Motor preparation is modulated by the resolution of the response timing information

Abstract

In the present experiment, the temporal predictability of response time was systematically manipulated to examine its effect on the time course of motor pre-programming and release of the intended movement by an acoustic startle stimulus. Participants performed a ballistic right wrist extension task in four different temporal conditions: 1) a variable foreperiod simple RT task, 2) a fixed foreperiod simple RT task, 3) a low resolution countdown anticipation-timing task, and 4) a high resolution anticipation-timing task. For each task, a startling acoustic stimulus (124dB) was presented at several intervals prior to the "go" signal ("go" -150ms, -500ms, and -1500ms). Results from the startle trials showed that the time course of movement pre-programming was affected by the temporal uncertainty of the imperative "go" cue. These findings demonstrate that the resolution of the timing information regarding the response cue has a marked effect on the timing of movement preparation such that under conditions of low temporal resolution, participants plan the movement well in advance in accordance with the anticipated probability of onset of the cue, whereas movement preparation is delayed until less than 500ms prior to response time when continuous temporal information is provided.

Figure 1

Mean (+/− 1 SE) initial agonist (AG1) EMG burst duration (black) and amplitude (grey) collapsed across stimulus time for each cuing condition. Note that as duration increased across timing conditions, amplitude appeared to decrease.

Figure 2

Mean (+/− 1 SE) proportion of early responses elicited by the SAS for each cuing condition (separate lines) at each stimulus time (time is with respect to imperative “go” stimulus).

Figure 3

Mean (+/− 1 SE) premotor reaction time (RT) for the Variable (grey) and Fixed (black) foreperiod RT cues for both the Control (separated points) and startle conditions at each stimulus time (joined points). Note that for the startle conditions RT refers to the time between the acoustic stimulus and any elicited response within 250 ms.

Figure 4

Mean (+/− 1SE) initial agonist (AG1) EMG onset (filled, solid lines) and angular displacement onset (open, dashed lines) for the Countdown and Clock cues in both the Control (grey squares) condition and when a startling acoustic stimulus was presented 150 ms prior to the target (Startle-150, black diamonds).

Figure 5

Mean (+/− 1SE) proportion of startle trials (124 dB acoustic stimulus) in which a startle response (EMG activity in SCM) was observed for each cuing condition (separate lines) and each stimulus time. Note the similarity to Figure 3.

Figure 6

A modified model of cortical motor preparatory activity in reaction time (panel A) and anticipation timing tasks (panel B). Panel A depicts a reaction time situation in which the earliest possible precue (light grey) occurs 3 sec prior to the imperative “go” signal, with related motor preparation initiated 1 sec later and taking 1 sec to reach a peak. The latest possible precue (dark grey) occurs 2 seconds prior to “go” with associated motor preparation 1 sec later. Mean motor preparatory activity between the two extremes shown with a dashed line. Panel B depicts cortical motor preparatory activity in the anticipation timing situations used in the current experiment. The Countdown condition is shown with motor preparation initiated following the last cue (white) and the Clock condition is shown with motor preparation initiated shortly before the “go” target. See text for further discussion.

Figure 7

Percentage of initial agonist EMG onsets observed at various times (30 ms bins) with respect to the target (time “0”) for the Countdown (top panel) and Clock (middle panel) cues. Grey lines are control trials, black lines are trials in which a 124 dB startling acoustic stimulus (SAS) was delivered 150 ms prior to the target (dashed line). Bottom panel is a schematic representation of how the EMG onset distribution is skewed by the presentation of a SAS. See text for further discussion.

Figure 8

Overhead schematic of positioning and task. Wrist starting position was flexed 10 deg (light dashed line), with the movement target located 20 deg of angular extension from the starting position (bold dashed line). The speaker that delivered the 124 dB acoustic stimulus was located 50 cm behind the ears of the participant.