We reassessed the response properties of peri-stimulus time histograms (PSTHs) in cases when a test stimulus was triggered by a motor-unit discharge with a constant delay time. In this experiment, single motor unit action potentials were recorded from the right tibialis anterior (TA) muscle of five healthy persons. A test stimulus of the common peroneal nerve with low intensity to activate only Ia afferents of the TA was applied through a bipolar stimulating electrode placed distal to the neck of the fibula. We obtained several PSTHs with various delay times and stimulus intensities in the same recording session for maintaining the background property as the same among the test situations. As a result, we confirmed three characteristics of PSTHs from observed data: (1) given the same delay time (the same background firing properties), a weaker stimulus intensity evokes a lessened effect on PSTHs, naturally; (2) delay time alters the induction balance of direct and indirect effects on PSTHs even if the stimulus intensity is the same because the background firing properties are different; and (3) response probabilities do not correspond directly to stimulus intensities when background firing properties are different; it is possible for a relatively strong intensity stimulus to produce a weaker effect than a weak stimulus. We concluded that comparisons of effects taken at different phases in the control distribution (and also effects taken from different control distributions or different motor units) can be misleading. Therefore, such comparisons should only be made within data obtained from the same phase in the same control distribution of the same motor unit.
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