Biomedical subjects

Kielan Yarrow

Publications and source records attributed to Kielan Yarrow.

10 recordsLinked to original sources

PubMed2006

Involvement of human basal ganglia in offline feedback control of voluntary movement.

Practice makes perfect, but the neural substrates of trial-to-trial learning in motor tasks remain unclear. There is some evidence that the basal ganglia process feedback-related information to modify learning in essentially cognitive tasks , but the evidence that these key motor structures are involved in offline feedback-related improvement of performance in motor tasks is paradoxically limited. Lesion studies in adult zebra finches suggest that the avian basal ganglia are involved in the transmission or production of an error signal during song . However, patients with Huntington's disease, in which there is prominent basal ganglia dysfunction, are not impaired in error-dependent modulation of future trial performance . By directly recording from the subthalamic nucleus in patients with Parkinson's disease, we demonstrate that this nucleus processes error in trial performance at short latency. Local evoked activity is greatest in response to smallest errors and influences the programming of subsequent movements. Accordingly, motor parameters are least likely to change after the greatest evoked responses so that accurately performed trials tend to precede other accurate trials. This relationship is disrupted by electrical stimulation of the nucleus at high frequency. Thus, the human subthalamic nucleus is involved in feedback-based learning.

Aged↗

PubMed2005

Modulation of beta oscillations in the subthalamic area during motor imagery in Parkinson's disease.

Activation of the basal ganglia has been shown during the preparation and execution of movement. However, the extent to which the activation during movement is related to efferent processes or feedback-related motor control remains unclear. We used motor imagery (MI), which eliminates peripheral feedback, to further investigate the role of the subthalamic area in the feedforward organization of movement. We recorded local field potential (LPF) activity from the region of the subthalamic nucleus (STN) in eight patients with Parkinson's disease off dopaminergic medication during performance of a warned reaction time task. Patients were instructed to either extend the wrist [motor execution (ME)], to imagine performing the same task without any overt movement (MI), or, in a subgroup, to perform a non-motor visual imagery (VI) task. MI led to event-related desynchronization (ERD) of oscillatory beta activity in the region of the STN in all patients that was similar in frequency, time course and degree to the ERD occurring during ME. The degree of ERD during MI correlated with the ERD in trials of ME and, like ME, was accompanied by a decrease in cortico-STN coherence, so that STN LFP activity during MI was similar to that in ME. The ERD in ME and MI were both significantly larger than the ERD in VI. In contrast, event-related synchronization (ERS) was significantly smaller in trials of MI, and even smaller in trials of VI, than during ME. The data suggest that the activity in the region of the human STN indexed by the ERD during movement is related to the feedforward organization of movement and is relatively independent of peripheral feedback. In contrast, sensorimotor feedback is an important factor in the ERS occurring in the STN area after completion of movement, consistent with a role for this region in trial-to-trial motor learning or the re-establishment of postural set following movements.

Cortical Synchronization↗

PubMed2005

Spatial consequences of bridging the saccadic gap.

We report six experiments suggesting that conscious perception is actively redrafted to take account of events both before and after the event that is reported. When observers saccade to a stationary object they overestimate its duration, as if the brain were filling in the saccadic gap with the post-saccadic image. We first demonstrate that this illusion holds for moving objects, implying that the perception of time, velocity, and distance traveled become discrepant. We then show that this discrepancy is partially resolved up to 500 ms after a saccade: the perceived offset position of a post-saccadic moving stimulus shows a greater forward mislocalization when pursued after a saccade than during pursuit alone. These data are consistent with the idea that the temporal bias is resolved by the subsequent spatial adjustment to provide a percept that is coherent in its gist but inconsistent in its detail.

Adult↗

PubMed2005

Lateralization of event-related beta desynchronization in the EEG during pre-cued reaction time tasks.

OBJECTIVE: Here, we investigate whether the event-related desynchronization (ERD) of spectral components of the cortical EEG in the beta (13-30 Hz) frequency range may, in part, index motor selection processes. Specifically, we sought evidence for a contralaterally dominant component of the beta ERD that is limited to trials in which motor selection is possible prior to any imperative cue to move, with attendant behavioural advantage. METHODS: We measured reaction time and assessed the lateralization of beta ERD in 12 healthy volunteers as they performed pre-cued choice reaction time tasks, in which warning S1 cues were either fully predictive about the laterality of a subsequent imperative S2 signal or provided no laterality information. We calculated 'lateralized ERD index' (LERDI), a parallel measure to the lateralized readiness potential in the time domain. RESULTS: Trials with 100% S1-S2 congruency produced significantly shorter reaction times than trials with 50% S1-S2 congruency, where laterality information was unreliable. Beta LERDI indicated significantly greater lateralisation of the ERD in the warning-go interval and of event-related synchronization (ERS) following movement in the 100% condition than in the 50% condition. The lateralization of the beta ERD with respect to hand persisted, even when subjects were instructed to make movements of opposite laterality to those prompted. CONCLUSIONS: Lateralized EEG changes occur in the beta band in the S1-S2 interval prior to movement, but only when informative warning cues allow early motor selection, as suggested by the shortening of reaction time. Furthermore, the enhanced contralateral ERS with 100% S1-S2 congruency suggests that this phenomenon is at least partly independent of afferent feedback, as the same movement was made in the 100 and 50% conditions. SIGNIFICANCE: Lateralized suppression of beta power prior to externally generated movements is associated with motor selection.

Adult↗

PubMed2004

Event-related beta desynchronization in human subthalamic nucleus correlates with motor performance.

Although the basal ganglia play an important role in self-generated movement, their involvement in externally paced voluntary movement is less clear. We recorded local field potentials (LFPs) from the region of the subthalamic nuclei of eight patients with Parkinson's disease during the performance of a warned reaction time task in which an imperative cue instructed the subject to move or not to move. In 'go' trials, LFP activity in the beta frequency band ( approximately 20 Hz) decreased prior to movement, with an onset latency that strongly correlated with mean reaction time across patients. This was followed by a late post-movement increase in beta power. In contrast, in 'nogo' trials the beta power drop following imperative signals was prematurely terminated compared with go trials and reversed into an early beta power increase. These differences were manifest as power increases when go trials were subtracted from nogo trials. In six patients these relative beta power increases in nogo-go difference trials were of shorter latency than the respective reaction time. The findings suggest that, firstly, the subthalamic nucleus is involved in the preparation of externally paced voluntary movements in humans and, secondly, the degree of synchronization of subthalamic nucleus activity in the beta band may be an important determinant of whether motor programming and movement initiation is favoured or suppressed.

Aged↗

PubMed2004

Action, arousal, and subjective time.

Saccadic chronostasis refers to the subjective temporal lengthening of the first visual stimulus perceived after an eye movement. It has been quantified using a duration discrimination task. Most models of human duration discrimination hypothesise an internal clock. These models could explain chronostasis as a transient increase in internal clock speed due to arousal following a saccade, leading to temporal overestimation. Two experiments are described which addressed this hypothesis by parametrically varying the duration of the stimuli that are being judged. Changes in internal clock speed predict chronostasis effects proportional to stimulus duration. No evidence for proportionality was found. Two further experiments assessed the appropriateness of the control conditions employed. Results indicated that the chronostasis effect is constant across a wide range of stimulus durations and does not reflect the pattern of visual stimulation experienced during a saccade, suggesting that arousal is not critical. Instead, alternative processes, such as one affecting the onset of timing (i.e., the time of internal clock switch closure) are implicated. Further research is required to select between these alternatives.

Adult↗

PubMed2004

Consistent chronostasis effects across saccade categories imply a subcortical efferent trigger.

Saccadic chronostasis refers to the subjective temporal lengthening of the first visual stimulus perceived after an eye movement, and is most commonly experienced as the "stopped clock" illusion. Other temporal illusions arising in the context of movement (e.g., "intentional binding") appear to depend upon the volitional nature of the preceding motor act. Here we assess chronostasis across different saccade types, ranging from highly volitional (self-timed saccades, antisaccades) to highly reflexive (peripherally cued saccades, express saccades). Chronostasis was similar in magnitude across all these conditions, despite wide variations in their neural bases. The illusion must therefore be triggered by a "lowest common denominator" signal common to all the conditions tested and their respective neural circuits. Specifically, it is suggested that chronostasis is triggered by a low-level signal arising in response to efferent signals generated in the superior colliculus.

Adult↗

PubMed2003

Behavioural cues are associated with modulations of synchronous oscillations in the human subthalamic nucleus.

The speed with which one reacts to an imperative signal depends on the extent to which preceding cues predict that command. When reliable warning cues are available, the processing of the imperative stimulus can be favoured and responses partially pre-prepared, leading to shorter reaction times. Here we seek evidence for involvement of the human basal ganglia in the exploitation of behaviourally relevant predictive cues. To this end, local field potentials (LFPs) were recorded in the region of the subthalamic nuclei of parkinsonian patients during the performance of a pre-cued reaction task in which the cue either predicted or failed to predict the demands of the imperative signal. We demonstrate that LFP activity in the beta frequency band ( approximately 20 Hz) is modulated by the behavioural relevance of the external cue. The findings suggest that, first, the subthalamic nucleus is involved in mediating or facilitating the response advantage derived from predictive cues in humans and, secondly, variations in synchronous neuronal activity in the beta band may contribute to this function in the subthalamic nucleus.

Aged↗

PubMed2003

Manual chronostasis: tactile perception precedes physical contact.

When saccading to a silent clock, observers sometimes think that the second hand has paused momentarily. This effect has been termed chronostasis and occurs because observers overestimate the time that they have seen the object of an eye movement. They seem to extrapolate its appearance back to just prior to the onset of the saccade rather than the time that it is actually fixated on the retina. Here, we describe a similar effect following an arm movement: subjects overestimate the time that their hand has been in contact with a newly touched object. The illusion's magnitude suggests backward extrapolation of tactile perception to a moment during the preceding reach. The illusion does not occur if the arm movement triggers a change in a continuously visible visual target: the time of onset of the change is estimated correctly. We hypothesize that chronostasis-like effects occur when movement produces uncertainty about the onset of a sensory event. Under these circumstances, the time at which neurons with receptive fields that shift in the temporal vicinity of a movement change their mappings may be used as a time marker for the onset of perceptual properties that are only established later.

Humans↗

PubMed2003

Changes in finger coordination and responses to single pulse TMS of motor cortex during practice of a multifinger force production task.

We investigated the changes in finger coordination and in finger force responses to transcranial magnetic stimulation (TMS) applied over the motor cortex associated with a single practice session of an accurate ramp force production task. Subjects pressed with their index, middle and ring fingers onto three force transducers fixed to a rigid platform that was balanced on a narrow pivot under the middle finger. The task was to produce a smoothly increasing ramp of total force from 0 to 25 N over 4 s following a visual target. Subjects performed three brief series of trials without TMS (12 trials each) in the beginning, in the middle, and in the end of the experiment. The main part of the experiment involved 173 trials, and in each of them at random times in the ramp a suprathreshold TMS pulse was applied over the hand area of the contralateral motor cortex in order to evoke a twitch in the finger flexor muscles. At the end of the experiment the subjects also performed 12 constant force production trials, and TMS was unexpectedly applied in each trial. During the ramp force trials the amplitude of the response to TMS was largely independent of the force exerted at the time of stimulation, whereas in static holding trials the amplitude of the response increased with higher levels of background contraction. Over time subjects improved their overall tracking performance: the variance of the force trajectory (VarF(TOT)), as computed over sets of unperturbed trials, declined by 60% after the first 100 trials, but there was little additional improvement after the second 100 trials. Variance in the force finger space related to the total moment with respect to the pivot also showed a decline during the first half of practice and minimal further changes during the second half. In contrast, finger force variance that did not affect either total force or total moment showed no changes after the first 100 trials and a decline during the second 100 trials. This variance component quantified per finger was significantly larger than those related to the total force and total moment. The mean size of the TMS-induced phasic force increment decreased by 12% over the course of the 200 trials. The forces evoked in the index and ring fingers gradually became more equal, reducing the total moment with respect to the pivot and improving balance. We speculate that development of a relatively low twitch force with low total moment on the pivot made it easier for subjects to continue tracking after the TMS pulse. Such changes could well be correlated with the degree of corticospinal involvement in the task. The results suggest task specific, practice-related plastic changes in neural structures involved in the responses to TMS.

Adult↗