Abstract
We recorded reaction times (RT) of participants who were asked to pick up an elongated object at various orientations with a precision grip. Although most of these orientations consistently elicited one of two grip postures, there was a region of orientations that, due to biomechanical constraints, elicited both grip postures to varying degrees. In the first experiment, we found a clear increase in RT as object orientations reached this ‘ambiguous’ region. Also, we found that randomizing the order of orientations—instead of presenting them in a clockwise or counterclockwise fashion—significantly lengthened RTs. To determine whether the difference in RT across different trial orders reflected explicit knowledge or simply trial history, we conducted a second experiment in which we presented the object at only two unambiguous orientations (each affording a different grip posture), in three different trial orders: blocked, alternated, and randomized. We found that the blocked and alternated trials led to significantly faster RTs than the randomized trials, suggesting that when switching between two unambiguous orientations, participants used their explicit knowledge about upcoming trial types. In a third experiment, we presented the object within the ambiguous region or within an unambiguous region, with the same trial order conditions that were used in the second experiment. As expected, RTs during blocked unambiguous trials were shorter than those during blocked ambiguous trials. Also, there was slight but significant difference between randomized ambiguous and unambiguous trials, probably reflecting trial history effects. Most surprisingly, we were unable to detect a difference in reaction time between the alternated ambiguous and unambiguous trials. Thus, while explicit knowledge of upcoming trial type provided an advantage when participants switched between two unambiguous grip postures, it failed to provide such an advantage when participants switched between ambiguous and unambiguous orientations, suggesting an interaction between explicit knowledge and ambiguity.