A Two-Stage Cascade Model of BOLD Responses in Human Visual Cortex
We measured responses to several types of grating stimuli varying in contrast. Responses of an example voxel are shown (subject 2, area V1, voxel 31150). The complex-cell energy (CC) model consists of V1 energy and spatial summation, and predicts that responses rise linearly with contrast. However, the actual responses exhibit saturation at low contrasts. To account for contrast saturation, we incorporated divisive normalization  into the model. The divisive normalization (DN) model fits the data accurately.