Biomedical subjects

Michael A Gresty

Publications and source records attributed to Michael A Gresty.

14 recordsLinked to original sources

PubMed2006

Cardio-respiratory responses evoked by transient linear acceleration.

INTRODUCTION: Predictive control of the responses to re-orientating stimuli and its interaction with vestibular signals may be an important factor in protecting against spatial disorientation. Here we evaluated the influence of stimulus predictability on the cardio-respiratory responses to transient fore-aft linear accelerations. METHODS: There were 13 normal subjects and 6 patients with bilateral loss of vestibular function who were exposed to linear acceleration of +0.26 Gx peak while seated upright and restrained on a motorized bogie. Accelerations were: (1) 'unpredictable', triggered by the experimenter either at the end of expiration or at the end of inspiration; or (2) 'predictable', triggered by the subjects pressing a button. The two conditions included false trials when nothing would happen. Respiratory frequency, electrocardiogram, and trunk acceleration were recorded. RESULTS: For unpredictable accelerations, in all subjects, the RR interval decreased within the first to second beat after acceleration onset. In normal subjects this decrease was maintained or more evident during the third, fourth, or fifth heart beat after onset. Cardiac responses were not significantly different when acceleration was triggered at the end of inspiration or at the end of expiration. Self-triggered acceleration also provoked prolonged, but attenuated, heart rate responses in healthy subjects, while responses were absent in the patients. All subjects responded with a consistent rapid inspiration to the onset of acceleration, whether predictable or not. DISCUSSION: A vestibulo-cardiac response is evoked by transient linear acceleration, independently from the phase of the breathing cycle and from the predictability of the stimulus. A vestibular signal of motion appears to be required to produce a prolonged increase in heart rate.

Acceleration↗

PubMed2005

Motion sickness.

PURPOSE OF REVIEW: The public's longstanding resigned tolerance to motion sickness threatens to change, due to the widespread introduction of nauseogenic tilting trains and the increasing use of virtual reality immersion. RECENT FINDINGS: Scientific effort over the last 5 years has focused on precise evaluation of the stimuli that provoke sickness and on the development of behavioural and new pharmacological interventions to suppress sickness. SUMMARY: The precise mechanical ride characteristics of vehicles that provoke sickness have been identified and this will lead to guidelines for future engineering design, especially for suspension systems that compensate for inertial tilt, and recommendations for passengers at risk. The frequency characteristics of motion provoking sickness have been defined with greater precision and identified with shifts in perception of motion versus orientation, and changes in the quality of reflex eye movements. Ability to modify readily the time constant of vestibular 'velocity store' has emerged as a potential candidate marker of successful motion sickness habituation. Behavioural 'autogenic' countermeasures to the development of sickness, such as controlled breathing, which can be implemented readily, are shown to have significant ameliorating effects on nausea and are of value for short term moderate exposures. New classes of pharmacological agents such as N-methyl-D-aspartate antagonists and 5HT1a receptor agonists show promise in animals but await trials in humans.

Antiemetics↗

PubMed2005

Effect of a novel motion desensitization training regime and controlled breathing on habituation to motion sickness.

To study whether controlled breathing, known to ameliorate motion sickness, speeds habituation to nauseogenic motion when used in a novel accelerated training regime, 30 participants (13 men, 17 women, ages 18-51 years) were randomized to either a Breathing group or a Control group. Randomization was balanced for rotation tolerance measured during a first exposure to off-vertical axis rotation (OVAR 72 degrees sec.(-1)). Participants subsequently received four exposures to OVAR in 1-hr. (accelerated training). The Breathing group followed taped instructions to control their breathing during training and when retested the next day. There was overall habituation with stimulus repetition, shown by an increase in tolerance for motion, a reduction in symptoms, and speeded recovery times on retesting. There was a tendency for greater habituation with controlled breathing.

Adolescent↗

PubMed2005

Motion sickness susceptibility fluctuates through the menstrual cycle.

INTRODUCTION: Surveys of transportation by sea, land, and air indicate that women are more susceptible to motion sickness than men, showing higher incidences of vomiting and reporting more symptoms such as nausea. The cause of this greater motion sickness susceptibility in women is unknown. One suggestion is that it is due to the functioning of the female endocrine system, but there are contradictions between existing studies. HYPOTHESIS: Motion sickness in women varies across the menstrual cycle. METHODS: There were 12 healthy women, age 24.4 +/- 6.6 yr (mean +/- SD), not using contraceptive medication, who were exposed to a provocative cross-coupled (Coriolis) motion continued to the point of moderate nausea. The motion stimulus was whole body rotation on a turntable, during which subjects executed head tilts of approximately 45 degrees in batches of eight spaced at 30-s intervals during a staircase profile of rotational velocity which commenced from stationary and incremented in steps of 3 degrees x s(-1) every 30 s. Subjects were tested on: A.) day 5 'menstruation'; B.) day 12 'ovulatory'; C.) day 19 'mid-luteal'; and D.) day 26 'pre-menstrual,' according to a design counter-balanced for order. RESULTS: Mean +/- SD sequences of head movements required to achieve moderate nausea were: A.) 13.7 +/- 2.8; B.) 14.3 +/- 4.5; C.) 15.4 +/- 6.6; and D.) 16.9 +/- 6.1. The difference between A and D was significant (p < 0.05, 2-tailed). CONCLUSIONS: There was a trend indicating that motion sickness susceptibility was maximal at day 5 'menstruation', decreasing through days 12 and 19 to a minimum at day 26 'pre-menstrual', to a small but significant extent. However, it is unlikely that this can fully account for the greater susceptibility in women since the magnitude of fluctuation in susceptibility across the cycle is only around one-third of the overall difference between male and female susceptibility.

Adolescent↗

PubMed2004

Simulator based rehabilitation in refractory dizziness.

UNLABELLED: Patients with chronic vestibular symptoms are common in neurological practice but the most effective treatment remains an open question. The purpose of our study was to conduct a controlled, between-group comparison of patients' responses to a customised exercise regime (Group C, for customised) versus treatment additionally incorporating simulator based desensitisation exposure (Group S, for simulator) integrating whole-body or visual environment rotators. Forty chronic peripheral vestibular patients who had previously undergone conventional vestibular rehabilitation without notable improvement were randomly assigned into Group C or S. Individuals attended therapy sessions twice weekly for eight weeks and were provided with a customised home programme. Response to treatment was assessed at four-week intervals with dynamic posturography, vestibular time constants, and questionnaires concerning symptoms, symptom-triggers and emotional status. At final assessment posturography and subjective scores had significantly improved for both groups, although Group S showed greater improvement. A statistically significant improvement for visual vertigo symptom scores was noted only for Group S (p < 0.01; total improvement 53.5 %). Anxiety and depression levels significantly decreased for both groups; improvements were significantly correlated particularly to improvements in visual vertigo (SCQ) (p < 0.01; r = 0.53 and r = 0.57, respectively). Significant differences were noted between groups (p = 0.02) for posturography scores. Vestibular time constants showed no notable change in either group. CONCLUSIONS: Both groups improved but exposure to simulator motion gave overall better results. These effects were also observed in psychological symptoms and partly relate to simulator effects on visual vertigo symptoms. Visual motion and visuo-vestibular conflict situations should be incorporated in the rehabilitation of patients with refractory dizziness.

Adult↗

PubMed2003

Posture and mental task performance when viewing a moving visual field.

We investigated the characteristics of standing posture and performance of concurrent cognitive tasks in subjects confronted by whole field visual motion. Movements of the head and centre of pressure (COP) were recorded in 12 subjects who performed modified Brooks spatial and verbal tasks when in quiet stance viewing a chequerboard pattern, planar, visual field, moving with uniform velocity (25 degrees /s, 50 degrees /s and 76 degrees /s). Eight subjects were also tested seated to control for the effect of stance. Task load was monitored by heart rate and eye movements were recorded to ensure viewing compliance. Subjects rated their quotidian susceptibility to visual disorientation on a validated scale. In both lateral and antero-posterior directions there were small amplitude but significant increases in COP sway path length and standard deviations of both COP and head sway during exposure to visual motion in proportion to visual flow speed. Performing cognitive tasks during visual motion attenuated sway S.D. The effects on sway of task and visual flow were independent. Visual motion induced a slight tilt and turn of the head and body in the direction of flow together with slight neck flexion. Errors on both verbal and spatial tasks increased >250% during visual motion both when standing and when seated. Ratings of subjects' susceptibility to disorientation were un-related to either verbal or spatial task error rates. A current hypothesis is that the enhancement of sway by visual motion is destabilisation. We propose an alternative explanation that sway enhancement could be exploratory 'testing of the ground' movements to check for self motion. Hence decrease in sway magnitude during a cognitive task could be caused by a reduction in exploratory movement because attention is diverted from postural control to a secondary task. Mere passive viewing of a moving visual field may interfere with cognitive tasks possibly because the threat of disorientation by whole field motion diverts attentional resources.

Adult↗

PubMed2003

Perception of self-motion from peripheral optokinetic stimulation suppresses visual evoked responses to central stimuli.

In a previous functional neuroimaging study we found that early visual areas deactivated when a rotating optical flow stimulus elicited the illusion of self-motion (vection) compared with when it was perceived as a moving object. Here, we investigated whether electrical cortical responses to an independent central visual probe stimulus change as a function of whether optical flow stimulation in the periphery induces the illusion of self-motion or not. Visual-evoked potentials (VEPs) were obtained in response to pattern-reversals in the central visual field in the presence of a constant peripheral large-field optokinetic stimulus that rotated around the naso-occipital axis and induced intermittent sensations of vection. As control, VEPs were also recorded during a stationary peripheral stimulus and showed no difference than those obtained during optokinetic stimulation. The VEPs during constant peripheral stimulation were then divided into two groups according to the time spans where the subjects reported object- or self-motion, respectively. The N70 VEP component showed a significant amplitude reduction when, due to the peripheral stimulus, subjects experienced self-motion compared to when the peripheral stimulus was perceived as object-motion. This finding supplements and corroborates our recent evidence from functional neuroimaging that early visual cortex deactivates when a visual flow stimulus elicits the illusion of self-motion compared with when the same sensory input is interpreted as object-motion. This dampened responsiveness might reflect a redistribution of sensorial and attentional resources when the monitoring of self-motion relies on a sustained and veridical processing of optic flow and may be compromised by other sources of visual input.

Adult↗

PubMed2003

Effect of breathing supplemental oxygen on motion sickness in healthy adults.

OBJECTIVE: To compare the effects of breathing supplemental oxygen vs air on alleviating motion sickness in healthy adults. SUBJECTS AND METHODS: Between April and July 2002, 20 healthy subjects were exposed to a provocative motion on 2 occasions (1-week interval) according to a randomized, counterbalanced, crossover design. During motion, subjects rated their nausea (1, no symptoms, to 4, moderate nausea) every 30 seconds. Once mild nausea occurred, subjects began breathing supplemental oxygen or air through a face mask. Motion ceased when moderate nausea occurred, but subjects continued breathing study gases for 5 minutes while recovering. Recovery was assessed for 20 minutes after motion. RESULTS: There were no significant differences in the rate of increase in symptom severity or the rate of recovery between the 2 conditions. CONCLUSION: Breathing supplemental oxygen had no advantage over breathing air in reducing motion sickness in healthy adults.

Adult↗

PubMed2003

Suppression of sickness by controlled breathing during mildly nauseogenic motion.

INTRODUCTION: Anecdotal reports from aviators indicate that controlled breathing is used as a countermeasure for motion sickness. We report a trial on the effectiveness of focused, regular breathing on motion sickness. METHODS: Experiment 1 compared the effects of controlled breathing vs. a counting task on subjective ratings of motion sickness and associated symptoms (n = 12). Experiment 2 investigated the effects of attention (n = 12) on these measures by comparing the counting task with no task. Motion sickness was induced by whole body pitch oscillations (0.2 Hz, +/- 20 degrees peak amplitude) while subjects viewed a video image of the environment oscillating in 180 degrees counter phase. Subjects performed controlled breathing, counting, or no task from the onset of mild nausea to an endpoint of moderate nausea or 30 min motion exposure, whichever came first. RESULTS: Experiment 1: The mean time to reach motion endpoint was significantly longer (p = 0.01) for controlled breathing (21.3 min) than for counting (15.1 min). Controlled breathing also prolonged the time tolerated with nausea and reduced the recovery time after motion cessation. Experiment 2: The counting task had no effect on baseline motion sickness susceptibility. CONCLUSION: Controlled breathing may be effective for controlling nausea.

Adult↗

PubMed2003

Vestibulo-autonomic control in man: Short- and long-latency vestibular effects on cardiovascular function.

We examined the hypothesis that vestibular signals may exert a rapid control on the heart adjusting cardiovascular function to maintain homoestasis during changes in body posture. Short- and long-latency effects of vestibular stimulation on heart rate (HR), systolic blood pressure (SP), diastolic pressure (DP) and digital blood flow (BF) were studied in fourteen normal controls (NC) and nine labyrinthine-defective subjects (LDS) exposed to abrupt head accelerations. Subjects lay supine with their head suspended in a sling whose 'release' was triggered at delays of 170 ms and 570 ms after an R-spike of the ECG. Release caused the head to fall with an acceleration approximately 0.8 g for approximately 140 ms. Three additional NC underwent head drops at 170 ms, 370 ms, 470 ms and 570 ms to determine response latencies with precision. In NC, the short-latency response to head drops timed 170 to 470 ms after an R-spike was to shorten the time to the next R-spike in comparison to pre-drop heartbeat cycles. Drops at 570 ms delay shortened only the succeeding post-stimulus RR-interval. In LDS, head drops timed 170 ms after a heartbeat failed to shorten the ongoing cardiac cycle. For both delays only the succeeding cardiac cycle was significantly shortened. In all subjects, SP rose slightly by approximately 1-3% and BF decreased by 7-24% after 3-4 heartbeats post-drop. The results are evidence for an excitatory vestibulo-cardiac reflex in man which accelerates heart rate at a latency circa 500-600 ms. SP and BF are affected at longer latencies of several heart beats. A delayed increase of heart rate in response to postural challenge may contribute to the autonomic distress experienced by patients with vestibular loss.

Acceleration↗

PubMed2002

Percept-related changes in horizontal optokinetic nystagmus at different body orientations in space.

Large-field motion of the visual environment is a powerful stimulus to induce the perception of contra-directional self-motion in a stationary observer. We investigated the interrelations between horizontal optokinetic nystagmus and subjective states of motion perception under variation of subjects' orientation with respect to gravity. Subjects were tested sitting upright and lying supine, and signalled transitions between object- and self-motion perception whilst viewing an optokinetic stimulus rotating about the subjects' longitudinal axis at a range of angular velocities. Optokinetic stimulation in the supine condition resulted in subjects perceiving a graviceptive conflict and the illusory perception of whole body tilt in a direction opposite to optokinetic stimulus rotation, whereas during upright viewing the axis of stimulus rotation was aligned with the direction of gravity and thus did not result in a conflict or perception of tilt. In both postures, self-motion perception coincided with an increased deviation of mean horizontal gaze position in the perceived direction of heading with a concurrent reduction in optokinetic nystagmus slow-phase gain. Slow-phase gain was also significantly reduced in the supine position as well as at increasing stimulus velocities. The results demonstrate that spontaneous transitions between the perception of object-motion and that of self-motion consistently coincide with spatial attentional and orientational strategies, shifting from passive monitoring to active oculomotor exploration and anticipation.

Adult↗

PubMed2002

Neural correlates of visual-motion perception as object- or self-motion.

Both self-motion and objects moving in our visual field generate visual motion by displacing images on the retina. Resolving this ambiguity may seem effortless but large-field visual-motion stimuli can yield perceptual rivalry between the real percept of object-motion and the illusory percept of self-motion (vection). We used functional magnetic resonance imaging to record brain activity in human observers exposed to constant-velocity roll-motion. This stimulus induced responses in areas reaching from calcarine to parieto-occipital and to ventral and lateral temporo-occipital cortex and the anterior insula. During vection, early motion-sensitive visual areas and vestibular parieto-insular cortex deactivated, whereas higher-order parieto- and temporo-occipital areas known to respond to optical flow retained identical activity levels. Within this sustained response, these latter areas displayed transient activations in response to each perceptual switch as identified in event-related analyses. Our results thus show that these areas are responsive to the type of visual motion stimulus and highly sensitive to its perceptual bistability. The only region to be more active during perceived self-motion was in, or close to, the cerebellar nodulus. This activation may correspond to the gain increase of torsional optokinetic nystagmus during vection and/or to changes in sensory processing related to the rotational percept. In conclusion, we identified neural correlates of perceiving self-motion from vision alone, i.e., in the absence of confirmatory vestibular or proprioceptive input. These functional properties preserve the organism's ability to move accurately in its environment by relying on visual cues under conditions when the other spatial senses fail to provide such information.

Adult↗

PubMed2002

Visual motion stimulation, but not visually induced perception of self-motion, biases the perceived direction of verticality.

Large-field torsional optokinetic stimulation is known to affect the perceived direction of gravity with verticality judgements deviating towards the direction of visual stimulus rotation. The present study aimed to replicate this effect and to examine it further by subjecting participants to optokinetic stimulation in roll, resulting in spontaneous alternations between the perception of object-motion and that of contradirectional self-motion (vection), as reported by the subjects. Simultaneously, subjects were oscillated laterally in a flight simulator and indicated their perception of postural verticality. Results confirmed that rotation of the visual environment in the frontal plane biases the perceived orientation of gravity towards the direction of visual stimulus motion. However, no differential effect of perceptual state on postural verticality was obtained when contrasting verticality judgements made during the perception of object-motion with those obtained during reported self-motion perception. This finding is likely to reflect a functional segregation of central nervous visual-vestibular subsystems that process the perception of self-tilt and that of self-rotation to some degree independently.

Adult↗

PubMedDate not supplied

Behavioral methods of alleviating motion sickness: effectiveness of controlled breathing and a music audiotape.

BACKGROUND: Behavioral countermeasures for motion sickness would be advantageous because of the side effects of antiemetic drugs, but few alternative treatments are available. The objective of this study was to compare the effectiveness of controlling breathing and listening to a music audiotape designed to reduce motion sickness symptoms, on increasing tolerance to motion-induced nausea. METHOD: Twenty-four healthy subjects were exposed to nauseogenic Coriolis stimulation on a rotating turntable under three conditions: whilst focusing on controlling breathing; listening to a music audiotape; or without intervention (control). The three conditions were performed by each subject according to a replicated factorial design at 1-week intervals at the same time of day. Ratings of motion sickness were obtained every 30 seconds. Once a level of mild nausea was reached subjects commenced controlling breathing or listened to the music audiotape. Motion was stopped after the onset of moderate nausea. RESULTS: Mean (+/- SD) motion exposure time in minutes tolerated before the onset of moderate nausea was significantly longer (p <.01) for controlling breathing (10.7 +/- 5.6 min) and longer (p <.01) for music (10.4 +/- 5.6 min) compared with control (9.2 +/- 5.9 min). CONCLUSIONS: Both controlling breathing and the music audiotape provided significant protection against motion sickness and with similar effectiveness. These nonpharmacologic countermeasures are only half as effective as standard doses of anti-motion sickness drugs, such as oral scopolamine; however, they are easy to implement and free of side effects.

Adult↗