Independence of Movement Preparation and Movement Initiation

Abstract

Initiating a movement in response to a visual stimulus takes significantly longer than might be expected on the basis of neural transmission delays, but it is unclear why. In a visually guided reaching task, we forced human participants to move at lower-than-normal reaction times to test whether normal reaction times are strictly necessary for accurate movement. We found that participants were, in fact, capable of moving accurately ∼80 ms earlier than their reaction times would suggest. Reaction times thus include a seemingly unnecessary delay that accounts for approximately one-third of their duration. Close examination of participants' behavior in conventional reaction-time conditions revealed that they generated occasional, spontaneous errors in trials in which their reaction time was unusually short. The pattern of these errors could be well accounted for by a simple model in which the timing of movement initiation is independent of the timing of movement preparation. This independence provides an explanation for why reaction times are usually so sluggish: delaying the mean time of movement initiation relative to preparation reduces the risk that a movement will be initiated before it has been appropriately prepared. Our results suggest that preparation and initiation of movement are mechanistically independent and may have a distinct neural basis. The results also demonstrate that, even in strongly stimulus-driven tasks, presentation of a stimulus does not directly trigger a movement. Rather, the stimulus appears to trigger an internal decision whether to make a movement, reflecting a volitional rather than reactive mode of control.

Keywords: movement initiation; movement preparation; reaching; reaction time; volitional movement.

Figure 1.

Experimental design and example data. A, Experimental setup. Participants made planar reaching movements to targets presented via a mirrored display. B, Free RT condition (left) and Forced RT condition (right). In the Free RT condition (left), a single target appeared at an unknown location at a predictable time (cued by a series of 4 tones), and participants were instructed to move as soon as the target appeared. In the Forced RT condition (right), participants were instructed to move synchronously with the fourth tone. RT was manipulated by varying the time that the target was presented relative to the time of movement onset. C, Behavior of a representative participant. Green histogram indicates the distribution of RTs in the Free RT condition (both correct and incorrect movements). Blue circles indicate RT (x-axis) and directional error (y-axis) for each trial in the Forced RT condition. Gray region indicates the range of directions considered to be accurate.

Figure 2.

Movement initiation lags movement preparation. A, Blue line, Moving average of the probability that a movement is successful for a given RT in the Forced RT condition, which corresponds to the cumulative probability. Green line, Cumulative distribution of RTs in the Free RT condition. Dashed lines indicate model fits based on maximum likelihood estimation of T_P and T_I distributions. B, Estimated distributions of T_P and T_I for the same example participant as shown in Figure 1. C, D, Same as A and B but showing mean behavior across 10 participants. Shaded regions indicate ±SEM. E, Mean T_I versus estimated mean T_P. Each point represents a single participant. F, Overall distribution of effective RTs in the Forced RT condition (across all participants). Green, Successful trials; red, failed trials; black, all trials.

Figure 3.

Distribution of guesses and alternative estimate of T_P. A, Circular histogram showing the distribution of initial movement directions in catch trials in the Forced RT condition in Experiment 1. Gray circles indicate potential target locations. Dashed black lines indicate classification boundaries used to obtain the discrete distribution p_guess(d), d = 1…8. B, Reach direction as a function of effective RT in the Forced RT condition in Experiment 1. Each point represents a single trial. Gray lines indicate target directions. C, Estimated time course of movement preparation, p(T_P < t). Blue line, Estimate based on improving success rate with RT, as in Figure 2, A and C. Pink line, Estimate based on the changing distribution of initial reach angles, seen in B. All plots in this figure are generated from data pooled across participants.

Figure 4.

Participants committed spontaneous errors at low RT. A, Initial directional error as a function of RT in the Forced RT (gray) and Free RT (green) conditions for a representative participant. B, The probability of an error for each trial was predicted based on the estimated distribution of T_P for that participant and the known RT for that trial. C, Comparison of actual error rate (total number of errors divided by total number of trials) versus predicted error rate (based on the average probability of an error in each trial, computed as shown in B).

Figure 5.

Motivation influenced RT but not T_P. A, RT as a function of trial number in each block for a representative participant. Gray circles indicate accurately initiated movements. Red circles indicate errors. Vertical gray lines indicate block boundaries. Horizontal black lines indicate initiation deadline. B, Blue line, Moving average of the probability of accurate movement initiation as a function of RT, determined from the Forced RT condition. Shaded region indicates ±SEM across 12 participants. Black lines indicate cumulative RT distribution in first and last (Free RT) blocks (solid, first; dashed, last). Green lines indicate cumulative RT distribution in each block in the Pressured RT condition (lighter, longer deadline; darker, shorter deadline). C, Mean RT across participants. Open circles indicate blocks with no RT deadline. Filled circles indicate blocks with a deadline. Black horizontal lines indicate deadlines (where this fits within axes). Shaded regions indicates ±SEM across participants. D, Error rate. E, Variability in RT. F, True versus predicted error rate for each block, for each participant.