Long-Term Recording of Subthalamic Aperiodic Activities and Beta Bursts in Parkinson's Disease.

BACKGROUND: Local field potentials (LFPs) represent the summation of periodic (oscillations) and aperiodic (fractal) signals. Although previous studies showed changes in beta band oscillations and burst characteristics of the subthalamic nucleus (STN) in Parkinson's disease (PD), how aperiodic activity in the STN is related to PD pathophysiology is unknown. OBJECTIVES: The study aimed to characterize the long-term effects of STN-deep brain stimulation (DBS) and dopaminergic medications on aperiodic activities and beta bursts. METHODS: A total of 10 patients with PD participated in this longitudinal study. Simultaneous bilateral STN-LFP recordings were conducted in six separate visits during a period of 18 months using the Activa PC + S device in the off and on dopaminergic medication states. We used irregular-resampling auto-spectral analysis to separate oscillations and aperiodic components (exponent and offset) in the power spectrum of STN-LFP signals in beta band. RESULTS: Our results revealed a systematic increase in both the exponent and the offset of the aperiodic spectrum over 18 months following the DBS implantation, independent of the dopaminergic medication state of patients with PD. In contrast, beta burst durations and amplitudes were stable over time and were suppressed by dopaminergic medications. CONCLUSIONS: These findings indicate that oscillations and aperiodic activities reflect at least partially distinct yet complementary neural mechanisms, which should be considered in the design of robust biomarkers to optimize adaptive DBS. Given the link between increased gamma-aminobutyric acidergic (GABAergic) transmission and higher aperiodic activity, our findings suggest that long-term STN-DBS may relate to increased inhibition in the basal ganglia. © 2022 International Parkinson and Movement Disorder Society.

Fronto-subthalamic phase synchronization and cross-frequency coupling during conflict processing.

Growing evidence suggests that both the medial prefrontal cortex (mPFC) and the subthalamic nucleus (STN) play crucial roles in conflict processing, but how these two structures coordinate their activities remains poorly understood. We simultaneously recorded electroencephalogram from the mPFC and local field potentials from the STN using deep brain stimulation electrodes in 13 Parkinson's disease patients while they performed a Stroop task. Both mPFC and STN showed significant increases in theta activities (2-8 Hz) in incongruent trials compared to the congruent trials. The theta activity in incongruent trials also demonstrated significantly increased phase synchronization between mPFC and STN. Furthermore, the amplitude of gamma oscillation was modulated by the phase of theta activity at the STN in incongruent trials. Such theta-gamma phase-amplitude coupling (PAC) was much stronger for incongruent trials with faster reaction times than those with slower reaction times. Elevated theta-gamma PAC in the STN provides a novel mechanism by which the STN may operationalize its proposed "hold-your-horses" role. The co-occurrence of mPFC-STN theta phase synchronization and STN theta-gamma PAC reflects a neural substrate for fronto-subthalamic communication during conflict processing. More broadly, it may be a general mechanism for neuronal interactions in the cortico-basal ganglia circuits via a combination of long-range, within-frequency phase synchronization and local cross-frequency PAC.

Acute low frequency dorsal subthalamic nucleus stimulation improves verbal fluency in Parkinson's disease.

BACKGROUND: Parkinson's disease (PD) is a common neurodegenerative disorder that results in movement-related dysfunction and has variable cognitive impairment. Deep brain stimulation (DBS) of the dorsal subthalamic nucleus (STN) has been shown to be effective in improving motor symptoms; however, cognitive impairment is often unchanged, and in some cases, worsened particularly on tasks of verbal fluency. Traditional DBS strategies use high frequency gamma stimulation for motor symptoms (∼130 Hz), but there is evidence that low frequency theta oscillations (5-12 Hz) are important in cognition. METHODS: We tested the effects of stimulation frequency and location on verbal fluency among patients who underwent STN DBS implantation with externalized leads. During baseline cognitive testing, STN field potentials were recorded and the individual patients' peak theta frequency power was identified during each cognitive task. Patients repeated cognitive testing at five different stimulation settings: no stimulation, dorsal contact gamma (130 Hz), ventral contact gamma, dorsal theta (peak baseline theta) and ventral theta (peak baseline theta) frequency stimulation. RESULTS: Acute left dorsal peak theta frequency STN stimulation improves overall verbal fluency compared to no stimulation and to either dorsal or ventral gamma stimulation. Stratifying by type of verbal fluency probes, verbal fluency in episodic categories was improved with dorsal theta stimulation compared to all other conditions, while there were no differences between stimulation conditions in non-episodic probe conditions. CONCLUSION: Here, we provide evidence that dorsal STN theta stimulation may improve verbal fluency, suggesting a potential possibility of integrating theta stimulation into current DBS paradigms to improve cognitive outcomes.

Interhemispheric interactions between the right angular gyrus and the left motor cortex: a transcranial magnetic stimulation study.

The interconnection of the angular gyrus of right posterior parietal cortex (PPC) and the left motor cortex (LM1) is essential for goal-directed hand movements. Previous work with transcranial magnetic stimulation (TMS) showed that right PPC stimulation increases LM1 excitability, but right PPC followed by left PPC-LM1 stimulation (LPPC-LM1) inhibits LM1 corticospinal output compared with LPPC-LM1 alone. It is not clear if right PPC-mediated inhibition of LPPC-LM1 is due to inhibition of left PPC or to combined effects of right and left PPC stimulation on LM1 excitability. We used paired-pulse TMS to study the extent to which combined right and left PPC stimulation, targeting the angular gyri, influences LM1 excitability. We tested 16 healthy subjects in five paired-pulsed TMS experiments using MRI-guided neuronavigation to target the angular gyri within PPC. We tested the effects of different right angular gyrus (RAG) and LM1 stimulation intensities on the influence of RAG on LM1 and on influence of left angular gyrus (LAG) on LM1 (LAG-LM1). We then tested the effects of RAG and LAG stimulation on LM1 short-interval intracortical facilitation (SICF), short-interval intracortical inhibition (SICI), and long-interval intracortical inhibition (LICI). The results revealed that RAG facilitated LM1, inhibited SICF, and inhibited LAG-LM1. Combined RAG-LAG stimulation did not affect SICI but increased LICI. These experiments suggest that RAG-mediated inhibition of LAG-LM1 is related to inhibition of early indirect (I)-wave activity and enhancement of GABAB receptor-mediated inhibition in LM1. The influence of RAG on LM1 likely involves ipsilateral connections from LAG to LM1 and heterotopic connections from RAG to LM1.NEW & NOTEWORTHY Goal-directed hand movements rely on the right and left angular gyri (RAG and LAG) and motor cortex (M1), yet how these brain areas functionally interact is unclear. Here, we show that RAG stimulation facilitated right hand motor output from the left M1 but inhibited indirect (I)-waves in M1. Combined RAG and LAG stimulation increased GABAB, but not GABAA, receptor-mediated inhibition in left M1. These findings highlight unique brain interactions between the RAG and left M1.

Bimanual but not unimanual finger movements are triggered by a startling acoustic stimulus: evidence for increased reticulospinal drive for bimanual responses.

The relative contributions of reticulospinal versus corticospinal pathways for movement production are thought to be dependent on the type of response involved. For example, unilateral distal movements involving the hand and finger have been thought to be primarily driven by corticospinal output, whereas bilateral responses are considered to have greater reticulospinal drive. The current study investigated whether a difference in the relative contribution of reticulospinal drive to a bimanual versus unimanual finger movement could be assessed using a StartReact protocol. The StartReact effect refers to the early and involuntary initiation of a prepared movement when a startle reflex is elicited. A decreased response latency on loud stimulus trials where a startle reflex is observed in sternocleidomastoid (SCM+ trials) confirms the StartReact effect, which is attributed to increased reticulospinal drive associated with engagement of the startle reflex circuitry. It was predicted that a StartReact effect would be absent for the predominantly corticospinal-mediated unimanual finger movement but present for the bimanual finger movement due to stronger reticulospinal drive. Results supported both predictions as reaction time was statistically equivalent for SCM+ and SCM- trials during unimanual finger movements but significantly shorter for SCM+ trials during bimanual finger movements. These results were taken as strong and novel evidence for increased reticulospinal output for bimanual finger movements.NEW & NOTEWORTHY The relative contributions of reticulospinal and corticospinal pathways to movement initiation are relatively unknown but appear to depend on the involved musculature. Here, we show that unimanual finger movements, which are predominantly initiated via corticospinal pathways, are not triggered at short latency by a startling acoustic stimulus (SAS), while bimanual finger movements are triggered by the SAS. This distinction is attributed to increased reticulospinal drive for bilateral responses.

Deep brain stimulation and recordings: Insights into the contributions of subthalamic nucleus in cognition.

Recent progress in targeted interrogation of basal ganglia structures and networks with deep brain stimulation in humans has provided insights into the complex functions the subthalamic nucleus (STN). Beyond the traditional role of the STN in modulating motor function, recognition of its role in cognition was initially fueled by side effects seen with STN DBS and later revealed with behavioral and electrophysiological studies. Anatomical, clinical, and electrophysiological data converge on the view that the STN is a pivotal node linking cognitive and motor processes. The goal of this review is to synthesize the literature to date that used DBS to examine the contributions of the STN to motor and non-motor cognitive functions and control. Multiple modalities of research have provided us with an enhanced understanding of the STN and reveal that it is critically involved in motor and non-motor inhibition, decision-making, motivation and emotion. Understanding the role of the STN in cognition can enhance the therapeutic efficacy and selectivity not only for existing applications of DBS, but also in the development of therapeutic strategies to stimulate aberrant circuits to treat non-motor symptoms of Parkinson's disease and other disorders.

High-intensity transcranial magnetic stimulation reveals differential cortical contributions to prepared responses.

Corticospinal output pathways have typically been considered to be the primary driver for voluntary movements of the hand/forearm; however, more recently, reticulospinal drive has also been implicated in the production of these movements. Although both pathways may play a role, the reticulospinal tract is thought to have stronger connections to flexor muscles than to extensors. Similarly, movements involuntarily triggered via a startling acoustic stimulus (SAS) are believed to receive greater reticular input than voluntary movements. To investigate a differential role of reticulospinal drive depending on movement type or acoustic stimulus, corticospinal drive was transiently interrupted using high-intensity transcranial magnetic stimulation (TMS) applied during the reaction time (RT) interval. This TMS-induced suppression of cortical drive leads to RT delays that can be used to assess cortical contributions to movement. Participants completed targeted flexion and extension movements of the wrist in a simple RT paradigm in response to a control auditory go signal or SAS. Occasionally, suprathreshold TMS was applied over the motor cortical representation for the prime mover. Results revealed that TMS significantly increased RT in all conditions. There was a significantly longer TMS-induced RT delay seen in extension movements than in flexion movements and a greater RT delay in movements initiated in response to control stimuli compared with SAS. These results suggest that the contribution of reticulospinal drive is larger for wrist flexion than for extension. Similarly, movements triggered involuntarily by an SAS appear to involve greater reticulospinal drive, and relatively less corticospinal drive, than those that are voluntarily initiated. NEW & NOTEWORTHY Through the use of the transcranial magnetic stimulation-induced silent period, we provide novel evidence for a greater contribution of reticulospinal drive, and a relative decrease in corticospinal drive, to movements involuntarily triggered by a startle compared with voluntary movements. These results also provide support for the notion that both cortical and reticular structures are involved in the neural pathway underlying startle-triggered movements. Furthermore, our results indicate greater reticulospinal contribution to wrist flexion than extension movements.

A Timeline of Motor Preparatory State Prior to Response Initiation: Evidence from Startle.

Response preparation in simple reaction time (RT) tasks has been modeled as an increase in neural activation to a sub-threshold level, which is maintained until the go-signal. However, the amount of time required for response preparation following a warning signal (WS) is currently unclear, as experiments typically employ long foreperiods to ensure maximal preparation. The purpose of the present experiments was to examine the time course of motor preparation in a simple RT task when given a limited amount of time to engage in preparatory processing. In Experiment 1, participants completed wrist extension movements in a simple RT paradigm with a short (500 ms) fixed foreperiod, and a long (8.5-10.5 s) inter-trial interval. To probe response preparation, a startling acoustic stimulus (SAS), which involuntarily triggers the release of sufficiently prepared responses, was randomly presented during the foreperiod at one of six equally spaced time points between 0 and 500 ms prior to the go-signal. Results showed that the long inter-trial interval was not always effective at preventing participants from engaging in preparatory processing between trials; thus, in Experiment 2 participants performed wrist flexion or extension movements in an instructed delay paradigm, where the required movement was cued by the WS. Results showed that the proportion of startle trials where the intended response was elicited by the SAS at short latency significantly increased until 100 ms prior to the go-signal, indicating response preparation can take up to 300-400 ms following the WS in a simple RT task with a short fixed foreperiod.

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

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.

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.

Foreknowledge of an impending startling stimulus does not affect the proportion of startle reflexes or latency of StartReact responses.

During a simple reaction time (RT) task, movements can be initiated early and involuntarily through presentation of a loud startling acoustic stimulus (SAS), a phenomenon termed the StartReact effect. In order to infer that activity in startle-related structures led to the early response triggering, it is important to observe a concurrent startle reflex in sternocleidomastoid. It is generally accepted that to consistently elicit a startle reflex, the SAS must be both intense and unpredictable. However, it remains unclear what effect explicit foreknowledge of an impending SAS has on the effectiveness of a SAS to elicit a startle reflex when preparing a motor response. To test this, participants completed two separate blocks of a simple RT task (counterbalanced order), where the control auditory go-signal was replaced with a SAS on 20 % of trials. In an unwarned block, knowledge of the trial type (SAS vs. control) was not provided in advance, while in a warned block, the trial type was forewarned. Results revealed that while foreknowledge of an impending SAS reduced the magnitude of the startle reflex, it did not affect the proportion of startle reflexes elicited or the magnitude of the StartReact effect. An increase in control trial RT was observed during the unwarned block, but only when it was performed first. These results indicate that preparation of a motor response leads to sufficiently increased activation in startle-related neural structures such that even with explicit knowledge of an upcoming SAS, participants are unable to proactively gate the upcoming sensory input.

Go-activation endures following the presentation of a stop-signal: evidence from startle.

It has been proposed that, in a stop-signal task (SST), independent go- and stop-processes "race" to control behavior. If the go-process wins, an overt response is produced, whereas, if the stop-process wins, the response is withheld. One prediction that follows from this proposal is that, if the activation associated with one process is enhanced, it is more likely to win the race. We looked to determine whether these initiation and inhibition processes (and thus response outcomes) could be manipulated by using a startling acoustic stimulus (SAS), which has been shown to provide additional response activation. In the present study, participants were to respond to a visual go-stimulus; however, if a subsequent stop-signal appeared, they were to inhibit the response. The stop-signal was presented at a delay corresponding to a probability of responding of 0.4 (determined from a baseline block of trials). On stop-trials, a SAS was presented either simultaneously with the go-signal or stop-signal or 100, 150, or 200 ms following the stop-signal. Results showed that presenting a SAS during stop-trials led to an increase in probability of responding when presented with or following the stop-signal. The latency of SAS responses at the stop-signal + 150 ms and stop-signal + 200 ms probe times suggests that they would have been voluntarily inhibited but instead were involuntarily initiated by the SAS. Thus results demonstrate that go-activation endures even 200 ms following a stop-signal and remains accessible well after the response has been inhibited, providing evidence against a winner-take-all race between independent go- and stop-processes. NEW & NOTEWORTHY: In this study, a startling acoustic stimulus (SAS) was used to determine whether response outcome could be manipulated in a stop-signal task. Results revealed that presenting a SAS during stop-signal trials led to an increase in probability of responding even when presented 200 ms following the stop-signal. The latency of SAS responses indicates that go-activation remains accessible and modifiable well after the response is voluntarily inhibited, providing evidence against an irrevocable commitment to inhibition.

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.

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.

Reduced motor preparation during dual-task performance: evidence from startle.

Previous studies have used a secondary probe reaction time (RT) task to assess attentional demands of a primary task. The current study used a startling acoustic stimulus (SAS) in a probe RT paradigm to test the hypothesis that attentional resources would be directly related to limitations in response preparation. Participants performed an easy or difficult version of a continuous primary task that was either primarily motor in nature (pursuit tracking) or cognitive (counting backward). Concurrently, participants responded to an auditory cue as fast as possible by performing a wrist extension secondary movement. On selected trials, the auditory cue was replaced with a SAS (120 dB), which is thought to involuntarily trigger a prepared response and thus bypass any response initiation bottleneck that may be present when trying to perform two movements. Although startle trials were performed at a shorter latency, both non-startle and startle probe trials resulted in a delayed RT, as compared to single-task trials, consistent with reduced preparation of the secondary task. In addition, analysis of SAS trial RT when a startle indicator was present versus absent provided evidence that the secondary task was at a lowered state of preparation when engaged in the cognitive primary task as compared to a motor primary task, suggesting a facilitative effect on preparatory activation when both the primary and secondary tasks are motoric in nature.

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.

Anodal tDCS over SMA decreases the probability of withholding an anticipated action.

Previous research has shown that the supplementary motor area (SMA) is critical in movement inhibition. Recently it was shown that applying transcranial direct current stimulation (tDCS) over SMA affected participants' ability to inhibit their movement in a stop-signal reaction time task (Hsu et al. [11]). Of interest in the current study was whether modulating SMA excitability using tDCS would have similar effects in an anticipation-timing stop-signal task. Participants performed 2 sessions each consisting of a pre- and post-tDCS block of 160 trials in which they were instructed to extend their wrist concurrently with the arrival of a pointer to a target (i.e., a clock hand reaching a set position). In 20% of trials (stop trials) the pointer stopped 80, 110, 140, 170, or 200 ms prior to the target, and on these trials participants were instructed to inhibit their movement if possible. Anodal and cathodal tDCS (separated by at least 48 h) was applied for each participant between the pre- and post-tDCS blocks. No change in the proportion of successfully inhibited movements on stop trials was found following cathodal tDCS (p>.05). However, anodal tDCS resulted in a decreased proportion of successfully inhibited movements on stop trials (p=002), and an earlier movement onset on control trials (p<.01). This suggests that the SMA may be more involved in initiation than in inhibition of anticipatory movements. Furthermore these data suggest that differences in initiation and inhibitory processes exist between stop-signal reaction time and anticipation-timing stop-signal tasks.

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.