Abstract
Recent influential theory suggests that when confronted with multiple potential reach targets, we maintain competing motor plans in parallel before deciding which one to execute (Cisek & Kalaska 2010). Although neurophysiological recordings from non-human primates have revealed results consistent with this controversial hypothesis (Cisek & Kalaska, 2005), unambiguous behavioural evidence remains sparse. Here we show, by exploiting the tendency of individuals to fixate an internal aimpoint when reaching to a single target under a visuomotor rotation (Rand & Rentsch, 2015), that gaze behaviour conveys details about the actions specified, but not necessarily executed, prior to target selection. Following a fixed preview period (2 or 4 s), either a single target or one of the two potential targets was filled in, signifying participants to initiate their reach. Targets were displayed on a visible ring and visuomotor rotations were applied to the targets, requiring participants to reach towards a location rotated away from the target to move the cursor from a central start position to the cued target. As expected, when only one target was presented, participants, in addition to fixating the visible target, reliably fixated an internal aimpoint during the preview period, indicative of movement planning. Critically, in two-target trials participants also fixated the aimpoints of the potential targets, as well as the visible targets during the preview period, indicating that they prepared competing reach movements prior to target selection. These findings provide compelling evidence for the influential, yet controversial idea that individuals specify, in advance of movement, competing motor plans under target uncertainty.Acknowledgments: Funded by NSERC and CIHR