Augmentation of induced visual gamma activity by increased task complexity.

Recently the study of induced gamma band oscillations has focused on their modulation by top-down processes, mainly attention. Numerous studies have observed an increase in induced gamma band energy with increases in covert selective attention and visual perception. The current study investigated the modulation of visually induced gamma band oscillations by top-down processes associated with task complexity. Fourteen human subjects performed a reaction time task under two experimental conditions that differed in task complexity. In one, subjects simply had to press one of four buttons that corresponded to a colour stimulus shown to the subject. In the second, the stimulus response mapping was altered by the implementation of a rule, thus increasing task complexity. Cortical electrical activity was recorded using a 65 electrode whole scalp electroencephalographic (EEG) net. The EEG activity was analysed using Morlet wavelets to produce time-frequency maps. Although induced gamma band activity was observed in both conditions, there was significantly greater energy during the rule-operation condition at approximately 276 ms after the appearance of the stimulus. This increase was localized to electrodes overlying the right-central parietal scalp. The results of this study show that top-down processes modulate the level of induced gamma band activity. We discuss these findings in terms of the role of gamma oscillations in the construction of a sensory representation useful for a correct motor response.