Dissociating the influence of response selection and task anticipation on corticospinal suppression during response preparation

Abstract

Motor behavior requires selecting between potential actions. The role of inhibition in response selection has frequently been examined in tasks in which participants are engaged in some advance preparation prior to the presentation of an imperative signal. Under such conditions, inhibition could be related to processes associated with response selection, or to more general inhibitory processes that are engaged in high states of anticipation. In Experiment 1, we manipulated the degree of anticipatory preparation. Participants performed a choice reaction time task that required choosing between a movement of the left or right index finger, and used transcranial magnetic stimulation (TMS) to elicit motor evoked potentials (MEPs) in the left hand agonist. In high anticipation blocks, a non-informative cue (e.g., fixation marker) preceded the imperative; in low anticipation blocks, there was no cue and participants were required to divide their attention between two tasks to further reduce anticipation. MEPs were substantially reduced before the imperative signal in high anticipation blocks. In contrast, in low anticipation blocks, MEPs remained unchanged before the imperative signal but showed a marked suppression right after the onset of the imperative. This effect occurred regardless of whether the imperative had signalled a left or right hand response. After this initial inhibition, left MEPs increased when the left hand was selected and remained suppressed when the right hand was selected. We obtained similar results in Experiment 2 except that the persistent left MEP suppression when the left hand was not selected was attenuated when the alternative response involved a non-homologous effector (right foot). These results indicate that, even in the absence of an anticipatory period, inhibitory mechanisms are engaged during response selection, possibly to prevent the occurrence of premature and inappropriate responses during a competitive selection process.

Keywords: Action selection; Competition resolution; Impulse control; Inhibition; Motor preparation; Movement; Transcranial magnetic stimulation.

Figure 1

A: Trial events in Experiment 1. In the SINGLE TASK blocks, a fixation marker (100 ms) signalled the start of the trial. 900 ms later, an uninformative preparatory cue was presented for 900 ms. The imperative signal was then added on the left or right side (350 ms), indicating a left or right index finger abduction movement. In the DUAL TASK blocks, the fixation marker and the preparatory cue were not shown. The soccer trials were intermingled with another task in which the imperative signal consisted of a word depicting an object or an animal. On these trials, participants responded with a left foot extension movement when the word was an animal. No response was required when the word depicted an object. B: Sequence and timing of trial events. The example depicts a right hand SINGLE TASK trial. A TMS pulse was applied over the right M1 at one of six possible times, selected to assess left hand CS excitability during a baseline probe (TMS_baseline), a pre-imperative probe (TMS_preImp) and four different post-imperative probes (TMS_postImp, see methods for details). FDI = First Dorsal Interosseous (index finger abductor). MEP = Motor Evoked Potential.

Figure 2

A: Group means with SE bars of Experiment 1 reaction times (RT, ms) on trials in which TMS was applied before the imperative signal (TMS_baseline and TMS_preImp) for left (white fill) and right (dashed fill) index finger responses in SINGLE TASK BLOCKS (left side) and DUAL TASK BLOCKS (right side). B: RT data for DUAL TASK BLOCKS of Experiment 2 on trials in which TMS was applied at TMS_baseline. * = p < 0.05 for the comparisons indicated.

Figure 3

Group means and SE bars (Experiment 1, n=10) of MEP amplitudes (mV), recorded from the left first dorsal interosseous (FDI) and the left adbuctor digiti minimi (ADM) following TMS of right M1 in the SINGLE (A) and DUAL TASK (B) blocks. TMS_baseline are from MEPs elicited at the onset of the fixation cross. TMS_preImp are from MEPs elicited just prior to the onset of the imperative signal (vertical dashed line). TMS_postImp are from MEPs elicited after the imperative signal, with the MEPs divided into bins in which the pulse occurred 160 ms to 100 ms (TMS_{postImp_early}) or from 100 ms to 20 ms (TMS_{postImp_late}) before EMG onset for left index finger responses (left side, uniform fill) or right index finger responses (right side, dashed fill). MEPs are shown as raw data but were log-transformed for statistical analyses. * = p < 0.05 for the comparisons indicated. ұ = p < 0.05 when compared with MEPs elicited at TMS_baseline. FDI = First Dorsal Interosseous (task-relevant muscle: index finger abductor). ADM = Abductor Digiti Minimi (task-irrelevant muscle: pinkie abductor).

Figure 4

Group means and SE bars (Experiment 1, n=10) of left FDI and ADM MEP amplitudes, expressed with respect to baseline (horizontal dashed line), elicited (A) prior to the imperative signal (TMS_preImp) or (B) during the interval 100 ms to 20 ms (TMS_{postImp_late}) prior to the onset of the EMG for left index finger (selected) or the right index finger (nonselected) trials. MEPs are shown as raw data (% Baseline) but were log-transformed for statistical analyses. * = p < 0.05 for the comparisons indicated.

Figure 5

Group means and SE bars of left FDI MEP amplitudes in the DUAL TASK blocks of Experiment 2. A: Mean MEPs (mV) at TMS_baseline and TMS_{postImp_early} for the selected (uniform fill) and nonselected (dashed fill) conditions. Data here are collapsed over the Finger-Finger and Finger-Foot conditions. MEPs are shown as raw data but were log-transformed for statistical analyses. B: MEPs at TMS_{postImp_late} expressed relative to baseline for the Finger-Finger and Finger-Foot conditions. ұ = p < 0.05 when compared with MEPs elicited at TMS_baseline. * = p < 0.05 for the comparisons indicated.