What is 'anti' about anti-reaches? Reference frames selectively affect reaction times and endpoint variability

Abstract

Reach movement planning involves the representation of spatial target information in different reference frames. Neurons at parietal and premotor stages of the cortical sensorimotor system represent target information in eye- or hand-centered reference frames, respectively. How the different neuronal representations affect behavioral parameters of motor planning and control, i.e. which stage of neural representation is relevant for which aspect of behavior, is not obvious from the physiology. Here, we test with a behavioral experiment if different kinematic movement parameters are affected to a different degree by either an eye- or hand-reference frame. We used a generalized anti-reach task to test the influence of stimulus-response compatibility (SRC) in eye- and hand-reference frames on reach reaction times, movement times, and endpoint variability. While in a standard anti-reach task, the SRC is identical in the eye- and hand-reference frames, we could separate SRC for the two reference frames. We found that reaction times were influenced by the SRC in eye- and hand-reference frame. In contrast, movement times were only influenced by the SRC in hand-reference frame, and endpoint variability was only influenced by the SRC in eye-reference frame. Since movement time and endpoint variability are the result of planning and control processes, while reaction times are consequences of only the planning process, we suggest that SRC effects on reaction times are highly suited to investigate reference frames of movement planning, and that eye- and hand-reference frames have distinct effects on different phases of motor action and different kinematic movement parameters.

Fig. 1

Spatial layout of the generalized pro-/anti-reach task. a General task design in standard and generalized trials. Squares depict positions where eye- and/or hand-fixation points (F) were presented. Gray circles depict the positions where the spatial cue (C) was presented. Dotted circles depict the positions of the reach goal (G). In the lower right panel, the 5 cm raster is illustrated at which the fixation points, cues, and goals could be positioned. For simplicity, each panel only shows one out of four possible spatial configurations (fixation left/right, cue left/right) for each of the four task conditions (pro/anti × standard/generalized). The task design consists of a total of 16 conditions. In a, black arrows indicate the direction of the spatial cue relative to eye fixation, and the gray arrows the direction of the reach goal relative to hand fixation. In pro-trials (per definition) both arrows point in the same direction, whereas in anti-trials, they point in opposite directions. b Four example trials in the generalized task condition, which illustrate the 2 × 2 variations of the SR compatibility in the eye-reference frame (black arrows) and in the hand-reference frame (gray arrows). The open arrows show the direction of the spatial cue, the solid arrows show the direction of the reach goal

Fig. 2

Timeline of the generalized pro-/anti-reach task. The subject had to direct gaze to a small red square throughout the trial. A flashed context cue (C-CUE, frame around the eye-fixation) instructed whether to prepare a pro- or anti- reach. In the experiment, a green frame instructed a pro-reach and a blue frame instructed an anti-reach. The reach goal (dotted circle, not visible to the subject) was defined by the combination of the context cue and a spatial cue (S-CUE, white circle), which was flashed left or right of the eye fixation after a variable memory period (MEM). Visual feedback (FDB) appeared only after the subject touched the correct reach goal on the screen. Pro- and anti-reaches were defined as a reaches relative to hand fixation (symbolized by white arrow, not shown to subjects) in the same (pro) or opposite (anti) direction as the spatial cue was relative to eye fixation (symbolized by black arrow, not shown to subjects). The example shows a generalized pro-trial

Fig. 3

Influence of SR compatibility in eye- and hand-reference frames on reach reaction times (RTs). a Average (mean ± SEM) RTs for the different combinations of compatibility/incompatibility in the hand-reference frame (C_H (triangles)/I_H (circles)) and compatibility/incompatibility in the eye-reference frame (C_E (light gray)/I_E (black)). RTs in the standard trials are plotted separately (dashed, dark gray curve). Note that for the standard trials eye and hand compatibility are identical. b Average inter-subject difference between the compatible and incompatible trials in the standard condition (1st bar) and between all possible combination of compatibility conditions in generalized trials. *P < 0.05; **P < 0.01, paired t test, Bonferroni corrected

Fig. 4

Influence of SR compatibility in eye- and hand-reference frames on reach movement times (MTs). Conventions are the same as for Fig. 3

Fig. 5

Influence of eye- and hand-reference frames on reach endpoint variability (EV). Conventions are the same as in Fig. 3