Sensory-evoked high-frequency (gamma-band) oscillating potentials in somatosensory cortex of the unanesthetized rat

A 64-channel epipial electrode array was used to investigate high-frequency (gamma-band) oscillations in somatosensory cortex of the unanesthetized and unrestrained rat. Oscillations were evoked by manual stimulation of the vibrissae and mystacial pad. Stimulation of the contralateral vibrissae resulted in a significant increase in gamma-power during 128-ms epochs taken just following stimulus onset compared to the prestimulus baseline. Stimulation of the ipsilateral vibrissae was completely ineffective in evoking gamma-oscillations in any animals. Sensory evoked gamma-oscillations were constrained to primary (SI) and secondary (SII) somatosensory cortex. When averaged to an arbitrary reference of peak times in one of the channels, these oscillations exhibited a systematic temporal organization, propagating from the rostral portion of SI to the barrel field proper, and finally to SII. These spatiotemporal characteristics were probably produced by intracortical pathways within rodent somatosensory cortex. The rostrocaudal propagation of gamma-oscillations within the barrel field may also reflect whisking patterns observed when the vibrissae are used as a sensory array, suggesting that synchronized gamma-oscillations may play a role in assembling punctate afferent information provided by the vibrissae into a coherent representation of a somatosensory stimulus.