Frontal gamma band enhancement during a dynamic reversal pattern

42

MAPPING BRAIN

Abstracts /International Journal of Psychophysiology 25 (1997) 17-84

COGNITIVE

PROCESSES

IN THE HUMAN

RSJ Frackowiak* Wellcome Department of Cognitive Neurology, Institute of Neurology, 12 Queen Square, London WClN 3BG, UK This plenary lecture reviews methods for the non-invasive mapping of human brain functions. The two principal methods providing spatial information about the distribution of brain activity associated with cognitive function are positron emission tomography and functional magnetic resonance imaging. PET uses a tracer or perfusion, that is an indirect index of local glucose consumption and hence synaptic activity. Functional MRI uses a mismatch between the extent of change in perfusion and change in oxygen consumption associated with local brain activity. This results in a local fall in the oxygen extraction factor and hence an increase in local oxyhaemoglobin resulting in an increase of fMR1 signals. The spatiotemporal characteristics of brain activity require methods with millisecond time resolution such as event related potentials, EEG and MEG mapping. Informative experimentation depends on well formulated models of brain activity and well designed experiments to test these models. A number of examples will be used to illustrate these points. ERP DIFFERENCES BETWEEN SPONTANEOUS FIGURE REVERSALS AND EXTERNALLY INDUCED PERCEPTUAL SWITCHES D. Striiber*, C. Bagar-Eroglu and M. Stadler Institute of Psychology and Cognition Research, University of Bremen, P.O. Box 33 04 40, D-28334 Bremen, Germany We compared visual event related potentials (ERP) in two conditions: 1. ERPs induced by a spontaneous, i.e., not stimulus-driven figure reversal as it is the case when viewing ambiguous patterns. 2. ERPs induced by an external event, i.e., stimulus-driven perceptual switching. In the figure-reversalcondition subjects viewed at a dynamic ambiguous pattern which consists of four spots positioned in an upright rectangle. Two diagonally positioned spots are always flashed simultaneously and after a short pause replaced by the flashing of the other two spots. In this display the impression of two horizontally moving light-points alternates spontaneously with seen vertical apparent motion of the light-points. In the stimulusdriven-condition a simultaneous flashing of the horizontal neighboured light-points alternates with a simultaneous flashing of the vertical neighboured light-points. In this condition the switching between horizontal and vertical apparent motion of the light-points is controlled externally be means of computer. Thus, the spatio-temporal ratios in both experiments are identical. In both conditions subjects had to press a button when they perceived the switching. EEG was recorded from F3, F4, Cz, C3, C4, P3, P4, 01, 02 in 14 subjects. The following results were obtained: In both conditions we found a

positive wave in the EEG during perceptual switching at all locations which was most prominent in the parietocentral area. In the figure-reversal-condition the positive waves show a significantly smaller amplitude and longer duration than the positive wave in the stimulus-driven-condition, respectively. As revealed by single sweep analysis this difference is not a consequence of better time locking in the stimulus-driven-condition. The results are interpreted that the discrimination of the spontaneous figure reversal is more difficult and requires more stimulus evaluation time than the perception of the externally induced switching. FRONTAL GAMMA BAND ENHANCEMENT DURING A DYNAMIC REVERSAL PATTERN Canan Bazar-Eroglu*, Daniel Striiber and M. Stadler Institute of Psychology and Cognition Research, University of Bremen, Postfach 330440, 28334 Bremen, Germany Recent measurements of gamma band activity at the cellular level stress high level functional correlates such as binding of features. Whereas, field potential measurements in both human and animal demonstrate that gamma band responses may have multifold functions both sensory and in cognitive processing. Many stimulus patterns are accompanied by sudden non-voluntary perceptual reversals. EEG-measurements in the reversal and non-reversal phase can give information about the localization and the frequency distribution of endogeneous brain processes during perception. The present study describes the significant gamma band activity increase in EEG during states of perceptual switching (reversal state). In our experiments the multistability was induced with an ambiguous stimulus pattern, known as stroboscopic alternative motion (SAM). The investigations carried out in eleven subjects included a measuring strategy with three different experimental conditions. i) Recording of spontaneous EEG as baseline, ii) Recording of the EEG during naive observation of the ambiguous pattern, iii) Recording of EEG during active observation of SAM. The results show that the multistable perception is one of the multifold cognitive processes giving rise to gamma band enhancement in the entire cortex. The most significant gamma band enhancements were measured in frontal areas and can reach increases of 40 to 50% in states of naive and active observations of SAM respectively in comparison to control-EEG. (11 C. Bagar-Eroglu, D. Striiber, P. Kruse, E. Bagar and M. Stadler (1996) Frontal gamma band enhancement during multistable visual perception. Int. J. Psychophysiol (in press) DRIVE-REINFORCEMENT LEARNING AND HIERARCHICAL NETWORKS OF CONTROL SYSTEMS AS MODELS OF NERVOUS SYSTEM FUNCTION A. Harry Klopf, Wright Laboratory, Wright-Patterson Force Base, Ohio 45433-7318, USA

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