Functional mapping of somatosensory cortices in the human brain

Abstract

Tactile perception is a remarkable ability that enables us to recognise objects and handle tools. Cortical processing during tactile and shape perception in humans is not fully understood. The perception of the shape of an object using touch can be divided into different submodalities such as the motion due to the indentation of the object-skin contact, the position of the exploring fingers, surface deviations and shape primitives (curvature and edges). Positron emission tomography (PET) was used in a series of experiments to measure changes in regional cerebral blood flow (rCBF) during discrimination of these different types of modalities.

The experimental set-up included investigating neuronal populations engaged during discrimination of differences in brush stroke velocity (paper I), sustained pressure, spring strengths during active compression, and the change in position of a limb (paper II). Studies number III and IV were designed to investigate how the somatosensory cortex computes shape. Surface curvature changes are suggested to be a major determinant for the perception of shape, hence the objective of study III was to investigate whether objects differing only in the spectra of surface curvatures activated somatosensory areas differently. Study number IV was designed to investigate whether any cortical areas are specifically activated by discrimination of objects differing in shape as opposed to other mechanoreceptive stimuli. Finally, changes in rCBF in visual areas during discrimination of somatosensory stimuli were examined (paper V).

Changes in rCBF were compared to post mortem brain representations of cytoarchitectonically defined areas in the postcentral gyrus (areas 3a, 3b, I and 2), the primary motor cortex, and the primary and secondary visual areas. The main findings of the experiments showed that cytoarchitecturally defined areas 3b and I were activated by discrimination of all types of stimuli in all conditions tested, with no selectivity (paper I-IV). Area 3a was only active in the conditions in which motor activity occurred (paper 11 and IV). Area 2 was activated by all mechanical stimuli tested, but showed statistical preference for surface curvature changes (study 111) and shape stimuli (study IV).

Two regions were identified that responded specifically when discriminating object stimuli as opposed to brush velocity (study IV). One was located to the anterior part of the supramarginal gyrus (ASM) and the other was located to the cortex lining the intraparietal sulcus (IPA). These regions were only activated during discrimination of the shapes of objects and not when other mechanical stimuli were discriminated. This could, in the context of shape processing, be in accordance with a hierarchical organisation. Areas 3b and I activated by all types of stimuli at the first level, area 2 at a second level specifically activated by curvature changes and shape stimuli and IPA and ASM at yet a higher level, activated by shape discrimination only. A number of cortical motor areas were found activated in the absence of motor activity during somatosensory stimulation (paper I, II and IV). This is in accordance with electrophysiological data showing activation in motor areas during processing of somatosensory stimuli. The anterior cerebellum was the only region found to distinguish active from passive touch (paper IV), Discriminations of somatosensory stimuli were accompanied by a general decrease in blood flow in primary and secondary visual cortices (paper V).