Figure 3.

Electrophysiological and oxygenation response to contrast changes. A, Time–frequency representations averaged over all 7 s segments of each contrast from a single subject (pooled electrodes O1 and O2) showing a response in the upper gamma frequency range (∼75 Hz). GBR is strongest for the highest contrast and decreases with lower contrasts. Note that gamma activity is sustained over the full length of stimulation. B, Fluctuations of occipital gamma activity (black; pooled electrodes O1 and O2) and HbR (red; pooled from colocated occipital probes) from a single subject for a period of ∼16 min. The background shading (yellow–green) depicts the changes in contrast strength. The close coupling between gamma and HbR changes is clearly seen in the unaveraged data; both parameters follow the contrast changes showing highest values for the largest contrast (yellow shading) and lowest values for the 0% contrast baseline (dark green shading). Note that contrast changes and gamma power are convolved with a HRF to allow for a comparison with the hemodynamic response.