Top-down and bottom-up mechanisms as reflected by beta and gamma oscillations in speech perception: An individual-difference approach

Recent neurophysiological studies have proposed distinct roles of beta and gamma oscillations in implementing top-down and bottom-up processes. The present study aims to test this hypothesis in the domain of speech perception. We examined beta and gamma oscillations elicited to a tone contrast in a passive oddball paradigm, and their relationships with discrimination sensitivity d' and RT from two groups of healthy adults who showed high and low discrimination sensitivity to the contrast. The low-sensitivity group showed a significant reduction in beta, which was further related to d'. Individual differences in RT were related to different frequency bands in the two groups, with a RT-beta correlation in the low-sensitivity group, and a RT-gamma relation in the high-sensitivity group. Based on these findings, we suggest that beta, implicated in top-down processing, reflects individual differences in phonological representations, and that gamma, involved in bottom-up processing, reflects individual differences in acoustic encoding.