Supralinear and supramodal integration of visual and tactile signals in rats: psychophysics and neuronal mechanisms

To better understand how object recognition can be triggered independently of the sensory channel through which information is acquired, we devised a task where rats judged the orientation of a raised, black/white grating. They learned to recognize two categories of orientation: 0+/-45 degrees ("horizontal") and 90+/-45 degrees ("vertical"). Each trial required a visual (V), tactile (T), or else visual-tactile (VT) discrimination; VT performance was better than that predicted by optimal linear combination of V and T signals, indicating synergy between sensory channels. We examined posterior parietal cortex (PPC) and uncovered key neuronal correlates of the behavioral findings: PPC carried both graded information about object orientation and categorical information about the rat's upcoming choice; single neurons exhibited identical responses under the three modality conditions. A linear classifier of neuronal population firing replicated the behavioral findings. Is the distinction between the cardinal orientations of horizontal and vertical - by use of a 45-degree boundary - a hardwired classification, or is the boundary flexible? To answer this, we employed boundaries between 15 and 30 degrees and between 60 and 75 degrees. Rats easily learned such a classification and could flexibly shift boundary. Neuronal analysis during the plasticity condition is underway. Taken together, these findings point to PPC's involvement in the supramodal processing of shape information.