Changes in Spinal Reflex Excitability in a Countermanded Timed Response Task

On the Nature of Stopping an Earlier Intended Voluntary Action

The ability to inhibit an earlier intended action in a stop-signal task is commonly assessed using the measures of latency and probability. The usual findings from stop-signal trials of lower response probabilities and shorter reaction latencies at reduced stop-signal delays were reported, as described in previous studies in terms of an independent race between stochastic processes. In addition, using the less common measure of amplitude, a continuum of reductions in surface EMG onsets was reported. Weakened motor discharges have yet to be explained in terms of a mechanism of inhibitory control. Using computer simulations of neural functioning, the properties of independence and non-independence were examined for their effects on motor pool output in terms of probability, latency, and EMG onsets. The data provided support to question the requirement of independent processes for a theory of inhibitory control.

Against a final ballistic process in the control of voluntary action: evidence using the Hoffmann reflex

Reduced surface electromyogram (EMG) onsets, observed in some cases when trying to stop an earlier intended action before it begins, were taken to suggest a control process that remains open to stopping right up to the point of motor discharge. This interpretation argues against a final ballistic (involuntary) process in the control of voluntary action. That a final ballistic process may receive reduced input shortly before its manifestation as reduced output (i.e., motor discharge), however, provides for an alternative interpretation of these same data. In this study we used the Hoffmann (H) reflex to further investigate for stopping effects in the brief interval before voluntary motor discharge. Late stopping effects on the facilitated H-reflex within the time window that a final ballistic process would otherwise be expected (i.e, shortly before EMG onset) were observed in some instances. We conclude from these data good evidence against a final ballistic process in the control of voluntary action.

Inhibitory motor control in stop paradigms: review and reinterpretation of neural mechanisms

What is the neurophysiological locus of inhibition when preparation for a manual response is countermanded? This paper evaluates data and models that pertain to inhibitory mechanisms operating in stop paradigms. In a model of De Jong, Coles and Logan (1995), (Strategies and mechanisms in nonselective and selective inhibitory motor control. Journal of Experimental Psychology: Human Perception and Performance, 21, 3, 498-511), a mechanism for nonselective inhibition operates peripheral to the motor cortex, while a selective mechanism operates at a central cortical level. We argue, however, that a peripheral mechanism of inhibition is incorrectly inferred from inhibition data available to date. Neurophysiological and psychophysiological data suggest that inhibitory processes always involve the cortex, and inhibitory effects are exerted upstream from the primary motor cortex. The prefrontal cortex and basal ganglia are candidate agents of response inhibition, whereas possible sites of inhibition are the thalamus and motor cortex.

Movement-related potentials associated with motor inhibition under different preparatory states during performance of two visual stop signal paradigms in humans

The motor-inhibition process was examined in humans by monitoring reaction times (RTs), electromyograms (EMGs), and movement-related potentials. Eight subjects performed two types of visual stop signal paradigm, one with ongoing motor activity and the other without background motor activity. NO-GO-specific negative potentials were consistently recorded from subjects with constant onset and duration relative to the onset of the NO-GO stimulus in both tasks and the spatial distribution of amplitudes was also invariant between the tasks. Sustained background muscle activity was not influenced by the inhibitory command. The constant features of NO-GO-specific potentials during performance of both tasks imply that temporally and spatially similar inhibition processes might specifically suppress the execution of a new motor process without any significant effect on the ongoing motor programme, regardless of differences in preparatory states.