The Journal of Neurophysiology Vol. 82 No. 6 December 1999, pp. 3095-3107
Copyright ©1999 by the American Physiological Society

Visual Stimulus-Dependent Changes in Interhemispheric EEG Coherence in Humans

 ¹Research Institute of Developmental Physiology, 119121 Pogodinskaya 8-2, Moscow, Russia;  ²Institut de Biologie Cellulaire et de Morphologie, Université de Lausanne, 1005 Lausanne, Switzerland;  ³Smith-Kettlewell Eye Research Institute, San Francisco, California 94115;  ⁴Department of Neurology, Centre Hospitalier Universitaire Vaudois, 1011 Lausanne, Switzerland; and  ⁵Division of Neuroanatomy and Brain Development, Department of Neuroscience, Karolinska Institutet, S-17177 Stockholm, Sweden

Knyazeva, M. G., D. C. Kiper, V. Y. Vildavski, P. A. Despland, M. Maeder-Ingvar, and G. M. Innocenti. Visual Stimulus-Dependent Changes in Interhemispheric EEG Coherence in Humans. J. Neurophysiol. 82: 3095-3107, 1999. We analyzed the coherence of electroencephalographic (EEG) signals recorded symmetrically from the two hemispheres, while subjects (n = 9) were viewing visual stimuli. Considering the many common features of the callosal connectivity in mammals, we expected that, as in our animal studies, interhemispheric coherence (ICoh) would increase only with bilateral iso-oriented gratings located close to the vertical meridian of the visual field, or extending across it. Indeed, a single grating that extended across the vertical meridian significantly increased the EEG ICoh in normal adult subjects. These ICoh responses were obtained from occipital and parietal derivations and were restricted to the gamma frequency band. They were detectable with different EEG references and were robust across and within subjects. Other unilateral and bilateral stimuli, including identical gratings that were effective in anesthetized animals, did not affect ICoh in humans. This fact suggests the existence of regulatory influences, possibly of a top-down kind, on the pattern of callosal activation in conscious human subjects. In addition to establishing the validity of EEG coherence analysis for assaying cortico-cortical connectivity, this study extends to the human brain the finding that visual stimuli cause interhemispheric synchronization, particularly in frequencies of the gamma band. It also indicates that the synchronization is carried out by cortico-cortical connection and suggests similarities in the organization of visual callosal connections in animals and in man.