Early gamma response is sensory in origin: a conclusion based on cross-comparison of results from multiple experimental paradigms

The study investigates the functional correlates of the early, time-locked gamma response. The study utilized a unique experimental strategy which involved the utilization of a series of experimental paradigms to which all subjects (n = 20) were exposed to in the same recording session. These paradigms induced an increasingly complex configuration of processes for their respective task performance and also required different levels of attention allocation. In their order of administration, the paradigms were single stimulus (SS), mismatch negativity (MMN), evoked potential (EP), easy oddball (OB-EZ) and hard oddball (OB-HD). Auditory stimuli were used in the study (10 ms r/f time, 50 ms duration, 65 dB SPL) with the standards as 1000 Hz or 1900 Hz and deviants as 2000 Hz. The early gamma showed a frontocentral topography. The difference between Fz and Pt recording sites were statistically significant. A comparative analysis of the gamma responses showed that the gamma that was obtained at the early time-window of 0-150 ms as a time-locked activity occurred irrespective of experimental paradigm; the early gamma did not vary with the degree of task complexity or with attentional allocation. It was concluded from these findings that the early gamma is basically a sensory phenomenon. Various studies have previously shown that under perceptual/cognitive tasks, gamma response is obtained as a non-phase-locked activity in the late time-windows. These studies concluded that the gamma response is basically perceptual/cognitive in function. However, in these studies the early sensory gamma was also present in the data. Collectively taken, these findings may lead to the conclusion that the gamma response is a multifunctional phenomenon, with the early portion representing sensory and the late portion perceptual/cognitive processing. (C) 1998 Published by Elsevier Science B.V. All rights reserved.