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Biomedical subjects

J G Colebatch

Publications and source records attributed to J G Colebatch.

At least 19 recordsLinked to original sources
PubMed2005

Vestibular-evoked extraocular potentials produced by stimulation with bone-conducted sound.

OBJECTIVE: To investigate the origin, whether ocular or extraocular, of the short latency frontal potential (N15) reported by following vestibular stimulation. METHODS: Fourteen subjects with low VEMP thresholds (V(T)) and 9 patients with vestibular or ocular disorders were stimulated at the mastoid with bone-conducted tone bursts (500 Hz, 8 ms) above vestibular threshold, using a B71 bone vibrator. Surface potentials were recorded from Fpz and around the eyes and referred to linked earlobes. RESULTS: The N15 was present at Fpz, but was largest around the eyes (mean amplitude 2.6 microV, peak latency 13.4 ms, with stimulation at +18 dB above threshold) and was generally in phase above and below the eyes. The response was vestibular-dependent and modulated by alteration of gaze direction. The potentials were delayed in a patient with Miller Fisher syndrome and were larger in patients with superior canal dehiscence than in controls. CONCLUSIONS: We report a new vestibular-evoked extraocular potential. Its properties are not consistent with an eye movement. It is likely to be produced, mainly or exclusively, by synchronous activity in extraocular muscles (i.e. a myogenic potential). SIGNIFICANCE: Vestibular-evoked extraocular potentials extend the range of vestibular pathways that can be assessed electrophysiologically, and may be a useful additional test of vestibular function.

Adult↗
PubMed2005

Vestibular responses to sound.

Research into vestibular responses to sound has evolved in four stages. The first, largely the work of Tullio in the 1920s, involved inspection of the eye, head, and postural responses to sound of alert animals with surgical fenestrae into various parts of the bony labyrinth. The second, begun in 1964 by Bickford and his group and continued by our group and then by others in the last 10 years, involves the measurement of evoked myogenic potentials to air-conducted and bone-conducted clicks and tones in normal humans. The third, begun by Mikaelian at about the same time as Bickford and continued by McCue, our group, and others, involves electrophysiological recordings of primary vestibular afferent neuron responses to sound in anesthetized animals. The fourth involves measurements of vestibulo-ocular responses to sound in humans with the Tullio phenomenon. It was begun by Minor and his group in 1998 with the observation that sound-induced nystagmus in humans, the Tullio phenomenon, aligned with the rotation axis of the superior semicircular canal. They then showed a defect in the temporal bone between the apex of the superior semicircular canal and the middle cranial fossa, which was the cause of most, if not all, cases of sound-induced nystagmus. Here some of the key observations made in each of these four stages are reviewed.

Acoustic Stimulation↗
PubMed2004

Movement related potentials in acutely induced weakness and stroke.

Weakness is a common symptom of neurological illness, and recovery may occur via restorative or compensatory mechanisms. Functional imaging studies have shown varied patterns of activity in motor areas following recovery from stroke. Movement related potentials (MRP) reflect the activity in primary and non-primary motor areas. We recorded MRPs in association with index finger abduction in six normal volunteers before and after induced weakness of a hand muscle and in six stroke patients with subcortical lesions and weakness affecting the arm. In both groups of subjects the greatest change was observed in the motor potential component of the MRP. On average, the motor potential had its greatest amplitude and duration at the time of the greatest weakness and became smaller with recovery. In artificially-induced weakness, the MRP had an earlier onset latency (P=0.042) and a greater early BP component (P=0.05) for the weakened finger. For the stroke subjects overall, the peak and NS' amplitudes were largest for the initial study and declined thereafter. Similar but smaller changes were present for movements of the clinically unaffected side. The increased motor potential is therefore consistent with increased activity in the motor cortex, and this may occur as compensation for weakness in both normals and after stroke.

Adult↗
PubMed2004

The acoustic startle reflex in ischemic stroke.

The authors recorded the acoustic startle response in 32 patients with stroke, 6 patients with incomplete cervical cord lesions, and 26 controls. Increased startle occurred in about one quarter of both stroke and spinal cord injury patients. The response in biceps demonstrated the greatest deviation from normal, with less marked changes in tibialis anterior. Increased startle in spinal cord injury suggests that changes at the segmental level may contribute. Symptomatic increased startle occurred only in pontine lesions.

Acoustic Stimulation↗
PubMed2004

Galvanic evoked vestibulospinal and vestibulocollic reflexes in stroke.

OBJECTIVE: Following stroke, the startle reflex, mediated via the reticulospinal tract, is often facilitated. Vestibulospinal reflexes are another bulbospinal reflex, abnormalities of which may contribute to impaired body posture and stance following stroke. We recorded galvanic evoked vestibulospinal and vestibulocollic reflexes to assess whether these showed similar changes to those for startle following stroke affecting the pons and above. METHODS: Twenty-four stroke subjects (aged 40-82) were studied in the vestibulospinal part of the study, 21 stroke subjects (aged 40-81 years) were studied in the vestibulocollic part, including 18 studied in both. Transmastoid galvanic (DC) current was used to stimulate the vestibular nerve. Vestibulocollic responses were recorded from the sternocleidomastoid muscles and vestibulospinal responses from over soleus in standing subjects. RESULTS: Vestibulocollic reflex amplitudes and latencies showed no significant differences between the two sides. Similarly short latency (SL) and medium latency (ML) vestibulospinal reflexes did not differ significantly in frequency, latency or amplitude between the affected and unaffected legs. CONCLUSIONS: Vestibular reflexes are not facilitated by stroke at or above the pontine level. The exaggeration of startle by stroke may be specific to this reflex.

Adult↗
PubMed2004

Motor unit excitability changes mediating vestibulocollic reflexes in the sternocleidomastoid muscle.

OBJECTIVE: To investigate the patterns of motor unit firing in single motor units from the sternocleidomastoid (SCM) muscles in response to stimuli previously shown to be capable of evoking vestibulocollic reflexes (loud clicks and electrical stimulation) and to relate these to the previously described surface potentials (VEMPs, vestibular evoked myogenic potentials) evoked by the same stimuli. METHODS: Eleven male subjects (30-43-years-old) were studied. Local anaesthetic was used to block the SCM and confirm that the surface potentials (p13n23) arose from it. At another time, fine wire or needle electrodes were used to record single motor unit activity and peristimulus time histograms were constructed. RESULTS: Local anaesthetic block reduced or abolished the p13n23 response in 5 of 6 subjects. A total of 94 histograms of motor unit discharges were recorded. The excitability changes seen were always small. Loud click stimuli given ipsilaterally evoked short latency (mean 14.2 ms, uncorrected for triggering delays), and short duration (mean 3.6 ms) inhibition. Contralaterally, the effect was excitatory (latency 11.9 ms, uncorrected, mean duration 2.3 ms). For electrical stimulation, short latency inhibition occurred ipsilateral to the cathode (average latency 14.0 ms, uncorrected, mean duration 2.9 ms) and excitation occurred contralaterally. CONCLUSIONS: We confirmed that the SCM is the sole or dominant source of the VEMPs recorded from electrodes over it. Short latency, short duration motor unit firing changes are evoked in SCM by loud clicks and electrical stimulation, stimuli known to be capable of evoking vestibulocollic reflexes. VEMPs beginning with a positivity correspond to inhibition of the underlying motor unit firing and those starting with a negativity correspond to an underlying excitation, findings consistent with intracellular recordings of otolith effects. Qualitative explanations of how the surface potentials are generated by these excitability changes and relating to other properties of the surface responses are proposed. SIGNIFICANCE: This study has shown consistent patterns of single motor unit firing which underlie VEMPs evoked by both clicks and short duration galvanic stimulation.

Acoustic Stimulation↗
PubMed2003

EMG responses evoked by the termination of galvanic (DC) vestibular stimulation: 'off-responses'.

OBJECTIVE: Vestibular responses in soleus electromyography (EMG) evoked by the sudden onset of galvanic (DC) stimulation ('on-responses') have been described in detail previously. The aim of the present study was to describe responses in soleus triggered by the termination of galvanic stimulation ('off-responses'). METHODS: In 10 healthy human subjects, we studied responses to transmastoid (bilateral) stimuli of 200 ms and 2 s average duration and 3 or 4 mA intensity. We obtained both on- and off-responses using the same raw data. EMG activity was recorded onto tape while current pulses of systematically varying duration were delivered. Averaged on-responses were obtained by triggering from the beginning of the current pulses. Averaged off-responses were obtained by triggering from the termination of the current pulses. RESULTS: Short-latency (SL) and medium latency (ML) off-responses were both obtained in all but one study. The SL and the ML components of the off-responses were present and had similar latencies and amplitudes, but opposite excitability, to the on-responses obtained with the same stimuli. CONCLUSIONS: Off-responses to galvanic vestibular stimulation can be recorded from soleus EMG. Our findings imply that vestibular SL and ML reflex responses in the legs are dependent on the change in the rate of vestibular nerve discharge, not its absolute level. Both on- and off-responses have properties appropriate to a role in maintaining body stability.

Adult↗
PubMed2003

Vestibular activation by bone conducted sound.

OBJECTIVE: To examine the properties and potential clinical uses of myogenic potentials to bone conducted sound. METHODS: Myogenic potentials were recorded from normal volunteers, using bone conducted tone bursts of 7 ms duration and 250-2000 Hz frequencies delivered over the mastoid processes by a B 71 clinical bone vibrator. Biphasic positive-negative (p1n1) responses were recorded from both sternocleidomastoid (SCM) muscles using averaged unrectified EMG. The best location for stimulus delivery, optimum stimulus frequency, stimulus thresholds, and the effect of aging on evoked response amplitudes and thresholds were systematically examined. Subjects with specific lesions were studied. Vestibular evoked myogenic potentials (VEMP) to air conducted 0.1 ms clicks, 7 ms/250-2000 Hz tones, and forehead taps were measured for comparison. RESULTS: Bone conducted sound evoked short latency p1n1 responses in both SCM muscles. Ipsilateral responses occurred earlier and were usually larger. Mean (SD) p1 and n1 latencies were 13.6 (1.8) and 22.3 (1.2) ms ipsilaterally and 14.9 (2.1) and 23.7 (2.7) ms contralaterally. Stimuli of 250 Hz delivered over the mastoid process, posterosuperior to the external acoustic meatus, yielded the largest amplitude responses. Like VEMP in response to air conducted clicks and tones, p1n1 responses were absent ipsilaterally in subjects with selective vestibular neurectomy and preserved in those with severe sensorineural hearing loss. However, p1n1 responses were preserved in conductive hearing loss, whereas VEMP to air conducted sound were abolished or attenuated. Bone conducted response thresholds were 97.5 (3.9) dB SPL/30.5 dB HL, significantly lower than thresholds to air conducted clicks (131.7 (4.9) dB SPL/86.7 dB HL) and tones (114.0 (5.3) dB SPL/106 dB HL). CONCLUSIONS: Bone conducted sound evokes p1n1 responses (bone conducted VEMP) which are a useful measure of vestibular function, especially in the presence of conductive hearing loss. For a given perceptual intensity, bone conducted sound activates the vestibular apparatus more effectively than air conducted sound.

Adult↗
PubMed2002

Movement-related potentials associated with self-paced, cued and imagined arm movements.

Self-paced movements, movement to a cue and imagined movement have all been reported to be preceded by a prolonged negativity on averaged electroencephalograph (EEG) recordings. Considerable evidence supports an important contribution from the supplementary motor area (SMA) to this potential and all three types of movement have been shown to be associated with SMA activation. This study was designed to compare the premovement component of these movement-related potentials (MRPs) in a group of subjects who performed each of these three types of movement. In addition, in view of the greater SMA activation in association with proximal arm movements, we studied movements at multiple joints in the right arm. All the potentials were largest at Cz. Self-paced movements were preceded by a negativity (mean onset 1.2 s prior to electromyographic activity) with two distinct phases - an early slow increase (early BP, Bereitschaftspotential) and a later, steeper phase (NS', negative slope). Proximal movements were associated with a larger peak amplitude (mean peak amplitude for shoulder 11.6 micro V, finger movement 9.0 micro V at Cz, n=14) due to a bigger NS' phase. Movements to a regular cue, but not to a randomly timed cue, were also preceded by a long duration negativity, but the NS' phase began earlier and was less distinct than for self-paced movements (mean peak amplitude for shoulder movement 9.1 micro V, finger 8.2 micro V at Cz, n=12). Imagining the movements to a regular cue was associated with a slow negativity, with no clear NS' phase (mean peak amplitude for shoulder movement 6.5 micro V, finger 6.2 micro V at Cz). Our results indicate that the MRPs prior to the three types of movement have distinct characteristics, most notably for the NS' phase. The MRP associated with movement to a regular cue may be analogous to the S2-related negativity of the contingent negative variation (CNV). We discuss the findings in the light of current evidence from functional imaging as to the cortical areas activated in similar movements.

Adult↗
PubMed2002

Selective effects of ageing on vestibular-dependent lower limb responses following galvanic stimulation.

OBJECTIVES: To examine the influence of ageing on electromyographic (EMG) responses to galvanic vestibular stimulation (GVS) in the lower limbs and to define normal values. METHODS: EMG responses to 4 mA/20 ms transmastoid binaural GVS were recorded from the soleus muscles of 70 healthy adults aged 24-85 years. Short (SL) and medium latency (ML) vestibular-dependent reflex latencies and amplitudes were measured from averaged rectified EMG. Side to side differences were examined in 30 subjects, using monaural stimulation, and expressed as an asymmetry ratio (AR). RESULTS: SL reflexes to transmastoid binaural stimulation were absent in 10 of the 25 subjects over the age of 60. ML responses were present in all subjects. SL reflex amplitudes decreased from 24.4+/-12.4% (mean+/-SD) in the 3rd decade to 7.6+/-6.2% in the 8th and 9th decades. The ML reflex amplitudes showed a tendency to increase with age. The mean AR for SL reflex amplitudes increased from 12.1% in the 3rd decade to 80.7% in the 8th and 9th decades but that for ML amplitudes did not change significantly. The average SL and ML response onset latencies were 56.5+/-7.5 and 97.2+/-9.4 ms. SL onset latencies were significantly delayed with age. Both SL and ML responses were highly reproducible between experiments. CONCLUSIONS: SL and ML responses behave differently in response to age. The decrease in SL reflex amplitudes correlates well with previously described age-related morphological changes. Preservation of the ML reflex may reflect central adaptation to reduced afferent input. SL and ML responses are potentially useful measures of vestibular-spinal function, which may have a role in the assessment of older subjects with dysequilibrium. These reflexes need to be interpreted in the context of age.

Adult↗
PubMed2002

Differential effect of current rise time on short and medium latency vestibulospinal reflexes.

OBJECTIVES: To investigate the effect of varying current rise time on galvanic-evoked short (SL) and medium (ML) latency vestibulospinal reflexes. METHODS: We recorded the soleus EMG of standing subjects in response to 3 mA direct current transmastoid stimulation with a series of current ramps with rise times of 0-300 ms. RESULTS: Longer current rise times significantly delayed the onset of both SL (P<<0.001) and ML (P<<0.001) vestibulospinal responses, by approximately 20 and 39 ms, respectively. The SL response amplitude was reduced with increasing rise time (P<<0.001), whereas the ML response amplitude was relatively unaffected by stimulus rise time. With very slow rise times a prolonged ML response alone was evoked. CONCLUSIONS: Both SL and ML reflexes can be evoked by changes in vestibular activity produced by transmastoid galvanic stimulation with a ramp onset. We found a differential effect of current rise time on SL and ML vestibulospinal reflexes, suggesting different potential functional roles for the two reflexes. SL reflexes can participate in the response to abrupt disturbances only. ML reflexes are evoked by both fast and slow changes in vestibular discharge and may be particularly effective for slowly-changing disturbances.

Adult↗
PubMed2001

Motor imagery in Parkinson's disease: a PET study.

We used positron emission tomography (PET) with 15O-labelled water to record patterns of cerebral activation in six patients with Parkinson's disease (PD), studied when clinically "off" and after turning "on" as a result of dopaminergic stimulation. They were asked to imagine a finger opposition movement performed with their right hand, externally paced at a rate of 1 Hz. Trials alternating between motor imagery and rest were measured. A pilot study of three age-matched controls was also performed. We chose the task as a robust method of activating the supplementary motor area (SMA), defects of which have been reported in PD. The PD patients showed normal degrees of activation of the SMA (proper) when both "off" and "on." Significant activation with imagining movement also occurred in the ipsilateral inferior parietal cortex (both "off" and when "on") and ipsilateral premotor cortex (when "off" only). The patients showed significantly greater activation of the rostral anterior cingulate and significantly less activation of the left lingual gyrus and precuneus when performing the task "on" compared with their performance when "off." PD patients when imagining movement and "off" showed less activation of several sites including the right dorsolateral prefrontal cortex (DLPFC) when compared to the controls performing the same task. No significant differences from controls were present when the patients imagined when "on." Our results are consistent with other studies showing deficits of pre-SMA function in PD with preserved function of the SMA proper. In addition to the areas of reduced activation (anterior cingulate, DLPFC), there were also sites of activation (ipsilateral premotor and inferior parietal cortex) previously reported as locations of compensatory overactivity for PD patients performing similar tasks. Both failure of activation and compensatory changes are likely to contribute to the motor deficit in PD.

Aged↗
PubMed2001

Vestibulospinal reflexes: quantitative effects of sensory feedback and postural task.

Vestibulospinal and vestibulocollic reflexes evoked by galvanic stimulation were studied in 20 normal volunteers. In an initial "baseline" study, subjects stood unsupported on a flat surface and a narrow base with their eyes shut and with their heads rotated to the left. The effects of vision, external support and increasing stance width were examined both individually and in combination. In a second series, the effects of the same factors were examined while subjects stood on a compliant surface. Short latency (SL) and medium latency (ML) vestibulospinal reflex responses were evoked using 4 mA/20 ms galvanic vestibular stimulation (GVS) and measured from the right soleus muscle. Vestibulocollic reflexes to short duration galvanic stimulation (4 mA/2 ms) were also measured under similar conditions. Both SL and ML vestibulospinal reflexes decreased significantly and to a similar degree with vision, external support and increasing stance width on a flat surface. On the compliant surface, stance width did not result in a decrease in the ML reflex. Reflex amplitudes further decreased in a non-linear fashion with each additional sensory modality ("factor") that was made available; the degree of attenuation due to the addition of a second and third factor closely approximated the product of the effect of each factor in isolation. Standing on a compliant surface resulted in enhancement of average SL and ML reflexes under all conditions. Vestibulocollic reflexes in contrast showed no significant modulation with vision, external support and stance width. The dissociation between vestibulocollic and vestibulospinal reflexes indicates that the modulation of vestibulospinal reflexes with task occurs proximal to the primary vestibular afferents. Vestibulospinal reflexes were largest when subjects stood on a narrow base, on a compliant surface, deprived of vision and external support, consistent with the importance of vestibular function under these conditions. Although attenuated, vestibulospinal reflexes were preserved in most subjects even when vision and external support were available and a wider stance width was adopted. The combination of different factors caused a multiplicative attenuation of the initial response.

Adult↗
PubMed2001

Vestibulocollic reflexes: normal values and the effect of age.

OBJECTIVES: To define normal values and examine the influence of ageing on vestibulocollic reflexes (VCR). METHODS: Vestibulocollic responses to 100 dB (normal hearing level; NHL) clicks, forehead taps and galvanic stimulation were measured in 70 healthy adults aged 25-85 years. RESULTS: Click- and galvanic-evoked responses were present bilaterally in all subjects below 60. Average click-evoked response amplitudes decreased with age, with a pronounced decline of 25-30% per decade from the 6th decade. The average click thresholds increased from 85 dB in the third decade to 96.5 dB in the 8th and 9th decades. Average galvanic-evoked VCR amplitudes decreased sharply from the seventh decade. Tap-evoked reflex amplitudes showed a milder decrease. When side to side differences in amplitude were expressed as asymmetry ratios (AR) in subjects below the age of 60, values of up to 35 and 46% were obtained for click amplitudes corrected and uncorrected for background electromyogram (EMG), up to 61% for both corrected and uncorrected tap response amplitudes, and up to 41 and 55% for corrected and uncorrected galvanic-evoked responses. CONCLUSIONS: A normative range of values can be specified for click- and galvanic-evoked VCRs for subjects up to the age of 60. Click- and galvanic-evoked VCR amplitudes decrease rapidly thereafter while tap-evoked responses are less affected. These changes are probably due to morphological changes in the vestibular system occurring with ageing and are more marked than in several previous reports of age-related changes in caloric responses and vestibulo-ocular reflexes.

Acoustic Stimulation↗
PubMed2001

Vestibular evoked potentials.

Loud clicks, short tone bursts, head taps and short duration transmastoid currents are all capable of activating vestibular receptors and evoking reflex changes in tonic electromyogram activity within the sternocleidomastoid muscles. Because they derive from averaged electromyograms, the responses are termed 'vestibular evoked myogenic potentials'. The earliest response ipsilateral to a loud click, p13n23, is dependent upon vestibular activation, specifically saccular afferents. These new techniques are beginning to be applied clinically. An important application is in suspected cases of the Tullio phenomenon, a condition that is characterized by a pathological reduction in click threshold. The techniques have also been applied in the assessment of otolith function, vestibular neuritis, Meniere's disease and vestibular nerve tumours (acoustic neuromas).

Acoustic Stimulation↗
PubMed2001

Characteristics of tone burst-evoked myogenic potentials in the sternocleidomastoid muscles.

HYPOTHESIS: Optimum stimulus parameters for tone burst-evoked myogenic responses can be defined. These optimized responses will be similar to those evoked by clicks in the same subjects. BACKGROUND: Loud tones give rise to myogenic responses in the anterior neck muscles, similar to click-evoked potentials, and are likely to be saccular in origin. METHODS: Tone burst-evoked and click-evoked myogenic potentials were measured from the sternocleidomastoid muscles of 12 normal subjects (6 men, 6 women) during tonic activation. The effects of tone burst frequency and duration were systematically investigated. Thresholds were measured and compared with click thresholds for the same subjects. Patients with specific lesions were studied using both stimuli. RESULTS: Tone burst-evoked responses showed frequency tuning, with the largest reflex amplitudes at either 500 Hz or 1 kHz. As the stimulus duration was increased, using a constant repetition rate, there was an increase in the reflex amplitudes followed by a decline. The overall optimum stimulus duration was 7 milliseconds. The mean tone burst threshold was 114.4-dB sound pressure level. Stimulus thresholds for click-evoked and tone burst-evoked responses were significantly correlated. Tone burst-evoked and click-evoked responses were present after stimulation of the affected ears of subjects with profound sensorineural hearing loss. Four subjects who had previously undergone vestibular neurectomy had an absence of click and tone burst-evoked responses on the side of the lesion, confirming their vestibular dependence. CONCLUSION: Tone burst-evoked myogenic responses are similar to click-evoked responses but require lower absolute stimulus intensities. To be certain of an optimum response, a stimulus duration of 7 milliseconds, an adequate intensity, and frequencies of both 500 Hz and 1 kHz should be used.

Acoustic Stimulation↗