Biomedical subjects

W E McIlroy

Publications and source records attributed to W E McIlroy.

At least 19 recordsLinked to original sources

PubMed2006

Changes in gait variability during different challenges to mobility in patients with traumatic brain injury.

Postural stability may be compromised in patients who have sustained a traumatic brain injury (TBI). The purpose of the present study was to examine dynamic stability during gait by measuring spatial and temporal variability of foot placement, and to determine the effect of increased difficulty of the walking task on gait variability in patients with TBI. It was hypothesized that patients with TBI will show increased variability in step time, step length, and step width in comparison to healthy controls and that such differences would be accentuated by increased task difficulty. Participants (patients: n=20, controls: n=20) were asked to walk across a pressure sensitive mat at their preferred pace (PW), as fast as possible (FW), and with their eyes closed (EC). In accordance with the hypotheses, patients had significantly greater variability in step time and step length in comparison to healthy controls, and when the complexity of the gait task increased (FW and EC tasks). Although step width variability showed no significant difference between the groups, both control and patient groups had increased step width variability in the EC task. It is proposed that such increases in variability reflect greater challenges to maintaining dynamic stability during gait among individuals with TBI and when performing more difficult tasks.

Adolescent↗

PubMed2004

The effect of directional compatibility on the response latencies of ocular and manual movements.

Visuomotor coordination is essential for the successful performance of everyday activities, and it could be affected by the directional compatibility between ocular and manual movements. Many tasks, such as driving or operating devices in the workplace, require a variety of coordination patterns with different levels of compatibility between the eyes and the hand. For example, the movement of the eyes and the arm can be coupled when both effectors point towards the same direction whereas in other tasks the movement of the eyes and the arm can be dissociated, for instance when a peripheral object is foveated while a button press response is executed concurrently. The objective of this study was to examine the latency of ocular and manual movements in tasks characterized by variations in directional compatibility. Four tasks were used to manipulate compatibility: 1. point and look at a peripheral stimulus (POINT AND LOOK)--high directional compatibility; 2. point to a peripheral stimulus while fixating in the center (POINT AND FIXATE)--low directional compatibility; 3. press a button while looking at a peripheral stimulus (PRESS AND LOOK)--low directional compatibility; and 4. press a button while fixating in the center (PRESS AND FIXATE)--no directional motor requirement. We hypothesized that the latency of (1) manual and (2) ocular responses would be faster in the task with high directional compatibility compared with the tasks with low compatibility or the task with no directional motor component. Ten healthy participants executed pointing and pressing movements with and without concurrent eye movement to randomly presented visual stimuli. In agreement with the first hypothesis, results showed that in a task with high directional compatibility, manual responses were initiated significantly faster when compared with the tasks with low compatibility or a task with no directional motor component: 1. pointing while looking was initiated 22 ms faster on average than pointing while fixating; 2. pointing while looking was initiated 91 ms faster than pressing accompanied by an eye movement; and 3. pointing while looking was initiated 102 ms faster than pressing while fixating. The second hypothesis was partially supported by data which showed that eye movements directed toward peripheral stimuli were initiated significantly more slowly (30 ms on average) when accompanied by pressing in comparison with the latency of eye movements in the high-compatibility task. In contrast with the hypothesis, eye movements that were accompanied by pointing were not initiated faster than those in a task which required looking without pointing. In summary, these data suggest that directional compatibility is an important aspect of motor control. The effects of directional compatibility are discussed in a conceptual framework that considers the neurophysiological substrates that might be involved in mediating these effects.

Adolescent↗

PubMed2004

Cortical activation following a balance disturbance.

Although recent work suggests that cortical processing can be involved in the control of balance responses, the central mechanisms involved in these reactions remain unclear. We presently investigated the characteristics of scalp-recorded perturbation-evoked responses (PERs) following a balance disturbance. Eight young adults stabilized an inverted pendulum using their ankle musculature while seated. When perturbations were applied to the pendulum, subjects were instructed to return (active condition) or not return (passive condition) the pendulum to its original stable position. Primary measures included peak latency and amplitude of early PERs (the first negative peak between 100 and 150 ms, N1), amplitude of late PERs (between 200 and 400 ms) and onset and initial amplitude of ankle muscle responses. Based on the timing of PERs, we hypothesized that N1 would represent sensory processing of the balance disturbance and that late PERs would be linked to the sensorimotor processing of balance corrections. Our results revealed that N1 was maximal over frontal-central electrode sites (FCz and Cz). Average N1 measures at FCz, Cz, and CPz were comparable between active and passive tasks ( p>0.05). In contrast, the amplitude of late PERs at Cz was less positive for the active condition than for the passive ( p<0.05). The similarity in N1 between tasks suggests a sensory representation of early PERs. Differences in late PERs may represent sensorimotor processing related to the execution of balance responses.

Adult↗

PubMed2003

Tactile stimulus predictability modulates activity in a tactile-motor cortical network.

Manipulating objects in the hand requires the continuous transformation of sensory input into appropriate motor behaviour. Using a novel vibrotactile device combined with fMRI, the cortical network associated with tactile sensorimotor transformations was investigated. Continuous tactile stimuli were delivered in a random or predictable pattern to the second digit on the right hand of all subjects. To better distinguish sensory and motor processes, subjects were instructed to make proportionate motor gripping responses with their left hand. A consistent cortical network of activation was revealed that included the supplementary motor, dorsal and ventral premotor, posterior parietal, primary and secondary somatosensory and primary motor cortex. Tracking the unpredictable versus predictable tactile stimulus led to greater delays in motor responses and to increased performance errors. Cortical effects due to stimulus predictability were observed in several components of the network, though it was most evident as increased cortical activation in frontal motor regions during tracking of unpredictable tactile stimuli. In contrast to the proposed hypotheses, primary and secondary somatosensory cortices contralateral to tactile input did not reveal enhanced responses during unpredictable tracking. Facilitation during unpredictable tracking was also observed in primary somatosensory cortex contralateral to motor responses, the receptive site for movement-related afference. The present study provides a novel and controlled approach to investigate the loci associated with tactile-motor processing and to measure the task-specific effect of stimulus predictability on network components.

Adult↗

PubMed2002

Adaptation in the motor cortex following cervical spinal cord injury.

BACKGROUND: The nature of the adaptive changes that occur in the cerebral cortex following injury to the cervical spinal cord are largely unknown. OBJECTIVE: To investigate these adaptive changes by examining the relationship between the motor cortical representation of the paretic right upper extremity compared with that of the tongue. The tongue was selected because the spinal cord injury (SCI) does not affect its movement and the cortical representation of the tongue is adjacent to that of the paretic upper extremity. METHODS: FMRI was used to map cortical representations associated with simple motor tasks of the right upper extremity and tongue in 14 control subjects and 9 patients with remote (>5.5 months) cervical SCI. RESULTS: The mean value for the site of maximum cortical activation during upper limb movement was identical between the two groups. The site of maximum left hemispheric cortical activation during tongue movement was 12.8 mm (p < 0.01) medial and superior to that of control subjects, indicating the presence of a shift in cortical activation. CONCLUSION: The findings indicate that the adult motor cortex does indeed adapt following cervical SCI. The nature of the adaptation and the underlying biological mechanisms responsible for this change require further investigation.

Adaptation, Physiological↗

PubMed2001

Cognitive demands of executing postural reactions: does aging impede attention switching?

A new dual-task paradigm was used to investigate age-related differences in attentional dynamics during rapid balancing reactions evoked by small, unpredictable antero-posterior platform movements. The perturbations were delivered while subjects performed a continuous visuo-motor pursuit-tracking task. Onset of significant deviation in tracking was inferred to indicate switching of attentional resources between tracking and balancing tasks. Although tracking deviation was equally likely to occur subsequent to postural perturbation in healthy young and older adults, deviation onset was delayed, on average, by 67% (123 ms) in the older subjects. Delay in onset of tracking deviation correlated with subsequent delay in generating the peak stabilizing postural response at the ankle. These results suggest that impaired attentional dynamics may exacerbate postural instability in older adults.

Adult↗

PubMed2001

Medio-lateral balance adjustments preceding reflexive limb withdrawal are modified by postural demands.

We have recently observed medio-lateral balance adjustments (BA) preceding reflexive stepping elicited by noxious stimulation. While task specific modulation is evident for BA prior to voluntary leg movement, it is unclear whether rapid BA reactions (prior to 'reflexive' stepping) represent a generic response to evoked limb withdrawal or can be modified to suit task-conditions. This study was designed to establish whether the CNS is able to modify rapid onset latency BAs to match task conditions. Reflexive stepping was evoked by applying a noxious stimulus (50 ms stimulus train, 1 ms pulses, 300 Hz, 4 x perceptual threshold) to the plantar surface of the either the left or right foot. Task conditions were varied prior to stimulation by having subjects maintain one of three different static positions: (1) lean left (70% body weight (BW) on left), (2) neutral (50% BW both sides), (3) lean right (70% BW on right). BAs were denoted by centre-of-pressure (CoP) excursions towards the swing foot after the onset of noxious stimulation (average onset latency of 128 ms). There was a significant increase in frequency of occurrence and a significant increase in magnitude of CoP shift when the stimulation was applied to a loaded limb (leaning with 70% BW on the stimulated foot) as compared to an unloaded limb (30% BW). In addition, 78% of loaded trials featured steps taken with the unstimulated foot, which delayed removal of the stimulated foot. Collectively, the results indicate modifiability of the very rapid onset balance adjustments that precede the onset of limb withdrawal revealing complex control of balance exists even over very brief latencies.

Adult↗

PubMed2001

Passive and active lower-limb movements delay upper-limb balance reactions.

This study investigated the influence of rhythmic lower-limb activity on the timing of upper-limb balance reactions. Compensatory grasping reactions were evoked in healthy subjects by rapid sagittal tilts of a chair under three conditions: (1) active leg pedaling, (2) passive (motor-driven) leg pedaling, and (3) no lower-limb movement (control task). Compared with control trials, both active and passive pedaling resulted in similar delays in the initiation (43-47 ms) and execution (12-17 ms) of grasping reactions. The similarity between effects due to active and passive movement suggests that the conditioning arose predominantly from sensory discharge associated with lower-limb movement. These results may have important implications for understanding the influence of locomotion or other ongoing movement on the control of stability.

Adult↗

PubMed2001

Bilateral movement enhances ipsilesional cortical activity in acute stroke: a pilot functional MRI study.

Functional MRI was performed in two acute stroke patients and six control subjects performing unilateral and bilateral repetitive gripping tasks. Patients were tested at three time points during recovery. Initially, bilateral movement enhanced activation in the primary motor cortex (M1) of the affected hemisphere compared with unilateral paretic hand movement. With recovery, activation of M1 in the affected hemisphere did not differ between unilateral paretic hand and bilateral movement. These preliminary data may have potential implications for acute stroke motor rehabilitation.

Acute Disease↗

PubMed2001

The gain of initial somatosensory evoked potentials alters with practice of an accurate motor task.

The gain of somatosensory afferent paths from the lower limb to the cerebral cortex was investigated during the acquisition of one target location during plantar flexion. Sensory gain was measured as the magnitude of somatosensory evoked potentials (SEPs) following electrical stimulation of a peripheral nerve in the lower limb, and was recorded from the scalp. We hypothesized gain attenuation of SEPs from sensory paths serving the limb segment responsible for target acquisition. SEP gain was studied as subjects plantar flexed about the anide to a target that was 15 degrees beyond the occurrence of a cutaneous stimulus (cue) to the lateral border of the foot. The "cue" was either fixed in one location or could appear at one of three positions in space. SEP gain was tested during practice and with task acquisition. Electroencephalographic (EEG) recordings were made of primary and secondary complexes of cortical SEPs from sural and tibial nerve stimulation, with 30-40 samples averaged per subject-condition. Electromyographic (EMG) records were made of soleus muscle H-reflexes and M-waves. Target acquisition was recorded as percent correct hits. The results showed significant attenuation in sural and tibial nerve primary SEPs with task acquisition when the cue was fixed or varied in movement space (P<0.05). Secondary SEPs from tibial nerve followed this pattern. Spinal H-reflexes only attenuated with movement per se. We conclude that the CNS preferentially reduces the cerebral inflow of sensory information once such a motor task has been successfully acquired.

Adult↗

PubMed2001

New devices to deliver somatosensory stimuli during functional MRI.

A new class of devices are described for improving investigation of somatosensory neuronal activation using fMRI. Dubbed magnetomechanical vibrotactile devices (MVDs), the principle of operation involves driving wire coils with small oscillatory currents in the large static magnetic field inherent to MRI scanners. The resulting Lorentz forces can be oriented to generate large vibrations that are easily converted to translational motions as large as several centimeters. Representative data demonstrate the flexibility of MVDs to generate different well-controlled vibratory and tactile stimuli to activate special proprioceptive and cutaneous somatosensory afferent pathways. The implications of these data are discussed with respect to the literature on existing devices for producing sensorimotor activation, as well as expanding the scope of current fMRI investigations.

Adult↗

PubMed2001

Quantifying head motion associated with motor tasks used in fMRI.

In functional magnetic resonance imaging (fMRI) studies, long experiment times and small intensity changes associated with brain activation frequently lead to image artifacts due to head motion. Methods to minimize and correct for head motion by restraint, fast imaging, and retrospective image registration are typically combined but do not completely solve the problem, particularly for specific patient populations. As an initial step toward optimizing future designs of head restraints and improving motion correction techniques, the head motion characteristics of groups of stroke subjects, age-matched controls, and young adults were investigated with the aid of an MR simulator and a highly accurate position tracking system. Position measurements were recorded during motor tasks involving either the hand or the foot. Head motion was strongly dependent on the subject group and less upon the task conditions based on ANOVA calculations (P < 0.05). The stroke subjects exhibited approximately twice the head motion compared to that of age-matched controls, and the latter's head motion was about twice that of young adults. Moreover, the range of head motion in stroke subjects over all tasks was approximately 2 +/- 1 mm, with the motion occurring predominantly as translation in the superior-inferior direction and pitch rotation (nodding). These results lead to several recommendations on the design of fMRI motor experiments and suggest that improved motion correction strategies are required to examine such patient populations comprehensively.

Adult↗

PubMed2001

Cortical representation of whole-body movement is modulated by proprioceptive discharge in humans.

Previous studies have revealed the influence of ongoing sensory discharge on modulating the central representation of muscle afferents from individual limbs. In the present study, we explored the potential for such modulatory influence on the afferent discharge arising from induced whole-body movement. Vestibular and somato-sensory inputs arise from such whole-body movement. The convergence of these two modalities is important in motor control, especially for the maintenance of postural stability. We hypothesised that transmission of proprioceptive and vestibular information to the cortex would be reduced as a result of muscle-spindle discharge in knee extensor muscles. Perturbation-evoked responses (PERs), recorded from central scalp electrodes (C3, CZ, C4), were evoked through rapid translations of subjects who were seated in a chair on a movable platform. PERs were recorded during passive linear translations alone and preceded by vibration of the patellar tendon. The PER was characterised by a slow, negative potential peaking at approximately 150 ms (N150) following displacement of the chair. The amplitude of the PER was reduced following vibration to 56% of the control. Such reduction of PERs was comparable to the attenuation of somatosensory evoked potentials and soleus H-reflex magnitudes from tibial-nerve stimulation. We conclude that muscle-spindle discharge in knee extensor muscles leads to gating of both of these afferent pathways. These results have potential implications to the understanding of the CNS control of stability during ongoing movement.

Adult↗

PubMed2000

The afferent origin of the secondary somatosensory evoked potential from the lower limb in humans.

The afferent origin of the secondary somatosensory evoked potential elicited from stimulation of the sural and tibial nerves was investigated as the limb was cooled. It was hypothesized that the peak of this potential is initiated from primary afferents in the A alpha group. We conclude that the peak of the secondary SEP arises from an afferent source whose diameter is of similar size to that of large diameter A alpha afferents.

Adult↗

PubMed2000

The role of plantar cutaneous mechanoreceptors in the control of compensatory stepping reactions evoked by unpredictable, multi-directional perturbation.

The role of plantar pressure sensation in controlling compensatory stepping was explored via hypothermic anesthesia of the foot soles, in 10 healthy young adults. Stepping reactions were evoked by unpredictable platform translation in forward, backward and lateral directions. The findings suggest three specific direction- and phase-dependent roles for the plantar cutaneous afferents: (1) sensing posterior stability limits during initiation of backward steps, (2) sensing and controlling heel-contact and subsequent weight transfer during termination of forward steps, and (3) maintaining stability during the prolonged swing phase of lateral crossover steps.

Adult↗

PubMed2000

Task-relevant selective modulation of somatosensory afferent paths from the lower limb.

Leg movement attenuates initial somatosensory evoked potentials (SEPS) from both cutaneous and muscle afferent origin. To date, as different sensory inputs become relevant for task performance, selective facilitation from such movement-related gating influences has not been shown. We hypothesized that initial SEP amplitudes from cutaneous (sural nerve, SN) and muscle afferent (tibial nerve, TN) sources are dependent on the relevance of the specific afferent information to task performance. SEPs were obtained at rest and during three movement conditions. In each movement condition, the left foot was passively moved episodically and additional cutaneous 'codes' of sensory information were applied to the dorsum of the left foot. Subjects were instructed to: simply relax (passive), or to make a response following the cessation of movement, dependent either on the cutaneous code (cutaneous task), or the passive movement trajectory of the left foot (position task). Passive movement, with no required subsequent response, attenuated initial TN and SN SEPs to approximately 40% of that at rest (p < 0.05). Versus passive movement, when cutaneous inputs provided the relevant cue for the task, mean SN SEPs significantly increased (p < 0.05), and when the proprioceptive inputs provided the relevant cue for the task, mean TN SEPs significantly increased (p < 0.05). We conclude that specific relevancy of sensory information selectively facilitates somatosensory paths from movement-related attenuation.

Adult↗

PubMed2000

Thresholds for step initiation induced by support-surface translation: a dynamic center-of-mass model provides much better prediction than a static model.

The need to initiate a step in order to recover balance could, in theory, be predicted by a static model based solely on displacement of the center of mass (COM) with respect to the base of support (BOS), or by a dynamic model based on the interaction between COM displacement and velocity. The purpose of this study was to determine whether the dynamic model provides better prediction than the static model regarding the need to step in response to moving-platform perturbation. The COM phase plane trajectories were determined for 10 healthy young adults for trials where the supporting platform was translated at three different acceleration levels in anterior and posterior directions. These trajectories were compared with the thresholds for step initiation predicted by the static and dynamic COM models. A single-link-plus-foot biomechanical model was employed to mathematically simulate termination of the COM movement, without stepping, using the measured platform acceleration as the input. An optimization routine was used to determine the stability boundaries in COM state space so as to establish the dynamic thresholds where a compensatory step must be initiated in order to recover balance. In the static model, the threshold for step initiation was reached if the COM was displaced beyond the BOS limits. The dynamic model showed substantially better accuracy than the static model in predicting the need to step in order to recover balance: 71% of all stepping responses predicted correctly by the dynamic model versus only 11% by the static model. These results support the proposition that the central nervous system must react to and control dynamic effects, i.e. COM velocity, as well as COM displacement in order to maintain stability with respect to the existing BOS without stepping.

Adult↗

PubMed2000

Upper limb H reflexes and somatosensory evoked potentials modulated by movement.

In the human lower limb, the magnitudes of both Hoffmann (H) reflexes and primary somatosensory evoked potentials (SEPs) from scalp electrodes, are reduced by active and/or passive movement. We surmised that similar effects occur for the upper limb and specifically hypothesised that amplitudes of median nerve induced flexor carpii radialis H reflexes and cortical SEPs are reduced with passive movement about the wrist or elbow. The results showed (P<0. 05) that either movement significantly attenuated mean magnitudes of SEPs elicited from stimulation at elbow or wrist and that reflex magnitudes attenuated with wrist movement. Thus, the upper limb shows similar movement-induced modulation to the lower limb. These attenuations of fast conducting sensory paths consequent to movement per se, may be a basic level of motor control, initiated from muscle mechanoreceptor discharge. Upon this basic level, more complex modulations then may be laid as appropriate for the particular characteristics of active motor tasks.

Adult↗