De Havas, JA; (2016) Principles of involuntary vs. voluntary control of human action: investigations using the Kohnstamm phenomenon. Doctoral thesis , UCL (University College London).

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Abstract

Psychological accounts of human action control strongly distinguish between voluntary and involuntary movements. In the Kohnstamm phenomenon, a sustained voluntary contraction of a muscle is followed by sustained, involuntary aftercontraction of the same muscle. This offers a useful experimental model of the voluntary/involuntary distinction, because aftercontractions physically resemble voluntary movements, while feeling subjectively very different. Despite 100 years of study, many basic questions remain unanswered about the Kohnstamm phenomenon. This thesis presents several experiments addressing these questions, and using the phenomenon to shed light on the voluntary/involuntary distinction. First, the recruitment of the Kohnstamm generator was explored by systematically varying the muscle contractions and task goal during the initial voluntary activity that induces the Kohnstamm phenomenon. This revealed that the Kohnstamm generator is a low frequency integrator. Next, experiments on physical obstruction of the involuntarily rising arm showed that afferent input can temporarily gate output from the Kohnstamm generator. Subjective estimates of contact force against the obstacle were higher than for matched voluntary movements, suggesting that the generator does not produce efference copies. In a further experiment, resistive and assistive perturbations during a horizontal Kohnstamm aftercontraction produced EMG responses, consistent with principles of negative position feedback control operating during voluntary movements, but with lower gains. Experiments in which participants were instructed to inhibit the aftercontraction showed that, though involuntary, Kohnstamm movements could nevertheless be voluntarily controlled, suggesting the novel concept of a “negative motor command”. Such voluntary inhibition caused a strange subjective experience of upward force, again suggesting a lack of efference copy for the aftercontraction. A model is presented that shows how the Kohnstamm phenomenon is generated and controlled. This systematic study of the control principles of the Kohnstamm phenomenon sheds important new light on the classical distinction between involuntary and voluntary movement.

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