Perception and Consciousness

Introduction

Fast, gamma rhythms range from 30 to 100 Hz, and may vary in frequency during a response. The 20-100 Hz range we consider here overlaps the beta band (15 to 30 Hz), but we will ignore the finer points of EEG classification here. The natural history and functional roles of synchronous gamma oscillations have been reviewed recently [5,10,12,22]. Below is a potted history.

Gamma rhythms occur in humans and other mammals following sensory stimuli. They often occur in brief runs in these responses. "Induced rhythms" at 50-60 Hz were first described in olfactory bulb by Adrian [1]. They have since been found in: olfactory [4], visual [3a,3b,6-8,11,22], auditory [13,16], somatosensory [2] and motor cortex [17,19,21]. Gamma oscillations also occur in the hippocampus [3,24], where the link with external sensory stimuli is less direct, but may still exist in the multimodal inputs it receives from higher order sensory cortices. Hippocampal gamma tends to occur during the theta (4-12 Hz) EEG that is a prominent feature of the hippocampus in vivo [3,23], especially during exploration.

In Man the auditory response includes brief "40 Hz transient responses" [18,25], which increase when the subject pays attention and which disappear with loss of consciousness during anaesthesia [14]. Repetitive auditory stimulation at ~40 Hz generates a large "40 Hz steady state response" [9]. MEG recordings in Man suggest that gamma rhythms can be very widespread [20], both during waking and dream states. Other MEG measurements in Man suggest that gamma rhythms may be organised to sweep across the whole brain, perhaps providing "temporal binding .... into a single cognitive experience" [15].

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