Biology subjects

Corneil, B. D.

Publications and source records attributed to Corneil, B. D..

At least 19 recordsLinked to original sources

bioRxiv2025

Subcortical control of reaching in humans

Accurate visually guided reaching requires transformation of target-related photoreceptor responses into precisely coordinated activation of trunk and arm muscles. The cerebral cortex is widely believed to compute the requisite kinematic and musculoskeletal dynamics strategies in humans 1-3, even though vertebrates lacking a cerebral cortex achieve sophisticated visuomotor control 4-6, and brainstem circuits executing coordinated eye and head gaze shifts perform analogous sensorimotor computations in non-human primates 7. Here we used a visuomotor reaching task that yields extremely rapid, "express", target-directed muscle activations 8-10 to test whether a putative subcortical sensorimotor network can compute musculoskeletal dynamics to initiate reaching in humans. We found coordinated express visuomotor responses (EVRs) in task-relevant shoulder, elbow, and bi-articular muscles that reflected both starting posture and target direction in similar patterns to longer latency, presumably cortically mediated, muscle responses. When the task goal was to reach away from the stimulus (i.e. an "anti-reach"; 11) the EVR involved coordinated muscle activation to initiate the hand toward the stimulus location, opposite to the subsequent goal-directed response. The results suggest a unified theory of visuomotor control for reaching and gaze shifts, in which subcortical systems compute musculoskeletal dynamics based on sensory target information and cortically derived context. The results imply that the transformation from motor goals in extrapersonal space into musculoskeletal dynamics can be performed by neural circuitry in humans that does not involve the sensorimotor cortex.

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bioRxiv2025

Effects of aging on upper body express visuomotor responses while reaching under varying postural demands

Humans can react remarkably quickly to novel or displaced visual stimuli when time is of the essence. Such movements are thought to be initiated by a subcortical fast visuomotor network, but it is unclear how this network declines with age. Past work in the upper limb has detailed delayed reaching corrections to jumped visual stimuli in the elderly, but the underlying mechanisms contributing to these changes of the fast visuomotor network are poorly understood. Conversely, work in the lower limb has reported delayed muscle recruitment during obstacle avoidance, but such findings may be confounded by age-related challenges in postural control. The output of the fast visuomotor network can be quantified by measuring express visuomotor responses (EVRs), which are the earliest and very short-latency bursts of muscle activity that follow visual target presentation. Here, we compare the prevalence, latency, and magnitude of EVRs in elderly (58-80 years old) and younger (18-25 years old) participants performing visually-guided reaches. We also investigated the impact of postural stability by having participants reach either while seated on a stable chair, or on a wobble stool. Both the elderly and younger cohorts expressed EVRs, but EVRs in the elderly were comparatively less frequent, and had longer latencies and smaller magnitude. Postural instability had no effects on these outcomes. Our results suggest age-related declines in the fast visuomotor network, potentially resulting from deterioration of underlying circuits and a prioritization of stability over speed. This study serves as an important standard for future research investigating clinical populations. HighlightsO_LIDo aging and postural stability affect express visuomotor responses (EVRs)? C_LIO_LIEVRs were recorded from the pectoralis muscle during rapid goal-directed reaches C_LIO_LIEVRs were smaller and delayed, and movement was slower, in elderly participants C_LIO_LIAge-related differences were independent of our postural stability manipulation C_LIO_LIOur results suggest age-related declines of the fast visuomotor network C_LI

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bioRxiv2025

Continuous theta-burst stimulation of the prefrontal cortex in the macaque monkey: no evidence for within-target inhibition or cross-hemisphere disinhibition of neural activity

BackgroundContinuous theta-burst stimulation (cTBS) can perturb neural activity and behavior by inducing effects that persist beyond the relatively short stimulation period. Although widely used in basic research and clinical settings, there lacks an understanding of the neurophysiological and behavioural effects of cTBS. Objectives/HypothesisTwo assumptions motivating the use of cTBS are that it will i) inhibit neural activity in the targeted area, and ii) consequently disinhibit neural activity in the mirroring region in the contralateral cortex. Here, we test these assumptions in the oculomotor system of healthy rhesus macaques. MethodsIn two macaques, we delivered cTBS between blocks of trials where they performed a delayed pro-/anti-saccade task, delivered cTBS to the right PFC (areas 8Ar and 46, which includes the frontal eye fields; 32 cTBS-PFC sessions), to the air as a SHAM control (27 cTBS-SHAM sessions), or to the nearby primary motor cortex as a brain control (21 cTBS-M1 sessions). Across these different types of sessions, we compared changes in oculomotor behavior (reaction times, error rates, peak saccade velocity), and changes in neural activity recorded from the left, contralateral PFC. ResultsDespite multiple lines of evidence consistent with TMS influencing neural activity in the cTBS-PFC and cTBS-M1 sessions, we found no behavioral evidence for inhibition of the right PFC in the cTBS-PFC sessions, nor any evidence for contralateral disinhibition in the left PFC. ConclusionsOur results call into question some of the fundamental assumptions underlying the application of cTBS. HighlightsO_LIcTBS widely used in lab and clinic to rebalance activity across cortex C_LIO_LIWe tested this in a monkey model, delivering cTBS to the prefrontal cortex C_LIO_LINo behavioural evidence for inhibition of brain area targeted by cTBS C_LIO_LINo evidence for disinhibition of spiking activity in mirroring, contralateral cortex C_LIO_LIResults question key assumptions about how cTBS influences network activity C_LI

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bioRxiv2025

Are online corrections really a distinct class of movement?

Humans have a remarkable capacity to adjust reaching movements rapidly and accurately when visual targets jump to a new location. The short latency of such online corrections has led to the hypothesis that they constitute a distinct class of movement and arise from an automatic pilot that is selectively engaged only during ongoing movements. Here, we test this idea by measuring muscle recruitment, force, and kinematics in a jumping target reaching task. In separate blocks of trials, participants were instructed to respond to target jumps by (1) following the jumped target, (2) stopping the on-going movement, or (3) ignoring the jumped target. This allowed us to establish the automaticity and timing of responses to target jumps and to compare such measures to the original reaching movement initiated from rest. We find that the earliest phase of muscle recruitment elicited by the jumped target corresponds to a subcortical reflex, beginning at [~]80ms and ending by [~]120ms, preceding the onset of voluntary recruitment at [~]130ms. This reflex inexorably drives a reaching adjustment towards the new target in all three blocks; it is only somewhat reduced in the stop and ignore blocks. Critically, this earliest phase of muscle recruitment was also present at the exact same latency (80ms) for the original reaching movement initiated from rest. Thus, rather than supporting the model of online corrections as distinct class of movement that is mediated by an automatic pilot, our results suggest that all reaches, whether adjusted in mid-flight or initiated from rest, arise from a common nested control system featuring subcortical and cortical components whose influence can be strategically preset by task demands. Our results also reinforce the importance of considering movement biomechanics when interpreting kinematic latency differences across movements made in different situations.

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bioRxiv2024

Cortical dynamics underlying initiation of rapid steps with contrasting postural demands

Our ability to flexibly initiate rapid visually-guided stepping movements can be measured in the form of express visuomotor responses (EVRs), which are short-latency ([~]100ms), goal-directed bursts of lower-limb muscle activity. Interestingly, we previously demonstrated that recruitment of anticipatory postural adjustments (APAs) interacted with the subcortically-generated EVRs in the lower limb, suggesting context-dependent top-down modulation. We investigated the associated cortical dynamics prior to and during rapid step initiation towards a salient visual target in twenty-one young, healthy individuals. We adopted two contrasting postural conditions by manipulating the stepping direction. Anterolateral steps involved low postural demands, whereas anteromedial stepping involved high postural demands. We recorded high-density EEG, surface electromyography from gluteus medius and ground-reaction forces. Independent component analysis and time-frequency statistics revealed significant, yet relatively modest differences between conditions in preparatory cortical dynamics, most evidently in primary motor areas. Following target presentation, we observed stronger theta and alpha power enhancement in the supplementary motor area, and stronger alpha and beta power decrease in primary motor, parietal and occipital clusters during APA recruitment that preceded steps under high postural demands. We found no differences in (pre)frontal areas associated with the observed EVR suppression in the high postural demand condition. Together, our findings point towards greater cortical involvement in step initiation under high postural demands as compared to more reflexive, stimulus-driven steps. This notion may be particularly relevant for populations where postural control is impaired by age or disease, as more cortical resources may need to be allocated during stepping.

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bioRxiv2024

Lower-limb express visuomotor responses are spared in Parkinson's Disease during step initiation from a stable position

While motor impairments have been extensively studied in Parkinsons Disease, rapid visuomotor transformations for flexible interaction with the environment have received surprisingly little attention. In recent years, such rapid visuomotor transformations have been studied in the form of express visuomotor responses (EVRs), which are goal-directed bursts of muscle activity that are thought to originate from superior colliculus, reaching the periphery via the tecto-reticulospinal pathway. Here, we examined EVRs in the lower limbs during goal-directed step initiation in 20 people with Parkisons Disease (PwPD) and 20 age-matched healthy control participants (HC). As lower-limb EVRs in the young have been shown to interact with postural control - which is often affected in PwPD - we manipulated the postural demands by varying initial stance width and target location. In the low postural demand condition, EVRs were robustly present in both the PwPD (17/20) and HC (16/20) group. However, when postural demands were high, EVRs were largely absent in both groups and, instead, strong anticipatory postural adjustments (APAs) were required prior to foot off. EVR magnitudes were, on average, stronger in PwPD compared to HC, but they decreased with increasing disease severity, suggesting that the EVR network may become compromised or down-regulated in later stages of the disease. While APA magnitudes were smaller in PwPD compared to HC, subsequent stepping performance (step reaction time, duration, size, velocity) was remarkably similar between the two groups. We suggest that the EVR network may be upregulated in the early stages of Parkinsons disease in order to compensate for some of the emerging motor deficits experienced in daily life.

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bioRxiv2024

Parkinson's Disease affects the contextual control, but not the expression, of a rapid visuomotor response that initiates visually-guided reaching: Evidence for multiple, interacting motor pathways and implications for motor symptoms in Parkinson's Disease

Despite significant deficits in voluntary motor control, patients with Parkinsons disease (PD) can generate reflexive or stimulus-driven movements. How are such spared capabilities realized? Here, we recorded upper limb muscle activity in patients with PD and age-matched healthy controls (HCs) as they reached either toward or away from a visual stimulus. The task promoted express visuomotor responses (EVRs), which are brief bursts of muscle recruitment time-locked (<100 ms) to stimulus presentation that are thought to originate from the midbrain superior colliculus. Across two experiments, we observed a remarkable sparing of the latency and magnitude of EVRs in patients with PD, but a decreased ability for patients with PD to contextually modulate the EVR depending on trial instruction. EVR Magnitudes were strikingly strongly correlated with PD Reaction times and Error rates, despite compromised levels of electromyographic (EMG) recruitment in subsequent phases of muscle activity, which predicted lower Peak velocities. Our results are consistent with a differential influence of PD on parallel-but-interacting subcortical and cortical pathways that converge onto brainstem and spinal circuits during reaching. This differential influence is discriminable even within a single trial in the selective sparing of stimulus-aligned but not movement-aligned muscle recruitment, and has implications for our understanding of the motor and cognitive deficits seen in PD.

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bioRxiv2024

Startling acoustic stimuli hasten choice reaching tasks by strengthening, but not changing the timing of, express visuomotor responses

Responding to an external stimulus takes [~]200 ms, but this can be shortened to as little as [~]120 ms with the additional presentation of a startling acoustic stimulus. This phenomenon is hypothesized to arise from the involuntary release of a prepared movement (a StartReact effect). However, a startling acoustic stimulus also expedites rapid mid-flight, reactive adjustments to unpredictably displaced targets which could not have been prepared in advance. We surmise that for such rapid visuomotor transformations, intersensory facilitation may occur between auditory signals arising from the startling acoustic stimulus and visual signals relayed along a fast subcortical network. To explore this, we examined how a startling acoustic stimulus shortens reaction times in a task that produces express visuomotor responses, which are brief bursts of muscle activity that arise from a fast tectoreticulospinal network. We measured express visuomotor responses on upper limb muscles in humans as they reached either toward or away from a stimulus in blocks of trials where movements could either be fully prepared or not, occasionally pairing stimulus presentation with a startling acoustic stimulus. The startling acoustic stimulus reliably produced larger but fixed-latency express visuomotor responses in a target-selective manner, and also shortened reaction times, which were equally short for prepared and unprepared movements. Our results provide insights into how a startling acoustic stimulus shortens the latency of reactive movements without full motor preparation. We propose that the reticular formation is the likely node for intersensory convergence during the most rapid transformations of vision into targeted reaching actions. KEY POINTSO_LIA startling acoustic stimulus (SAS) shortens reaction times by releasing fully prepared motor programs (the StartReact effect), but can also hasten responses in reflexive tasks without any movement preparation C_LIO_LIHere we measure the effect of a SAS on reaction times and upper limb muscle recruitment in a reflexive reaching task, focusing on express visuomotor responses that are evoked by visual target presentation and demarcate activity along a subcortical tectoreticulospinal pathway C_LIO_LIA SAS robustly increased the magnitude of express visuomotor responses without changing their timing, and this increase was tightly related to the subsequent reaction time even in the absence of motor preparation C_LIO_LIOur results attest to intersensory facilitation within the tectoreticulospinal pathway, which provides the shortest pathway mediating visuomotor transformations for reaching C_LIO_LIThese results reconcile discrepant findings by emphasizing the importance of intersensory facilitation in SAS-induced hastening of reaction times in reflexive tasks C_LI

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bioRxiv2023

Rapid integration of face detection and task set in visually guided reaching

The superior colliculus (SC) has been increasingly implicated in the rapid processing of evolutionarily relevant visual stimuli like faces, but the behavioural relevance of such processing is not clear. The SC has also been implicated in the generation of upper-limb Express Visuomotor Responses (EVRs) on upper limb muscles, which are very short-latency (within [~]80 ms) bursts of muscle activity time-locked to visual target presentation. This reasoning led us to investigate the influence of faces on EVRs. We recorded upper limb muscle activity from young healthy participants as they reached toward left or right targets in the presence of a distractor stimulus presented on the opposite side. Across blocks of trials, we varied the instruction as to which stimulus served as the target or distractor. Doing so allowed us to assess the impact of instruction on muscle recruitment by examining trials when the exact same stimuli required a reach to either the left or right. We found that EVRs were uniquely modulated in tasks involving face selection, promoting reaches toward or away from faces depending on instruction. Follow-up experiments confirmed that this phenomenon required highly salient repeated faces, and was not observed to non-facial salient stimuli nor to faces expressing different affect. We conclude that our results attest to an integration of top-down task set and bottom-up feature detection to promote rapid motor responses to faces at latencies that match or precede the arrival of face information in human cortex. STATEMENT OF SIGNIFICANCEThe tecto-reticulo-spinal pathway is hypothesized to mediate the express visuomotor response (EVR). This study extends this hypothesis by demonstrating that face detection in the subcortex impacts low-latency movement via the EVR at latencies preceding cortical activity for face perception. To date, this constitutes the most direct evidence for direct behavioural relevance of rapid face detection in the brainstem. Further, we find that this response can be modulated by task context, allowing for different instruction-based responses given the exact same visual stimulus and implicating top-down cortical control of the EVR.

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bioRxiv2023

Controlling focality and intensity of non-invasive deep brain stimulation using multipolar temporal interference in non-human primates and rodents

Temporal interference (TI) is a method of non-invasive brain stimulation using transcutaneous electrodes that allows the targeting and modulation of deeper brain structures, not normally associated with non-invasive simulation, while avoiding unwanted stimulation of shallower cortical structures. The properties of TI have been previously demonstrated, however, the problem of decoupling stimulation focality from stimulation intensity has not been addressed. In this paper, we provide a possible novel solution, multipolar TI (mTI), which allows increased independent control over both the size of the stimulated region and the stimulation intensity. The mTI method uses multiple carrier frequencies to create multiple overlapping amplitude-modulated envelopes, rather than using one envelope as in standard TI. The study presents an explanation of the concept of mTI along with experimental data gathered from Rhesus macaques and mice. We improved the focality at depth in anesthetized mice and monkeys, and using the new focality in awake monkeys, evoked targeted activity at depth in the superior colliculus. The mTI method could be an interesting and potentially useful new tool alongside other forms of non-invasive brain stimulation. Teaser: Multipolar Temporal Interference Stimulation can produce a more focal brain stimulation at depth compared to Temporal Interference.

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bioRxiv2023

Done in 65 ms: Express visuomotor responses in upper limb muscles in Rhesus Macaques

How rapidly can the brain transform vision into action? Work in humans has established that the transformation for visually-guided reaching can be remarkably rapid, with the first phase of upper limb muscle recruitment, the express visuomotor response, beginning within less than 100 ms of visual target presentation. Such short-latency responses limit the opportunities for extensive cortical processing, leading to the hypothesis that they are generated via the subcortical tectoreticulospinal pathway. Here, we examine if non-human primates (NHPs) exhibit express visuomotor responses. Two male macaques made visually-guided reaches in a behavioral paradigm known to elicit express visuomotor responses in humans, while we acquired intramuscular recordings from the deltoid muscle. Across several variants of this paradigm, express visuomotor responses began within 65 ms (range 48-91 ms) of target presentation. Although the timing of the express visuomotor response did not co-vary with reaction time, larger express visuomotor responses tended to precede shorter latency reaches. Finally, the magnitude of the express visuomotor response was muted on trials where NHPs withheld a reach to one stimulus in order to move to a stimulus appearing 34 ms later in the opposite direction. Overall, the response properties and contextual control of express visuomotor responses in NHPs resemble those in humans. Our results establish a new benchmark for visuomotor transformations underlying visually-guided reaches, setting the stage for experiments that can directly compare the role of cortical and subcortical areas in reaching when time is of the essence. Significance statementExpress visuomotor responses in upper limb muscles are brief periods of recruitment preceding visually-guided reaches. Such responses begin [~]90 ms after visual target presentation in humans, and potentially arise from signaling along the tecto-reticulo-spinal pathway. Here, we show that express visuomotor responses in macaques upper limb muscles resemble those in humans, excepting that they evolve [~]65 ms after target onset, consistent with shorter responses latencies in macaques versus humans. Our results clock the completion of the visuomotor transformation for rapid reaching, and set the stage for experiments to directly test the underlying substrates.

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bioRxiv2022

A complex reach direction rule that delays reaction time causes alternating excitation and inhibition in express muscle responses and corticospinal excitability

The dynamics of muscle activation during fast visually guided reaching are suggestive of two neural control signals; an early signal that acts at "express" latencies in response to the visual stimulus, and a longer latency signal that executes a strategic reach plan. Here we developed a task designed to temporally isolate the express visuomotor response from the longer latency muscle response, and to characterize the time course of corticospinal excitability changes in the express response time window when the late voluntary response is delayed. We tested this by measuring electromyograms (EMG) and changes in Motor Evoked Potential (MEP) amplitudes following Transcranial Magnetic Stimulation (TMS) of the motor cortex, as participants reached either towards or away from visual targets. Crucially, the information about the task rule was provided by the luminance of the target itself, and so was unknown to the subject until the instant of target presentation. This feature delayed reaction times, likely because additional (presumably cortical) processing was required to interpret and apply the rule before formation of a goal directed reach plan. The earliest EMG responses to target presentation occurred with a 70-105 ms time window, and were oriented to bring the hand toward the location of the target. However, there was also a slightly later response that was also time-locked to target appearance in a 105-140 ms time window. This second response was "reciprocal" to the first, such that it was oriented to take the hand in the direction opposite from the target. In some participants, additional oscillating cycles were apparent after the first two target-related responses. These multiphasic express visuomotor responses were nearly identical in both pro- and anti-reach conditions. These muscle activity responses were generally reflected in the temporal pattern of corticospinal excitability modulations in experiment two. Indeed, the MEP and background EMG responses showed an alternating pattern similar to that in experiment one, although the effect was clearer in the anti-reach than the pro-reach condition. Overall, the data show that the express and voluntary responses are indeed distinct neural control signals, which supports the hypothesis that at least two separate neural pathways (one slow and one fast) contribute to the control of visually guided reaching. The properties of the fast pathway are consistent with a tecto-reticulospinal pathway, while those of the slow pathway are consistent with a transcortical loop.

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bioRxiv2022

A subcortical origin for rapid, target-oriented corticospinal excitability changes during visually guided reaching

During visually guided reaching, proximal limb muscles can be activated within 80 ms of target appearance. Such "express" visuomotor responses are temporally aligned with target appearance rather than movement onset, and invariably tuned towards the direction of the visual target regardless of the instructed reach direction. These features prompt the hypothesis that express visuomotor responses are driven by a subcortical pathway. We tested this by measuring the changes in Motor Evoked Potential (MEP) size following Transcranial Magnetic Stimulation (TMS) or Transcranial Electrical Stimulation (TES) of the motor cortex, as participants reached either towards or away from visual targets. We found that 70-80 ms after target presentation, MEPs in a primary shoulder flexor muscle (pectoralis major) were oriented towards the target direction regardless of whether the participant subsequently reached towards or away from the target. Similar target-oriented MEP modulations occurred in posterior deltoid and biceps brachii muscles, whereas MEPs in a finger muscle were affected neither by target nor reach direction. Critically, there were no significant differences in modulation of responses to TMS versus TES across all reaching conditions, which suggests that the target-oriented modulation occurs downstream of the motor cortex output neurons. Combined, our results are consistent with a subcortical rather than cortical origin for the earliest changes in corticospinal excitability following visual target onset. A prime candidate for such subcortical modulation includes the superior colliculus and reticular formation.

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bioRxiv2022

Express visuomotor responses reflect knowledge of both target location and conscious intent during reaches of different amplitudes

When humans reach to visual targets, extremely rapid (~90 ms) bursts of activity can be observed on task-relevant proximal muscles. Such express visuomotor responses are inflexibly locked in time and space to the target and have been proposed to reflect rapid visuomotor transformations conveyed subcortically via the tecto-reticulo-spinal pathway. Previously, we showed that express visuomotor responses are sensitive to explicit cue-driven information about the target, suggesting that the express pathway can be modulated by cortical signals affording contextual pre-stimulus expectations. Here, we show that the express visuomotor system incorporates information about the veridical target-directed reaching metrics and contextual instructions during visuospatial tasks requiring different movement amplitudes. In one experiment, we recorded the activity from two shoulder muscles as participants reached toward targets that appeared at different distances. Longer hand-to-target distances led to larger and more prevalent express visuomotor responses than short-reach targets. This suggests that both the direction and distance of veridical hand-to-target reaches are encoded along the putative subcortical express pathway. In a second experiment, we modulated the movement amplitude by asking the participants to deliberately undershoot, overshoot, or stop (control) at the target. The overshoot and undershoot tasks impaired the generation of large and frequent express visuomotor responses, consistent with the inability of the express pathway to generate responses directed toward non-veridical targets (e.g. anti-reach tasks). Our findings appear to reflect strategic, cortically-driven modulation of the express visuomotor circuit to facilitate rapid and effective response initiation during target-directed actions. SIGNIFICANCE STATEMENTExpress (~90 ms) arm muscle responses that are consistently tuned toward the location of visual stimuli suggest a subcortical contribution to target-directed visuomotor behaviour in humans, potentially via the tecto-reticulo-spinal pathway. This study shows that express muscle responses are modulated appropriately to reach targets at different distances, but generally suppressed when the task required non-veridical responses to overshoot/undershoot the real target. The data suggest that the tecto-reticulo-spinal pathway can be exploited strategically by the cerebral cortex to facilitate rapid initiation of effective responses during a visuospatial task.

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bioRxiv2022

Evidence for an Intricate Relationship Between Express Visuomotor Responses, Postural Control and Rapid Step Initiation in the Lower Limbs

Recent work has described express visuomotor responses (EVRs) on the upper limb. EVRs are directionally-tuned bursts of muscle activity that occur within 100 ms of visual stimulus appearance, facilitating rapid reaching. Rapid stepping responses are also important in daily life, and while there is evidence of EVR expression on lower limbs, it is unknown whether lower-limb EVRs are influenced by increased postural demands. Here, we investigate the interaction between stepping-related EVRs and anticipatory postural adjustments (APAs) that typically precede step initiation. 16 healthy young subjects rapidly stepped towards visual targets presented in front of the left or right foot. We recorded bilateral surface EMG of gluteus medius (GM), a muscle involved in both APAs and stepping, and bilateral ground reaction forces. Two conditions were introduced: an anterolateral or anteromedial stepping condition with reduced or increased postural demands, respectively. In the anterolateral stepping condition, EVRs were robustly and strongly present in stance-side GM, and ground reaction forces revealed strongly decreased expression of APAs. Larger EVRs preceded shorter RTs, consistent with EVRs facilitating step initiation. In contrast, in the anteromedial stepping condition, EVRs were largely absent, and ground reaction forces revealed the consistent expression of APAs. When occasionally present, EVRs in the anteromedial stepping condition preceded larger APAs and longer RTs. Thus, while EVRs in lower limbs can facilitate rapid stepping, their expression is normally suppressed when postural stability is low. Failing to appropriately suppress EVRs in such situations disrupts postural stability, necessitating larger compensatory APAs and leading to longer stepping RTs. Key PointsO_LIExpress visuomotor responses (EVRs) are directionally tuned bursts of muscle activity that aid the rapid initiation of a goal-directed movement. They are thought to be relayed to the motor periphery along a rapid subcortical pathway involving the superior colliculus. C_LIO_LIWhile EVRs have predominantly been studied in reaching, it is unclear whether EVRs extend to the lower extremities and if so, whether increasing the postural demands of a stepping task interfere with lower-limb EVR expression. C_LIO_LIWe found that when postural demands were low, strong EVRs in the hip abductor muscle gluteus medius facilitated a rapid stepping response. Conversely, when postural demands were high, EVRs hindered a fast stepping response, as they necessitated larger, compensatory postural adjustments prior to step onset. C_LIO_LIThese results help us better understand the interaction between ultra-rapid visuomotor transformations in the EVR network, the postural demands of a given stepping task, and subsequent step initiation. C_LI

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bioRxiv2022

Cue-driven motor planning facilitates express visuomotor responses in human arm muscles

Humans can produce "express" ([~]100ms) arm muscle responses that are inflexibly locked in time and space to the presentation of a visual target, consistent with subcortical visuomotor transformations via the tecto-reticulo-spinal pathway. These express visuomotor responses are sensitive to explicit cue-driven expectations, but it is unclear at what stage of sensory-to-motor transformation such modulation occurs. Here, we recorded electromyographic activity from shoulder muscles as participants reached toward one of four virtual targets whose physical location was partially predictable from a symbolic cue. In an experiment in which targets could be veridically reached, express responses were inclusive of the biomechanical requirements for reaching the cued locations and not systematically modulated by cue validity. In a second experiment, movements were restricted to the horizontal plane so that the participants could perform only rightward or leftward reaches, irrespective of target position on the vertical axis. Express muscle responses were almost identical for targets that were validly cued in the horizontal direction, regardless of cue validity in the vertical dimension. Together, these findings suggest that the cue-induced enhancements of express responses are dominated by effects at the level of motor plans and not solely via facilitation of early visuospatial target processing. Notably, direct cortico-tectal and cortico-reticular projections exist that are well-placed to modulate pre-stimulus motor preparation state in subcortical circuits. Our results appear to reflect a neural mechanism by which contextually relevant motor plans can be stored within subcortical visuomotor nodes and rapidly released in response to compatible visual inputs. NEW & NOTEWORTHYExpress arm muscle responses to suddenly appearing visual targets for reaching rapid have been attributed to the tecto-reticulo-spinal pathway in humans. We demonstrate that symbolic cues before target presentation can modulate such express arm muscle responses compatibly with the biomechanics of the cued reaching direction and the cue validity. This implies cortically mediated modulation of one or more sensorimotor transformation nodes of the subcortical express pathway.

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bioRxiv2021

Express arm Responses Appear Bilaterally on Upper-limb Muscles in Arm Choice Reaching Task

When required, humans can generate very short latency reaches towards visual targets, like catching a falling cellphone. During such rapid reaches, express arm responses are the first wave of upper limb muscle recruitment, occurring ~80-100 ms after target appearance. There is accumulating evidence that express arm responses arise from signaling along the tecto-reticulo-spinal tract, but the involvement of the reticulo-spinal tract has not been well-studied. Since the reticulospinal tract projects bilaterally, we studied whether express arm responses would be generated bilaterally. Human participants (n = 14; 7 female) performed visually guided reaches in a modified emerging target paradigm where either arm could intercept the target. We recorded electromyographic activity bilaterally from the pectoralis major muscle. Our analysis focused on target locations where participants reached with the right arm on some trials, and the left arm on others. In support of the involvement of the reticulospinal tract, express arm responses persisted bilaterally regardless of which arm reached to the target. The latency and magnitude of the express arm response did not depend on whether the arm was chosen to reach or not. However, on the reaching arm, the magnitude of the express arm response was correlated to the level of anticipatory activity. The bilateral generation of express arm responses supports the involvement of the reticulo-spinal tract. We surmise that the correlation between anticipatory activity and the magnitude of express arm responses on the reaching arm arises from convergence of cortically-derived signals with a parallel subcortical pathway mediating the express arm response. New and NoteworthyExpress arm responses have been proposed to arise from the tecto-reticulo-spinal tract originating within the superior colliculus, but the involvement of the reticulo-spinal tract has not been well studied. Here, we show these responses appear bilaterally in a task where either arm can reach to a newly appearing stimulus. Our results suggest that the most rapid visuomotor transformations for reaching are performed by a subcortical pathway.

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bioRxiv2021

Trial-by-trial modulation of express visuomotor responses induced by symbolic or barely detectable cues

Human cerebral cortex can produce visuomotor responses that are modulated by contextual and task-specific constraints. However, the distributed cortical network for visuomotor transformations limits the minimal response time of that pathway. Notably, humans can generate express visuomotor responses that are inflexibly tuned to the target location and occur 80-120ms from stimulus presentation (stimulus-locked responses, SLRs). This suggests a subcortical pathway for visuomotor transformations involving the superior colliculus and its downstream reticulo-spinal projections. Here we investigated whether cognitive expectations can modulate the SLR. In one experiment, we recorded surface EMG from shoulder muscles as participants reached toward a visual target whose location was unpredictable in control conditions, and partially predictable in cue conditions by extrapolating a symbolic cue (75% validity). Valid symbolic cues led to faster and larger SLRs than control conditions; invalid symbolic cues produced slower and smaller SLRs than control conditions. This is consistent with a cortical top-down modulation of the putative subcortical SLR-network. In a second experiment, we presented high-contrast targets in isolation (control) or ~24ms after low-contrast stimuli, which could appear at the same (valid cue) or opposite (invalid cue) location as the target, and with equal probability (50% cue validity). We observed faster SLRs than control with the valid low-contrast cues, whereas the invalid cues led to the opposite results. These findings may reflect exogenous priming mechanisms of the SLR network, potentially evolving subcortically via the superior colliculus. Overall, our results support both top-down and bottom-up modulations of the putative subcortical SLR network in humans. NEW & NOTEWORTHYExpress visuomotor responses in humans appear to reflect subcortical sensorimotor transformation of visual inputs, potentially conveyed via the tecto-reticulo-spinal pathway. Here we show that the express responses are influenced both by symbolic and barely detectable spatial cues about stimulus location. The symbolic cue-induced effects suggest cortical top-down modulation of the putative subcortical visuomotor network. The effects of barely detectable cues may reflect exogenous priming mechanisms of the tecto-reticulo-spinal pathway.

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