Complementary roles of cortical oscillations in automatic and controlled processing during rapid serial tasks
Cognitive control may involve adjusting behaviour by inhibiting or altering habitual actions, requiring rapid communication between sensory, cognitive, and motor systems of the brain. Cognitive control may be achieved using top-down processing from frontal areas to inhibit prepared responses, likely mediated through frontal theta (4-8 Hz) oscillations. However there is conflicting evidence for mechanisms of response inhibition, where global and selective inhibition are either considered separate processes, or frontal areas maintain and execute goal-directed actions, including inhibition. In the current study we measured neuromagnetic oscillatory brain activity in twelve adults responding to rapidly presented visual cues. We used two tasks in the same subjects that required inhibition of a habitual "go" response. Presentation of infrequent "target" cues required subjects to completely inhibit responding (go/no-go task) or to perform an alternate response (go/switch task). Source analysis of oscillatory brain activity was compared for correct no-go and switch trials as well as error trials ("go" responses to targets). Frontal theta activity was similar in cortical location, amplitude and time course for correct no-go and switch responses reflecting an equivalent role in both global and selective response inhibition. Error-related frontal theta activity was also observed but was different in source location (errors vs correct, both tasks: p<0.005) and power (go/switch>go/no-go error, correct switch power, p=0.01). We additionally observed sensorimotor high gamma (60-90 Hz) activity accompanying motor responses, which was markedly stronger for correct switch and error responses compared with go responses, and was delayed for errors (p<0.01). These results suggest that gamma signals in the motor cortex may function to integrate inhibitory signals with sensorimotor processing, and may represent a mechanism for the overriding of habitual behaviours, as errors were predicted by a delay in gamma onset. This study supports a role for frontal areas in maintaining and executing goal-directed actions, and demonstrates that frontal theta activity and sensorimotor gamma oscillations have distinct yet complementary functional roles in monitoring and modifying habitual motor plans.