Abstract
The forebrain somatic sensory locus for input from sensors on the surface of an active prosthesis is an important component of the Brain Machine Interface. We now review the neuronal responses to controlled cutaneous stimuli and the sensations produced by Threshold Stimulation at Microampere current levels (TMIS) in such a locus, the human thalamic Ventral Caudal nucleus (Vc). The responses of these neurons to tactile stimuli mirror those for the corresponding class of tactile mechanoreceptor fiber in the peripheral nerve, and TMIS can evoke sensations like those produced by the stimuli that optimally activate each class. These neuronal responses show a somatotopic arrangement from lateral to medial in the sequence: leg, arm, face and intraoral structures. TMIS evoked sensations show a much more detailed organization into anterior posteriorly oriented rods, approximately 300 microns diameter, that represent smaller parts of the body, such as parts of individual digits. Neurons responding to painful and thermal stimuli are most dense around the posterior inferior border of Vc, and TMIS evoked pain sensations occur in one of two patterns: (i) pain evoked regardless of the frequency or number of spikes in a burst of TMIS; and (ii) the description and intensity of the sensation changes with increasing frequencies and numbers. In patients with major injuries leading to loss of somatic sensory input, TMIS often evokes sensations in the representation of parts of the body with loss of sensory input, e.g., the phantom after amputation. Some patients with these injuries have ongoing pain and pain evoked by TMIS of the representation in those parts of the body. Therefore, thalamic TMIS may produce useful patterned somatotopic feedback to the CNS from sensors on an active prosthesis that is sometimes complicated by TMIS evoked pain in the representation of those parts of the body.
Highlights
Cutaneous receptors and sensations play an important role in manual tasks and behaviors that are significant components of human behavior
We described a response pattern defined as Multi Receptive (MR) for cells that showed responses to mechanical stimuli from the non-painful into the painful range that were not graded with intensity
Threshold Stimulation at Microampere current levels (TMIS) of the Ventral Caudal nucleus (Vc) complex of evoke sensations like those produced by tactile, thermal and painful stimuli and could be produced by TMIS transmitting signals generated by a tactile sensor
Summary
Cutaneous receptors and sensations play an important role in manual tasks and behaviors that are significant components of human behavior. There are some mismatches between peripheral nerve versus thalamic TMIS evoked sensations, and between the mechanoreceptor fibers versus the corresponding Vc neuronal responses to mechanical stimuli. These results demonstrate surprising fidelity of activity of the mechanoreceptor like Vc neurons to that of mechanoreceptor fibers, in spite of the axonal spans, and the (limited) circuitry of the dorsal root ganglion and dorsal column nuclei between them It was not clear how these responses were matched to the sensations evoked by TMIS adjacent to the recording site, except in the case of RA like neurons. The frequent mismatch sites may have resulted from activation of axons that stream through thalamic nuclei and are afferent to or efferent from the Vc complex These fibers of passage often signal different RFs and peripheral stimuli than the adjacent neuronal somata. These differences in PF location may reflect differences in the distribution of mechanoreceptors and may result from and influence the sensory functions in different parts of the body
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