Gamma-band activity over early sensory areas predicts detection of changes in audiovisual speech stimuli

Oscillatory activity in the gamma-band range in human magneto- and electroencephalogram is thought to reflect the oscillatory synchronization of cortical networks. Findings of enhanced gamma-band activity (GBA) during cognitive processes like gestalt perception, attention and memory have led to the notion that GBA may reflect the activation of internal object representations. However, there is little direct evidence suggesting that GBA is related to subjective perceptual experience. In the present study, magnetoencephalogram was recorded during an audiovisual oddball paradigm with infrequent visual (auditory /ta/ + visual /pa/) or acoustic deviants (auditory /pa/ + visual /ta/) interspersed in a sequence of frequent audiovisual standard stimuli (auditory /ta/ + visual /ta/). Sixteen human subjects had to respond to perceived acoustic changes which could be produced either by real acoustic or illusory (visual) deviants. Statistical probability mapping served to identify correlations between oscillatory activity in response to visual and acoustic deviants, respectively, and the detection rates for either type of deviant. The perception of illusory acoustic changes induced by visual deviants was closely associated with gamma-band amplitude at approximately 80 Hz between 250 and 350 ms over midline occipital cortex. In contrast, the detection of real acoustic deviants correlated positively with induced GBA at approximately 42 Hz between 200 and 300 ms over left superior temporal cortex and negatively with evoked gamma responses at approximately 41 Hz between 220 and 240 ms over occipital areas. These findings support the relevance of high-frequency oscillatory activity over early sensory areas for perceptual experience.