The neural mechanisms of reliability weighted integration of shape information from vision and touch

Behaviourally, humans have been shown to integrate multisensory information in a statistically-optimal fashion by averaging the individual unisensory estimates according to their relative reliabilities. This form of integration is optimal in that it yields the most reliable (i.e. least variable) multisensory percept. The present study investigates the neural mechanisms underlying integration of visual and tactile shape information at the macroscopic scale of the regional BOLD response. Observers discriminated the shapes of ellipses that were presented bimodally (visual-tactile) or visually alone. A 2 × 5 factorial design manipulated (i) the presence vs. absence of tactile shape information and (ii) the reliability of the visual shape information (five levels). We then investigated whether regional activations underlying tactile shape discrimination depended on the reliability of visual shape. Indeed, in primary somatosensory cortices (bilateral BA2) and the superior parietal lobe the responses to tactile shape input were increased when the reliability of visual shape information was reduced. Conversely, tactile inputs suppressed visual activations in the right posterior fusiform gyrus, when the visual signal was blurred and unreliable. Somatosensory and visual cortices may sustain integration of visual and tactile shape information either via direct connections from visual areas or top-down effects from higher order parietal areas.