Default motor preparation under conditions of response uncertainty.

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. When advance information about a particular aspect of a movement is provided (precued), several different strategies might be used to prepare the motor system during the foreperiod. For example, in studies where response preparation time was manipulated, precues were provided specifying the required arm and direction but movement amplitude was left uncertain. In this case it was shown that a default movement was preprogrammed whose amplitude was intermediate between the alternatives (Favilla et al. in Exp Brain Res 75(2):280-294, 1989, Exp Brain Res 79(3):530-538, 1990; Ghez et al. in Exp Brain Res 115(2):217-233, 1997). However, this strategy did not appear to be used in a RT task since there was an absence of online adjustments to movement. Therefore, it appeared movements were not initiated until all parameters had been correctly specified and programmed by the nervous system (Bock and Arnold in Exp Brain Res 90:(1):209-216, 1992). The present study reinvestigated the notion of a default movement preparation strategy in a choice RT paradigm, employing the triggering effect of a startling acoustic stimulus. On control trials (80 dB imperative stimulus), the movements were initiated toward the correct targets. Providing a startle stimulus (124 dB) resulted in the early initiation of a "default" movement whose amplitude fell in between the potential response alternatives. Thus, the current experiment found behavioral evidence of default intermediate-amplitude movement preparation as a strategy under conditions of response amplitude uncertainty.