β-Band correlates of the fMRI BOLD response.

Oscillatory activity in the β-band (15-30 Hz) has been studied in detail in the sensorimotor cortex. It has been postulated that β-activity acts as a localized gating of cortical activity. Here, the induced oscillatory response in the β-band is measured by magnetoencephalography, and the hemodynamic response is measured by fMRI. We assess the linearity of the responses to stimuli of varying duration in the primary motor cortex and to a sinusoidal drifting grating of varying contrast amplitude and drift frequency in the visual cortex. In this way, we explore the nature of β-oscillations and their relationship with hemodynamic effects. Excellent spatial colocalization of BOLD and β-activity in both central and lateral (MT) visual areas and sensorimotor areas suggests that the two are intimately related. In contrast to the BOLD response, the level of β-desynchronization is not modulated by stimulus contrast or by stimulus duration, consistent with a gating role. The amplitude of β-desynchronization in the central visual area is however modulated by drift frequency, and this seems to parallel the modulation in BOLD amplitude at the same location.