P < 0.056). Also, it can be seen that the MMNs obtained in the reading and counting conditions were distributed in a similar manner.

The second row of Fig. 6 presents the "pu re" N2b as described for the preceding condition. As can be seen, the "pu re " N2b (the thick lines) is again somewhat more posterior than for the thin lines, which contain overlap between the MMN and N2b.

Alternation. The bot tom panels of Figs. 1 and 2 present the grand mean ERPs elicited by the standard (i.e., regularly alternating) and deviant (i.e., infrequently repeating) stimuli for the midline and coronal line recording sites, respectively. In contrast to the count oddball conditions, the ERPs of the deviant stimuli did not separate from the ERPs of the standards until around 120 msec, at which point they were negatively displaced until approximately 280 msec. The latter displacement peaked at about 200 msec (N2b) and was followed by a slow positive wave that was largest at Pz and peaked in the vicinity of 400 msec (P3b). There was no apparent P3a. The difference

wave forms obtained by subtracting the ERPs elicited by the standards from the ERPs elicited by the deviants are presented in Fig. 3 (dotted lines).

Table I1 shows that there was no significant activity in the latency region of the MMN for any of the recording sites analyzed. At R1, the result approached significance. For N2b, significant negative activity was found at Fz, Cz, L1 and R1.

The lower left panels of Figs. 4 and 5 present the scalp distributions of the activity in the latency ranges of the MMN and N2b. No significant interactions on 2-way ANOVAs were found between the activity in the relevant latency windows and electrode.

The third row of Fig. 6 indicates that there was virtually no difference between the difference wave form used to obtain a "pu re" N2b (the thick lines) and the difference wave form which would have contained an overlap between the MMN, had it been elicited, and N2b (the thin lines). The latter result provides further support for the conclusion that the MMN was not obtained in this condition.

Fir, e Tones. The ERPs and associated difference wave forms of the Five Tones condition are displayed in the bottom panels of Figs. 1 and 2, and Fig. 3 (the dotted lines), respectively. As with the count Alterna- tion condition, there was little or no indication of an MMN or P3a. A small N2b, which peaked later than in the other count conditions, can be seen at Fz, L1 and R2. This was followed by a large positive wave that was largest at Pz (P3b).

The mean peak latency of N2b at Cz across subjects for the various count conditions was 195 msec for Oddball (1 deviant), 186 msec for the 1050 Hz deviant and 190 msec for the 952 Hz deviant in Oddball (2 deviants), 208 msec for Alternation, and 262 msec for Five Tones. Comparison of these latencies with a 2-way, repeated measures A N O V A (with factors of condition and subject) yielded a significant result (F ( 4 / 3 6 ) = 13.4, P < 0.01). Subsequent Newman-Keuls tests showed that the only significant latency difference was between the Five Tones condition and all the other conditions ( P < 0.01).

The results of t tests used to establish the presence of the MMN and N2b are presented in Table II. In the latency region used to assess the presence of the MMN (180-240 msec), there was no significanty negative activity at Fz or Cz and no significant positive activity at the mastoids. There was a significant amount of positive activity at Oz. In the latency region used to assess the presence of N2b (240-300 msec), there was a significant amount of negativity at Fz and R1.

The scalp distributions of the activity in the 180-240 and 240-300 msec regions for the midline and coronal recordings are presented in the bottom right panels of Figs. 4 and 5, respectively. The topography of the activity in the two latency windows looked very similar,


DIFFERENCE: deviant-standard (counting)

Fz

6.~ ~G*~I' . " •

Cz Pz vl

OB1 OB2 (1050 Hz) OB2 (952 Hz)

ALT " " J • • 6eo ~

5T

Fig. 7. Grand mean difference wave forms (ERPs elicited by standards subtracted from ERPs elicited by deviants) obtained at Fz, Cz and Pz during the counting conditions. OB 1 = thin wave forms; OB 2 (1050 Hz) = small dotted wave forms; OB 2 (952 Hz) = large dotted wave forms;

ALT = thick wave forms; 5 T = dashed wave forms. Abbreviations as in Fig. 1.

suggesting that the activity in the 180-240 msec window (if reliable) was associated with the early portion of N2b. No significant interactions were obtained on 2-way ANOVAs between the activity in the relevant latency windows and electrode.

The 4th row of Fig. 6 again indicates that there was no difference between the difference wave form used to obtain the "pure" N2b (the thick lines) and the difference wave form that would have contained overlap between the MMN, had it occurred, and the N2b (the thin lines). The latter result provides additional evidence for the absence of the MMN in this condition.

In describing the results for the count conditions, it was remarked that N2b was followed by P3a in the two oddball conditions but not in the Alternation or Five Tones conditions. In order to clarify these results, Fig. 7 superimposes the difference wave forms of the 4 count conditions obtained at Fz, Cz and Pz by subtracting the ERPs elicited by the standards from the ERPs elicited by the deviants. Fig. 8 displays graphically the mean amplitude of the activity between 305-335 and 335-365 msec for Oddball (1 deviant) and Oddball (2 deviants), and 385-415 and 435-465 msec for Alterna- tion, and Five Tones. As can be seen, the positivity that followed N2b had a more frontal distribution in the oddball conditions than in the other two conditions. The more frontal distribtion appears to be due to an

overlap of P3a (which was somewhat earlier in latency) and P3b, whereas the more posterior distribution appears to contain P3b with no overlap from P3a. Two- way ANOVAs on scaled data yielded significant, or nearly significant, interactions between two latency regions and electrode for the oddball conditions but not for either of the other conditions (the statistical details are presented beneath each panel of Fig. 8). Thus, in the count conditions, the P3a appeared to behave in a similar manner as the MMN: it was robust when the eliciting stimulus was different than the stimulus on the preceding trial, and small or absent when the eliciting stimulus was the same as the preceding stimulus.

Discussion

Clear results with respect to the MMN and N2b were obtained for the stimulus arrangements used in the Oddball (1 deviant) and Oddball (2 deviants) conditions versus the Five Tones condition. In both oddball conditions, the MMN was elicited by the deviants in the ignore and count conditions, whereas the N2b was only elicited during the count conditions. In contrast, in the Five Tones condition, the MMN was not elicited by the infrequent, repeating tones in either the


0

4

Oddball (1 Deviant)

Amplitude [laV]

8 I I I - -

Fz Cz Pz

Electrode

F(2/18)=6.80, p=0.0175

.

.

_

_

8

Oddball

Amplitude ~V]

(2 Deviants)

I I

Fz Cz Pz

Electrode

1050 Hz: F(2/18)=4.78, p=0.0498 952 Hz: F(2/18)=4.01, p=0.0605


Amplitude [uV] Amplitude [pV] 0- 0

4

_

4

8 I I I 8 I I I

Fz Cz Pz Fz Cz Pz

Electrode Electrode

F(2/18)= 1.30, p=0.2926 F(2/18)=0.22, p=0.6847 Fig. 8. Scalp distribution of the mean amplitude (in p.Vs) across subjects of the positivity obtained along the midline for the subtractions depicted in Fig. 7. In Oddball (1 deviant) and Oddball (2 deviants) the thin lines represent the mean positivity between 305 and 335 msec and the thick lines represent the mean positivity obtained between 335 and 365 msec. The corresponding latency windows for the Alternation and Five Tones conditions were 385-415 msec and 435-465 msec, respectively. Beneath the panels are the statistical results obtained with 2-way ANOVAs for

the interaction of latency window and electrode.


ignore or count conditions. As before, the N2b was only elicited during the count condition.

These results suggest that the N2b can be associated with match as well as mismatch operations. The only way in which the N2b could have been elicited by a mismatch operation in the Five Tones condition would have been if, after each stimulus, templates of the pitch of each of the other 4 tones had been used. Were the next stimulus to be of the same pitch as the preceding stimulus, then it could elicit the N2b by mismatching all of the templates. Were the next stimulus to be of a different pitch than the preceding stimulus, however, there would have been a mismatch with 3 of the 4 templates. Clearly, the most likely and efficient strategy to detect targets would be to maintain a template of each stimulus as it occurred and determine whether the next stimulus matched that template. Thus, it seems fairly evident that in the Five Tones condition the N2b was associated with a matching operation.

Supporting data for the latter conclusion were obtained by Heinze et al. (1990). In their experiment, bilateral visual stimuli consisted of horizontal rows of 4 letters of the alphabet, and subjects at tended to either the 2 letters in the left visual field or the 2 letters in the right visual field. A total of 4 different letters were used. In a given visual field, on half of the trials the 2 letters were different and on half of the trials the 2 letters were identical. Each of the 4 letters used had an equal probability of constituting a matching pair in each visual field. The latter stimuli were designated targets to which subjects responded with a button press. The ERPs elicited by stimuli that contained matching letters in the at tended visual field were associated with an enhanced N2 relative to the ERPs elicited by stimuli that contained matching letters in the unat tended visual field. In this case it appears clear that the N2 was associated with a matching operation. An important similarity between their study and our Five Tones condition is that all of the stimuli employed had an equal probability of being a target.

Taken together, the data of the present study and that of Heinze et al. (1990) indicate that N2b is associated with higher order processing than is the MMN. The MMN appears to be solely associated with a mismatch operation, whereas the N2b can be associated with a match or mismatch operation depending on the experimental circumstances and the strategies used by the subjects. The flexibility with which the N2b can be associated with match or mismatch operationts sug- gests that it is associated with a controlled process. The data of our Five Tones condition also make clear that, whereas the latency of N2b is linked to that of the MMN when the latter is elicited by deviant tones (Novak et al. 1990), the auditory N2b can be elicited in the absence of the MMN, as proposed by N~t~it~inen and Gaillard (1983). What seems to be the common

factor in N2b elicitation is the infrequency of the eliciting event.

It is possible that the N2b obtained when it was associated with a matching operation had a different generator source than when it was associated with a mismatch operation. To examine this question, the scaled N2b data from the Five Tones condition was compared separately to that of each of the two oddball conditions (separately for the midline and coronal line recording sites), and no significant interactions were found in 2-way ANOVAs between condition and electrode site. While these results are consistent with the hypothesis that the generator of N2b in these conditions was similar, it does not rule out the possiblility that they were different.

The amplitude of the N2b was smaller in the Five Tones condition than in both of the oddball conditions. This was probably largely due to the number of errors committed in the Five Tones condition (24%) which was about 5 times that for the two oddball conditions (4-5%). Most of the errors in the Five Tones condition consisted of underestimations of the number of targets. Since the subject's task was to silently count the targets, it was not possible to average separately detected and undetected targets. Consequently, the averaged ERPs of the target tones in the Five Tones condition contained a large percentage of undetected targets. If the undetected targets were not associated with N2b, this would have reduced the amplitude of the measured N2b. An additional factor may have been a greater latency jitter of N2b on single trials due to this condition being more difficult than the other count conditions. Consistent with this view is that P3b was smallest and had the most flattened morphology in this condition (Fig. 1, bot tom panel).

A clear result concerning the MMN in the Alterna- tion conditions was not obtained. This is the paradigm with which Nordby et al. (1988) reported obtaining an MMN. In the present study, both when subjects ignored the stimuli and when they counted the infrequent repetitions, there was no significant negativity along the midline in the difference wave forms (Tables I and II). The only significant activity obtained in the difference wave forms was a positivity at the right mastoid when the stimuli were ignored. There is a suggestion of a negativity being present in the latency range of the MMN at Fz, L1 and R1 in the ignore condition (see the reading condition data in Fig. 3). The peak latency of this negativity was longer in latency in the Alternation condition (about 170 msec) than in the two oddball conditions (between about 120 and 140 msec). Moreover, examination of the difference wave forms in Fig. 3 will show that the negativity onset later in both the reading and count conditions of Alternation than in the oddball conditions. The longer latency is consistent with the performance data, which


indicated that the subjects found the Alternation condition more difficult than the oddball conditions (more than twice as many errors were made). Thus, it is possible that the negativity was the MMN, but it is also possible that it was N2b that was associated with occa- sional shifts of attention to the stimuli. The comparison of the present results with that of Nordby et al., however, is not straightforward. Nordby et al. used widely separated tones (500 and 1000 Hz), whereas the present study employed two tones that were closer to one another in frequency (1000 and 1050 Hz). In addition, in Nordby et al. the probability of stimulus repetition was 10%, whereas in the present study it was 20%. Both of these factors are important because the MMN is smaller for stimuli that are closer together as well as for deviant stimuli that are of greater probability (N~i~it~inen and Gaillard 1983). If no MMN to stimulus repetition (that is, a break in stimulus alternation) was obtained in the present study, this might be due to differences in our respective methods.

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Event-related potentials to repetition and change of auditory stimuli

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