Object shape differences reflected by somatosensory cortical activation.

Humans can easily by touch discriminate fine details of the shapes of objects. The computation of representations and the representations of objects differing in shape are, when the differences are not founded in different sensory cues or the objects belong to different categories, assumed to take place in a series of cortical areas, which only show differences at the single-neuron level. How the somatosensory cortex computes shape is unknown, but theoretically it should depend heavily on the curvatures of the object surfaces. We measured regional cerebral blood flow (rCBF) of normal volunteers with positron emission tomography (PET) as an index of neuronal activation. One group discriminated a round set of ellipsoids having a narrow spectrum of curvatures and an oblong set of ellipsoids having a broad spectrum of curvatures. Another group discriminated curvatures. When the rCBF from the conditions round and oblong ellipsoid discrimination was contrasted, part of the cortex lining the postcentral sulcus had significantly higher rCBF when ellipsoids having a broader spectrum of curvatures were discriminated. This cortex was also activated by curvature discrimination. The activation is therefore regarded as crucial for the computation of curvature and in accordance with curvature being a major determinant of object form; this cortex is also crucially active in somatosensory shape perception. A comparison of the activation with cytoarchitectural maps, in the anatomical format of the standard brain for both PET and cytoarchitectural brain images, revealed that this part of the cortex lining the postcentral sulcus is situated caudally from cytoarchitectural area 1 and may involve presumptive area 2 on the posterior bank of the sulcus.