Abstract
‘Gamma’ range activity is often proposed to support perceptual processes by coordinating neural activity within and between visual areas. Historically, studies using non-human primate electrophysiology have repeatedly observed narrowband gamma oscillations (30–70 Hz) within early visual cortex in response to visual stimuli. In contrast, studies using human intracranial electrophysiology have often emphasized a broadband ‘high-gamma’ range activity (e.g. 70–150 Hz). Growing evidence suggests that these two signals in the gamma range reflect different biophysical processes and display distinctive stimulus dependencies. To quantify these differences, we employed high-density intracranial recordings from human visual cortex. We presented participants (n=7) with large field static grating stimuli (1 cycle/degree) at three contrast levels (20, 50, 100%). A subset of participants (n=5) also viewed grey-scale natural images from several visual categories (faces, houses, etc.). Spectral analyses revealed that grating stimuli induced both narrow and broadband gamma activity. Broadband gamma activity was transiently increased at stimulus onset and offset, with a fast onset time (~80ms). Signals in the narrowband gamma range occurred later (~130ms) and were sustained throughout stimulus presentation. In addition, narrowband gamma showed a clear central frequency (spectral peak), which was dependent on stimulus contrast, and increased with higher contrast levels (36 Hz at 20%; 40 Hz at 50%; 44 Hz at 100%). However, mean responses to naturalistic images displayed large broadband gamma range increases, but lacked narrowband gamma spectral peaks. Overall, our findings further develop prior observations made in human and non-human primate visual cortex, suggesting highly dissociable and stimulus dependent properties of narrow and broadband gamma activity. These findings have important implications for the functional significance of gamma range activity and how signals in this range should be quantified and interpreted.
Acknowledgement: R00MH103479 R01MH116914