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Date of Award

12-2001

Degree Type

Thesis

Degree Name

Doctor of Philosophy (PhD)

Department

Philosophy

Supervisor

Professor Larry E. Roberts

Abstract

The basic problem addressed by the experiments of this thesis was how the human brain organizes the information transduced by the receptors of the somatosensory system (somatotopic maps), and how representations of that information are updated as a consequence of sensory experience (plasticity). The experiments of this thesis were designed to measure somatotopic maps of the digits in adult human subjects (experiment 1 & 2) and to measure the reorganization of these representations following discrimination training (experiment 3).

Experiment 1 of this thesis confirmed that somatotopic representations evoked by 1.5 Hz mechanical stimulation of the digits can be measured noninvasively by applying dipole analysis to EEG and MEG projected onto a region of postcentral gyrus and were in agreement with known anatomical landmarks near somatosensory area 3b. This experiment was the first to provide evidence of digit somatotopy by recording EEG responses to mechanical digit stimulation. However, the procedures used to map somatotopic representations were impracticably long (>5 minutes per digit), prompting the development of more efficient steady-state stimulus procedures investigated in experiment 2.

Experiment 2 compared somatotopic maps of the digits derived from the presentation of a 3 Hz "transient" stimulus with that of an 18 Hz "steady-state" stimulus. Results from this experiment showed that somatotopic maps observed using a steady-state stimulus were remarkably similar to those observed using a tansient stimulus and both maps were observed to be stable over repeated measurements. However, the steady-state maps were obtained in half the measurement time. The results from this experiment demonstrate that steady-state stimuli may be a more efficient procedure for mapping somatosensory representations.

Experiment 3 used steady-state stimulation to investigate whether somatotopic representations are statically fixed or whether they can be remodeled by temporally coherent stimulation during a tactile discrimination task. In this experiment, subjects were trained for three days to detect changes in the frequency of a 21 Hz tactile stimulus applied to the digits 2 + 3 + 4 (fusion condition) or 2 + 4 (segregation condition) of the right hand. Results from this experiment showed changes in the 21 Hz representation for digit 3 were obtained on the third day of training for a subset of subjects in the fusion condition and for the segregation group as a whole.

The results from this thesis build upon a body of research which employs noninvasive procedures to describe the organization of the human somatosensory cortex. These noninvasive measurements help to extend our basic understanding of how the body represents somatosensory stimuli in the brain. Further, this thesis adds to a growing literature which demonstrates that the adult somatosensory cortex is capable of reorganization, depending both on the pattern of peripheral stimulus and the adaptive relevance of the stimulus to the individual.

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