Neil M. Drummond

Motor preparation is delayed for both directly and indirectly cued movements during an anticipation-timing task

Previous investigations comparing direct versus indirectly cued movements have consistently shown that indirectly cued movements take longer to prepare (Neely and Heath, 2010. Brain Res. 1366, 129-140) and involve the recruitment of additional brain areas (Connolly et al., 2000. J. Neurophysiol. 84, 1645-1655). ). This increase in processing time has been associated with the additional cognitive transformations required of the task (Neely and Heath, 2010. Brain Res. 1366, 129-140).. In the present study we investigated whether differences between direct versus indirectly cued movements are also reflected in the time course of motor preparation. Participants performed a targeting task, moving directly to the location of a visual cue (i.e., directly cued movement) or to a location that differed by 60°, 90°, or 120° with respect to the visual cue provided (i.e., indirectly cued movements). Participants were instructed to initiate their movements concurrently with an anticipated go-signal. To examine the time course of motor preparation, a startling acoustic stimulus (SAS, 124dB) was randomly presented 150ms, 500ms, or 1000ms prior to the go-signal. Results from the startle trials revealed that the time course of motor preparation was similar regardless of the angle of rotation required and hence whether it was a direct or indirectly cued trial. Specifically, motor preparation was delayed until less than 500ms prior to movement initiation for both direct and indirectly cued movements. These findings indicate that similar motor preparation strategies are engaged for both types of cued movements, suggesting that the time to prepare a motor response may be similar regardless of whether a cognitive transformation is required.

Startle activation is additive with voluntary cortical activation irrespective of stimulus modality.

When a startling acoustic stimulus (SAS) is presented during a simple reaction time (RT) task, it can trigger the prepared response through an involuntary initiation pathway. Previous research modelling the effects of presenting a SAS at various intervals following a non-startling auditory imperative signal (IS) suggested that involuntary initiation-related neural activation is additive with the voluntary initiation processes. The current study tested the predictions of this additive model when the SAS and IS are of different modalities by using a visual rather than auditory go-signal. Because voluntary RT latencies are delayed for visual stimuli compared to acoustic stimuli, it was hypothesised that the time course of additive activation would be similarly delayed. Participants performed 150 RT trials requiring a targeted 20° wrist extension task with a SAS presented 0-125 ms following a visual go-signal. Results were not different to those predicted by an additive model (p=0.979), yet were significantly different to those predicted by a horse-race model (p=0.037), indicating a joint contribution of voluntary and involuntary activation, even when the IS and SAS are of different modalities. Furthermore, the results indicated that voluntary RT differences due to stimulus modality are attributable to processes that occur prior to the increase in initiation-related activation.

Effector-independent reduction in choice reaction time following bi-hemispheric transcranial direct current stimulation over motor cortex

Increased reaction times (RT) during choice-RT tasks stem from a requirement for additional processing as well as reduced motor-specific preparatory activation. Transcranial direct current stimulation (tDCS) can modulate primary motor cortex excitability, increasing (anodal stimulation) or decreasing (cathodal stimulation) excitability in underlying cortical tissue. The present study investigated whether lateralized differences in choice-RT would result from the concurrent modulation of left and right motor cortices using bi-hemispheric tDCS. Participants completed a choice-RT task requiring either a left or right wrist extension. In forced-choice trials an illuminated target indicated the required response, whereas in free-choice trials participants freely selected either response upon illumination of a central fixation. Following a pre-test trial block, offline bi-hemispheric tDCS (1 mA) was applied over the left and right motor cortices for 10 minutes, which was followed by a post-tDCS block of RT trials. Twelve participants completed three experimental sessions, two with real tDCS (anode right, anode left), as well as a sham tDCS session. Post-tDCS results showed faster RTs for both right and left responses irrespective of tDCS polarity during forced-choice trials, while sham tDCS had no effect. In contrast, no stimulation-related RT or response selection differences were observed in free-choice trials. The present study shows evidence of an effector-independent speeding of response initiation in a forced-choice RT task following bi-hemispheric tDCS and yields novel information regarding the functional effect of bi-hemispheric tDCS.

Offline continuous theta burst stimulation over right inferior frontal gyrus and pre-supplementary motor area impairs inhibition during a go/no-go task

ABSTRACT In a typical go/no‐go task a single imperative stimulus is presented each trial, either a go or no‐go stimulus. Participants are instructed to initiate a known response upon appearance of the go‐signal and withhold the response if the no‐go signal is presented. It is unclear whether the go‐response is prepared in advance of the imperative stimulus in a go/no‐go task. Moreover, it is unclear if inhibitory control processes suppress preparatory go‐activation. The purpose of the present experiment was 1) to determine whether the go‐response is prepared in advance of stimulus identification with the use of a startling acoustic stimulus (SAS), and 2) investigate the inhibitory role of the right inferior frontal gyrus (rIFG) and pre‐supplementary motor area (preSMA) during the performance of a go/no‐go task with the use of continuous theta burst stimulation (cTBS). The experiment consisted of three phases; a pre‐cTBS phase in which participants completed a go/no‐go and simple‐RT task, followed by offline cTBS to temporarily deactivate either rIFG or preSMA (with a sham control), then a post‐cTBS phase which was identical to the pre‐cTBS phase. Results revealed that stimulation to both cortical sites impaired participants’ ability to withhold movements during no‐go trials. Notably, rIFG or preSMA stimulation did not affect the latency of voluntary go‐responses and did not enable the SAS to involuntarily trigger responses. These findings suggest that preparation and initiation of the go‐response occurs after the imperative stimulus, with the rIFG and preSMA involved in inhibiting the go‐response once the stimulus is identified as a no‐go signal. HIGHLIGHTSOffline cTBS was applied over rIFG and preSMA to impair inhibitory control.cTBS only impaired response inhibition and not response initiation.Decreasing inhibition did not allow the involuntary triggering of the go‐response.The go‐response is prepared after the imperative stimulus and inhibited if required.

Inhibition of motor-related activation during a simple reaction time task requiring visuomotor mental rotation.

The present study investigated whether differences in reaction time (RT) between movements initiated to a visual cue (directly cued) versus movements initiated to a location other than the visual cue (indirectly cued) arise because of varying levels of inhibition within the motor system during response preparation. Unlike typical visuomotor mental rotation (VMR) experiments, this study employed a simple RT paradigm to allow response preparation to occur in advance of the imperative stimulus (IS). Participants responded to the IS by either moving directly to the location of a visual cue or to a location that required a mental transformation between the visual cue and the intended movement goal (i.e., a location 60, 90, or 120 degrees rotated with respect to the visual cue). To probe motor-related activation during response preparation, a startling acoustic stimulus (SAS, 124 dB) was randomly presented 500 ms, 1,000 ms, or 1,500 ms after visual cue onset, but before the IS. Results showed similar RTs during nonstartle control trials regardless of rotation angle and whether trials were completed in a random or blocked design. Additionally, SAS trials showed a low incidence of early response triggering across all time points regardless of whether the movement was directly or indirectly cued. In contrast, directly cued movements performed outside of the VMR context showed a high incidence of SAS response triggering. These results suggest that when a stimulus to target-goal transformation might be required, inhibitory suppression of motor-related activation arises regardless of whether the final movement is directly or indirectly cued.

Startle reveals decreased response preparatory activation during a stop-signal task

In a stop-signal task participants are instructed to initiate a movement in response to a go signal, but to inhibit this movement if an infrequent stop signal is presented after the go. Reaction time (RT) in a stop-signal task is typically longer compared with that in a simple RT task, which may be attributed to a reduced readiness to initiate the response caused by the possibility of having to inhibit the response. The purpose of this experiment was to probe the preparatory activation level of the motor response during a stop-signal task using a startling acoustic stimulus (SAS), which has been shown to involuntarily trigger sufficiently prepared responses at a short latency. Participants completed two separate tasks: a simple RT task, followed by a stop-signal RT task. During both tasks, an SAS (120 dB) was pseudorandomly presented concurrently with the go signal. As expected, RT during the simple RT task was significantly shorter than during the stop-signal task. A significant reduction in RT was noted when an SAS was presented during the simple RT task; however, during the stop-signal task, an SAS resulted in either a significant speeding or a moderate delay in RT. Additionally, the subset of SAS trial responses with the shortest RT latencies produced during the stop-signal task were also delayed compared with the short-latency SAS trial responses observed during the simple RT task. Despite evidence that a response was prepared in advance of the go signal during a stop-signal task, it appears that the amount of preparatory activation was reduced compared with that achieved during a simple RT task.

Increased response preparation overshadows neurophysiological evidence of proactive selective inhibition.

Humans are able to proactively inhibit a particular motor response when provided with a precue in a bimanual selective stopping task (e.g., Maybe Stop Right Hand). We investigated how preparation affects proactive selective inhibition, as previous experiments have been performed in a paradigm where the ability to prepare responses ahead of the go-signal was limited. Participants completed a simple selective stopping task (Experiment 1), in which the bimanual response was known in advance of the go-signal. In a control experiment (Experiment 2), participants completed the typical choice selective stopping task in which the bimanual response was indicated by the go-signal. Single-pulse transcranial magnetic stimulation was delivered prior to the go-signal to index corticospinal excitability in stop-cued and noncued limbs. In addition, a startling acoustic stimulus was delivered concurrent with the go-signal to probe independent stop-cued and noncued response preparation (Experiment 1 only). Consistent with previous findings, neurophysiological and behavioral evidence of proactive selective inhibition was found during the choice selective stopping task. In contrast, the simple selective stopping task revealed an overall increase in corticospinal excitability in both the stop-cued and noncued limbs but no difference between them despite behavioral evidence of selective inhibition. In line with the neurophysiological results, startle trials revealed that when a startle reflex was elicited, both noncued and stop-cued responses were initiated early and synchronously. Results suggest that increased preparatory activation of the stop-cued response overshadows the small selective inhibitory effect typically seen in choice selective stopping tasks with limited advance preparation.