Mary Faldon

Influence of whole-body pitch tilt and kinesthetic cues on the perceived gravity-referenced eye level

We investigated the effects of whole body tilt and lifting the arm against gravity on perceptual estimates of the Gravity-Referenced Eye Level (GREL), which corresponds to the subjective earth-referenced horizon. The results showed that the perceived GREL was influenced by body tilt, that is, lowered with forward tilt and elevated with backward tilt of the body. GREL estimates obtained by arm movements without vision were more biased by whole-body tilt than purely visual estimates. Strikingly, visual GREL estimates became more dependent on whole-body tilt when the indication of level was obtained by arm lifting. These findings indicate that active motor involvement and/or the addition of kinesthetic information increases the body tilt-induced bias when making GREL judgements. The introduction of motor/kinaesthetic cues may induce a switch from a semi-geocentric to a more egocentric frame of reference. This result challenges the assumption that combining non-conflicting multiple sensory inputs and/or using intermodal information provided during action should improve perceptual performance.

Reduced self-motion perception in patients with midline cerebellar lesions

Vestibular input to the cerebellum mediates balance and eye movement control. Recent functional MRI studies, however, show midline cerebellar activation during visually induced illusions of self-rotation, thus suggesting that the cerebellum may also contribute to self-motion perception. Here, we investigate self-motion perception directly in patients with vermal (or midline) cerebellar ataxia. Participants were rotated in the dark (90°/s velocity steps) and the time constant of decay of the postrotational angular velocity sensation was measured. The perceptual vestibular time constant in patients was considerably reduced (7.8 s) with respect to control values in this (25.6 s) and several previous studies. In addition to the processing of vestibular signals for motor control, the cerebellar vermis is involved in vestibular processing of self-motion perception.

Head Accelerations during Particle Repositioning Manoeuvres

Benign paroxysmal positional vertigo (BPPV) due to canalithiasis can be treated with particle repositioning manoeuvres, which aim to evacuate trapped particles from the semicircular canals (SCC). The movement of particles within the SCC is affected by gravity as well as by the accelerations of the head during the manoeuvres. Moreover, as experienced by the particles, gravity is indistinguishable from an upward acceleration of the SCC in free space. We used a set of three orthogonal linear accelerometers to measure the net three-dimensional linear acceleration vector acting on the head during the Hallpike manoeuvre and Epley and Semont particle repositioning manoeuvres (which are used to treat posterior canal BPPV). The projection of the net acceleration vector onto the SCC planes showed that both the Epley and Semont manoeuvres approximated to stepwise, 360°, backward rotations in the plane of the targeted posterior canal. Angular velocity measurements however showed that the rotational component during the central stages of these two manoeuvres is opposite in direction. A simple model of head rotations during particle repositioning manoeuvres was created which showed good agreement to the linear acceleration measurements. Analysis of modelled and measured data identified that speed of movement during the Semont manoeuvre should be critical to its clinical success.

Visual dependence and BPPV.

The increased visual dependence noted in some vestibular patients may be secondary to their vertigo. We examine whether a single, brief vertigo attack, such as in benign paroxysmal positional vertigo (BPPV), modifies visual dependency. Visual dependency was measured before and after the Hallpike manoeuvre with (a) the Rod and Frame and the Rod and Disc techniques whilst seated and (b) the postural sway induced by visual roll-motion stimulation. Three subject groups were studied: 20 patients with BPPV (history and positive Hallpike manoeuvre; PosH group), 20 control patients (history of BPPV but negative Hallpike manoeuvre; NegH group) and 20 normal controls. Our findings show that while both patient groups showed enhanced visual dependency, the PosH and the normal control group decreased visual dependency on repetition of the visual tasks after the Hallpike manoeuvre. NegH patients differed from PosH patients in that their high visual dependency did not diminish on repetition of the visual stimuli; they scored higher on the situational characteristic questionnaire (‘visual vertigo’ symptoms) and showed higher incidence of migraine. We conclude that long term vestibular symptoms increase visual dependence but a single BPPV attack does not increase it further. Repetitive visual motion stimulation induces adaptation in visual dependence in peripheral vestibular disorders such as BPPV. A positional form of vestibular migraine may underlie the symptoms of some patients with a history of BPPV but negative Hallpike manoeuvre. The finding that they have non adaptable increased visual dependency may explain visuo-vestibular symptoms in this group and, perhaps more widely, in patients with migraine.

Influence of target size on vertical gaze palsy in a pathologically proven case of progressive supranuclear palsy

We document a new oculomotor phenomenon in a patient with pathologically proven progressive supranuclear gaze palsy (PSP), namely that vertical gaze excursion improves with larger pursuit targets. We used computerised video‐oculography during vertical smooth pursuit eye movements (SPEM) of circular targets of diameter 0.16 degrees and 16 degrees, sinusoidally oscillating at 0.08 Hz (peak‐to‐peak amplitude 49 degrees). Increasing target size improved vertical gaze excursion from 10 degrees to 25 degrees. There was no concomitant increase in slow phase eye velocity. The findings could be explained by a potentiation of the position control mechanism of pursuit by target size due to increased activation of brainstem pursuit‐optokinetic pathways and to higher order attentional mechanisms. This observation may be useful in the clinical assessment of PSP patients with severe neck rigidity in whom the dolls head–eye manoeuvre cannot be performed by comparing the degree of vertical gaze palsy during smooth pursuit testing between at least two differently sized targets and observing whether there is a larger excursion in response to a large target such as a newspaper.

A New Device to Quantify Ocular Counterroll Using Retinal Afterimages

Background: The integrity of vertical semicircular canal and otolith function remains difficult to assess in the clinical setting, partly due to difficulties in recording ocular counterroll. Here, we quantify static ocular counterroll from head tilt using a new head-mounted device. Methods: The device consists of an LED positioned 42 cm in front of one eye and a striated lens which produces a streak of light on the retina. The LED is illuminated at full intensity (80 cd) to generate a retinal afterimage. Subsequently, in darkness, the subject’s head is tilted in the roll plane. Finally, the LED is illuminated dimly (0.2 cd) and the subject rotates the striated lens to superimpose the dim light streak onto the afterimage. An angular scale indicates the angle through which the lens is rotated, giving a measure of the ocular counterroll. To validate the device, we recorded binocular counterroll simultaneously with 3D computerised video-oculography of the other eye in 16 normal subjects; 2 patients with acquired bilateral loss of vestibular function were also tested. Results: In the normal subjects, there was no significant difference between the two techniques (p = 0.24) when recording ocular counterroll and the correlation between the two techniques was R2 = 0.78. The 2 avestibular patients essentially showed no ocular counterroll with both techniques. Conclusions: We have devised a non-invasive, quick and reliable test of ocular counterroll. The lack of response in the 2 avestibular patients indicates that this device is clinically applicable to assess otolith function.

Influence of pitch tilts on the perception of gravity-referenced eye level in labyrinthine defective subjects.

We investigate the role of vestibular information in judging the gravity-referenced eye level (i.e., earth-referenced horizon or GREL) during sagittal body tilt whilst seated. Ten bilateral labyrinthine-defective subjects (LDS) and 10 age-matched controls set a luminous dot to their perception of GREL in darkness, with and without arm pointing. Although judgements were linearly influenced by the magnitude of whole-body tilt, results showed no significant difference between LDS and age-matched controls in the subjective GREL accuracy or in the intra-subject variability of judgement. However, LDS performance without arm pointing was related to the degree of vestibular compensation inferred from another postural study performed with the same patients. LDS did not utilize upper limb input during arm pointing movements as a source of graviceptive information to compensate for the vestibular loss. The data suggest that vestibular cues are not of prime importance in GREL estimates in static conditions. The absence of difference between controls and LDS GREL performance, and the correlation between the postural task and GREL accuracy, indicate that somatosensory input may convey as much graviceptive information required for GREL judgements as the vestibular system.

Accessing Escalators: A Central Vestibular Disorder After Posterior Fossa Tumor Removal

About 20% of childhood tumors originate within the central nervous system. Progress in assessment and treatment of these lesions has led to improved survival rates. We describe a patient with a posterior fossa ependymoma who despite a remarkable recovery following treatment has been frustrated by difficulty in using escalators. Such symptom selectivity is explained by specific vertical visuomotor and high-frequency vestibular deficits disrupting the execution of this complex motor act.