Elaine Cornell

The basis for using bone-conducted vibration or air-conducted sound to test otolithic function.

Extracellular single neuron recordings of primary vestibular neurons in Scarpas ganglion in guinea pigs show that low‐intensity 500 Hz bone‐conducted vibration (BCV) or 500 Hz air‐conducted sound (ACS) activate a high proportion of otolith irregular neurons from the utricular and saccular maculae but few semicircular canal neurons. In alert guinea pigs, and humans, 500 Hz BCV elicits otolith‐evoked eye movements. In humans, it also elicits a myogenic potential on tensed sternocleidomastoid muscles. Although BCV and ACS activate both utricular and saccular maculae, it is possible to probe the functional status of these two sense organs separately because of their differential neural projections. Saccular neurons have a strong projection to neck muscles and a weak projection to the oculomotor system. Utricular afferents have a strong projection to eye muscles. So measuring oculomotor responses to ACS and BCV predominantly probes utricular function, while measuring neck muscle responses to these stimuli predominantly probes saccular function.

Neural basis of new clinical vestibular tests: otolithic neural responses to sound and vibration.

Extracellular single neuron recording and labelling studies of primary vestibular afferents in Scarpas ganglion have shown that guinea‐pig otolithic afferents with irregular resting discharge are preferentially activated by 500 Hz bone‐conducted vibration (BCV) and many also by 500 Hz air‐conducted sound (ACS) at low threshold and high sensitivity. Very few afferent neurons from any semicircular canal are activated by these stimuli and then only at high intensity. Tracing the origin of the activated neurons shows that these sensitive otolithic afferents originate mainly from a specialized region, the striola, of both the utricular and saccular maculae. This same 500 Hz BCV elicits vestibular‐dependent eye movements in alert guinea‐pigs and in healthy humans. These stimuli evoke myogenic potentials, vestibular‐evoked myogenic potentials (VEMPs), which are used to test the function of the utricular and saccular maculae in human patients. Although utricular and saccular afferents can both be activated by BCV and ACS, the differential projection of utricular and saccular afferents to different muscle groups allows for differentiation of the function of these two sensory regions. The basic neural data support the conclusion that in human patients in response to brief 500 Hz BCV delivered to Fz (the midline of the forehead at the hairline), the cervical VEMP indicates predominantly saccular function and the ocular VEMP indicates predominantly utricular function. The neural, anatomical and behavioural evidence underpins clinical tests of otolith function in humans using sound and vibration.

Prevalence of heterophoria and associations with refractive error, heterotropia and ethnicity in Australian school children.

Aims To establish the prevalence of heterophoria and its association with refractive error and ethnicity in a population-based study of Australian schoolchildren. Methods The Sydney Myopia Study is a stratified, random cluster (school-based) sample of 4093 students (examined: 2003–2005). Two samples aged 6 (n=1692) and 12 years (n=2289) without heterotropia were included. Prevalent heterophoria was assessed using cover un-cover and prism bar alternate cover testing at 33 cm and 6 m distance fixation. Cycloplegic autorefraction (1% cyclopentolate) was performed. Significant refractive error was defined as ≤−0.50SE and ≥+2.00SE. Results Exophoria was highly prevalent at near fixation (age 6: 58.3%, age 12: 52.2%). Orthophoria predominated at distance fixation (age 6: 85.4%, age 12: 90.9%). Hyperopia was associated with esophoria at near (age 6: OR 1.7, 95% CI 1.1 to 2.8, age 12: OR 2.9, CI 1.1 to 2.8) and distance fixation (age 6: OR 9.7, CI 3.5 to 26, age 12: 9.6 OR, CI 4.2 to 22). Myopia was associated with exophoria at near (OR 2.1, CI 1.5 to 2.7) and distance fixation (OR 3.1, CI 2.1 to 4.4) for 12-year-old children only. Exophoria was more frequent in children of East Asian than European Caucasian origins, even after adjusting for refraction; at near (age 6: OR 1.4, CI 1.0 to 2.0, age 12: OR 1.4, CI 1.0 to 1.9) and distance (age 12: OR 1.7, CI 1.1 to 2.7). Conclusion Contrary to other studies, exophoria, not orthophoria, was predominant for near. Exophoria was more prevalent in children of East Asian origin. Longitudinal studies are needed to establish if incident heterotropia is preceded by heterophoria.

Errors of binocular fixation are common in normal subjects during natural conditions.

Purpose. To investigate the accuracy of fixation after symmetrical vergence eye movements along the midline during natural full-field viewing conditions using a video method of eye position measurement. Methods. The accuracy of binocular fixation after symmetrical vergence eye movements during natural conditions was measured on 29 young adults using a precise head-mounted video eye movement measuring system. All subjects had normal binocular vision and good visual acuity. Measurements were taken for both near and far fixation after vergence changes of 5°, 10°, and 15° using three rates of change, approximately 0.25, 0.5, and 1 Hz. Results. The amplitude of the vergence movement tended to be hypometric, resulting in underconvergence for near fixations, and overconvergence for distance fixation. For far fixations, most errors (82%) were from −120 to +120 min arc, and for near, most errors (85%) were from −30 to +120 min arc. For far fixations, there was a significant effect of the size of vergence change (F1,28 = 61.8; p < 0.001), the rate of change (F1,28 = 7.08; p = 0.013), and the interaction between these two factors (F1,28 = 7.17; p = 0.012) on resulting errors, with the eyes showing greater overconvergence on the target for the larger and faster fixation changes. For near fixations, there was a significant effect (F1,28 = 15.9; p < 0.001) for the angle of change with the faster vergence changes producing relatively more convergence, thus reducing the mean vergence error. No subject reported diplopia during any conditions despite our measures showing vergence errors of up to 5°. Conclusions. Vergence errors of up to ±2°, without diplopia, were common in subjects with normal binocular single vision. Errors of 5° were rare but present. In all, the largest number of errors occurred as a failure of divergence for far fixations, consistent with previous studies that have suggested differences in the neural control of pathways for convergence and divergence, or possibly caused by differences in the anatomical properties of the medial and lateral rectus muscles and their associated fascia. The absence of diplopia during the period of fixation could only be partly associated with the visual suppression associated with vergence eye movements that has been reported by others because it was still present after the vergence movement was completed. The natural viewing conditions in this study that included a full visual field and multiple disparities may have contributed to this effect.

Vertical and horizontal eye movement responses to unilateral and bilateral bone conducted vibration to the mastoid.

Bone-conducted vibration (BCV) delivered to the skull causes linear acceleration of the mastoids and results in small potentials (oVEMPs) from the extraocular muscles in humans. Very small eye movements to both air- and bone-conducted sound with latencies of around 9 ms and amplitudes up to 16.5 mdeg have also been reported (where mdeg = millidegrees =0.001 degree). To clarify the otolith-ocular responses we investigated the effect of unilateral and bilateral BCV (500 Hz of the mastoid) on eye movements in human subjects, and the effect of gaze position on these movements using a video-based eye-movement recording system. We found very small, short-latency eye-movement responses similar to those previously reported, but these were followed by larger (up to 0.57 degrees) prolonged responses peaking at around 120 ms post-stimulus. For unilateral stimulation the eyes moved downwards when directed away from the stimulated mastoid and upwards when directed towards stimulation. Horizontal movements were towards the stimulated mastoid with little effect of gaze position. Bilateral stimulation enhanced the downward responses, suggesting that the depressor muscles are preferentially activated by BCV. Bilateral stimulation either eliminated the horizontal response or produced convergence of the eyes: the convergence suggests that the eye-movement response is probably primarily driven by the contralateral mastoid.

Convergence reduces ocular counterroll (OCR) during static roll-tilt

When humans are roll-tilted around the naso-occipital axis, both eyes roll or tort in the opposite direction to roll-tilt, a phenomenon known as ocular counterroll (OCR). While the magnitude of OCR is primarily determined by vestibular, somatosensory, and proprioceptive input, direction of gaze also plays a major role. The aim of this study was to measure the interaction between some of these factors in the control of OCR. Videooculography was used to measure 3D eye position during maintained whole body (en bloc) static roll-tilt in darkness, while subjects fixated first on a distant (at 130 cm) and then a near (at 30 cm) head-fixed target aligned with the subjects midline. We found that while converging on the near target, human subjects displayed a significant reduction in OCR for both directions of roll-tilt--i.e. the interaction between OCR and vergence was not simple addition or subtraction of torsion induced by vergence with torsion induced by roll-tilt. To remove the possibility that the OCR reduction may be associated with the changed horizontal position of the eye in the orbit during symmetric convergence, we ran an experiment using asymmetric convergence in which the distant and near targets were aligned directly in front of one eye. We found the magnitude of OCR in this asymmetric convergence case was also reduced for near viewing by about the same amount as in the symmetric vergence condition, confirming that the convergence command rather than horizontal position of the eye underlies the OCR reduction, since there was no horizontal movement of the aligned eye in the orbit between fixation on the distant and near targets. Increasing vergence from 130 to 30 cm reduced OCR gain by around 35% on average. That reduction was equal in both eyes and occurred in both the symmetric and asymmetric convergence conditions. These results demonstrate the important role vergence plays in determining ocular counterroll during roll-tilt and may support the contention that vergence acts to reduce the conflict facing a stereopsis-generating mechanism.

To be or to refuse to be? Exploring the concept of leisure as resistance for young people who are visually impaired.

This study explored the concept of leisure as resistance to social constraints of blindness. Leisure, because of its relative freedoms, has been conceptualised as a forum in which dominant cultural discourses and stereotypes can be challenged or resisted. A post-structural analysis of the leisure narratives of five young people who are blind revealed that they intentionally used aspects of leisure to resist some constraints and stereotypes of disability. Three strategies were identified: advocacy, redefinition and passing. Advocacy targeted other people’s beliefs; yet, some activities done to resist dominant discourses actually perpetuated them. Redefinition was a covert strategy to show themselves that they could be the same as everyone else. Passing was used in different ways. One young person used it when advocating, another to maintain social connections. However, maintaining social connections by passing was often at the cost of enjoyment, engagement and perceptions of ability. Empowerment through resistance in leisure related primarily to gains in self-confidence. These gains in self-confidence have the potential to contribute to social interactions and perhaps influence some of the discourses that operate as ‘truths’ about people who are blind.

Bone conducted vibration to the mastoid produces horizontal, vertical and torsional eye movements.

INTRODUCTION We have previously shown that bone-conducted vibration to each mastoid in humans results in small stimulus-locked horizontal and vertical eye movements suggesting that the ipsilateral superior oblique and the contralateral inferior rectus are activated. However, as torsional responses were not measured we have repeated the study by measuring torsional as well as horizontal and vertical responses in seven adult subjects during different directions of gaze. METHODS We measured stimulus-locked horizontal (H), vertical (V) and torsional (T) eye movements during brief stimulation to the mastoid unilaterally and bilaterally using a high-speed, high-resolution video technique. Bone-conducted vibrations were delivered to each mastoid separately as well as bilaterally using a Radioear B71 bone oscillator while the subjects viewed targets 2 metres away situated straight ahead and 10° to the right and left. RESULTS For unilateral stimulation the eyes moved downwards when directed away from the stimulated mastoid, along with bilateral counter-rolling away from the side of stimulation. Horizontal movements were small, with little effect of gaze position. Bilateral stimulation enhanced the downward responses. CONCLUSIONS Bone conducted vibration to the mastoid in humans produces vertical and torsional eye movements consistent with activation of the ipsilateral superior oblique muscle, and the contralateral inferior oblique muscle to produce counter rolling of the eyes away from the stimulated mastoid.