Proceedings of The Physiological Society

University of Oxford (2011) Proc Physiol Soc 23, PC89

Poster Communications

Measuring the Refractory Period in a visual-manual pursuit task

C. van de Kamp1, P. J. Gawthrop2, I. D. Loram1

1. IRM, Manchester Metropolitan University, Manchester, United Kingdom. 2. Department of Mechanical Engineering, Centre for Systems and Control, University of Glasgow, Glasgow, United Kingdom.


Some 60 years ago it has been shown that in a double stimulus tracking tasks the response to the second stimulus is generally slowed, with greater slowing the closer the two stimuli are presented in time. This slowing, a consequence of the Refractory Period (RP), limits the frequency at which one can execute a series of discrete movements without mutual interference and loss of accuracy. The finding that there is a maximum frequency (2-3 actions per second) at which humans are able to execute discrete responses to unpredictable stimuli can be explained in terms of an intermittent control model. Taking account of the refractory period, this model provides a framework to explain human behavior under a wide range of conditions. Here we present a methodology to systematically study the refractoriness in a visual-manual pursuit task. To investigate the RP, ten subjects, using a joystick interface, were asked to track unpredictable pairs of discrete step responses represented by the position of a dot displayed on an oscilloscope. To measure the RP, a novel methodology was developed in which we first modelled the closed loop relationship between the disturbance and the joystick position as an auto regressive moving average (ARMA) process. Subsequently, it was assessed to what value we had to build up the interval between the first and second stimulus in order to make the second response time identical to the first. In other words, the RP equals the inter-stimulus interval for which the reaction time to the first stimulus matches the reaction time to the second stimulus. Our method showed that in a zero order pursuit task, participants were refractory to the second stimulus for, on average, 0.284 (0.05 sd) s. Refractoriness is problematic for any linear time invariant model trying to fit human behavior. Intermittent open loop control, on the other hand, is an appropriate solution to refractoriness (and online time delays in general (cf. Gawthrop et al., 2011)). A refractory period of this size has been predicted to explain the upper frequency limit of significant coherence (the control bandwidth) in visual manual compensatory tracking (Loram et al 2011, J Physiol). Questions arising from this preliminary finding and previous research (Loram et al., 2009) are: 1) does this psychological refractory period generalize to other tasks? 2) Will we find the same values when varying the complexity (order) or stability of the load being controlled? 3) And is the refractory period related to the (physiological) visual feedback time delay in humans?

Where applicable, experiments conform with Society ethical requirements