Experimental study of EEG and BOLD responses to sinusoidal contrast modulation

This paper reports pilot experiments and modeling studies on the dynamics of the BOLD response to sinusoidal input. Steady-state visual evoked potentials were generated with a contrast-reversing grating. The absolute contrast of the grating was modulated sinusoidally, across a range of frequencies relevant to block and event-related designs. The amplitude of the neural response was found to track the stimulus closely, with some interesting differences at higher frequencies. The BOLD responses were quasi-sinusoidal, without the onset or offset transients typical in block designs. A hemodynamic model, based upon the balloon model with delayed compliance, and a linear model of inward flow, was fit to the BOLD data using the contrast waveform as input. The hemodynamic model fit the BOLD data well, but required different model parameters for each stimulus frequency. This implies that the differential equations comprising the hemodynamic model do not capture adequately the dynamics of the BOLD response across multiple time scales.