The neural basis of illusory motion perception evoked from static images has not been established well. We examined changes in neural activity in motion sensitive areas of the human visual cortex by using functional magnetic resonance imaging (fMRI) technique when a static illusory-motion image (‘Rotating Snakes’) was presented to participants. The blood-oxygenation-level dependent (BOLD) signal changes were compared between the test stimulus that induced illusory motion perception and the control stimulus that did not. Comparison was also made between those stimuli with and without eye movements. Signal changes for the test stimulus were significantly larger than those for the control stimulus, if accompanied by eye movements. On the other hand, the difference in signal changes between test and control stimuli was smaller, if steady fixation was required. These results support the empirical finding that this illusion is related to some component of eye movements.

The variability of color-selective neurons in human visual cortex is considered more diverse than cone-opponent mechanisms. We addressed this issue by deriving histograms of hue-selective voxels measured using fMRI with a novel stimulation paradigm, where the stimulus hue changed continuously. Despite the large between-subject difference in hue-selective histograms, individual voxels exhibited selectivity for intermediate hues, such as purple, cyan, and orange, in addition to those along cone-opponent axes. In order to rule the possibility out that the selectivity for intermediate hues emerged through spatial summation of activities of neurons selectively responding to cone-opponent signals, we further tested hue-selective adaptations in intermediate directions of cone-opponent axes, by measuring responses to 4 diagonal hues during concurrent adaptation to 1 of the 4 hues. The selective and unidirectional reduction in response to the adapted hue lends supports to our argument that cortical neurons respond selectively to intermediate hues.

Colors are represented in the cone-opponent signals, L-M versus S cones, at least up to the level of inputs to the primary visual cortex. We explored the hue selective responses in early cortical visual areas through recordings of steady-state visual evoked potentials (SSVEPs), elicited by a flickering checkerboard whose color smoothly swept around the hue circle defined in a cone-opponent color space. If cone opponency dominates hue representation in the source of SSVEP signals, SSVEP amplitudes as a function of hue should form a profile that is line-symmetric along the cardinal axes of the cone-opponent color space. Observed SSVEP responses were clearly chromatic ones with increased SSVEP amplitudes and reduced response latencies for higher contrast conditions. The overall elliptic amplitude profile was significantly tilted away from the cardinal axes to have the highest amplitudes in the “lime-magenta” direction, indicating that the hue representation in question is not dominated by cone-opponency. The observed SSVEP amplitude hue profile was better described as a summation of a perceptual response and cone-opponent responses with a larger weight to the former. These results indicate that hue representations in the early visual cortex, measured by the SSVEP technique, are possibly related to perceptual color contrast.