grantor:
University of TorontoRepresentational plasticity has been well documented in experimental animals. However in humans, somatotopographic reorganization has only been anecdotally observed. Recent studies have suggested that cortical representation of body parts can change in humans; however the extent of reorganization is unknown and no attempts have been made to determine the contribution of subcortical structures to cortical plasticity. The aim of this thesis was to determine (i) whether plasticity exists in the human at the thalamic level and (ii) what the possible mechanisms involved are. 'I'. Long-term deafferentation was associated with different patterns of thalamic organization as determined by receptive (RF) and projected field (PF) mapping. In patients with leg/foot deafferentation there was an expansion of "trunk" representation and 4 of 5 limb amputees displayed an enlarged stump representation. Patients with face deafferentation had a significantly larger region of thalamus from which microstimulation evoked PFs on the face. 'II'. Two mechanisms have been proposed to account for such alterations in representation: sprouting of new connections and unmasking of previously present, relatively ineffective connections. The unmasking hypothesis was investigated by temporarily deafferenting cells with RFs located on the lidocaine-blocked digit. Most of these neurons displayed an improvement in, or an appearance of, new responses to mechanical stimulation of adjacent digits. 'III'. To identify the relatively ineffective connections responsible for this phenomenon, the RFs of single units and nearby sites were subjected to electrical stimulation (ES) and in some cases repetitive mechanical stimulation (RMS). Responses from within the RF were excitatory and of short latency (SLRs, 20 ms). Most cells (87%) responded to ES and RMS applied outside their RF and these responses were of long latency (LLRs, 42 ms), although some showed both SLRs and LLRs. 'IV'. Excessive cutaneous stimulation of one skin surface has also resulted in alterations in representation in the cortex of experimental animals. Patients with tremor, or increased afferent input to movement-related neurons, were found to have a larger region of movement-related representation. Despite limited thalamic exploration and the inherent problems with data collection in patients, when I-IV are examined together and interpreted in light of animal models, one can conclude that plasticity is indeed possible in the human somatosensory system and present at the thalamic level. The clinical importance of plasticity may be two-fold: both beneficial in that it may aid recovery from injury and detrimental in its possible maintenance of pain and tremor. (Abstract shortened by UMI.)Ph.D
