INTER-HEMISPHERIC AND INTRA-HEMISPHERIC SPATIOTEMPORAL ORGANIZATION OF SPONTANEOUS ELECTROCORTICAL OSCILLATIONS

1. Two 64-channel epipial electrode arrays were positioned on homologo us locations of the right and left hemisphere, covering most of primar y and secondary auditory and somatosensory cortex in eight lightly ane sthetized rats. Array placement was verified with the use of cytochrom e oxidase histochemistry. 2. Middle-latency auditory and somatosensory evoked potentials (MAEPs and MSEPs, respectively) and spontaneous osc illations in the frequency range of 20-40 Hz (gamma oscillations) were recorded and found to be spatially constrained to regions of granular cortex, suggesting that both phenomena are closely associated with se nsory information processing. 3. The MAEP and MSEP consisted of an ini tial biphasic sharp wave in primary auditory and somatosensory cortex, respectively, and a similar biphasic sharp wave occurred similar to 4 -8 ms later in secondary sensory cortex of the given modality. Average d gamma oscillations also revealed asynchronous activation of sensory cortex, but with a shorter 2-ms delay between oscillations in primary and secondary regions. Although the long latency shift of the MAEP and MSEP may be due in part to asynchronous activation of parallel thalam ocortical projections to primary and secondary sensory cortex, the muc h shorter shift of gamma oscillations in a given modality is consisten t with intracortical coupling of these regions. 4. Gamma oscillations occurred independently in auditory and somatosensory cortex within a g iven hemisphere. Furthermore, time series averaging revealed that ther e was no phase-locking of oscillations between the sensory modalities. 5. Gamma oscillations were loosely coupled between hemispheres; oscil lations occurring in auditory or somatosensory cortex of one hemispher e were often associated with lower-amplitude oscillations in homologou s contralateral sensory cortex. Yet, the fact that time series averagi ng revealed no interhemispheric phase-locking suggests that the corpus callosum may not coordinate the bilateral gamma oscillations, and tha t a thalamic modulatory influence may be involved.