These new experiments seek to dissociate and study peripheral sensory mechanisms underlying the percept of direction of motion across the skin, and to evaluate central neural interactions involving information provided by these mechanisms in health and after nerve injury. In the first study, the responses evoke in individual low-threshold mechanoreceptors in the healthy human inferior alveolar, median and redial nerves by three novel modes of moving stimulus delivery will be studied. One mode will most closely a natural object (the stimulus both translates across and laterally stretches the skin). The other two modes will involves translation (one continuous, and one, discontinuous) across the skin but insignificant lateral skin stretch. The three modes are hypothesized to differ in the mechanoreceptor channels (rapidly and slowly adapting) recruited, and in the peripheral mechanisms by which information about direction is encoded. Rigorous quantitative analyses of the neural responses will be conducted to evaluate this hypothesis. In a second study, the three modes of stimulation will be used to estimate the threshold length for direction discrimination on the face and hand/arm of normal human subjects. Artificial neural networks will be constructed from the neural responses to determine whether population will be constructed from the neural responses to determine whether populations models based on the spatiotemporal sequence of mechanoreceptor activation or on discharge intensity better predict the impact of velocity, test site, and their interaction on normal human capacity to discriminate direction of motion. In a third study, direct discrimination will be studied across the border of sensory alteration in pain-free, trigeminal nerve-injured patients with stable patterns of impairment. The three modes of moving stimulus delivery are hypothesized to be unequally useful in detecting and characterizing the sensory impairment of these patients. A fourth study will explore the feasibility of quantitively characterizing the subjective intensity and unpleasantness of moving tactile stimuli in trigeminal nerve-injured patients who developed allodynia or discomfort to touch. The findings of the four studies are anticipated to better clarify the relationship among different modes of tactile stimulation, peripheral neural mechanisms that encode information about movement across the skin, and central selection/utilization of this information in health and after injury to the peripheral nervous system.
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