Discrete and continuous planning of hand movements and isometric force trajectories

Abstract

We have previously demonstrated that, in preparing themselves to aim voluntary impulses of isometric elbow force to unpredictable targets, subjects selected default values for amplitude and direction according the range of targets that they expected. Once a specific target appeared, subjects specified amplitude and direction through parallel processes. Amplitude was specified continuously from an average or central default; direction was specified stochastically from one of the target directions. Using the same timed response paradigm, we now report three experiments to examine how the time available for processing target information influences trajectory characteristics in two-degree-of-freedom forces and multijoint movements. We first sought to determine whether the specification of force direction could also take the form of a discrete stochastic process in pulses of wrist muscle force, where direction can vary continuously. With four equiprobable targets (two force amplitudes in each of two directions separated by 22 degrees or 90 degrees), amplitude was specified from a central default value for both narrow and wide target separations as a continuous variable. Direction, however, remained specified as a discrete variable for wide target separations. For narrow target separations, the directional distribution of default responses suggested the presence of both discrete and central values. We next examined point-to-point movements in a multijoint planar hand movement task with targets at two distances and two directions but at five directional separations (from 30 degrees to 150 degrees separation). We found that extent was again specified continuously from a central default. Direction was specified discretely from alternative default directions when target separation was wide and continuously from a central default when separation was narrow. The specification of both extent and direction evolved over a 200-ms time period beginning about 100 ms after target presentation. As in elbow force pulses, extent was specified progressively in both correct and wrong direction responses through a progressive improvement in the scaling of acceleration and velocity peaks to the target. On the other hand, movement time and hand path straightness did not change significantly in the course of specification. Thus, the specification of movement time and linearity, global features of the trajectories, are given priority over the specific values of extent and direction. In a third experiment, we varied the distances between unidirectional target pairs and found that movement extent is specified discretely, like direction, when the disparity in distances is large. The implications of these findings for contextual effects on trajectory planning are discussed. The independence of extent and direction specification and the prior setting of response duration and straightness provide critical support for the hypothesis that point-to-point movements are planned vectorially.

Basal ganglia network mediates the control of movement amplitude

“…Relevant here, precues for movement extent and direction can reduce RTs additively, implying thereby that movement direction and amplitude are planned separately (Rosenbaum, 1980;Lepine et al, 1989;Bock & Arnold, 1992). This idea is also supported by disparate additional observations: speci®cation of extent and direction follows different timecourses (Ghez et al, 1997); visuomotor gains are learnt more readily and generalize more widely than directional rotations (Krakauer et al, 2000;Vindras & Viviani, 2002); learning of rotations and gains activates different cortical and subcortical networks (Krakauer et al, 2004); and, direction and extent errors vary independently (Gordon et al, 1994;Desmurget et al, 1997;Vindras & Viviani, 1998;Desmurget et al, 1999). This perspective should not be taken to imply that extent is controlled in isolation, but rather that, at certain stages of motor planning, a constellation of parameters related to the extent of movement are controlled independently from parameters related to movement direction.…”

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

“…Using these methods it has been found that motor plans are initially established using information available from task instructions, prior experience with the task, and perception of the task layout (Ghez et al, 1997;Ghez et al, 1990;Haith et al, 2015;Hudson et al, 2007;Schutte and Spencer, 2007). 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).…”

“…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).…”

“…Incorporating new information into an existing motor plan appears rapid, but not instantaneous: following presentation of new information during the reaction time, new task parameters are not reflected in the resulting movement for hundreds of milliseconds (e.g., Ghez et al, 1997;Marinovic et al, 2010;van Sonderen et al, 1988;van Sonderen et al, 1989). However, it is uncertain to what degree estimates of the time-costs of motor plan updating are inflated by simultaneous processing demands related to task instructions (e.g.…”

“…However, it is uncertain to what degree estimates of the time-costs of motor plan updating are inflated by simultaneous processing demands related to task instructions (e.g. pay attention to a sequence of tones to start moving, Ghez et al, 1997;Ghez et al, 1990;Haith et al, 2015), which might interfere with the ability to attend to and incorporate new target information. Data from double step RT experiments, in which no temporal demands are imposed upon the participants, provide similar estimates of the time cost of plan updating (van Sonderen et al, 1988;van Sonderen et al, 1989).…”