The Coding of Color, Motion, and Their Conjunction in the Human Visual Cortex

Background: Color and motion serve as the prime examples of segregated processing in the visual brain, giving rise to the question how color-motion conjunctions are represented. This problem is also known as the "binding problem.". Results: Human volunteers viewed visual displays containing colored dots rotating around the center. The dots could be red or green and rotate clockwise or counterclockwise, leading to four possible stimulus displays. Superimposed pairs of such stimuli provided two additional displays, each containing both colors and both directions of motion but differing in their feature conjunctions. We applied multivariate classifiers to voxel-activation patterns obtained while subjects viewed such displays. Our analyses confirm the presence of directional-motion information across visual cortex and provide evidence of hue coding in all early visual areas except V5/MT⁺. Within each cortical area, information on color and motion appeared to be coded in distinct sets of voxels. Furthermore, our results demonstrate the explicit representation of feature conjunctions in the primary visual cortex and beyond. Conclusions: The results show that conjunctions can be decoded from spatial activation patterns already in V1, indicating an explicit coding of conjunctions at early stages of visual processing. Our findings raise the possibility that the solution of what has been taken as the prime example of the binding problem engages neural mechanisms as early as V1.