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Biomedical subjects

Brian L Day

Publications and source records attributed to Brian L Day.

At least 19 recordsLinked to original sources
PubMed2006

Craniocentric body-sway responses to 500 Hz bone-conducted tones in man.

Whole-body responses evoked by bone-conducted sound, a stimulus known to activate vestibular afferents, were recorded in standing subjects deprived of vision. With the head facing forward, unilateral mastoid vibration (500 Hz, 2 s, 136 dB force level) produced an oblique body sway with a consistent lateral component away from the stimulated ear and an average forward component. The side of stimulation had a powerful influence on the direction but not the magnitude of sway. Individuals' mean response directions were significantly clustered between subjects, as well as within subjects for 12 of 16 subjects when tested on five occasions. Single trial analysis did not reveal any habituation of the response. To investigate whether muscle spindle activation might be responsible for the response, vibration was applied directly over posterior and anterior neck muscles and tendons. This generally produced responses that were smaller and with different direction characteristics than with mastoid vibration. In contrast, stimulation over the temporal fossa produced responses similar in magnitude and direction to mastoid stimulation. When the head was turned in yaw to face in different directions the sway response changed direction by the same amount but with no change in magnitude, suggesting response organization in a craniocentric reference frame. Whole-body sway evoked by 500 Hz vibration delivered over sites close to the ear is thus likely to represent a vestibular-evoked balance response. When compared with sway responses evoked by 500 Hz vibration of the left temporal fossa, responses to 1 mA left cathodal galvanic vestibular stimulation were of similar magnitude, yet significantly different in direction, suggesting differences in the end organ afferents activated by these two stimuli. This may enable investigation of previously inaccessible aspects of vestibular function in intact freely behaving human subjects.

Acoustic Stimulation↗
PubMed2006

Resolving head rotation for human bipedalism.

Alignment of the body to the gravitational vertical is considered to be the key to human bipedalism. However, changes to the semicircular canals during human evolution suggest that the sense of head rotation that they provide is important for modern human bipedal locomotion. When walking, the canals signal a mix of head rotations associated with path turns, balance perturbations, and other body movements. It is uncertain how the brain uses this information. Here, we show dual roles for the semicircular canals in balance control and navigation control. We electrically evoke a head-fixed virtual rotation signal from semicircular canal nerves as subjects walk in the dark with their head held in different orientations. Depending on head orientation, we can either steer walking by "remote control" or produce balance disturbances. This shows that the brain resolves the canal signal according to head posture into Earth-referenced orthogonal components and uses rotations in vertical planes to control balance and rotations in the horizontal plane to navigate. Because the semicircular canals are concerned with movement rather than detecting vertical alignment, this result shows the importance of movement control and agility rather than precise vertical alignment of the body for human bipedalism.

Adult↗
PubMed2006

A method for quantifying directional strength and motor control of the trunk.

We describe a new apparatus for studying trunk motor control. Quasi-isometric trunk forces are exerted in any horizontal direction while seated, with or without the pelvis stabilised. Trunk force magnitude and direction are measured using a novel transducing system. In addition, reaction forces between the buttocks and the seat and between each foot and the ground are measured with three force plates. Twenty-four healthy subjects exerted maximal forces in eight directions, with and without pelvis fixation. Accuracy of trunk force measurements was assessed by comparing them with reaction force measurements. The agreement analysis showed a small mean bias of the new system of -3.0 N for force magnitude and 0.72 degrees for force direction. The trunk force measurement system was sensitive enough to detect effects of movement direction and pelvis fixation on maximum force magnitude. When subjects repeated the test at a later date there were no significant differences between the two sessions. In conclusion, the new device allows measurement of static and dynamic aspects of voluntary trunk activation in multiple directions while seated and provides reproducible measurements for application to longitudinal studies. The method should prove useful for quantifying deficits and recovery of trunk motor control following neurological damage.

Adult↗
PubMed2005

Visual guidance of the human foot during a step.

When the intended foot placement changes during a step, either due to an obstacle appearing in our path or the sudden shift of a target, visual input can rapidly alter foot trajectory. However, previous studies suggest that when intended foot placement does not change, the path of the foot is fixed after it leaves the floor and vision has no further influence. Here we ask whether visual feedback can be used to improve the accuracy of foot placement during a normal, unperturbed step. To investigate this we measured foot trajectory when subjects made accurate steps, at fast and slow speeds, to stationary floor-mounted targets. Vision was randomly occluded in 50% of trials at the point of foot-off. This caused an increase in foot placement error, reflecting lower accuracy and higher variability. This effect was greatest for slow steps. Trajectory heading analysis revealed that visually guided corrections occurred as the foot neared the target (on average 64 mm away). They occurred closer to the target for the faster movements thus allowing less time and space to execute corrections. However, allowing for a fixed reaction time of 120 ms, movement errors were detected when the foot was approximately halfway to the target. These results suggest that visual information can be used to adjust foot trajectory during the swing phase of a step when stepping onto a stationary target, even for fast movements. Such fine control would be advantageous when environmental constraints place limitations on foot placement, for example when hiking over rough terrain.

Adult↗
PubMed2005

Vestibular reafference shapes voluntary movement.

The vestibular organs in the inner ear are commonly thought of as sensors that serve balance, gaze control, and higher spatial functions such as navigation. Here, we investigate their role in the online control of voluntary movements. The central nervous system uses sensory feedback information during movement to detect and correct errors as they develop. Vestibular organs signal three-dimensional head rotations and translations and so could provide error information for body movements that transport the head in space. To test this, we electrically stimulated human vestibular nerves during a goal-directed voluntary tilt of the trunk. The stimulating current waveform was made identical to the angular velocity profile of the head in the roll plane. With this, we could proportionally increase or decrease the rate of vestibular nerve firing, as if the head were rotating faster or slower than it actually was. In comparison to movements performed without stimulation, subjects tilted their trunk faster and further or slower and less far, depending upon the polarity of the stimulus. The response was negligible when identical stimulus waveforms were replayed to stationary subjects. We conclude that the brain uses vestibular information for online error correction of planned body-movement trajectories.

Biomechanical Phenomena↗
PubMed2005

Virtual head rotation reveals a process of route reconstruction from human vestibular signals.

The vestibular organs can feed perceptual processes that build a picture of our route as we move about in the world. However, raw vestibular signals do not define the path taken because, during travel, the head can undergo accelerations unrelated to the route and also be orientated in any direction to vary the signal. This study investigated the computational process by which the brain transforms raw vestibular signals for the purpose of route reconstruction. We electrically stimulated the vestibular nerves of human subjects to evoke a virtual head rotation fixed in skull co-ordinates and measure its perceptual effect. The virtual head rotation caused subjects to perceive an illusory whole-body rotation that was a cyclic function of head-pitch angle. They perceived whole-body yaw rotation in one direction with the head pitched forwards, the opposite direction with the head pitched backwards, and no rotation with the head in an intermediate position. A model based on vector operations and the anatomy and firing properties of semicircular canals precisely predicted these perceptions. In effect, a neural process computes the vector dot product between the craniocentric vestibular vector of head rotation and the gravitational unit vector. This computation yields the signal of body rotation in the horizontal plane that feeds our perception of the route travelled.

Adult↗
PubMed2005

Expectation and the vestibular control of balance.

Recent experiments have shown that the visual channel of balance control is susceptible to cognitive influence. When a subject is aware that an upcoming visual disturbance is likely to arise from an external agent, that is, movement of the visual environment, rather than from self-motion, the whole-body response is suppressed. Here we ask whether this is a principle that generalizes to the vestibular channel of balance control. We studied the whole-body response to a pure vestibular perturbation produced by galvanic vestibular stimulation (GVS; 0.5 mA for 3 sec). In the first experiment, subjects stood with vision occluded while stimuli were delivered either by the subject himself (self-triggered) or by the experimenter. For the latter, the stimulus was delivered either without warning (unpredictable) or at a fixed interval following an auditory cue (predictable). Results showed that GVS evoked a whole-body response that was not affected by whether the stimulus was self-triggered, predictable, or unpredictable. The same results were obtained in a second experiment in which subjects had access to visual information during vestibular stimulation. We conclude that the vestibular-evoked balance response is automatic and immune to knowledge of the source of the perturbation and its timing. We suggest the reason for this difference between visual and vestibular channels stems from a difference in their natural abilities to signal self-motion. The vestibular system responds to acceleration of the head in space and therefore always signals self-motion. Visual f low, on the other hand, is ambiguous in that it signals object motion and eye motion, as well as self-motion.

Adult↗
PubMed2004

Otolith and canal reflexes in human standing.

We used galvanic vestibular stimulation (GVS) to identify human balance reflexes of the semicircular canals and otolith organs. The experiment used a model of vestibular signals arising from GVS modulation of the net signal from vestibular afferents. With the head upright, the model predicts that the GVS-evoked canal signal indicates lateral head rotation while the otolith signal indicates lateral tilt or acceleration. Both signify body sway transverse to the head. With the head bent forward, the model predicts that the canal signal indicates body spin about a vertical axis but the otolith signal still signifies lateral body motion. Thus, we compared electromyograms (EMG) in the leg muscles and body sway evoked by GVS when subjects stood with the head upright or bent forward. With the head upright, GVS evoked a large sway in the direction of the anodal electrode. This response was abolished with the head bent forward leaving only small, oppositely directed, transient responses at the start and end of the stimulus. With the head upright, GVS evoked short-latency (60-70 ms), followed by medium-latency (120 ms) EMG responses, of opposite polarity. Bending the head forward abolished the medium-latency but preserved the short-latency response. This is compatible with GVS evoking separate otolithic and canal reflexes, indicating that balance is controlled by independent canal and otolith reflexes, probably through different pathways. We propose that the short-latency reflex and small transient sway are driven by the otolith organs and the medium-latency response and the large sway are driven by the semicircular canals.

Adult↗
PubMed2004

Predictive control of body mass trajectory in a two-step sequence.

During a step, the body enters a state of falling as a foot is lifted from the ground. This has important implications for the control of body trajectory when stepping in different directions. A model has been proposed in which the central nervous system controls body trajectory by a predictive, ballistic "throw" of the body occurring just before the stepping foot leaves the ground. Here we investigate this model and ask how far into the future the body trajectory is predetermined by this initial ballistic throw. We measured body centre-of-mass (CoM) and foot trajectories during two-step sequences involving stepping onto illuminated targets, one for each foot. The targets were varied spatially such that leading foot placement could be dissociated from final CoM position. The results showed that the body throw was altered when stepping in different directions. However, the throw varied only with leading foot placement and not with final CoM position. Thus, provided the leading foot was placed identically, the same throw was used for steps in which body trajectories would later diverge. Furthermore, these trajectories did not diverge while the stepping foot was in the air but occurred after it touched down. The results are consistent with the ballistic stepping model. However, they suggest that the throw is limited in its predictive capacity, being concerned only with where the stepping foot is to be placed rather than final CoM position. This constitutes a one-step-at-a-time strategy that allows the body to be safely "caught" between steps.

Biomechanical Phenomena↗
PubMed2004

The functional neuroanatomy of temporal discrimination.

Two identical stimuli, such as a pair of electrical shocks to the skin, are readily perceived as two separate events in time provided the interval between them is sufficiently long. However, as they are presented progressively closer together, there comes a point when the two separate stimuli are perceived as a single stimulus. Damage to posterior parietal cortex, peri-supplementary motor area (peri-SMA), and basal ganglia can disturb this form of temporal discrimination. Our aim was to establish, in healthy subjects, the brain areas that are involved in this process. During functional magnetic resonance imaging scanning, paired electrical pulses, separated by variable inter-stimulus intervals (5-110 msec), were delivered to different sites on one forearm (8-64 mm from the midline). Subjects were required to simply detect the stimulus (control task) or to identify a stimulus property. For temporal discrimination (TD), subjects reported whether they felt one or two stimuli. For spatial discrimination, they reported whether the stimuli were located on the right or left side of the forearm. Subjects reported their choice by pressing a button with the opposite hand. Our results showed that discrimination, as opposed to simply detection, activated several brain areas. Most were common to both discrimination tasks. These included regions of prefrontal cortex, right postcentral gyrus and inferior parietal lobule, basal ganglia, and cerebellum. However, activation of pre-SMA and anterior cingulate was found to be specific to the TD task. This suggests that these two frontal regions may play a role in the temporal processing of somatosensory events.

Adult↗
PubMed2004

Is the spinal cord the generator of 16-Hz orthostatic tremor?

Primary orthostatic tremor is characterized by 16-Hz motor activity that is coherent between muscles. It has been suggested that this tremor originates in the brain. This view is questioned by findings from a patient with complete paraplegia who experiences intermittent leg spasms at rest. The EMG activity within the spasms showed a 16-Hz component that was coherent between muscles unilaterally and bilaterally. This raises the possibility that the spinal cord could be the source of orthostatic tremor.

Electromyography↗
PubMed2004

Probing the human vestibular system with galvanic stimulation.

Galvanic vestibular stimulation (GVS) is a simple, safe, and specific way to elicit vestibular reflexes. Yet, despite a long history, it has only recently found popularity as a research tool and is rarely used clinically. The obstacle to advancing and exploiting GVS is that we cannot interpret the evoked responses with certainty because we do not understand how the stimulus acts as an input to the system. This paper examines the electrophysiology and anatomy of the vestibular organs and the effects of GVS on human balance control and develops a model that explains the observed balance responses. These responses are large and highly organized over all body segments and adapt to postural and balance requirements. To achieve this, neurons in the vestibular nuclei receive convergent signals from all vestibular receptors and somatosensory and cortical inputs. GVS sway responses are affected by other sources of information about balance but can appear as the sum of otolithic and semicircular canal responses. Electrophysiological studies showing similar activation of primary afferents from the otolith organs and canals and their convergence in the vestibular nuclei support this. On the basis of the morphology of the cristae and the alignment of the semicircular canals in the skull, rotational vectors calculated for every mode of GVS agree with the observed sway. However, vector summation of signals from all utricular afferents does not explain the observed sway. Thus we propose the hypothesis that the otolithic component of the balance response originates from only the pars medialis of the utricular macula.

Afferent Pathways↗
PubMed2003

Spinally generated electromyographic oscillations and spasms in a low-thoracic complete paraplegic.

We have measured some oscillatory properties of severe lower limb spasms experienced by a low-thoracic complete paraplegic during assisted standing. Electromyograms (EMG) were recorded from the leg muscles while the patient stood passively in a standing frame. The patient also stood using functional electrical stimulation (FES) while ground and handle reaction force vectors were measured together with EMG activity. During passive standing, spasms appeared simultaneously in all leg muscle groups on one side. The interval between spasms varied between 3 and 30 seconds. Within the spasms, there was a tendency of repetitive grouped discharge of motor units as well as a strong 10-Hz component in the EMG that was coherent across ipsilateral muscle groups. Thus, the spasms were inherently oscillatory. During FES-assisted standing, clinically similar spasms were observed. However, the interspasm interval became relatively fixed at around 16 seconds, which may indicate entraining of the spasm cycle by FES. There are similarities between this patient's spasms and the pathological motor activities seen in other movement disorders that may also be of spinal origin.

Adult↗
PubMed2003

Theories of developmental dyslexia: insights from a multiple case study of dyslexic adults.

A multiple case study was conducted in order to assess three leading theories of developmental dyslexia: (i) the phonological theory, (ii) the magnocellular (auditory and visual) theory and (iii) the cerebellar theory. Sixteen dyslexic and 16 control university students were administered a full battery of psychometric, phonological, auditory, visual and cerebellar tests. Individual data reveal that all 16 dyslexics suffer from a phonological deficit, 10 from an auditory deficit, four from a motor deficit and two from a visual magnocellular deficit. Results suggest that a phonological deficit can appear in the absence of any other sensory or motor disorder, and is sufficient to cause a literacy impairment, as demonstrated by five of the dyslexics. Auditory disorders, when present, aggravate the phonological deficit, hence the literacy impairment. However, auditory deficits cannot be characterized simply as rapid auditory processing problems, as would be predicted by the magnocellular theory. Nor are they restricted to speech. Contrary to the cerebellar theory, we find little support for the notion that motor impairments, when found, have a cerebellar origin or reflect an automaticity deficit. Overall, the present data support the phonological theory of dyslexia, while acknowledging the presence of additional sensory and motor disorders in certain individuals.

Adult↗
PubMed2002

Galvanic vestibular stimulation evokes sensations of body rotation.

Psychophysical experiments identified effects of galvanic vestibular stimulation (GVS) on the perception of whole-body angular rotation. Subjects lay supine on a platform that could rotate about a vertical axis through the vestibular axis so that linear movements were excluded. Movements were applied sufficiently above perception threshold to enable a reliable report of direction and movement size. In some trials, binaural GVS was applied concurrently at 1-2 mA. When GVS that was incongruent with the movement was applied, subjects reported lesser spin, on average cancelling the movement perception. When the GVS and movement were congruent, subjects reported greater spin. We conclude that GVS produces a vestibular signal of rotation, probably though an effect on semicircular canals.

Adult↗
PubMed2002

Short-latency eye movements evoked by near-threshold galvanic vestibular stimulation.

To investigate whether the primary planes of eye and body responses to galvanic vestibular stimulation (GVS) are congruent, we have measured the binocular, three-dimensional eye movements (scleral coil technique) to bilateral bipolar GVS in six normal human subjects. Stimulation intensities were kept deliberately low in order to characterize the response to near-threshold intensities of stimulation (0.1-0.9 mA) that had been used previously to characterise body postural responses. Stimuli were applied for 4 s, but only the early responses that occurred within the initial 300 ms of turning the current on or off were measured. At intensities of 0.1-0.7 mA the 'on' response consisted almost exclusively of a torsional slow phase eye movement in which the top of the eyes rotated towards the anode. The latency of the torsional response was ca. 46 ms. A weak polarity-dependent disconjugate response was also observed in which the intorting eye elevated and the extorting eye depressed ('skew eye deviation'). When the current was turned off similar responses occurred in the reverse direction. Removal of the visual fixation light-emitting diode (LED) had no consistent effect on the short-latency ocular responses. The direction of the ocular response was similar to that of the postural response and is compatible with GVS inducing an apparent dynamic roll-tilt of the head towards the cathode. However, weak horizontal eye movements, which became more prominent as the stimulus intensity was increased to 0.9 mA, were also observed. This suggests that an additional weak rotational component about the yaw axis, or a component of lateral translation in the frontal plane, is contained in the GVS-evoked signal. The overall pattern of eye movement suggests that semicircular canal afferents contribute to these low-intensity GVS responses.

Electric Stimulation↗
PubMed2002

Position and velocity responses to galvanic vestibular stimulation in human subjects during standing.

Galvanic vestibular stimulation (GVS) in animals modulates the firing of otolith and semicircular canal afferents alike. Here, we look for postural responses evoked by GVS from the otolith organs and semicircular canals. To minimise the modifying effects of somatosensory input on the response, low-intensity (0.3-0.5 mA) GVS was applied for 8 s while subjects stood on foam rubber with the feet together and strapped to the floor. The response had three phases: (i) a rapid movement during the first second, (ii) a slower movement that persisted throughout the stimulus, and (iii) a rapid partial return movement after GVS stopped. The three movement velocities were significantly different. The GVS response therefore appears to be the sum of a step response that returns to the starting point when the stimulus stops, and a constant-velocity ramp response for the duration of the stimulus without a return movement. Subjects' responses differed in size and profile, some with the step or ramp responses almost exclusively but most with a combination of both. The 'step-plus-ramp' model was tested by comparing the three velocities. If the responses add, the initial velocity should not be different from the sum of the velocities during the ramp-only period and the step-only period at offset. ANOVA and pairwise comparisons confirmed this. It is concluded that postural responses to GVS arise through stimulation of both otolith and canal afferents.

Adult↗