Neurophysiological methods for studies of the motor system in freely moving human subjects

In this paper, the following experimental methods for studies of the motor system in freely moving human subjects will be considered: (i) eliciting the H-reflex and understanding its use as a test response, (ii) methods to measure reciprocal inhibition between antagonist muscles, (iii) methods to measure presynaptic inhibition of Ia-afferent terminals in the spinal cord, (iv) certain aspects of the interpretation of peri-stimulus time histograms (PSTH) of single motor unit discharge, and finally, (v) stimulation of the motor cortex and the measurement of response parameters that may reflect task dependent changes. Two closely related ideas bearing directly on these methods will be emphasized--the influence of the background level of motor activity on input output properties of the neural pathway investigated and the operating point on the input-output curves at which the experimental variable is measured. Finally, in the discussion a simple model that is easily understandable in geometric terms is presented to help predict and interpret the outcome of these sorts of experiments.