Biomedical subjects

Daniel L Wardman

Publications and source records attributed to Daniel L Wardman.

4 recordsLinked to original sources

PubMed2003

Absence of short-term vestibular modulation of muscle sympathetic outflow, assessed by brief galvanic vestibular stimulation in awake human subjects.

There is evidence in experimental animals for a potent vestibulosympathetic reflex, but its existence in humans is controversial. Static head-down neck flexion and off-vertical axis rotation have been shown to increase muscle sympathetic nerve activity (MSNA), but not skin sympathetic nerve activity (SSNA), whereas horizontal linear acceleration decreases MSNA in humans. However, both forms of stimuli also activate other receptors. To examine the effects of a pure vestibular stimulus on MSNA and SSNA, and its potential interaction with the baroreceptors, we used galvanic vestibular stimulation (GVS) in 12 healthy seated subjects. MSNA was recorded in ten subjects via a percutaneous microelectrode in the peroneal nerve; ECG, blood pressure, respiration, skin blood flow and sweating were also recorded. GVS (2 mA, 1 s pulse) was delivered via surface electrodes over the mastoid processes at unexpected times, triggered from the R-wave with a delay of 0, 200, 400 or 600 ms. In addition to causing robust postural illusions, GVS caused cutaneous vasoconstriction and sweat release in all subjects (due to a short-latency increase in SSNA, three subjects), but no significant change in MSNA. The failure of GVS to elicit a change in muscle sympathetic nerve activity, as documented by averaging, suggests that the vestibular system is not engaged in short-term modulation of muscle sympathetic activity. Conversely, phasic vestibular inputs do excite cutaneous sympathetic neurones, consistent with the observation that motion sickness is accompanied by pallor and sweating.

Action Potentials↗

PubMed2003

Effects of galvanic vestibular stimulation on human posture and perception while standing.

This study examines three hypotheses that have been proposed to explain the effects of galvanic vestibular stimulation (GVS) in standing human subjects. The first assumes realignment to an altered representation of vertical. GVS-evoked body tilt produced under conditions of different stability was compared with perceptions of the vertical which subjects indicated by two means, a visual line and a manipulandum. GVS produced body tilt that increased in unstable conditions but there were no differences in the perceived vertical in any condition. The second hypothesis is that the altered vestibular signal is interpreted as a tilt of the support surface. The postural response evoked by tilting the support surface was compared with the GVS response under conditions of varying stability. These responses were different, particularly for the lower body where movements were oppositely directed. Standing on foam augmented GVS responses whereas standing with feet apart augmented platform-tilt responses. The third hypothesis is that GVS produces an illusion of movement, and this causes a reaction in the opposite direction. Perception of movement during GVS was determined in standing and immobilised subjects. Although immobilised subjects experienced illusions of movement in the direction opposite the sway response, this only happened after long periods of stimulation and never for standing where subjects accurately reported the true direction of sway. Thus, the results do not support any of these proposals. Instead, they and other observations support a simpler interpretation that the GVS signal is consistent with head movement and evokes an automated response to stabilise the head in space.

Adult↗

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↗

PubMed2002

What does galvanic vestibular stimulation stimulate?

The technique of galvanic vestibular stimulation (GVS) has been used for a long time. The stimulus produces stereotyped automatic postural and ocular responses. The mechanisms underlying these responses are not understood although they are commonly attributed to altered otolith output. Based on animal studies, it seems reasonable to assume that vestibular afferents from the otoliths and semicircular canals are affected similarly by GVS. With this assumption, and anatomical knowledge of the vestibular apparatus, a model is developed to describe the expected responses of vestibular afferents to percutaneous GVS and the physiological implications of this altered sensory signal. Bilateral bipolar GVS, the most commonly used technique, should produce a canal signal consistent with a strong ear-down roll towards the cathodal side, a smaller nose-to-cathode yaw, but no pitch signal. Bilateral bipolar GVS should also produce an otolith signal consistent with tilt towards the cathodal side or a translational acceleration towards the anodal side. The expected responses for other configurations of GVS are also described. The model appears consistent with published data on the ocular and postural responses to GVS, and suggests other testable hypotheses concerning postural, ocular and perceptual responses to GVS.

Animals↗