Touching upon tactile processing: Output dependent somatosensory functions in the brain

When a fly lands on your hand, one is both able to identify the exact location on the skin of the hand (perceptual recognition), as well as to hit the fly with the other hand (action). The research reported in this thesis aimed to study whether the processing of tactile information depends on the type of response one performs on it (action or perceptual recognition). Furthermore we were interested in the relationship between lower-level and higher-order somatosensory (tactile and kinaesthetic) processes and whether this relationship can be modulated by the required response. Using various experimental paradigms in healthy participants and stroke patients we were able to ascertain that higher order stored somatosensory knowledge influences bottom-up perception in several ways. We observed that these influences were most salient when bottom-up information was processed in order to consciously identify or recognize the tactile information. On the contrary, the processing of information for the computation of a goal directed motor response was less affected by higher-order processes. An important test that can account for such conclusion was provided by comparing the performances of two stroke patients, revealing a classical double dissociation. That is, one patient performed relatively well when programming a motor response directed towards a touched location on his hand, but was impaired when indicating this location on a drawing of a hand which requires stored knowledge about general body characteristics. In contrast, a second patient showed the reverse pattern of results (impaired motor response with intact perceptual recognition). This suggests a relative independence of tactile processing that underlies action and perception. Further support for the assumption that influences of higher-order somatosensory information are most pronounced in perceptual tasks is provided by a study on tactile imagery. Here we found that participants were able to use higher-order knowledge about the tactile world to imagine tactile information such as provided by a key ring. Also, this mental image influenced subsequently presented real tactile information (such as a short tap on the fingers), suggesting that the processing of real and imagined stimuli (partly) overlaps. This finding might be relevant for future investigation of the rehabilitation of tactile disorders. In sum, although the studies in this thesis show that tactile information is processed differently when it is used for the programming and control of an action as compared to when it is used for conscious recognition, this does not entail that these systems operate without communication. It could therefore be that the processing of somatosensory input may be better explained in terms of complex interactions between the two tasks. These interactions need to be specified in more detail and future studies should be aimed at exploring them.