Planning face, hand, and leg movements: anatomical constraints on preparatory inhibition

Motor-evoked potentials (MEPs), elicited by transcranial magnetic stimulation (TMS) over the motor cortex, are reduced during the preparatory period in delayed response tasks. In this study we examined how MEP suppression varies as a function of the anatomical organization of the motor cortex. MEPs were recorded from a left index muscle while participants prepared a hand or leg movement in experiment 1 or prepared an eye or mouth movement in experiment 2. In this manner, we assessed if the level of MEP suppression in a hand muscle varied as a function of the anatomical distance between the agonist for the forthcoming movement and the muscle targeted by TMS. MEP suppression was attenuated when the cued effector was anatomically distant from the hand (e.g., leg or facial movement compared with finger movement). A similar effect was observed in experiment 3 in which MEPs were recorded from a muscle in the leg and the forthcoming movement involved the upper limb or face. These results demonstrate an important constraint on preparatory inhibition: it is sufficiently broad to be manifest in a muscle that is not involved in the task, but it is not global, showing a marked attenuation when the agonist muscle belongs to a different segment of the body. NEW & NOTEWORTHY Using transcranial magnetic stimulation, we examined changes in corticospinal excitability as people prepared to move. Consistent with previous work, we observed a reduction in excitability during the preparatory period, an effect observed in both task-relevant and task-irrelevant muscles. However, this preparatory inhibition is anatomically constrained, attenuated in muscles belonging to a different body segment than the agonist of the forthcoming movement.

Double dissociation of single-interval and rhythmic temporal prediction in cerebellar degeneration and Parkinson's disease

Predicting the timing of upcoming events is critical for successful interaction in a dynamic world, and is recognized as a key computation for attentional orienting. Temporal predictions can be formed when recent events define a rhythmic structure, as well as in aperiodic streams or even in isolation, when a specified interval is known from previous exposure. However, whether predictions in these two contexts are mediated by a common mechanism, or by distinct, context-dependent mechanisms, is highly controversial. Moreover, although the basal ganglia and cerebellum have been linked to temporal processing, the role of these subcortical structures in temporal orienting of attention is unclear. To address these issues, we tested individuals with cerebellar degeneration or Parkinson's disease, with the latter serving as a model of basal ganglia dysfunction, on temporal prediction tasks in the subsecond range. The participants performed a visual detection task in which the onset of the target was predictable, based on either a rhythmic stream of stimuli, or a single interval, specified by two events that occurred within an aperiodic stream. Patients with cerebellar degeneration showed no benefit from single-interval cuing but preserved benefit from rhythm cuing, whereas patients with Parkinson's disease showed no benefit from rhythm cuing but preserved benefit from single-interval cuing. This double dissociation provides causal evidence for functionally nonoverlapping mechanisms of rhythm- and interval-based temporal prediction for attentional orienting, and establishes the separable contributions of the cerebellum and basal ganglia to these functions, suggesting a mechanistic specialization across timing domains.

tDCS to premotor cortex changes action verb understanding: Complementary effects of inhibitory and excitatory stimulation

Processing the meaning of action language correlates with somatotopic activity in premotor cortex (PMC). A previous neurostimulation study supported a causal contribution of PMC activity to action verb understanding, but the direction of the effect was unexpected: inhibiting PMC made participants respond faster to action verbs. Here we investigated the effects of PMC excitation and inhibition on action verb understanding using tDCS. Right-handed participants received tDCS stimulation with the anodal electrode (presumed to be excitatory) and cathodal electrode (presumed to be inhibitory) placed over left and right PMC, respectively, or with the reverse configuration. After completing the stimulation protocol, participants made lexical decisions on unimanual action verbs (e.g., throw) and abstract verbs (e.g., think). tDCS configuration selectively affected how accurately participants responded to unimanual action verbs. When the anode was positioned over left PMC we observed a relative impairment in performance for right-hand responses (i.e. the hand with which these participants typically perform unimanual actions). By contrast, when the cathode was positioned over left PMC we observed a relative improvement. tDCS configuration did not differentially affect responses to abstract verbs. These complementary effects of excitatory and inhibitory tDCS clarify the functional role of premotor hand areas in understanding action language.

Invariant errors reveal limitations in motor correction rather than constraints on error sensitivity

Implicit sensorimotor adaptation is traditionally described as a process of error reduction, whereby a fraction of the error is corrected for with each movement. Here, in our study of healthy human participants, we characterize two constraints on this learning process: the size of adaptive corrections is only related to error size when errors are smaller than 6°, and learning functions converge to a similar level of asymptotic learning over a wide range of error sizes. These findings are problematic for current models of sensorimotor adaptation, and point to a new theoretical perspective in which learning is constrained by the size of the error correction, rather than sensitivity to error.

The cerebellum does more than sensory prediction error-based learning in sensorimotor adaptation tasks

Individuals with damage to the cerebellum perform poorly in sensorimotor adaptation paradigms. This deficit has been attributed to impairment in sensory prediction error-based updating of an internal forward model, a form of implicit learning. These individuals can, however, successfully counter a perturbation when instructed with an explicit aiming strategy. This successful use of an instructed aiming strategy presents a paradox: In adaptation tasks, why do individuals with cerebellar damage not come up with an aiming solution on their own to compensate for their implicit learning deficit? To explore this question, we employed a variant of a visuomotor rotation task in which, before executing a movement on each trial, the participants verbally reported their intended aiming location. Compared with healthy control participants, participants with spinocerebellar ataxia displayed impairments in both implicit learning and aiming. This was observed when the visuomotor rotation was introduced abruptly (experiment 1) or gradually (experiment 2). This dual deficit does not appear to be related to the increased movement variance associated with ataxia: Healthy undergraduates showed little change in implicit learning or aiming when their movement feedback was artificially manipulated to produce similar levels of variability (experiment 3). Taken together the results indicate that a consequence of cerebellar dysfunction is not only impaired sensory prediction error-based learning but also a difficulty in developing and/or maintaining an aiming solution in response to a visuomotor perturbation. We suggest that this dual deficit can be explained by the cerebellum forming part of a network that learns and maintains action-outcome associations across trials.NEW & NOTEWORTHY Individuals with cerebellar pathology are impaired in sensorimotor adaptation. This deficit has been attributed to an impairment in error-based learning, specifically, from a deficit in using sensory prediction errors to update an internal model. Here we show that these individuals also have difficulty in discovering an aiming solution to overcome their adaptation deficit, suggesting a new role for the cerebellum in sensorimotor adaptation tasks.

The Cerebellum: Adaptive Prediction for Movement and Cognition

Over the past 30 years, cumulative evidence has indicated that cerebellar function extends beyond sensorimotor control. This view has emerged from studies of neuroanatomy, neuroimaging, neuropsychology, and brain stimulation, with the results implicating the cerebellum in domains as diverse as attention, language, executive function, and social cognition. Although the literature provides sophisticated models of how the cerebellum helps refine movements, it remains unclear how the core mechanisms of these models can be applied when considering a broader conceptualization of cerebellar function. In light of recent multidisciplinary findings, we examine how two key concepts that have been suggested as general computational principles of cerebellar function- prediction and error-based learning- might be relevant in the operation of cognitive cerebro-cerebellar loops.

Physiological Markers of Motor Inhibition during Human Behavior

Transcranial magnetic stimulation (TMS) studies in humans have shown that many behaviors engage processes that suppress excitability within the corticospinal tract. Inhibition of the motor output pathway has been extensively studied in the context of action stopping, where a planned movement needs to be abruptly aborted. Recent TMS work has also revealed markers of motor inhibition during the preparation of movement. Here, we review the evidence for motor inhibition during action stopping and action preparation, focusing on studies that have used TMS to monitor changes in the excitability of the corticospinal pathway. We discuss how these physiological results have motivated theoretical models of how the brain selects actions, regulates movement initiation and execution, and switches from one state to another.

Characteristics of Implicit Sensorimotor Adaptation Revealed by Task-irrelevant Clamped Feedback

Sensorimotor adaptation occurs when there is a discrepancy between the expected and actual sensory consequences of a movement. This learning can be precisely measured, but its source has been hard to pin down because standard adaptation tasks introduce two potential learning signals: task performance errors and sensory prediction errors. Here we employed a new method that induces sensory prediction errors without task performance errors. This method combines the use of clamped visual feedback that is angularly offset from the target and independent of the direction of motion, along with instructions to ignore this feedback while reaching to targets. Despite these instructions, participants unknowingly showed robust adaptation of their movements. This adaptation was similar to that observed with standard methods, showing sign dependence, local generalization, and cerebellar dependency. Surprisingly, adaptation rate and magnitude were invariant across a large range of offsets. Collectively, our results challenge current models of adaptation and demonstrate that behavior observed in many studies of adaptation reflect the composite effects of task performance and sensory prediction errors.

Individual differences in implicit motor learning: task specificity in sensorimotor adaptation and sequence learning

In standard taxonomies, motor skills are typically treated as representative of implicit or procedural memory. We examined two emblematic tasks of implicit motor learning, sensorimotor adaptation and sequence learning, asking whether individual differences in learning are correlated between these tasks, as well as how individual differences within each task are related to different performance variables. As a prerequisite, it was essential to establish the reliability of learning measures for each task. Participants were tested twice on a visuomotor adaptation task and on a sequence learning task, either the serial reaction time task or the alternating reaction time task. Learning was evident in all tasks at the group level and reliable at the individual level in visuomotor adaptation and the alternating reaction time task but not in the serial reaction time task. Performance variability was predictive of learning in both domains, yet the relationship was in the opposite direction for adaptation and sequence learning. For the former, faster learning was associated with lower variability, consistent with models of sensorimotor adaptation in which learning rates are sensitive to noise. For the latter, greater learning was associated with higher variability and slower reaction times, factors that may facilitate the spread of activation required to form predictive, sequential associations. Interestingly, learning measures of the different tasks were not correlated. Together, these results oppose a shared process for implicit learning in sensorimotor adaptation and sequence learning and provide insight into the factors that account for individual differences in learning within each task domain.

NEW & NOTEWORTHY

We investigated individual differences in the ability to implicitly learn motor skills. As a prerequisite, we assessed whether individual differences were reliable across test sessions. We found that two commonly used tasks of implicit learning, visuomotor adaptation and the alternating serial reaction time task, exhibited good test-retest reliability in measures of learning and performance. However, the learning measures did not correlate between the two tasks, arguing against a shared process for implicit motor learning.

Patients with focal cerebellar lesions show reduced auditory cortex activation during silent reading

Functional neuroimaging studies consistently report language-related cerebellar activations, but evidence from the clinical literature is less conclusive. Here, we attempt to bridge this gap by testing the effect of focal cerebellar lesions on cerebral activations in a reading task previously shown to involve distinct cerebellar regions. Patients (N=10) had lesions primarily affecting medial cerebellum, overlapping cerebellar regions activated during the presentation of random word sequences, but distinct from activations related to semantic prediction generation and prediction error processing. In line with this pattern of activation-lesion overlap, patients did not differ from matched healthy controls (N=10) in predictability-related activations. However, whereas controls showed increased activation in bilateral auditory cortex and parietal operculum when silently reading familiar words relative to viewing letter strings, this effect was absent in the patients. Our results highlight the need for careful lesion mapping and suggest possible roles for the cerebellum in visual-to-auditory mapping and/or inner speech.

Taking Aim at the Cognitive Side of Learning in Sensorimotor Adaptation Tasks

Sensorimotor adaptation tasks have been used to characterize processes responsible for calibrating the mapping between desired outcomes and motor commands. Research has focused on how this form of error-based learning takes place in an implicit and automatic manner. However, recent work has revealed the operation of multiple learning processes, even in this simple form of learning. This review focuses on the contribution of cognitive strategies and heuristics to sensorimotor learning, and how these processes enable humans to rapidly explore and evaluate novel solutions to enable flexible, goal-oriented behavior. This new work points to limitations in current computational models, and how these must be updated to describe the conjoint impact of multiple processes in sensorimotor learning.

Competition between movement plans increases motor variability: evidence of a shared resource for movement planning

Various lines of evidence indicate that multiple movements can be prepared in parallel. Here, we show that preparing more than one movement comes with a cost: a movement plan is more variable if it is prepared simultaneously with another plan. This suggests that the representations of movement plans share a common neural resource and implies that the number of alternative plans is constrained by noise.

Do movement plans, like representations in working memory, share a limited pool of resources? If so, the precision with which each individual movement plan is specified should decrease as the total number of movement plans increases. To explore this, human participants made speeded reaching movements toward visual targets. We examined if preparing one movement resulted in less variability than preparing two movements. The number of planned movements was manipulated in a delayed response cueing procedure that limited planning to a single target (experiment 1) or hand (experiment 2) or required planning of movements toward two targets (or with two hands). For both experiments, initial movement direction variability was higher in the two-plan condition than in the one-plan condition, demonstrating a cost associated with planning multiple movements, consistent with the limited resource hypothesis. In experiment 3, we showed that the advantage in initial variability of preparing a single movement was present only when the trajectory could be fully specified. This indicates that the difference in variability between one and two plans reflects the specification of full motor plans, not a general preparedness to move. The precision cost related to concurrent plans represents a novel constraint on motor preparation, indicating that multiple movements cannot be planned independently, even if they involve different limbs.

Individual differences in GABA content are reliable but are not uniform across the human cortex

¹H magnetic resonance spectroscopy (MRS) provides a powerful tool to measure gamma-aminobutyric acid (GABA), the principle inhibitory neurotransmitter in the human brain. We asked whether individual differences in MRS estimates of GABA are uniform across the cortex or vary between regions. In two sessions, resting GABA concentrations in the lateral prefrontal, sensorimotor, dorsal premotor, and occipital cortices were measured in twenty-eight healthy individuals. GABA estimates within each region were stable across weeks, with low coefficients of variation. Despite this stability, the GABA estimates were not correlated between regions. In contrast, the percentage of brain tissue per volume, a control measure, was correlated between the three anterior regions. These results provide an interesting dissociation between an anatomical measure of individual differences and a neurochemical measure. The different patterns of anatomy and GABA concentrations have implications for understanding regional variation in the molecular topography of the brain in health and disease.

Dissociation of Spatial and Temporal Coupling in the Bimanual Movements of Callosotomy Patients

The neural mechanisms of limb coordination were investigated by testing callosotomy patients and normal control subjects on bimanual movements Normal subjects produced deviations in the trajectories when spatial demands for the two hands were different, despite temporal synchrony in the onset of bimanual movements Callosotomy patients did not produce spatial deviations, although their hands moved with normal temporal synchrony Normal subjects but not callosotomy patients exhibited large increases in planning and execution time for movements with different spatial demands for the two hands relative to movements with identical spatial demands for the two hands This neural dissociation indicates that spatial interference in movements results from callosal connections, whereas temporal synchrony in movement onset does not rely on the corpus callosum

Eyeblink Classical Conditioning and Awareness Revisited

Dual-task performance was assessed in 140 adults during eyeblink classical conditioning (EBCC) and one of several secondary tasks (timed-interval tapping, recognition memory choice reaction time, video viewing) Four groups received paired-EBCC stimulus presentation and three groups received explicitly unpaired EBCC stimuli Although the subjects were not told about the conditioning task, they acquired conditioned responses (CRs) at normal levels Postsession interviews probed participants' awareness of EBCC stimulus contingencies and production of CRs Reported awareness of paired-EBCC stimulus contingencies and CR production was not related to actual EBCC performance Twenty-seven percent of the participants receiving explicitly unpaired stimuli reported a stimulus contingency when none existed The dissociation between awareness and performance provides additional support for the categorization of simple EBCC as a form of nondeclarative learning

Hemispheric Differences in Auditory Perception Are Similar to Those Found in Visual Perception

In a pitch discrimination task, subjects were faster and more accurate in judging low-frequency sounds when these stimuli were presented to the left ear, compared with the right ear. In contrast, a right-ear advantage was found with high-frequency sounds. The effect was in terms of relative frequency and not absolute frequency, suggesting that the effect arises from postsensory mechanisms. A simitar laterality effect has been reported in visual perception with stimuli varying in spatial frequency. These multimodal laterality effects may reflect a general computational difference between the two cerebral hemispheres, with the left hemisphere biased for processing high-frequency information and the right hemisphere biased for processing low-frequency information.

Timing in Eyeblink Classical Conditioning and Timed-Interval Tapping

The cerebellum is implicated in interval timing for diverse tasks including eyeblink classical conditioning (EBCC) and repetitive tapping. We examined performance on both tasks across identical intervals ranging from 325 to 550 ms. In five weekly sessions, 23 participants used a different interval each week, both as the target for tapping and as the delay interval in EBCC. Changes in variability as a function of the tapping or delay interval were assessed using regression analyses. The slope for repetitive tapping was comparable to two measures of temporal acuity in EBCC, onset and peak latency of the conditioned response. Each of 80 additional participants was assessed in one session at one of four tapping and delay intervals. Results were similar to those observed in the repeated measures group. These findings provide further evidence that EBCC and repetitive tapping utilize common mechanisms for representing temporal information.

Transcranial Direct Current Stimulation Does Not Influence the Speed-Accuracy Tradeoff in Perceptual Decision-making: Evidence from Three Independent Studies

In perceptual decision-making tasks, people balance the speed and accuracy with which they make their decisions by modulating a response threshold. Neuroimaging studies suggest that this speed-accuracy tradeoff is implemented in a corticobasal ganglia network that includes an important contribution from the pre-SMA. To test this hypothesis, we used anodal transcranial direct current stimulation (tDCS) to modulate neural activity in pre-SMA while participants performed a simple perceptual decision-making task. Participants viewed a pattern of moving dots and judged the direction of the global motion. In separate trials, they were cued to either respond quickly or accurately. We used the diffusion decision model to estimate the response threshold parameter, comparing conditions in which participants received sham or anodal tDCS. In three independent experiments, we failed to observe an influence of tDCS on the response threshold. Additional, exploratory analyses showed no influence of tDCS on the duration of nondecision processes or on the efficiency of information processing. Taken together, these findings provide a cautionary note, either concerning the causal role of pre-SMA in decision-making or on the utility of tDCS for modifying response caution in decision-making tasks.

Cerebellar contributions to motor control and language comprehension: searching for common computational principles

The past 25 years have seen the functional domain of the cerebellum extend beyond the realm of motor control, with considerable discussion of how this subcortical structure contributes to cognitive domains including attention, memory, and language. Drawing on evidence from neuroanatomy, physiology, neuropsychology, and computational work, sophisticated models have been developed to describe cerebellar function in sensorimotor control and learning. In contrast, mechanistic accounts of how the cerebellum contributes to cognition have remained elusive. Inspired by the homogeneous cerebellar microanatomy and a desire for parsimony, many researchers have sought to extend mechanistic ideas from motor control to cognition. One influential hypothesis centers on the idea that the cerebellum implements internal models, representations of the context-specific dynamics of an agent's interactions with the environment, enabling predictive control. We briefly review cerebellar anatomy and physiology, to review the internal model hypothesis as applied in the motor domain, before turning to extensions of these ideas in the linguistic domain, focusing on speech perception and semantic processing. While recent findings are consistent with this computational generalization, they also raise challenging questions regarding the nature of cerebellar learning, and may thus inspire revisions of our views on the role of the cerebellum in sensorimotor control.

Taxonomies of Timing: Where Does the Cerebellum Fit In?

Recent models of interval timing have emphasized local, modality-specific processes or a core network centered on a cortico-thalamic-striatal circuit, leaving the role of the cerebellum unclear. We examine this issue, using current taxonomies of timing as a guide to review the association of the cerebellum in motor and perceptual tasks in which timing information is explicit or implicit. Evidence from neuropsychological, neurophysiological, and neuroimaging studies indicates that the involvement of the cerebellum in timing is not restricted to any subdomain of this taxonomy. However, an emerging pattern is that tasks in which timing is done in cyclic continuous contexts do not rely on the cerebellum. In such scenarios, timing may be an emergent property of system dynamics, and especially oscillatory entrainment. The cerebellum may be necessary to time discrete intervals in the absence of continuous cyclic dynamics.

Impact of task-related changes in heart rate on estimation of hemodynamic response and model fit

The blood oxygen level dependent (BOLD) signal, as measured using functional magnetic resonance imaging (fMRI), is widely used as a proxy for changes in neural activity in the brain. Physiological variables such as heart rate (HR) and respiratory variation (RV) affect the BOLD signal in a way that may interfere with the estimation and detection of true task-related neural activity. This interference is of particular concern when these variables themselves show task-related modulations. We first establish that a simple movement task reliably induces a change in HR but not RV. In group data, the effect of HR on the BOLD response was larger and more widespread throughout the brain than were the effects of RV or phase regressors. The inclusion of HR regressors, but not RV or phase regressors, had a small but reliable effect on the estimated hemodynamic response function (HRF) in M1 and the cerebellum. We next asked whether the inclusion of a nested set of physiological regressors combining phase, RV, and HR significantly improved the model fit in individual participants' data sets. There was a significant improvement from HR correction in M1 for the greatest number of participants, followed by RV and phase correction. These improvements were more modest in the cerebellum. These results indicate that accounting for task-related modulation of physiological variables can improve the detection and estimation of true neural effects of interest.

Delayed feedback during sensorimotor learning selectively disrupts adaptation but not strategy use

In sensorimotor adaptation tasks, feedback delays can cause significant reductions in the rate of learning. This constraint is puzzling given that many skilled behaviors have inherently long delays (e.g., hitting a golf ball). One difference in these task domains is that adaptation is primarily driven by error-based feedback, whereas skilled performance may also rely to a large extent on outcome-based feedback. This difference suggests that error- and outcome-based feedback may engage different learning processes, and these processes may be associated with different temporal constraints. We tested this hypothesis in a visuomotor adaptation task. Error feedback was indicated by the terminal position of a cursor, while outcome feedback was indicated by points. In separate groups of participants, the two feedback signals were presented immediately at the end of the movement, after a delay, or with just the error feedback delayed. Participants learned to counter the rotation in a similar manner regardless of feedback delay. However, the aftereffect, an indicator of implicit motor adaptation, was attenuated with delayed error feedback, consistent with the hypothesis that a different learning process supports performance under delay. We tested this by employing a task that dissociates the contribution of explicit strategies and implicit adaptation. We find that explicit aiming strategies contribute to the majority of the learning curve, regardless of delay; however, implicit learning, measured over the course of learning and by aftereffects, was significantly attenuated with delayed error-based feedback. These experiments offer new insight into the temporal constraints associated with different motor learning processes.

Comparison of the two cerebral hemispheres in inhibitory processes operative during movement preparation

Neuroimaging and neuropsychological studies suggest that in right-handed individuals, the left hemisphere plays a dominant role in praxis, relative to the right hemisphere. However hemispheric asymmetries assessed with transcranial magnetic stimulation (TMS) has not shown consistent differences in corticospinal (CS) excitability of the two hemispheres during movements. In the current study, we systematically explored hemispheric asymmetries in inhibitory processes that are manifest during movement preparation and initiation. Single-pulse TMS was applied over the left or right primary motor cortex (M1LEFT and M1RIGHT, respectively) to elicit motor-evoked potentials (MEPs) in the contralateral hand while participants performed a two-choice reaction time task requiring a cued movement of the left or right index finger. In Experiments 1 and 2, TMS probes were obtained during a delay period following the presentation of the preparatory cue that provided partial or full information about the required response. MEPs were suppressed relative to baseline regardless of whether they were elicited in a cued or uncued hand. Importantly, the magnitude of these inhibitory changes in CS excitability was similar when TMS was applied over M1LEFT or M1RIGHT, irrespective of the amount of information carried by the preparatory cue. In Experiment 3, there was no preparatory cue and TMS was applied at various time points after the imperative signal. When CS excitability was probed in the cued effector, MEPs were initially inhibited and then rose across the reaction time interval. This function was similar for M1LEFT and M1RIGHT TMS. When CS excitability was probed in the uncued effector, MEPs remained inhibited throughout the RT interval. However, MEPs in right FDI became more inhibited during selection and initiation of a left hand movement, whereas MEPs in left FDI remained relatively invariant across RT interval for the right hand. In addition to these task-specific effects, there was a global difference in CS excitability across experiments between the two hemispheres. When the intensity of stimulation was set to 115% of the resting threshold, MEPs were larger when the TMS probe was applied over the M1LEFT than over M1RIGHT. In summary, while the latter result suggests that M1LEFT is more excitable than M1RIGHT, the recruitment of preparatory inhibitory mechanisms is similar within the two cerebral hemispheres.