Automatic control during hand reaching at undetected two-dimensional target displacements

Abstract

1. The aim of this study was to demonstrate that goal-directed pointing movements, executed at normal speed to a small visual target, but without vision of the movement, do not rely on preprogrammed commands (open-loop process); by contrast these responses are under the control of a feedback loop, which compares the ongoing response and the goal (or its internal representation). When the location of this goal is changed at the onset of the movement, an automatic correction of the path occurs. Modification of the goal was obtained by presenting a target in the peripheral visual field that the subject had to look at and point at as quickly and accurately as possible. When the orienting ocular saccade reached its peak velocity, statistically corresponding to the hand movement onset, the target was suddenly shifted 10 degrees in a random direction. This perturbation was undetected by the subject because of the absence of perception during the saccade. For the compensation to occur, the initial orientation of the movement and also its extent had to be modified. The results revealed 1) a nearly complete compensation of the movement path and a 66- to 80-ms duration lengthening; 2) relatively short reaction times to the perturbations (from 145 to 174 ms, with effective reaction times even 40 ms shorter); 3) nearly identical spatiotemporal movement characteristics to the perturbations, regardless of whether vision of the hand was allowed, suggesting that corrections were subserved by the same mechanisms. 2. The spatiotemporal characteristics of these unconscious corrections were similar to those observed in the classical double-step experiments investigating the intentional modifications of ongoing movements and suggest that they might share some common low-level mechanisms. That is, they could rely on visuokinesthetic feedback loops, which compare the updated information provided by the eye at the end of the saccade and the proprioceptive information of the end point effector (the fingertip here); they could also rely on feed-forward processes detecting the discrepancy between an efference copy of the movement and the new goal; or they could rely on a combination of those two main processes.

Functional Anatomy of Nonvisual Feedback Loops during Reaching: A Positron Emission Tomography Study

“…This modification triggered a change in hand trajectory that deviated early and smoothly from its initial path to reach the new target location. The occurrence and characteristics of these deviations were identical whether or not vision of the moving limb was allowed, indicating that nonvisual feedback loops represent the key mechanism for early hand trajectory control, even when vision of the moving limb is available (Prablanc and Martin, 1992).…”

“…As reported in several studies, this signal is not entirely accurate (Prablanc et al, 1979;Bock, 1993). At the end of the ocular saccade, which roughly corresponds to hand movement onset (Prablanc and Martin, 1992), the target location is recomputed on the basis of perifoveal information. The updated visual signal is then used by the nervous system to adjust the ongoing trajectory .…”

“…The existence of these loops was demonstrated initially in simple psychophysical experiments showing that reaching movements performed without vision of the moving limb were significantly less accurate when the target was turned off after hand movement onset . Further evidence was provided by subliminal target displacement experiments Prablanc and Martin, 1992;Desmurget et al, 1999a). In these "double-step" experiments, subjects were required to "look and point," in the dark, to visual targets displayed in the peripheral visual field.…”

Prehension movements in a patient (AC) with posterior parietal cortex damage and posterior callosal section

“…Also, the trajectory of the hand can be adjusted early in the movement when the spatial location of a target has been modiWed at movement onset (Prablanc & Martin, 1992). In the current study, this adjustment to perturbation is reXected by a slower transport velocity, longer movement time and latencies as well as a larger grasp aperture.…”

Unexpected acoustic stimulation during action preparation reveals gradual re-specification of movement direction

“…Where the target is not initially specified, the initial planning state represents the information available concerning all potential targets (Cisek and Kalaska, 2002;Favilla et al, 1990;Findlay, 1982;Gallivan et al, 2015;Ghez et al, 1997;Haith et al, 2015;He and Kowler, 1989;Hudson et al, 2007;Stewart et al, 2014), and many forms for this representation have been proposed (Cisek and Kalaska, 2005;Erlhagen and Schoner, 2002;Gallivan et al, 2015;Haith et al, 2015;Kopecz and Schoner, 1995;Stewart et al, 2014). Incorporation of new information into the motor plan can occur at any time prior to initiation of descending motor commands Ghez et al, 1997), and indeed there may be little or no distinction between the processes that underlie this plan updating and those responsible for feedback corrections of ongoing movements (Flanagan et al, 1993;4 Flash and Henis, 1991;Hudson et al, 2007;Nashed et al, 2014;Prablanc and Martin, 1992; van Sonderen et al, 1989).…”