Differential effects of startle on reaction time for finger and arm movements

Abstract

Recent studies using a reaction time (RT) task have reported that a preprogrammed response could be triggered directly by a startling acoustic stimulus (115-124 dB) presented along with the usual "go" signal. It has been suggested that details of the upcoming response could be stored subcortically and are accessible by the startle volley, directly eliciting the correct movement. However, certain muscles (e.g., intrinsic hand) are heavily dependent on cortico-motoneuronal connections and thus would not be directly subject to the subcortical startle volley in a similar way to muscles whose innervations include extensive reticular connections. In this study, 14 participants performed 75 trials in each of two tasks within a RT paradigm: an arm extension task and an index finger abduction task. In 12 trials within each task, the regular go stimulus (82 dB) was replaced with a 115-dB startling stimulus. Results showed that, in the arm task, the presence of a startle reaction led to significantly shorter latency arm movements compared with the effect of the increased stimulus intensity alone. In contrast, for the finger task, no additional decrease in RT caused by startle was observed. Taken together, these results suggest that only movements that involve muscles more strongly innervated by subcortical pathways are susceptible to response advancement by startle.

FIG. 1.

Illustration of the finger movement task. The right hand was placed with the index finger extended and relaxed, resting on a switch. Participants were instructed to make a rapid finger abduction movement in the upward direction (indicated by the arrow) following the acoustic stimulus.

FIG. 2.

Example raw data from a single representative participant. Left panels contain data from the arm task; right panels from the finger task. Top 2 panels show control trials (82 dB), middle panels show startle trials (115 dB) where no sternocleidomastoid (SCM) activity was observed (SCM−), and bottom panels show startle trials with SCM activity (SCM+). Time 0 is stimulus onset. Arm task panels show displacement data, raw triceps, biceps, and SCM EMG activity. Finger task panels show displacement onset (moment of lift off switch), and raw FDI and SCM EMG activity. Dashed line shows prime mover EMG onset in the SCM+ condition allowing for comparison to the other conditions.

FIG. 3.

Mean premotor reaction time (RT; ±SE) in the arm extension task (A) and the finger abduction task (B) for each stimulus condition: control (82 dB), SCM− (115 dB, no sternocleidomastiod activity observed), and SCM+ (115 dB, sternocleidomastiod activity observed).

FIG. 4.

Premotor RT (PMT) distributions in the arm extension task and the finger abduction task. Data are proportions of the total number of trials observed across all participants in 10-ms PMT bins for each stimulus condition: control (82 dB), SCM− (115 dB, no sternocleidomastiod activity observed), and SCM+ (115 dB, sternocleidomastiod activity observed). PMT bin values are upper limits for that bin.