Visually guided, aimed movements can be unaffected by stimulus-response uncertainty.

Response times generally increase linearly with the logarithm of the number of potential stimulus-response alternatives (e.g., Hick's law). The ubiquity and theoretical importance of this generalization make exceptions particularly interesting. Recently, Kveraga et al. (Exp Brain Res 146:307, 2002) added a third to the two previously known exceptions, demonstrating that saccade latencies were unaffected by stimulus-response uncertainty. They suggest that visually guided saccades are exceptional, because these movements can be automatically selected using a privileged pathway: the topographically organized regions in superior colliculus that convert spatially coded visual activity into spatially coded motor commands. We report that visually guided, aimed hand movements also are unaffected by both stimulus-response uncertainty and stimulus-response repetition. A second experiment demonstrated that this lack of an uncertainty effect persists for equiluminant stimuli. This result suggests that posterior parietal cortex is not the privileged pathway eliminating stimulus-response uncertainty for hand movements. Because hand movements are not guided by mechanisms in the superior colliculus, our results cast doubt on the privileged-pathway hypothesis, at least for hand movements. Instead, the absence of stimulus-response uncertainty may occur only in tasks that do not require the stimulus to be associated with a response effector and that have high stimulus-response compatibility.