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344 Gamma-oscillations during perception of illusory contours
Abstracts
/International
Journal
Seventeen right-handed subjects were presented reversal of checkerboard pattern as slimuli at right visual field or left visual field, and EEG was recorded at 01, 02, P3, P4. The grand-averaged VEPs were transformed to the frequency domain by means of the fast Fourier transform to obtain the amplitude frequency characteristics. Band pass filters were chosen adequately according to tuning frequencies indicated by clear peaks in the amplitude frequency characteristics. The chosen band pass filters (4-8 Hz, 8-15 Hz, 15-20 Hz, 20-32 Hz) were applied to the VEP of subjects, and obtained 4 different components of VEP for each VEP. The latency of PlOO and N160 of unfiltered VEP was measured. In the band-pass digital filter applied VEPs, positive and negative peaks, which are consistent with PlOO and N160, were measured for each subject. Latency differences between hemispheres for digitally unfiltered and filtered VEPs were computed to estimate IHIT. Callosal transfer was found to be faster in theta band (4-8 Hz) from right-to-left than left to-right. Furthermore, one direction transfer from right-to-left in beta 2 band (20-32 Hz) was found. Results suggested that the callosal fibers might differentiate in dimension, depending on the direction. *This study was supported by a grant horn The Jiilich Research Centre in Germany and from TUBITAK in Turkey.
342 INTERHEMISPHERE SYNCHRONY OF SHORTTERM VARIATIONS IN HUMAN EEG ALPHA POWER CORRELATES WITH SELF-ESTIMATES OF FUNCTIONAL STATE S.L. Shishkin*, B.S. Darkhovsky, Al. A. Fingelkurts, An. A. Fingelkurts and A.Ya. Kaplan Department of Human Physiology, Biological Faculty, Moscow State University, Moscow 119899, Russia It is well known that the spectral power of spontaneous alpha rhythm estimated in intervals of tens of seconds or longer depends on general activation and cognitive loading. Fluctuations of alpha activity within shorter time intervals and their relation to psychological state are far less studied. We suggested that the synchrony of changes in alpha rhythm in different EEG derivations can reflect the synchrony in functioning of different brain areas, and developed a procedure for estimating an Index of the Coincidence of Changes in EEG alpha power (alpha ICC) based on nonparametric statistical approach to the detection of change-points in a random sequence. This index differs from the common measures such as coherency, because only prominent changes in alpha power dynamics are taken into account for the computation of the ICC. The present study investigated the sensitivity of alpha ICC to cognition state and general functional state. The EEGs were recorded monopolarly in 12 male volunteers when they performed a memory test. Each subject participated in 2 sessions on different days. Alpha ICCs were computed for the interhemisphere electrode pairs 01-02, P3-P4, C3-C4, and
of Psychophysiology
30 (1998)
95-271
133
F3-F4. These indices showed no significant differences between 3 stages of the memory test requiring different cognitive activity, while the averaged alpha power differed significantly in most derivations. However, parietal and central alpha ICCs in each stage and occipital alpha ICC in 2 of 3 stages correlated significantly with the scores of self-estimated activation produced by Doeskin functional state test (r = -.53..-.73) and with the scores of Spielberger state anxiety test (r = + .46..+ .70), showing that weaker interhemisphere synchrony was related to “better” subjective estimates of the functional state. The results demonstrate the usefulness of the proposed index and provide preliminary evidence of the existence of a factor determining the synchrony of fluctuations in EEG alpha activity, which is stable to cognitive loading and sensitive to general functional state.
343 EPISODIC FRONTAL GAMMA ACTIVITY ON HUMAN SCALP EEG DURING IMMEDIATE MEMORY TASKS Takahiro Matsuoka*, Ryoichi Toyoshima, Hideharu lida, Hiroshi Aikawa, Toshio Ota and Toshio Yamauchi Department of Neuropsychiatry, Saitama Medical School, Saitama 350-0495, Japan To investigate whether episodic frontal gamma synchronization occurs during immediate memory tasks in human brain, we studied event-related gamma-band (30-50 Hz) power changes in multichannel EEGs from 3 healthy subjects performing visuo-spatial immediate memory tasks. First imperative stimulus for spatial discrimination (S2) appeared 3000 ms after warning stimulus (Sl), and second imperative stimulus for motor response (S3) appeared 1000 ms after S2 presentation. Event-related power analysis was performed for each of 100 ms segments shifting in steps of 25 ms from the beginning to the end of the 6000 ms periods that spanned -1000 ms to 5000 ms starting from the Sl onset. An episodic increase in gamma-band power occurred in frontal area between 500-1000 ms after Sl presentation concomitant with contingent negative variation. An episodic increase in gamma-band power also occurred between 500-750 ms after S2 presentation, and the increase was prominent in ‘GO’ condition rather than ‘No Go’ condition. The present study suggests that episodic synchronous gamma activity enhanced episodically in frontal area during human immediate memory tasks.
344 GAMMA-OSCILLATIONS OF ILLUSORY CONTOURS
DURING
PERCEPTION
C. Herrmann*, E. Pfeifer and A. Me&linger Max-Planck-Institute of Cognitive Neuroscience, Inselstr. 22, 04103 Leipzig, Germany
134
Abstracts
/International
Journal
We examined event-related potentials and gamma-range EEG activity a visual classification task using Kanisza figures. Ten subjects silently counted the occurrence of rare Kanisza squares (targets) among Kanisza triangles and non-Kanisza figures (standards). In order to detect phase-locked and nonphase-locked gamma activity, a time-frequency analysis based on a special wavelet transform was developed. The following results were obtained: .
.
.
We found phase-locked gamma oscillations with a central topography in the same latency range as the Nl/P2 complex. This response was obtained for Kanisza and nonKanisza figures. Phase-locked gamma oscillations in the range of 30-45 Hz (mostly 35-40 Hz) with an occipital topography in a later time-window (200-350ms) were found for Kanisza figures only, irrespective of the target condition. In addition, we found gamma oscillations in the range of 70-80 Hz, with a clearly occipital topography and much higher amplitudes in case of Kanisza figures than nonKanisza figures.
This pattern of results extends evidence of late 40 Hz oscillations specific to Kanisza figures found only in nonphase-locked activity and without a clear topography. The occipital topography of our late 40 Hz component suggests that this activity is actually related to visual processing and might reflect some aspect of the bindinpprocess. Since there is no difference in brightness, contrast etc. between Kanisza and non-Kanisza figures, the 70-80 Hz activity is probably not just a monitor artefact. Its stimulus-dependency rather suggests that it is also associated with the binding process.
345 PRESTIMULUS ALPHA OSCILLATIONS SORIMOTOR WORKING MEMORY TASK
IN A SEN-
A. Brand*, H. Hildebrandt, A. Cabuk and C. Basar-Eroglu Krankenhaus Bremen Ost, Zentrum fiir Psychiatric und Psychotherapie I, Ziiricher Str. 40, 28325 Bremen, Germany Baddeley’s model of working memory (1986) contains the central executive and two domain specilic subsystems: the phonological and the visuo-spatial loop. Other cognitive functions, however, require integration across time, too. We recorded event-related potentials (ERPs) in order to investigate sensory-motor working memory. 15 healthy volunteers participated in this study. In this paper we present results for prestimulus EEG. There were three experimental conditions for each subject: a choice reaction task as control task, a working memory task with low demands and another one with high demands. The working memory task requires the subjects to keep in mind the right- or left-hand used for button pressing. The recorded ERPs were analysed by means of FFI. The frequency analysis computed from prestimulus EEG showed a prominent maximum in the 10 Hz frequency range
of Psychophysiology
30 (1998)
95-271
at the occipital location. Following this procedure, the prestimulus EEG were filtered in the 8-15 Hz range. The results showed that during the high demand memory task the amplitude of alpha waves were highly increased. Furthermore we observed a prolongation of alpha synchronization. Our results support the results of Klimesch (1997) and dynamic memory model of Basar et al. (19971, who demonstrated that the alpha activity is strongly correlated with short term memory, working memory and probably with long term memory engrams.
346 MULTI-CHANNEL EEG: CHOOSING AN METHOD
EOG CORRECTION OF THE APPROPRIATE REGRESSION
Rodney J. Croft and Robert J. Barry Department of Psychology, University of Wollongong, Wollongong, Australia In order to remove eye movement-related contamination from the EEG, EOG correction procedures are used. These estimate the proportion of EOG present in the EEG, and remove it. This estimation relies on the regression procedure. When more than one EOG channel is used in the correction, the appropriate type of regression equation to use is not known. To resolve this matter, computer simulations were employed to compare the simultaneous, multiple-stage and single channel regression methods of correction. Two-channel EOG propagation was modeled on prior findings, under conditions of varying. EOG/EEG ratio and VEOG/HEOG correlation. The dependent variable was the correlation between the uncontaminated and the corrected EEG. It was found that the simultaneous regression procedure gave the best correction, with its advantage increasing as a function of both EOG/EEG ratio and VEOG/HEOG correlation. It is therefore recommended that the simultaneous regression procedure be used for multi-channel EOG correction of the EEG.
347 EFFECTS OF COGNITIVE LOAD ON THE ONGOING-EEG
AND PHYSICAL WORK-
Gebhard Sammer IPV, University of Giessen, Otto-Behaghel Germany
Str. 10, D-35394
Frontally, an increased theta-activity can be seen with mental load, which is suggested to indicate focused-attention. In this study it was investigated if physical load and cognitive load can be discriminated by measures of the ongoing-EEG. Ten subjects performed three tasks; a physical task (push/pull a lever), a cognitive task (both detection and mental addition of numerical target stimuli), and a dual task (combination of the physical/cognitive tasks). Stimuli were presented visually with fixed ISls (= 1.3 set). Lever movements were required at the same rate. Subjects performed 4 blocks x 3 tasks (physical, mental, dual).
/International
Journal
Seventeen right-handed subjects were presented reversal of checkerboard pattern as slimuli at right visual field or left visual field, and EEG was recorded at 01, 02, P3, P4. The grand-averaged VEPs were transformed to the frequency domain by means of the fast Fourier transform to obtain the amplitude frequency characteristics. Band pass filters were chosen adequately according to tuning frequencies indicated by clear peaks in the amplitude frequency characteristics. The chosen band pass filters (4-8 Hz, 8-15 Hz, 15-20 Hz, 20-32 Hz) were applied to the VEP of subjects, and obtained 4 different components of VEP for each VEP. The latency of PlOO and N160 of unfiltered VEP was measured. In the band-pass digital filter applied VEPs, positive and negative peaks, which are consistent with PlOO and N160, were measured for each subject. Latency differences between hemispheres for digitally unfiltered and filtered VEPs were computed to estimate IHIT. Callosal transfer was found to be faster in theta band (4-8 Hz) from right-to-left than left to-right. Furthermore, one direction transfer from right-to-left in beta 2 band (20-32 Hz) was found. Results suggested that the callosal fibers might differentiate in dimension, depending on the direction. *This study was supported by a grant horn The Jiilich Research Centre in Germany and from TUBITAK in Turkey.
342 INTERHEMISPHERE SYNCHRONY OF SHORTTERM VARIATIONS IN HUMAN EEG ALPHA POWER CORRELATES WITH SELF-ESTIMATES OF FUNCTIONAL STATE S.L. Shishkin*, B.S. Darkhovsky, Al. A. Fingelkurts, An. A. Fingelkurts and A.Ya. Kaplan Department of Human Physiology, Biological Faculty, Moscow State University, Moscow 119899, Russia It is well known that the spectral power of spontaneous alpha rhythm estimated in intervals of tens of seconds or longer depends on general activation and cognitive loading. Fluctuations of alpha activity within shorter time intervals and their relation to psychological state are far less studied. We suggested that the synchrony of changes in alpha rhythm in different EEG derivations can reflect the synchrony in functioning of different brain areas, and developed a procedure for estimating an Index of the Coincidence of Changes in EEG alpha power (alpha ICC) based on nonparametric statistical approach to the detection of change-points in a random sequence. This index differs from the common measures such as coherency, because only prominent changes in alpha power dynamics are taken into account for the computation of the ICC. The present study investigated the sensitivity of alpha ICC to cognition state and general functional state. The EEGs were recorded monopolarly in 12 male volunteers when they performed a memory test. Each subject participated in 2 sessions on different days. Alpha ICCs were computed for the interhemisphere electrode pairs 01-02, P3-P4, C3-C4, and
of Psychophysiology
30 (1998)
95-271
133
F3-F4. These indices showed no significant differences between 3 stages of the memory test requiring different cognitive activity, while the averaged alpha power differed significantly in most derivations. However, parietal and central alpha ICCs in each stage and occipital alpha ICC in 2 of 3 stages correlated significantly with the scores of self-estimated activation produced by Doeskin functional state test (r = -.53..-.73) and with the scores of Spielberger state anxiety test (r = + .46..+ .70), showing that weaker interhemisphere synchrony was related to “better” subjective estimates of the functional state. The results demonstrate the usefulness of the proposed index and provide preliminary evidence of the existence of a factor determining the synchrony of fluctuations in EEG alpha activity, which is stable to cognitive loading and sensitive to general functional state.
343 EPISODIC FRONTAL GAMMA ACTIVITY ON HUMAN SCALP EEG DURING IMMEDIATE MEMORY TASKS Takahiro Matsuoka*, Ryoichi Toyoshima, Hideharu lida, Hiroshi Aikawa, Toshio Ota and Toshio Yamauchi Department of Neuropsychiatry, Saitama Medical School, Saitama 350-0495, Japan To investigate whether episodic frontal gamma synchronization occurs during immediate memory tasks in human brain, we studied event-related gamma-band (30-50 Hz) power changes in multichannel EEGs from 3 healthy subjects performing visuo-spatial immediate memory tasks. First imperative stimulus for spatial discrimination (S2) appeared 3000 ms after warning stimulus (Sl), and second imperative stimulus for motor response (S3) appeared 1000 ms after S2 presentation. Event-related power analysis was performed for each of 100 ms segments shifting in steps of 25 ms from the beginning to the end of the 6000 ms periods that spanned -1000 ms to 5000 ms starting from the Sl onset. An episodic increase in gamma-band power occurred in frontal area between 500-1000 ms after Sl presentation concomitant with contingent negative variation. An episodic increase in gamma-band power also occurred between 500-750 ms after S2 presentation, and the increase was prominent in ‘GO’ condition rather than ‘No Go’ condition. The present study suggests that episodic synchronous gamma activity enhanced episodically in frontal area during human immediate memory tasks.
344 GAMMA-OSCILLATIONS OF ILLUSORY CONTOURS
DURING
PERCEPTION
C. Herrmann*, E. Pfeifer and A. Me&linger Max-Planck-Institute of Cognitive Neuroscience, Inselstr. 22, 04103 Leipzig, Germany
134
Abstracts
/International
Journal
We examined event-related potentials and gamma-range EEG activity a visual classification task using Kanisza figures. Ten subjects silently counted the occurrence of rare Kanisza squares (targets) among Kanisza triangles and non-Kanisza figures (standards). In order to detect phase-locked and nonphase-locked gamma activity, a time-frequency analysis based on a special wavelet transform was developed. The following results were obtained: .
.
.
We found phase-locked gamma oscillations with a central topography in the same latency range as the Nl/P2 complex. This response was obtained for Kanisza and nonKanisza figures. Phase-locked gamma oscillations in the range of 30-45 Hz (mostly 35-40 Hz) with an occipital topography in a later time-window (200-350ms) were found for Kanisza figures only, irrespective of the target condition. In addition, we found gamma oscillations in the range of 70-80 Hz, with a clearly occipital topography and much higher amplitudes in case of Kanisza figures than nonKanisza figures.
This pattern of results extends evidence of late 40 Hz oscillations specific to Kanisza figures found only in nonphase-locked activity and without a clear topography. The occipital topography of our late 40 Hz component suggests that this activity is actually related to visual processing and might reflect some aspect of the bindinpprocess. Since there is no difference in brightness, contrast etc. between Kanisza and non-Kanisza figures, the 70-80 Hz activity is probably not just a monitor artefact. Its stimulus-dependency rather suggests that it is also associated with the binding process.
345 PRESTIMULUS ALPHA OSCILLATIONS SORIMOTOR WORKING MEMORY TASK
IN A SEN-
A. Brand*, H. Hildebrandt, A. Cabuk and C. Basar-Eroglu Krankenhaus Bremen Ost, Zentrum fiir Psychiatric und Psychotherapie I, Ziiricher Str. 40, 28325 Bremen, Germany Baddeley’s model of working memory (1986) contains the central executive and two domain specilic subsystems: the phonological and the visuo-spatial loop. Other cognitive functions, however, require integration across time, too. We recorded event-related potentials (ERPs) in order to investigate sensory-motor working memory. 15 healthy volunteers participated in this study. In this paper we present results for prestimulus EEG. There were three experimental conditions for each subject: a choice reaction task as control task, a working memory task with low demands and another one with high demands. The working memory task requires the subjects to keep in mind the right- or left-hand used for button pressing. The recorded ERPs were analysed by means of FFI. The frequency analysis computed from prestimulus EEG showed a prominent maximum in the 10 Hz frequency range
of Psychophysiology
30 (1998)
95-271
at the occipital location. Following this procedure, the prestimulus EEG were filtered in the 8-15 Hz range. The results showed that during the high demand memory task the amplitude of alpha waves were highly increased. Furthermore we observed a prolongation of alpha synchronization. Our results support the results of Klimesch (1997) and dynamic memory model of Basar et al. (19971, who demonstrated that the alpha activity is strongly correlated with short term memory, working memory and probably with long term memory engrams.
346 MULTI-CHANNEL EEG: CHOOSING AN METHOD
EOG CORRECTION OF THE APPROPRIATE REGRESSION
Rodney J. Croft and Robert J. Barry Department of Psychology, University of Wollongong, Wollongong, Australia In order to remove eye movement-related contamination from the EEG, EOG correction procedures are used. These estimate the proportion of EOG present in the EEG, and remove it. This estimation relies on the regression procedure. When more than one EOG channel is used in the correction, the appropriate type of regression equation to use is not known. To resolve this matter, computer simulations were employed to compare the simultaneous, multiple-stage and single channel regression methods of correction. Two-channel EOG propagation was modeled on prior findings, under conditions of varying. EOG/EEG ratio and VEOG/HEOG correlation. The dependent variable was the correlation between the uncontaminated and the corrected EEG. It was found that the simultaneous regression procedure gave the best correction, with its advantage increasing as a function of both EOG/EEG ratio and VEOG/HEOG correlation. It is therefore recommended that the simultaneous regression procedure be used for multi-channel EOG correction of the EEG.
347 EFFECTS OF COGNITIVE LOAD ON THE ONGOING-EEG
AND PHYSICAL WORK-
Gebhard Sammer IPV, University of Giessen, Otto-Behaghel Germany
Str. 10, D-35394
Frontally, an increased theta-activity can be seen with mental load, which is suggested to indicate focused-attention. In this study it was investigated if physical load and cognitive load can be discriminated by measures of the ongoing-EEG. Ten subjects performed three tasks; a physical task (push/pull a lever), a cognitive task (both detection and mental addition of numerical target stimuli), and a dual task (combination of the physical/cognitive tasks). Stimuli were presented visually with fixed ISls (= 1.3 set). Lever movements were required at the same rate. Subjects performed 4 blocks x 3 tasks (physical, mental, dual).