Default motor preparation under conditions of response uncertainty

Abstract

In a choice reaction time (RT) paradigm, providing partial advance information (a precue) about the upcoming response has been shown to decrease RT, presumably due to preprogramming of the precued parameters. If arm and direction are precued, but amplitude is rendered uncertain, a number of different preparation strategies have been suggested to occur during the foreperiod. Initial preprogramming of a default movement lying between response alternatives has been suggested in studies whereby response preparation time was manipulated (Favilla et al. 1989; Favilla et al. 1990; Ghez et al 1997). However in a RT paradigm, this strategy was discounted due to the absence of online adjustments to movement, therefore it appeared movements were not initiated until after all parameters had been correctly specified and programmed by the nervous system (Bock and Arnold 1992). The present study reinvestigated the validity of default motor preparation as a strategy in a choice RT paradigm, employing the triggering effects of a startling acoustic stimulus. On control trials (80 dB imperative stimulus), the movements were performed to the correct targets. Presenting a startle stimulus (124 dB) resulted in the early trigger of a default movement whose amplitude fell in between the potential response alternatives. Thus, the present study found behavioural evidence of default movement preparation as a strategy under conditions of uncertainty regarding response amplitude.

Acknowledgments: This study was supported by a grant from the Natural Sciences and Engineering Research Council of Canada (NSERC) awarded to I.M.F. and an NSERC Undergraduate Student Research Award (USRA) awarded to C.J.F.