Hierarchical processing of tactile shape in the human brain.

It is not known exactly which cortical areas compute somatosensory representations of shape. This was investigated using positron emission tomography and cytoarchitectonic mapping. Volunteers discriminated shapes by passive or active touch, brush velocity, edge length, curvature, and roughness. Discrimination of shape by active touch, as opposed to passive touch, activated the right anterior lobe of cerebellum only. Areas 3b and 1 were activated by all stimuli. Area 2 was activated with preference for surface curvature changes and shape stimuli. The anterior part of the supramarginal gyrus (ASM) and the cortex lining the intraparietal sulcus (IPA) were activated by active and passive shape discrimination, but not by other mechanical stimuli. We suggest, based on these findings, that somatosensory representations of shape are computed by areas 3b, 1, 2, IPA, and ASM in this hierarchical fashion.