A functional gamma-band defined by stimulus-dependent synchronization in area 18 of awake behaving cats

During the past decade, numerous studies have demonstrated stimulus-specific synchronization of neuronal activity in the gamma-frequency range. However, it appears that the different analyses are based on widely different assumptions about which frequency range to investigate. Therefore, the term "gamma-synchronization" refers to an inhomogeneous spectrum of definitions and corresponding frequency bands. Moreover, most studies have been performed in anesthetized animals or in awake animals by use of fixation paradigms. Thus, it is difficult to relate these results to alert animals behaving under natural conditions. Here, we investigate stimulus specific synchronization in primary visual cortex of awake cats in a tracking paradigm. We record local field potentials and multiunit activity simultaneously from multiple electrodes. (1) We demonstrate that visual stimulation induces neuronal synchronization in a broad frequency range reaching well above 100 Hz. (2) We derive a functional gamma-band based on an objective criterion: We show that synchronization of neuronal activity is optimally orientation-tuned when a broad frequency band is considered. This band starts above 40 Hz, a frequency that is typically related to the term gamma-synchronization, and extends to very high frequencies. Interestingly, the frequency of maximum synchronization is different from the frequency at which synchronization is most stimulus specific. (3) We demonstrate synchronization of neuronal activity in a distinct low-frequency band with different properties suggesting separate functional roles of low- and high-frequency synchronization.