The ongoing neural activity in human primary somatosensory cortex (SI) is characterized by field potential oscillations in the 7–13 Hz range known as the mu rhythm. Recent work has shown that the magnitude of the mu oscillation immediately preceding the onset of a weak stimulus has a significant impact on its detection. The neural mechanisms mediating this impact remain not well understood. In particular, whether and how somatosensory mu rhythm is modulated by executive areas prior to stimulus onset for improved behavioral performance has not been investigated. We addressed these issues by recording 128-channel scalp electroencephalogram from normal volunteers performing a somatosensory perception experiment in which they reported the detection of a near-threshold electrical stimulus (∼50% detection rate) delivered to the right index finger. Three results were found. First, consistent with numerous previous reports, the N1 component (∼140 msec) of the somatosensory-evoked potential was significantly enhanced for perceived stimulus compared to unperceived stimulus. Second, the prestimulus mu power and the evoked N1 amplitude exhibited an inverted-U relationship, suggesting that an intermediate level of prestimulus mu oscillatory activity is conducive to stimulus processing and perception. Third, a Granger causality analysis revealed that the prestimulus causal influence in the mu band from prefrontal cortex to SI was significantly higher for perceived stimulus than for unperceived stimulus, indicating that frontal executive structures, via ongoing mu oscillations, exert cognitive control over posterior sensory cortices to facilitate somatosensory processing.

Response time (RT) is an important behavioral measure of the overall efficacy of sensorimotor processing and is known to vary significantly from trial to trial. Past work on how stimulus evoked cortical responses contribute to RT variability has helped delineate the stages of neuronal information processing. Much less is known about how the state of the brain immediately preceding the stimulus onset (prestimulus) affects RT. We addressed this problem by analyzing data from three macaque monkeys trained to perform a visuomotor pattern discrimination task. Local field potentials were recorded from up to 16 bipolar surface-to-depth electrodes widely distributed over one cerebral hemisphere in each monkey. The degree of linear correlation between RT and prestimulus spectral power was determined over a wide range of frequencies. In the prefrontal cortex, prestimulus power in the beta range (14–30 Hz) was negatively correlated with RT in two monkeys, suggesting a possible role of activity in this frequency range in the mediation of top-down control of visuomotor processing. In the sensorimotor cortex, prestimulus power in the beta range was positively correlated with RT in two monkeys, consistent with the hypothesis that oscillations in this range support the maintenance of steady-state motor output. In visual occipital and temporal lobe areas, prestimulus power in the alpha/low beta range (8–20 Hz) showed positive correlations with RT in three monkeys, possibly reflecting a spatially specific disengagement of visual anticipatory attention. Through measurement of prestimulus spectral coherence, it was further determined that sites showing similar patterns of correlation between spectral power and RT were also linked together in synchronized networks.