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Abstract:
Colour and motion serve as the prime examples of segregated processing in the visual brain, giving rise to the question how conjunctions of these features are represented. This problem has been termed the ‘binding problem’. Human volunteers viewed visual displays containing coloured dots rotating around a central fixation cross. The dots could be red or green, and rotate clockwise or counter-clockwise, leading to four possible stimulus displays. Superimposed pairs of such stimuli provided two additional displays, each containing both colours and both directions of motion, but differing exclusively in their feature-conjunctions. We used fMRI and multivariate classifier analysis to examine voxel activation patterns obtained whilst subjects viewed such displays. Our analyses confirm the presence of directionally specific motion information across visual cortex and also provide evidence of spatially structured hue representations in all early visual areas except V5/MT+. Within each cortical area, the most informative voxels coding for colour and for motion appeared to be spatially separate and to exhibit a high degree of invariance to changes in a corresponding secondary feature. Notably, our results also demonstrate the explicit representation of feature conjunctions in primary visual cortex and beyond. 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.
