A Two-Stage Cascade Model of BOLD Responses in Human Visual Cortex
(A) Second-order contrast improves model fits. We fit the CSS model to the spatial stimuli (shown in Figure 4) and evaluated how well the model predicts responses to the grating stimuli (shown in Figure 3). Results for an example voxel are shown (subject 2, area V2, voxel 42608). The CSS model substantially overestimates the grating responses. To improve performance, we incorporated computation of second-order contrast into the model. The second-order contrast (SOC) model fits the data accurately. (B) Additional demonstration of second-order effect. We measured responses to noise patterns varying in the amount of separation between the contours composing the patterns. At low separation levels, the stimuli contain little variation in contrast-energy across space and evoke weak responses, as expected (same voxel in panel A).