Relationship between Orientation- and Direction-Selective Responses in the Human Visual Cortex

The human visual system effectively processes the direction of motion, irrespective of the form that defines the motion. We investigated the neural basis of this form-invariant motion processing, by comparing cortical responses to moving gratings and random dot patterns using functional magnetic resonance imaging (fMRI) in conjunction with multivariate pattern classification. In the gratings experiment, observers viewed sinusoidal gratings moving in one of four possible directions, in which the spatial orientations of the gratings were perpendicular to the direction of motion. In separate fMRI runs, observers viewed random-dot patterns moving in the same four possible directions at 100% coherence, except here the spatial orientations, induced by motion streaks, were parallel to the direction of motion. We found that activity patterns throughout early visual areas V1 to MT+ could reliably discriminate the direction of motion defined by moving gratings and by random dot patterns, indicating that direction-selective neural responses are prevalent in all early visual areas. However, generalization analysis between two types of moving stimuli revealed that classification of BOLD activity patterns was more strongly influenced by the spatial orientation of stimuli than motion direction for early visual areas V1 and V2, and that motion-based classification was more prominent in relatively higher visual areas. These findings indicate that both orientation and direction information is preserved in all visual areas, and second, that patterns of BOLD activity are dominated by orientation-selective responses at early stages of visual processing and direction-selective responses become dominant at the later stages of visual processing