- Email: [email protected]
Effect of frequency separation and stimulus rate on the mismatch negativity: an examination of the issue of refractoriness in humans
Neuroscience Letters 287 (2000) 167±170
www.elsevier.com/locate/neulet
Effect of frequency separation and stimulus rate on the mismatch negativity: an examination of the issue of refractoriness in humans Diana Deacon a,*, Hilary Gomes b, Jo Manette Nousak b, Walter Ritter b, Daniel Javitt c a Department of Psychology, The City College of the City University of New York, NY, USA Department of Neuroscience, Albert Einstein College of Medicine, Bronx, New York, NY, USA c Nathan Kline Institute for Psychiatric Research, Orangeburg, NY, USA
b
Received 28 October 1999; received in revised form 25 February 2000; accepted 14 March 2000
Abstract Refractoriness of the generators of the mismatch negativity (MMN) was examined in two experiments in which two deviant tones occurred in a row. In Experiment 1, the size of the MMN elicited by the ®rst deviant was manipulated by using deviants that were close to or far from the standard in frequency. In Experiment 2, the time between two identical deviants was varied. It was found that neither the size of the MMN elicited by the ®rst deviant, nor the time between two deviants, affected the amplitude of the MMN elicited by the second deviant. It was concluded that refractoriness played no role in the amplitude of the MMN for the parameters used. q 2000 Published by Elsevier Science Ireland Ltd. Keywords: Mismatch negativity; Refractoriness; Event-related potentials
Research concerning the mismatch negativity (MMN), a component of event-related potentials (ERPs), has revealed a system that detects changes in the acoustic environment on an automatic basis (see Ref. [5] for a review). The MMN is an electric signal generated in auditory cortex [1] when change is detected. The main hypothesis concerning the manner in which the system works is that when stimuli or attributes of the acoustic input repeat, representations of invariant characteristics are maintained in memory for a period of about 10 s [2,9]. Aspects of subsequent stimulation that do not match these representations elicit the MMN. An hypothesis held for a number of years is that the amplitude of the MMN can be used as a measure of the strength of the memory for speci®c stimuli or attributes, e.g. [7]. More recently, Winkler et al. [11] have reviewed old and new data, which the hypothesis can not account for. An alternative suggested by them is that the amplitude of the MMN re¯ects the degree with which acoustic events are `predicted' by the deviance detection system. By prediction is not meant conscious prediction, but rather the degree to which the system is `set' for given stimuli. The pattern of previous stimuli generates sets for some stimuli and not
* Corresponding author. Tel.: 11-413-528-6586; fax: 11-413528-4681. E-mail address: [email protected] (D. Deacon).
others, and when subsequent stimuli are different than what the system is set for, a MMN is elicited. In a recent paper [7], we used the amplitude of the MMN to provide data to support the hypothesis that the memory stores features of stimuli independently. The impetus for the study stemmed from the ®nding that if two identical deviants are presented in a row, the amplitude of the MMN elicited by the second deviant is about half the size of that elicited by the ®rst deviant [8]. In terms of the memory strength hypothesis, the result suggested that the reason why the second deviant elicited a smaller MMN was because the ®rst deviant weakened the strength of the memory for the standard. According to the set hypothesis, the ®rst deviant lessened the degree to which the system was set for the standard. In our study, the two deviants that occurred in a row differed from the standard on different features. The result was that the presence of the ®rst deviant had no effect on the amplitude of the MMN elicited by the second deviant. The interpretation offered was that the ®rst deviant did not weaken the memory for the standard per se, but, rather, weakened the memory of the feature by which the ®rst deviant differed from the standard. In terms of the set hypothesis, the ®rst deviant altered the set for the feature by which the ®rst deviant differed from the standard, but had no effect on the set for other features. Whichever interpretation is used, it appears that the system can handle features independently of one another (variations in the system's
0304-3940/00/$ - see front matter q 2000 Published by Elsevier Science Ireland Ltd. PII: S03 04 - 394 0( 0 0) 01 17 5- 7
168
D. Deacon et al. / Neuroscience Letters 287 (2000) 167±170
representation or set for one feature can occur independently of the state of the representations or set for other features). There is, however, another possible interpretation. It could be that when a MMN occurs the system becomes refractory. This could directly account for why the MMN is smaller for the second of two identical deviants in a row. It could also account for the lack of an effect when two different deviants occur in a row on the grounds that neural ensembles, which implement the two MMNs, are different. In support of the latter, it has been found that there are different generators of the MMNs for deviants that differ from the standards on different features [3]. Thus, the system could be refractory with regard to one feature and not be refractory for other features. The present study tested the refractory hypothesis in two ways: (1) by varying the magnitude of the MMN elicited by the ®rst of two consecutive deviants and (2) by varying the time between consecutive deviants. In Experiment 1, the ®rst of two deviants that differed from the standard on the same feature was either physically close to or far from the standard. It has been shown that the amplitude of the MMN becomes larger as the separation between the standard and deviant is increased [5]. If the magnitude of the MMN elicited by the ®rst deviant is positively related to the degree of refractoriness produced by the ®rst deviant, then the MMN elicited by the second deviant should be smaller following ®rst deviants that produce larger than smaller MMNs. In Experiment 2, two identical deviants were presented in a row, with the amount of time between deviants varied across conditions. If the system were in a refractory state following the ®rst deviant, then increasing the time between deviants should produce a larger MMN for the second deviant. In Experiment 1, 15 young adults (ten women) were paid for their participation. The subjects sat in a comfortable chair and ignored the stimuli, reading a book of their choice. The stimuli were pure tones of 100 ms duration (5% rise/fall times), 75 dB pe SPL, presented binaurally via insert earphones. The tone sequences regularly alternated between ten standards followed by only one deviant (condition 1) or by two deviants in a row (conditions 2, 3 and 4). The standards were always 1000 Hz. For condition 1, the deviant was 952 Hz. For conditions 2, 3 and 4, the ®rst deviant was 952, 1050 and 3000 Hz, respectively, in each case the second deviant was 952 Hz. Stimulus onset asynchrony (SOA) was constant at 600 ms. Each condition included four runs of 500 standards. Brain electrical activity was recorded with DC-coupled ampli®ers with low pass ®ltering at 40 Hz along the midline at Fpz, Fz, Cz and Pz. Lateral electrodes were placed along a coronal chain from Fz to each of the mastoids consisting of electrodes one-third of the distance (LC1 and RC1 for the left and right coronal chain, respectively), two-thirds of the distance (LC2 and RC2) and the left (LM) and right (RM) mastoids. The reference was the nose. Ocular potentials were monitored with bipolar electrodes placed at the outer
canthi and above and below the left eye. All impedances were maintained below 5 kV. The digitization rate was 256 Hz. Each epoch was baseline corrected across the entire epoch before artifact rejecting and averaging. The averages from each block were baseline corrected again using the average amplitude of the entire prestimulus portion of the epoch. Trials on which electrical activity exceeded ^100 mV at all but the horizontal EOG recordings were automatically rejected. All recordings were subsequently assessed visually for residual artifact. Recordings began 100 ms prior to stimulus onset and extended 450 ms post-stimulus. The runs were combined for each subject for each condition. Grand mean ERPs, averaged across subjects, were obtained for display purposes and to select latency windows for amplitude measurements for each condition. MMN peak latencies for the grand means were identi®ed in difference waveforms obtained by subtracting the ERPs elicited by the standard from the ERPs elicited by the deviant. The windows chosen for average amplitude measurements were the 50 ms surrounding the peak latency (25 ms on each side of the peak). For each subject, the amplitude of the MMN was measured in the standard and deviant waveforms as the mean voltage between 160 and 210 ms for deviants one and two of condition 2; 170±220 ms for deviant one and 145±195 ms for deviant two of condition 3; 110±160 ms for deviant one and 140±190 ms for deviant two of condition 4. To establish the presence of the MMN for each of the deviants, orthogonal planned comparisons [4] were calculated to determine whether the mean amplitudes of the standards and deviants in the latency region of the MMN differed at each recording site. The mean square error terms used for these comparisons were calculated separately for the deviants in the ®rst position and the second condition. The alpha level was 0.05. Fig. 1 presents the grand mean difference waveforms obtained in the various conditions for ®rst and second deviants (since condition 1 did not have deviants that occurred in a row, it had only ®rst deviants). All deviants elicited MMNs as evidenced by a negative wave at Fz (thick line) and positive waves at a similar latency at the mastoids (thin lines), typical of frequency deviants. For each deviant, there was a signi®cant difference in mean voltage between the standard and deviant ERPs in the latency range of the MMN at Fz, Cz and the two mastoids. Since any potential differences between the conditions were expected to be small, a composite MMN measure was used for the amplitude analysis. The composite MMN was the sum of the mean amplitude of the MMN at Fz and the average mean amplitude of the MMN at the mastoids ([LM 1 RM]/2). A two-way ANOVA yielded a signi®cant interaction with factors of condition and position (F 2;28 13:6, P , 0:001). Follow-up Tukey post-hoc tests indicated, as expected, that the MMN elicited by the ®rst deviant that was distant from the standard in frequency (condition 4) was larger in amplitude than the MMNs elicited by the other ®rst deviants. Most importantly, an ANOVA indicated
D. Deacon et al. / Neuroscience Letters 287 (2000) 167±170
Fig. 1. Grand mean difference waveforms associated with the ®rst deviant in conditions 1±4 and with the second deviant in conditions 2±4. Thick lines are recordings at Fz and thin lines are recordings at the mastoids. Stimulus onset at time zero.
there was no signi®cant differences among the MMNs elicited by the second deviants of conditions 2, 3 and 4. Ten young adults (seven women) were paid for their participation in Experiment 2. Eight of the subjects also participated in Experiment 1. As in condition 3 of Experiment 1, the standard was 1000 Hz and the ®rst and second deviants were
169
1050 and 952 Hz, respectively. There were two conditions, one with an SOA of 600 ms (as in Experiment 1), and one with an SOA of 2 s. For the latter condition, there were 12 runs with 170 standards per run. Otherwise, the procedures were the same as in Experiment 1. The amplitude of the MMN was measured in the latency windows of 155±205 and 135±185 ms for the ®rst and second deviants for the short and long SOAs, respectively. The presence of the MMN was established as in Experiment 1. Fig. 2 presents the grand mean difference waveforms at Fz (thick lines) and the mastoids (thin lines). Signi®cant differences between the standard and deviant ERPs for the ®rst deviants in the latency range of the MMN were obtained at Fz, Cz in both conditions and at LM for condition 1. As expected, the second deviants had smaller MMNs than the ®rst deviants. Using the composite measure of the MMN, a two-way ANOVA with factors of condition and position yielded a signi®cant main effect of position (F1;9 20:0, P , 0:002). More importantly, there was no signi®cant main effect of condition (F1;9 1:36, P 0:274), indicating that increasing the time between deviants did not increase the size of the MMN. In summary, in Experiment 1 the magnitude of the MMN elicited by the ®rst frequency deviant had no effect on the amplitude of the MMN elicited by the second frequency deviant. In Experiment 2, an increase in the SOA from 600 to 2000 ms had no effect on the amplitude of the MMNs elicited by second frequency deviants. Hence, the data provide no support for the hypothesis that refractoriness accounts for the decrease in the amplitude of the MMNs from the ®rst to the second deviants in both experiments. Instead, the results suggest that a frequency deviant produces a change in the set for the frequency of the standard, whatever the speci®c frequency of the ®rst deviant (Experiment 1), which was not affected by the temporal interval between successive deviants employed (Experiment 2). Refractoriness of the MMN generators has been consid-
Fig. 2. Grand mean difference waveforms associated with the ®rst and second deviants in the short and long SOA conditions. Thick lines are recordings at Fz and thin lines are recordings at the mastoids. Stimulus onset at time zero.
170
D. Deacon et al. / Neuroscience Letters 287 (2000) 167±170
ered before. SchroÈger [10] pointed out that the refractory period of the generators must be short. In his experiment, reducing the time between successive frequency deviants to 4 s was associated with a reduction in the size of N2 and P3, but not MMN. NaÈaÈtaÈnen et al. [6] used a 51 ms SOA and an average rate of two deviants per s, and were surprised to ®nd that there was no refractoriness of the MMN generator. In both of these studies, however, standards intervened between successive deviants. The present experiment is a stronger test of refractoriness because no stimuli intervened between deviants. It is possible that shorter intervals between deviants may yield smaller MMNs for the second of two successive deviants. However, the present experiment indicates that our earlier report [7], that the MMN elicited by the second of two deviants in a row (each of which differed from the standard on different features) was not affected by the ®rst deviant, can not be accounted for by the ®rst deviant differentially producing refractoriness for MMN generators associated with the feature by which it differed from the standard. A more viable interpretation is that the ®rst deviant had no affect on the set regarding the feature by which the second deviant differed from the standard. This research was supported by National Institute of Health grant NS30029. [1] Alho, K., Cerebral generators of mismatch negativity (MMN) and its magnetic counterpart (MMNm) elicited by sound changes, Ear Hear., 16 (1995) 38±51. [2] Cowan, N., Winkler, I., Teder, W. and NaÈaÈtaÈnen, R., Memory
[3]
[4] [5] [6]
[7] [8] [9]
[10]
[11]
prerequisites of the mismatch negativity in the auditory event-related potential (ERP), J. Exp. Psychol.: Learn. Mem. Cogn., 19 (1993) 909±921. Giard, M.H., Lavikainen, J., Reinikainen, K., Perrin, F., Bertrand, O., Thevenent, M., Perneir, J. and NaÈaÈtaÈnen, R., Separate representation of frequency, intensity, and duration in auditory sensory memory: an event-related potential and dipole model analysis, J. Cogn. Neurosci., 7 (1995) 133± 143. Hays, W.L., Statistics for Psychologists, Hold, Rinehart & Winston, New York, 1963. NaÈaÈtaÈnen, R., Attention and Brain Function, Erlbaum, Hillsdale, 1992, p. 494. NaÈaÈtaÈnen, R., Paavilainen, P., Alho, K., Reinikainen, M. and Sams, M., Interstimulus interval and the mismatch negativity, In C. Barber and T. Blum (Eds.), Evoked Potentials III, The Third International Potentials Symposium, Butterworths, London, UK, 1987, pp. 392±397. Nousak, J.M.K., Deacon, D., Ritter, W. and Vaughan Jr., H.G., Storage and comparison of information in transient auditory memory, Cogn. Brain Res., 4 (1996) 305±317. Sams, M., Alho, K. and NaÈaÈtaÈnen, R., Short-term habituation and dishabituation of the mismatch negativity of the ERP, Psychophysiology, 21 (1984) 434±441. Sams, M., Hari, R., Rif, J. and Knuutila, J., The human auditory sensory memory trace persists about 10 seconds: neuromagnetic evidence, J. Cogn. Neurosci., 5 (1993) 363±370. SchroÈger, E., The in¯uence of stimulus intensity and interstimulus interval on the detection of pitch and loudness changes, Electroenceph. clin. Neurophysiol., 100 (1996) 517±526. Winkler, I., Karmos, G. and NaÈaÈtaÈnen, R., Adaptive modeling of the unattended acoustic environment re¯ected in the mismatch negativity event-related potential, Brain Res., 742 (1996) 239±252.
www.elsevier.com/locate/neulet
Effect of frequency separation and stimulus rate on the mismatch negativity: an examination of the issue of refractoriness in humans Diana Deacon a,*, Hilary Gomes b, Jo Manette Nousak b, Walter Ritter b, Daniel Javitt c a Department of Psychology, The City College of the City University of New York, NY, USA Department of Neuroscience, Albert Einstein College of Medicine, Bronx, New York, NY, USA c Nathan Kline Institute for Psychiatric Research, Orangeburg, NY, USA
b
Received 28 October 1999; received in revised form 25 February 2000; accepted 14 March 2000
Abstract Refractoriness of the generators of the mismatch negativity (MMN) was examined in two experiments in which two deviant tones occurred in a row. In Experiment 1, the size of the MMN elicited by the ®rst deviant was manipulated by using deviants that were close to or far from the standard in frequency. In Experiment 2, the time between two identical deviants was varied. It was found that neither the size of the MMN elicited by the ®rst deviant, nor the time between two deviants, affected the amplitude of the MMN elicited by the second deviant. It was concluded that refractoriness played no role in the amplitude of the MMN for the parameters used. q 2000 Published by Elsevier Science Ireland Ltd. Keywords: Mismatch negativity; Refractoriness; Event-related potentials
Research concerning the mismatch negativity (MMN), a component of event-related potentials (ERPs), has revealed a system that detects changes in the acoustic environment on an automatic basis (see Ref. [5] for a review). The MMN is an electric signal generated in auditory cortex [1] when change is detected. The main hypothesis concerning the manner in which the system works is that when stimuli or attributes of the acoustic input repeat, representations of invariant characteristics are maintained in memory for a period of about 10 s [2,9]. Aspects of subsequent stimulation that do not match these representations elicit the MMN. An hypothesis held for a number of years is that the amplitude of the MMN can be used as a measure of the strength of the memory for speci®c stimuli or attributes, e.g. [7]. More recently, Winkler et al. [11] have reviewed old and new data, which the hypothesis can not account for. An alternative suggested by them is that the amplitude of the MMN re¯ects the degree with which acoustic events are `predicted' by the deviance detection system. By prediction is not meant conscious prediction, but rather the degree to which the system is `set' for given stimuli. The pattern of previous stimuli generates sets for some stimuli and not
* Corresponding author. Tel.: 11-413-528-6586; fax: 11-413528-4681. E-mail address: [email protected] (D. Deacon).
others, and when subsequent stimuli are different than what the system is set for, a MMN is elicited. In a recent paper [7], we used the amplitude of the MMN to provide data to support the hypothesis that the memory stores features of stimuli independently. The impetus for the study stemmed from the ®nding that if two identical deviants are presented in a row, the amplitude of the MMN elicited by the second deviant is about half the size of that elicited by the ®rst deviant [8]. In terms of the memory strength hypothesis, the result suggested that the reason why the second deviant elicited a smaller MMN was because the ®rst deviant weakened the strength of the memory for the standard. According to the set hypothesis, the ®rst deviant lessened the degree to which the system was set for the standard. In our study, the two deviants that occurred in a row differed from the standard on different features. The result was that the presence of the ®rst deviant had no effect on the amplitude of the MMN elicited by the second deviant. The interpretation offered was that the ®rst deviant did not weaken the memory for the standard per se, but, rather, weakened the memory of the feature by which the ®rst deviant differed from the standard. In terms of the set hypothesis, the ®rst deviant altered the set for the feature by which the ®rst deviant differed from the standard, but had no effect on the set for other features. Whichever interpretation is used, it appears that the system can handle features independently of one another (variations in the system's
0304-3940/00/$ - see front matter q 2000 Published by Elsevier Science Ireland Ltd. PII: S03 04 - 394 0( 0 0) 01 17 5- 7
168
D. Deacon et al. / Neuroscience Letters 287 (2000) 167±170
representation or set for one feature can occur independently of the state of the representations or set for other features). There is, however, another possible interpretation. It could be that when a MMN occurs the system becomes refractory. This could directly account for why the MMN is smaller for the second of two identical deviants in a row. It could also account for the lack of an effect when two different deviants occur in a row on the grounds that neural ensembles, which implement the two MMNs, are different. In support of the latter, it has been found that there are different generators of the MMNs for deviants that differ from the standards on different features [3]. Thus, the system could be refractory with regard to one feature and not be refractory for other features. The present study tested the refractory hypothesis in two ways: (1) by varying the magnitude of the MMN elicited by the ®rst of two consecutive deviants and (2) by varying the time between consecutive deviants. In Experiment 1, the ®rst of two deviants that differed from the standard on the same feature was either physically close to or far from the standard. It has been shown that the amplitude of the MMN becomes larger as the separation between the standard and deviant is increased [5]. If the magnitude of the MMN elicited by the ®rst deviant is positively related to the degree of refractoriness produced by the ®rst deviant, then the MMN elicited by the second deviant should be smaller following ®rst deviants that produce larger than smaller MMNs. In Experiment 2, two identical deviants were presented in a row, with the amount of time between deviants varied across conditions. If the system were in a refractory state following the ®rst deviant, then increasing the time between deviants should produce a larger MMN for the second deviant. In Experiment 1, 15 young adults (ten women) were paid for their participation. The subjects sat in a comfortable chair and ignored the stimuli, reading a book of their choice. The stimuli were pure tones of 100 ms duration (5% rise/fall times), 75 dB pe SPL, presented binaurally via insert earphones. The tone sequences regularly alternated between ten standards followed by only one deviant (condition 1) or by two deviants in a row (conditions 2, 3 and 4). The standards were always 1000 Hz. For condition 1, the deviant was 952 Hz. For conditions 2, 3 and 4, the ®rst deviant was 952, 1050 and 3000 Hz, respectively, in each case the second deviant was 952 Hz. Stimulus onset asynchrony (SOA) was constant at 600 ms. Each condition included four runs of 500 standards. Brain electrical activity was recorded with DC-coupled ampli®ers with low pass ®ltering at 40 Hz along the midline at Fpz, Fz, Cz and Pz. Lateral electrodes were placed along a coronal chain from Fz to each of the mastoids consisting of electrodes one-third of the distance (LC1 and RC1 for the left and right coronal chain, respectively), two-thirds of the distance (LC2 and RC2) and the left (LM) and right (RM) mastoids. The reference was the nose. Ocular potentials were monitored with bipolar electrodes placed at the outer
canthi and above and below the left eye. All impedances were maintained below 5 kV. The digitization rate was 256 Hz. Each epoch was baseline corrected across the entire epoch before artifact rejecting and averaging. The averages from each block were baseline corrected again using the average amplitude of the entire prestimulus portion of the epoch. Trials on which electrical activity exceeded ^100 mV at all but the horizontal EOG recordings were automatically rejected. All recordings were subsequently assessed visually for residual artifact. Recordings began 100 ms prior to stimulus onset and extended 450 ms post-stimulus. The runs were combined for each subject for each condition. Grand mean ERPs, averaged across subjects, were obtained for display purposes and to select latency windows for amplitude measurements for each condition. MMN peak latencies for the grand means were identi®ed in difference waveforms obtained by subtracting the ERPs elicited by the standard from the ERPs elicited by the deviant. The windows chosen for average amplitude measurements were the 50 ms surrounding the peak latency (25 ms on each side of the peak). For each subject, the amplitude of the MMN was measured in the standard and deviant waveforms as the mean voltage between 160 and 210 ms for deviants one and two of condition 2; 170±220 ms for deviant one and 145±195 ms for deviant two of condition 3; 110±160 ms for deviant one and 140±190 ms for deviant two of condition 4. To establish the presence of the MMN for each of the deviants, orthogonal planned comparisons [4] were calculated to determine whether the mean amplitudes of the standards and deviants in the latency region of the MMN differed at each recording site. The mean square error terms used for these comparisons were calculated separately for the deviants in the ®rst position and the second condition. The alpha level was 0.05. Fig. 1 presents the grand mean difference waveforms obtained in the various conditions for ®rst and second deviants (since condition 1 did not have deviants that occurred in a row, it had only ®rst deviants). All deviants elicited MMNs as evidenced by a negative wave at Fz (thick line) and positive waves at a similar latency at the mastoids (thin lines), typical of frequency deviants. For each deviant, there was a signi®cant difference in mean voltage between the standard and deviant ERPs in the latency range of the MMN at Fz, Cz and the two mastoids. Since any potential differences between the conditions were expected to be small, a composite MMN measure was used for the amplitude analysis. The composite MMN was the sum of the mean amplitude of the MMN at Fz and the average mean amplitude of the MMN at the mastoids ([LM 1 RM]/2). A two-way ANOVA yielded a signi®cant interaction with factors of condition and position (F 2;28 13:6, P , 0:001). Follow-up Tukey post-hoc tests indicated, as expected, that the MMN elicited by the ®rst deviant that was distant from the standard in frequency (condition 4) was larger in amplitude than the MMNs elicited by the other ®rst deviants. Most importantly, an ANOVA indicated
D. Deacon et al. / Neuroscience Letters 287 (2000) 167±170
Fig. 1. Grand mean difference waveforms associated with the ®rst deviant in conditions 1±4 and with the second deviant in conditions 2±4. Thick lines are recordings at Fz and thin lines are recordings at the mastoids. Stimulus onset at time zero.
there was no signi®cant differences among the MMNs elicited by the second deviants of conditions 2, 3 and 4. Ten young adults (seven women) were paid for their participation in Experiment 2. Eight of the subjects also participated in Experiment 1. As in condition 3 of Experiment 1, the standard was 1000 Hz and the ®rst and second deviants were
169
1050 and 952 Hz, respectively. There were two conditions, one with an SOA of 600 ms (as in Experiment 1), and one with an SOA of 2 s. For the latter condition, there were 12 runs with 170 standards per run. Otherwise, the procedures were the same as in Experiment 1. The amplitude of the MMN was measured in the latency windows of 155±205 and 135±185 ms for the ®rst and second deviants for the short and long SOAs, respectively. The presence of the MMN was established as in Experiment 1. Fig. 2 presents the grand mean difference waveforms at Fz (thick lines) and the mastoids (thin lines). Signi®cant differences between the standard and deviant ERPs for the ®rst deviants in the latency range of the MMN were obtained at Fz, Cz in both conditions and at LM for condition 1. As expected, the second deviants had smaller MMNs than the ®rst deviants. Using the composite measure of the MMN, a two-way ANOVA with factors of condition and position yielded a signi®cant main effect of position (F1;9 20:0, P , 0:002). More importantly, there was no signi®cant main effect of condition (F1;9 1:36, P 0:274), indicating that increasing the time between deviants did not increase the size of the MMN. In summary, in Experiment 1 the magnitude of the MMN elicited by the ®rst frequency deviant had no effect on the amplitude of the MMN elicited by the second frequency deviant. In Experiment 2, an increase in the SOA from 600 to 2000 ms had no effect on the amplitude of the MMNs elicited by second frequency deviants. Hence, the data provide no support for the hypothesis that refractoriness accounts for the decrease in the amplitude of the MMNs from the ®rst to the second deviants in both experiments. Instead, the results suggest that a frequency deviant produces a change in the set for the frequency of the standard, whatever the speci®c frequency of the ®rst deviant (Experiment 1), which was not affected by the temporal interval between successive deviants employed (Experiment 2). Refractoriness of the MMN generators has been consid-
Fig. 2. Grand mean difference waveforms associated with the ®rst and second deviants in the short and long SOA conditions. Thick lines are recordings at Fz and thin lines are recordings at the mastoids. Stimulus onset at time zero.
170
D. Deacon et al. / Neuroscience Letters 287 (2000) 167±170
ered before. SchroÈger [10] pointed out that the refractory period of the generators must be short. In his experiment, reducing the time between successive frequency deviants to 4 s was associated with a reduction in the size of N2 and P3, but not MMN. NaÈaÈtaÈnen et al. [6] used a 51 ms SOA and an average rate of two deviants per s, and were surprised to ®nd that there was no refractoriness of the MMN generator. In both of these studies, however, standards intervened between successive deviants. The present experiment is a stronger test of refractoriness because no stimuli intervened between deviants. It is possible that shorter intervals between deviants may yield smaller MMNs for the second of two successive deviants. However, the present experiment indicates that our earlier report [7], that the MMN elicited by the second of two deviants in a row (each of which differed from the standard on different features) was not affected by the ®rst deviant, can not be accounted for by the ®rst deviant differentially producing refractoriness for MMN generators associated with the feature by which it differed from the standard. A more viable interpretation is that the ®rst deviant had no affect on the set regarding the feature by which the second deviant differed from the standard. This research was supported by National Institute of Health grant NS30029. [1] Alho, K., Cerebral generators of mismatch negativity (MMN) and its magnetic counterpart (MMNm) elicited by sound changes, Ear Hear., 16 (1995) 38±51. [2] Cowan, N., Winkler, I., Teder, W. and NaÈaÈtaÈnen, R., Memory
[3]
[4] [5] [6]
[7] [8] [9]
[10]
[11]
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