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Abstract
The sense of touch is mediated by several separate tactile ‘channels’ defined by the classes of peripheral tactile receptors in the skin. How the information arising in these channels interacts during cortical processing is currently debated. This thesis explores the topic of tactile cross-channel interaction with a specific focus on the stimulus property of intensity in fast-adapting receptors. This is done using both neurophysiological and psychophysical methodologies: electrophysiological recordings using large multielectrode arrays in primary and secondary somatosensory cortices of the anaesthetised cat (Chapters 3 and 4); and psychophysical studies using human subjects (Chapter 5). Chapter 3 shows that some neurons in primary and secondary somatosensory cortex of the cat receive input from both fast-adapting channels of the glabrous skin, contrary to the prevailing model that argues for no cross-channel convergence. Chapter 4 describes the spatial extent of the region of primary and secondary somatosensory cortex activated by a vibrotactile stimulus of varying stimulus intensity. The results suggest that the area of activated cortex increases with increasing stimulus intensity, but that this is mostly due to a ‘filling in’ of a mosaic-like active region of cortex. Chapter 5 shows that cross-channel interaction between both fast-adapting glabrous tactile channels affects the detection threshold of a low-frequency vibrotactile stimulus. A high-frequency sinusoidal masking stimulus presented to the same location as a low-frequency sinusoidal stimulus resulted in an increased detection threshold of the low-frequency stimulus, whereas a high-frequency masker presented at a proximal location from the low-frequency stimulus does not produce an increase in detection threshold. The results obtained from all these studies support the claim for cross-channel interaction among the tactile channels, even for the processing of basic perceptual properties such as detection threshold that likely occur as early as primary and secondary somatosensory cortical regions.