High frequency (gamma-band) oscillating potentials in rat somatosensory and auditory cortex.

An 8 x 8 multichannel electrode array was used to record epipial field potentials, spontaneous gamma oscillations, and the interaction between single trial evoked potentials and ongoing gamma activity in rat somatosensory and auditory Cortex. Array placement over both these cortical regions was verified using cytochrome oxidase histochemistry. Replicating earlier findings, the epipial middle latency auditory and somatosensory evoked potentials (MAEP and MSEP, respectively) consisted of a stereotyped pattern of activation characterized by a spatially confined biphasic sharp wave followed by more diffuse slow wave components whose areal distribution adhered closely to established boundaries of primary and secondary sensory cortex. Spontaneous gamma activity, while exhibiting far more spatiotemporal variation, was also centered on primary and secondary sensory cortex and was significantly attenuated at intercalated dysgranular regions. A modality specificity of gamma activity was also demonstrated in the present study, where spindles occurred independently in auditory and somatosensory cortex. Furthermore, following presentation of a single click or vibrissal displacement, spontaneous gamma activity was suppressed and subsequently enhanced only in the modality stimulated. We conclude that in the lightly anesthetized rodent, spontaneous gamma oscillations are not a global neocortical phenomena, but are instead restricted to the same areas of sensory cortex participating in evoked potentials. However, unlike the MAEP and MSEP which are dominated by systematic activation of parallel thalamocortical projections, the marked spatiotemporal variability of gamma spindles suggests a more complex neurogenesis, probably including dominant contributions from intracortical neural circuitry.