PMC

Validation of the unitary stop model. A and B: compensation functions of the eye (A) and hand (B) derived from the eye redirect, hand redirect, and eye-hand redirect trial conditions in a typical subject. C: bar plot comparing the average Weibull mean across subjects calculated from the individual compensation functions during the eye redirect, hand redirect, and eye-hand redirect conditions for eye (left) and hand (right) effectors. D: bar plot comparing the mean TSRTs from the eye redirect, hand redirect, and eye-hand redirect conditions for eye (left) and hand (right) across subjects. E: bar plot comparing the frequency of EH-initial (left), EH-dissociated (center), and EH-final (right) trials during the eye redirect, hand redirect, and eye-hand redirect conditions for a typical subject. F: scatterplot comparing the frequency of trial types (EH-initial, EH-dissociated, and EH-final) observed during the eye-hand redirect condition relative to the frequencies observed during eye redirect (blue) and hand redirect (red) conditions across subjects. The points lying close to the unity line (dashed black) suggest that the frequencies of trial types observed across the 3 conditions were comparable across subjects.

Experimental paradigm. (A) Task configuration showing center start position (green), potential reach target (gray), and instructed reach target for that trial (yellow). The subject moved the hand to position a cursor on the target. Mid-reach redirect cues were given via vibrating motor, a blue circle, or microstimulation of. (B) Task Timeline. Sample horizontal hand trajectory across time during a direct reach task (black dashed line) and a redirect task (solid blue line) with the go cue represented at time 0. Horizontal bands represent targets displayed during the redirect task. Target presentation during the direct reach task was identical up to onset of the redirect cue (blue). Potential reach targets (gray) appear after the start target is acquired and remain visible. Position of the hand is marked at the redirect cue onset (white circle) and moment of reach inflection (red circle). (C) Horizontal hand velocity of the direct reach and redirect tasks. Response time to the redirect cue was measured as the time elapsed between redirect cue onset (white circle) and moment of reach inflection (red circle).

Summary of tasks, observed behavior and simulation results []. (A) Search redirect task: 60% of trials were no-step where the target did not shift, and 40% of trials were step trials where the target shifted. In each no-step trial, participants had to make movements to the odd colored target (red among green or green among red). In the step trials, after a delay called the Target Step Delay (TSD), the target shifted to a new location and the participants were now expected to move to the second target location instead. (B) Dual redirect task: 60% of trials were no-step where participants had to make an eye movement to the peripheral target and also make a hand movement to it if a tone was also presented. A total of 40% of the trials were step trials where a yellow target was subsequently presented after a TSD. In these trials, participants had to make an eye and hand movement to the yellow target. (C) Cumulative weibull fits for an exemplar participant for the eye (orange), and hand (red) in the Search redirect task (data, dots). The arrow indicates the means of the compensation functions, i.e., the TSD corresponding to the mean of the error (unsuccessful redirection). (D) Same as C but in the Dual redirect task. (E) Comparison of the difference between the means of the eye and hand compensation functions between the Search redirect (green) and Dual redirect (purple) tasks. Each dot represents a participant, while the bar and cross hair represent the mean ± s.e.m. across the population. (F) Comparison of the trial percentages behaviorally observed and that derived from the simulations of the unitary STOP model for the Search redirect task. The green dots represent the correct trials (both eye and hand movements to the second target), the red represents the error trials (both eye and hand movements to the first target), and the blue represents the dissociated trials (eye and hand movements to different locations). (G) Same as F but for the Separate STOPs model. Dotted circles indicate that predicted % does not match the observed %. (H) Same as I but for the Dual redirect task. (I) Same as G but for the Dual redirect task. *** p < 0.001.

Search-step task. The task consisted of three randomly interleaved trial types: no-step (30% of trials), redirect (40% of trials), and follow (30% of trials). Each trial lasted 4 s and began with a variable fixation period between 1000 and 2000 ms. On no-step and redirect trials, after the fixation period, an eight-element search array appeared with one red singleton among green distractors (T1). On no-step trials, this array remained on the screen for the remainder of the trial. On redirect trials, the red target jumped to a new location (T2) via an isoluminant color change at some delay after the initial array presentation (target step delay; TSD). On follow trials, the array appeared with two red targets and was visible until the end of the trial. On no-step and redirect trials, subjects were instructed to saccade to T1 as quickly as possible. They were instructed that if the target jumped to a new location (redirect trials), they should try to inhibit the saccade to T1 and to look as quickly as possible to T2. On follow trials, participants were instructed to look at each red target in succession (the order was irrelevant). Redirect trials in which the subject successfully looked immediately toward T2 were referred to as compensated trials. Redirect trials in which the subject erroneously made an initial saccade to T1 were referred to as noncompensated trials; these noncompensated trials were nearly always followed by a corrective saccade to the new target location. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)