Deep Learning Model for Static Ocular Torsion Detection Using Synthetically Generated Fundus Images

Purpose

The objective of the study is to develop deep learning models using synthetic fundus images to assess the direction (intorsion versus extorsion) and amount (physiologic versus pathologic) of static ocular torsion. Static ocular torsion assessment is an important clinical tool for classifying vertical ocular misalignment; however, current methods are time-intensive with steep learning curves for frontline providers.

Methods

We used a dataset (n = 276) of right eye fundus images. The disc-foveal angle was calculated using ImageJ to generate synthetic images via image rotation. Using synthetic datasets (n = 12,740 images per model) and transfer learning (the reuse of a pretrained deep learning model on a new task), we developed a binary classifier (intorsion versus extorsion) and a multiclass classifier (physiologic versus pathologic intorsion and extorsion). Model performance was evaluated on unseen synthetic and nonsynthetic data.

Results

On the synthetic dataset, the binary classifier had an accuracy and area under the receiver operating characteristic curve (AUROC) of 0.92 and 0.98, respectively, whereas the multiclass classifier had an accuracy and AUROC of 0.77 and 0.94, respectively. The binary classifier generalized well on the nonsynthetic data (accuracy = 0.94; AUROC = 1.00).

Conclusions

The direction of static ocular torsion can be detected from synthetic fundus images using deep learning methods, which is key to differentiate between vestibular misalignment (skew deviation) and ocular muscle misalignment (superior oblique palsies).

Translational Relevance

Given the robust performance of our models on real fundus images, similar strategies can be adopted for deep learning research in rare neuro-ophthalmologic diseases with limited datasets.

A simple marking system for accurate intraoperative monitoring and adjustment of cyclotorsion strabismus surgery

Ocular cyclotorsion is treatable only with surgery. The surgical procedure must be tailored individually to the specific etiologies causing the horizontal and vertical strabismus and its torsional components. An adjustable surgical approach is often used for postoperative or intraoperative adjustments. However, the methods currently used have some limitations. In this study, we propose a simple intraoperative marking system for all cyclotorsion correction surgery. The proposed marking system used three sets of surface markers: external horizontal markings, ocular horizontal markings, and surgical torsion markings, drawn in sequence. We retrospectively analyzed the surgical results using this novel marking system in this single-center, single-surgeon study. Fifteen patients with cyclotorsion who underwent treatment using the proposed marking system as an intraoperative aid between August 2019 and August 2021 were included. The medical charts were thoroughly reviewed, and the pre-and postoperative subjective and objective cyclotorsion were analyzed. Among the study subjects (10 males, 5 females; age range: 6-89 years), 13 had excyclotorsion and 2 incyclotorsion. Preoperative mean net subjective cyclotorsion measured by the double Maddox rod (DMR) test was 6.0° (standard deviation: 10.8°) and mean net disc-to-fovea angle (DFA) was 20.23° (13.21°). The postoperative net DMR and DFA were 0.2° (2.1°) and 14.09° (5.97°), respectively. The mean absolute net DMR and DFA being treated were 9.8° (4.8°) and 9.76° (4.61°). Overall, the proposed intraoperative marking system is a simple and quantitative method to assess, monitor, and adjust the torsional aspect for all strabismus surgeries.

Pattern Strabismus in a Tertiary Hospital in Southern China: A Retrospective Review

Background and objectives: To analyze demographic and clinical features of pattern strabismus patients and assess the relationship among these clinical variables and risk factors. Materials and Methods: Medical records of pattern strabismus patients who had undergone strabismus surgery at our center between 2014 and 2019 were retrospectively reviewed. Data collected included gender, age at onset, age at surgery, refraction, Cobb angle, pre- and post-operative deviations in the primary position, up- and downgaze, angle of ocular torsion, type/amount of pattern, grade of oblique muscle function and presence/grade of binocular function. To verify the clinical significance of the Cobb angle, 666 patients who had undergone surgery within one week after ocular trauma between 2015 and 2021 were enrolled as controls. Results: Of the 8738 patients with horizontal strabismus, 905 (507 males and 398 females) had pattern strabismus, accounting for 10.36%. Among these 905 patients, 313 showed an A-pattern and 592 showed a V-pattern. The predominant subtype was V-exotropia, followed by A-exotropia, V-esotropia and A-esotropia. Over half of these patients (54.6%) manifested an A- or V-pattern in childhood. The overall mean ± SD Cobb angle was 5.03 ± 4.06° and the prevalence of thoracic scoliosis was 12.4%, both of which were higher than that observed in normal controls (4.26 ± 3.36° and 7.8%). Within A-pattern patients, 80.2% had SOOA and 81.5% an intorsion, while in V-pattern patients, 81.5% had IOOA and 73.4% an extorsion. Patients with binocular function showed decreases in all of these percent values. Only 126 (13.9%) had binocular function, while 11.8% of A-pattern and 15.1% of V-pattern patients still maintained binocular function. Pre-operative horizontal deviation was negatively correlated with binocular function (r = −0.223, p < 0.0001), while the grade of oblique muscle overaction was positively correlated with the amount of pattern (r = 0.768, p < 0.0001) and ocular torsion (r = 0.794, p < 0.0001). There were no significant correlations between the Cobb angle and any of the other clinical variables. There were 724 patients (80.0%) who had received an oblique muscle procedure and 181 (20.0%) who received horizontal rectus muscle surgery. The most commonly used procedure consisted of horizontal rectus surgery plus inferior oblique myectomy (n = 293, 32.4%), followed by isolated horizontal rectus surgery (n = 122, 13.4%). Reductions of pattern were 14.67 ± 6.93 PD in response to horizontal rectus surgery and 18.26 ± 7.49 PD following oblique muscle surgery. Post-operative deviations were less in V- versus A-pattern strabismus. Post-operative binocular function was obtained in 276 of these patients (30.5%), which represented a 16.6% increase over that of pre-operative levels. The number of patients with binocular function in V-pattern strabismus was greater than that of A-pattern strabismus (p = 0.048). Conclusions: Of patients receiving horizontal strabismus surgery, 10.36% showed pattern strabismus. In these patients, 54.6% manifested an A- or V-pattern in childhood, and V-exotropia was the most frequent subtype. Pattern strabismus patients showed a high risk for developing scoliosis. Cyclovertical muscle surgery was performed in 724 of these patients (80.0%), and horizontal rectus surgery was effective in correcting relatively small levels of patterns. Binocular function represented an important factor as being involved with affecting the occurrence and development of pattern strabismus.

Bechterew's Phenomenon in Bilateral Sequential Vestibular Neuritis: A Report of Two Cases

The brain can compensate for the vestibular imbalance. When the unilateral labyrinthine function is lost, the asymmetry between the peripheral vestibular inputs is compensated centrally by readjusting the signal difference from both ears and regaining vestibular balance. If the other healthy labyrinth is destroyed, the vestibular nuclei become imbalanced again, creating spontaneous nystagmus even though there is no input to the vestibular nuclei from either labyrinth. This is called Bechterew's phenomenon; a rare and not widely recognized phenomenon that occurs in cases of bilateral sequential vestibular neuritis. This is of clinical importance because spontaneous nystagmus with bilaterally absent or diminished caloric responses may give a misleading impression of a central lesion rather than a second peripheral lesion superimposed upon the effects of central compensation for the first. Although well-documented in experimental animals, this phenomenon rarely occurs in human beings. The objective of this study is to highlight the characteristics and the progression of test results from two patients from our own experience. Along with careful history taking and physical examination, a complex interpretation of various vestibular function tests, including induced nystagmus, head impulse test, caloric test, and fundus photography, is needed to make an accurate diagnosis of bilateral sequential vestibular neuritis (BSVN).

Minimal detectable change of the disc-fovea angle for ocular torsion assessment

PURPOSE

The disc-fovea angle (DFA) is used as a relevant indicator of ocular torsion change in cyclovertical strabismus. However, interpretation of the variation in time must differentiate whether a real change has occurred or if the disparity is due to random measurement error. The aim of the study was to obtain the minimal detectable change (MDC) of the DFA. It represents the minimal variation between two measurements that may be considered a real ocular torsion change.

METHODS

A prospective cross-sectional study was conducted in San Carlos Clinical Hospital of Madrid, Spain. Sixty healthy right eyes from 60 patients (31 men and 29 women) were recruited. Three digital fundus photographs were obtained, and between measurements, the patient moved their head away from the head support and then returned. Two observers quantified the DFA with software designed with MATLAB. Test-retest and interrater reliability were calculated.

RESULTS

Mean participant age was 56.1 years (SD 16.6, range 25-85). Mean DFA was 8.1° (SD 3.5, range 1.3-18.5). Test-retest reliability for Observer 1 (Ob1), Observer 2 (Ob2) and interrater reliability were excellent (ICC 0.80, 0.83 and 0.95, respectively). Precision was 2.9° (Ob1) and 3.0° (Ob2), and the MDC₉₅ was 4.1° (Ob1) and 4.2° (Ob2). Bland-Altman analysis revealed an absence of bias and a homoscedastic distribution of the differences.

CONCLUSIONS

The MDC of the DFA in fundus photography was 4°, which represents the minimal change that may be considered a real change in ocular torsion. This result may improve the interpretation of ocular torsion changes in surgery and clinical scenarios.

Alertness and Visual Attention Impact Different Aspects of the Optokinetic Reflex

Purpose

Assessing visual attention and alertness is of great importance in visual and cognitive neuroscience, providing objective measures valuable for both researchers and clinicians. This study investigates how the optokinetic response differs between levels of visual attention in healthy adults while controlling for alertness.

Methods

Twelve healthy subjects (8 men and 4 women; mean age = 33 ± 9.36) with intact gaze-stability, visual acuity, and binocularity were recruited. Subjects viewed a rotating visual scene provoking torsional optokinetic nystagmus (OKN) while wearing a video eye tracker in a seated head-fixed position. Tasks requiring focused, neutral, and divided visual attention were issued to each subject and the OKN was recorded. Pupil sizes were monitored as a proxy for alertness.

Results

Pupil dilation was increased for both focused and divided visual attention. The number of nystagmus beats was highest for the focused condition and lowest for the divided attentional task. OKN gain was increased during both focused and divided attention. The distribution of nystagmus beats over time showed that only focused attention produced a reliable adaptation of the OKN.

Conclusions

Results consequently indicate that OKN frequency is adaptive to a viewer's level of visual attention, whereas OKN gain is influenced by alertness levels. This pattern offers insight into the neural processes integrating visual input with reflexive motor responses. For example, it contextualizes why attention to visual stimuli can cause dizziness, as the OKN frequency reflects activity of the velocity storage mechanism. Additionally, the OKN could offer a possible venue for differentiating between visual attention and alertness during psychometric testing.

Resolution of Diplopia and Torsion Following Yttrium Aluminum Garnet (YAG) Laser Capsulotomy in a Patient With the Ocular Tilt Reaction

The ocular tilt reaction is a rare neuro-ophthalmological phenomenon commonly occurring due to an injury to the vestibulo-ocular pathway, or a thalamic, brainstem, or cerebellar lesion. Most ocular tilt reactions are transient and demonstrate spontaneous recovery. This report documents the immediate resolution of diplopia and the patient’s ocular tilt reaction following visual recovery from left yttrium aluminum garnet laser capsulotomy.

Non-invasive recognition of eye torsion through optical imaging of the iris pattern in ocular proton therapy

The introduction of non-invasive imaging techniques such as MRI imaging for treatment planning and optical eye tracking for in-room eye localization would obviate the requirement of clips implantation for many patients undergoing ocular proton therapy. This study specifically addresses the issue of torsional eye movement detection during patient positioning. Non-invasive detection of eye torsion is performed by measuring the iris pattern rotations using a beams eye view optical camera. When handling images of patients to be treated using proton therapy, a number of additional challenges are encountered, such as changing eye position, pupil dilatation and illumination. A method is proposed to address these extra challenges while also compensating for the effect of cornea distortion in eye torsion computation. The accuracy of the proposed algorithm was evaluated against corresponding measurement of eye torsion using the clips configuration measured on x-ray images. This study involves twenty patients who received ocular proton therapy at Paul Scherrer Institute and it is covered by ethical approval (EKNZ 2019-01987).

Effect of unilateral inferior oblique weakening on fundus torsion in bilateral eyes of children with congenital superior oblique palsy

AIM

To study the change of torsion in both eyes after unilateral inferior oblique (IO) weakening on children with congenital superior oblique palsy (SOP).

METHODS

This retrospective study enrolled all patients diagnosed with unilateral congenital superior oblique palsy (UCSOP) accompanied by inferior oblique overaction (IOOA). A total of 120 eyes of 60 patients were divided into group 1 (more extorted paretic eye) and group 2 (more extorted nonparetic eye). The degree of fundus torsion was evaluated before and 1mo after the IO weakening procedure. The torsion of the fundus was recorded by measuring the disk-foveal angle (DFA) using fundus photography.

RESULTS

Group 1 included 26 cases and group 2 included 34 cases, thus the rate of extorsion was insignificantly higher in the nonparetic eye (P=0.10). The preoperative DFA in the paretic and nonparetic eyes was 13.21±5.95, 7.97±4.25 in group 1, and 4.65±3.79, 13.16±5.35 in group 2 (both P<0.001). The postoperative DFA in the paretic and nonparetic eyes was 8.57±4.87, 7.32±4.27 in group 1 (P=0.24), and 3.85±6.00 and 9.94±5.45 in group 2 (P<0.001). The amount of postoperative reduction of the DFA in the paretic and nonparetic eyes was 4.64±3.90, 0.65±0.76 in group 1 (P=0.002), and 0.80±0.81, 3.21±5.50 in group 2 (P=0.01). The difference in the amount of reduction of DFA in the more extorted eye in group 1 (paretic eye) vs group 2 (nonparetic eye) was insignificant (P=0.30).

CONCLUSION

Excyclotorsion in the nonparetic eye has a similar probability in the paretic eye in UCSOP children, and weakening of the ipsilateral IO has a more obvious effect on the decrement of extorsion in the more extorted eye regardless of which eye is paretic.

No handedness effect on spatial orientation or ocular counter-roll during lateral head tilts

Although vestibular inputs are bilaterally represented within the cerebral hemispheres, the higher level vestibular functions exhibit hemispheric asymmetries. Previous studies have suggested that such asymmetries are associated with handedness. Here, we studied the impact of handedness (i.e., hemispheric lateralization) on spatial orientation using a subjective visual vertical (SVV) task. We tested 22 right‐handed and 22 left‐handed subjects in upright position, during prolonged lateral head tilts of 20° (~15 min), and after the head returned to upright position. The corresponding changes in torsional eye position were measured simultaneously using video‐oculography. During lateral head tilts, both right‐ and left‐handers had initial SVV biases in the opposite direction of the head tilt (right‐handers: left tilt 3.0 ± 1.3°, right tilt −4.7 ± 1.5°; left‐handers: left tilt 3.4 ± 1.1°, right tilt −4.1 ± 1.0°). The SVV subsequently drifted in the direction of the head tilt, and there was an aftereffect in the same direction when the head was brought back upright. The ocular torsion initially changed in the opposite direction of the head tilt (right‐handers: left tilt 3.8 ± 0.4°, right tilt −3.8 ± 0.4°; left‐handers: left tilt 4.2 ± 0.5°, right tilt −4.5 ± 0.5°), and there were also drift and aftereffect in the same direction as the head tilt. The changes in upright perception and ocular torsion did not differ between right‐ and left‐handers. These findings show no functional laterality, neither in the higher level neural mechanisms that maintain spatial orientation, nor in the lower level mechanisms that generate the ocular torsion response during lateral head tilt.

Objective cyclodeviation measurement in normal subjects by means of Cyclocheck® application

PURPOSE

To evaluate the range of cyclodeviation in normal individuals by means of Cyclocheck^® application recently designed by the authors and freely available at www.cyclocheck.com.

METHODS

Healthy subjects with normal muscle balance, best-corrected visual acuity of ⩾0.8, and stereopsis on Randot charts of ⩽100 s of arc were included in the study. Two separate digital fundus photographs were taken of each eye of every patient. The disk-foveal angle was calculated using the Cyclocheck^® application. The average result of the disk-foveal angle measurements were considered for data analysis.

RESULTS

A total of 131 patients met inclusion criteria for the study population. The mean value of the disk-foveal angle in the whole study group (both right and left eye) was 6.39° ± 2.72° with 5.26° ± 2.56° (range from -0.4° to 12.55°) in the right eye and 7.52° ± 2.39° (range from 1.25° to 12.76°) in the left eye. The mean value of the disk-foveal angle of the left eye was greater by 2.26° than that of the right eye.

CONCLUSION

Cyclocheck^® software allows easy assessment of cyclodeviation. Normal individuals present with a positive value of the disk-foveal angle with a certain spread of the results. The analysis of obtained measurements revealed a significant asymmetry between both eyes with the left eye being more excyclodeviated in an otherwise orthotropic population, which remains a subject for further investigations.

Comparing Ocular Responses to Caloric Irrigation and Electrical Vestibular Stimulation in Vestibular Schwannoma

Electrical Vestibular Stimulation (EVS) is a non-invasive technique for activating the vestibular-ocular reflex, evoking mainly a torsional eye movement response. We have previously demonstrated that this response can be used to detect vestibular asymmetry in patients with vestibular schwannoma (VS). Here we perform a direct comparison of EVS with caloric irrigation in this patient group. We studied 30 patients with unilateral VS, alongside an equal number of aged-matched healthy control subjects. EVS current was delivered to the mastoid process in a monaural configuration using a sinusoidal stimulus (2 Hz; ± 2 mA; 10 s), with an electrode placed over the spinous C7 process. Evoked eye movements were recorded from the right eye in darkness using an infra-red sensitive camera while the subject sat relaxed with their head on a chinrest. Ocular torsion was subsequently tracked off-line using iris striations. Each subject separately underwent water caloric irrigation, in accordance with the British Society of Audiology guidelines. For the caloric test, eye movement was recorded in the yaw axis using electro-oculography. For both EVS and calorics, inter-aural response asymmetry was calculated to determine the extent of canal paresis. Both tests revealed impaired vestibular function in the ipsilesional ear of VS patients, with a mean asymmetry ratio of 15 ± 17% and 18 ± 16% for EVS and calorics, respectively. Overall, the caloric test results discriminated controls from patients slightly more effectively than EVS (Cohen's D effect size = 1.44 vs. 1.19). Importantly, there was a significant moderate correlation between the AR values produced by EVS and calorics (r = 0.53, p < 0.01), and no significant difference between mean AR estimates. When questioned, ≥85% of participants subjectively preferred the EVS experience, in terms of comfort. Moreover, it took ~15 min to complete, vs. ~1 h for caloric. These results confirm that the results of the EVS test broadly agree with those of caloric irrigation, in terms of detecting vestibular asymmetry. Furthermore, they suggest a higher degree of convenience and patient comfort.

Comparison of torsional amplitudes between emmetropes and myopes using after-image slides

Purpose:

To describe the influence of corrected refractive error on measured torsional fusional amplitudes (TA) by comparing the TA between emmetropes and spectacle corrected myopes, using the after-image slides of the synoptophore, as targets.

Methods:

Fifty emmetropes (Group I) and 50 myopes (Group II) with best-corrected acuity of 6/6 in each eye were included in the study. Near point of convergence (NPC), near point of accommodation (NPA), and horizontal fusional amplitudes (HFA) were assessed in all the subjects. After-image slides, both horizontally aligned, were used as targets (without the bright flashes). One of the slides was rotated inwards, till cyclo-diplopia was reported by the subject; the procedure was repeated with the slide rotated outwards. The sum of the two readings was taken as TA. NPC, NPA, HFA, and TA were analyzed.

Results:

There was no significant difference in the NPC, NPA, and HFA between the two groups. The emmetropic subjects had significantly better torsional amplitude (8.4 ± 1.4 degrees) compared to myopes (7.7 ± 1.5 degrees, P = 0.03). We postulate that this difference may be due to perceived image minification, which brings the edges of retinal image of the targets closer to the fovea, thus rendering the myopes lesser tolerant to cyclodiplopia than emmetropes.

Conclusion:

Refractive error, corrected with spectacles, influences the measured TA. Myopic subjects have lesser torsional fusional amplitude than emmetropes.

Ipsiversive Ocular Torsion, Skew Deviation, and Hearing Loss as Initial Signs of Anterior Inferior Cerebellar Artery Infarction

A 67-year-old man with hypertension and type 2 diabetes mellitus was admitted to our hospital because of left hearing loss and vertical diplopia. A neurological examination showed ocular torsion, skew deviation, and sensorineural hearing loss in the left ear. Brainstem and cerebellar neurological signs were not observed. Left middle cerebellar peduncle infarction was evident on magnetic resonance imaging. He was treated with antiplatelet, however, the infarct progressed after this administration. Ocular tilt reaction (OTR) involves the triad of ocular torsion, skew deviation, and head tilt. Ipsiversive OTR components associated with hearing loss can be early diagnostic signs of anterior inferior cerebellar artery infarction.

Beyond the Vestibulo-Ocular Reflex: Vestibular Input is Processed Centrally to Achieve Visual Stability

The current study presents a re-analysis of data from Zink et al. (1998, Electroencephalography and Clinical Neurophysiology, 107), who administered galvanic vestibular stimulation through unipolar direct current. They placed electrodes on each mastoid, and applied both right and left anodal stimulation. Ocular torsion and visual tilt were measured under different stimulation intensities. New modelling introduced here demonstrates that directly proportional linear models fit reasonably well to the relationship between vestibular input and visual tilt, but not to that between vestibular input and ocular torsion. Instead, an exponential model characterised by a decreasing slope and an asymptote fitted best. These results demonstrate that in the results presented by Zink et al., ocular torsion could not completely account for visual tilt. This suggests that vestibular input is processed centrally to stabilise vision when ocular torsion is insufficient. Potential mechanisms and seemingly conflicting literature are discussed.

Exploring the Role of Temporoparietal Cortex in Upright Perception and the Link With Torsional Eye Position

Upright perception is a key aspect of orientation constancy, as we maintain a stable perception of the world despite continuous movements of our eyes, head, and body. Torsional position of the eyes can impact perception of upright by changing orientation of the images on the retina relative to gravity. Here, we investigated the role of temporoparietal cortex in upright perception with respect to ocular torsion, by means of the inhibitory effect of continuous theta burst transcranial magnetic stimulation (TMS). We used a subjective visual vertical (SVV) paradigm to track changes in upright perception, and a custom video method to track ocular torsion simultaneously. Twelve participants were tested during a lateral head tilt of 20° to the left. TMS at the posterior aspect of the supramarginal gyrus (SMGp) resulted in an average SVV shift in the opposite direction of the head tilt compared to a sham stimulation (1.8°). Ocular torsion following TMS at SMGp showed no significant change compared to the sham stimulation (-0.1°). Thus, changes in upright perception at SMGp were dissociated from ocular torsion. This finding suggests that perception of upright at SMGp is primarily related to sensory processing for spatial orientation, as opposed to subcortical regions that have direct influence on ocular torsion.

Perception of Upright: Multisensory Convergence and the Role of Temporo-Parietal Cortex

We inherently maintain a stable perception of the world despite frequent changes in the head, eye, and body positions. Such "orientation constancy" is a prerequisite for coherent spatial perception and sensorimotor planning. As a multimodal sensory reference, perception of upright represents neural processes that subserve orientation constancy through integration of sensory information encoding the eye, head, and body positions. Although perception of upright is distinct from perception of body orientation, they share similar neural substrates within the cerebral cortical networks involved in perception of spatial orientation. These cortical networks, mainly within the temporo-parietal junction, are crucial for multisensory processing and integration that generate sensory reference frames for coherent perception of self-position and extrapersonal space transformations. In this review, we focus on these neural mechanisms and discuss (i) neurobehavioral aspects of orientation constancy, (ii) sensory models that address the neurophysiology underlying perception of upright, and (iii) the current evidence for the role of cerebral cortex in perception of upright and orientation constancy, including findings from the neurological disorders that affect cortical function.

Clinical measurement of compensatory torsional eye movement during head tilt

PURPOSE

To measure the degree of compensatory torsional eye movement during head tilt using a fundus photography method.

METHODS

We enrolled 55 healthy subjects who were 20-66 years of age. Fundus photographs were obtained in the presumed baseline position and in stepwise head tilt positions to evaluate ocular torsion using a non-mydriatic fundus camera. Horizontal marks on the nose were photographed simultaneously to evaluate head tilt. Images were analysed using Photoshop to measure the degree of ocular torsion and head tilt.

RESULTS

A consistent compensatory torsional eye movement was observed in all subjects during head tilt. The degree of compensatory torsional eye movement showed a positive correlation with the angle of head tilt. Ocular torsional disconjugacy was observed during head tilt, with larger excycloductional eye movement than incycloductional eye movement (4.88 ± 2.91° versus 4.50 ± 2.76°, p < 0.001). In multiple linear regression analysis, the degree of compensatory torsional eye movement was significantly associated with the degree of head tilt (β = 0.191, p < 0.001), and the direction of cycloduction (β = -0.548, p < 0.001).

CONCLUSIONS

The fundus photography method is a non-invasive, accurate and objective tool for measuring compensatory torsional eye movement. Considering the availability of fundus photography in clinical ophthalmology practice, the proposed method can be used as a clinical tool to measure compensatory torsional eye movement.

Upright Perception and Ocular Torsion Change Independently during Head Tilt

We maintain a stable perception of the visual world despite continuous movements of our eyes, head and body. Perception of upright is a key aspect of such orientation constancy. Here we investigated whether changes in upright perception during sustained head tilt were related to simultaneous changes in torsional position of the eyes. We used a subjective visual vertical (SVV) task, modified to track changes in upright perception over time, and a custom video method to measure ocular torsion simultaneously. We tested 12 subjects in upright position, during prolonged (~15 min) lateral head tilts of 20 degrees, and also after the head returned to upright position. While the head was tilted, SVV drifted in the same direction as the head tilt (left tilt: -5.4 ± 1.4° and right tilt: +2.2 ± 2.1°). After the head returned to upright position, there was an SVV aftereffect with respect to the pre-tilt baseline, which was also in the same direction as the head tilt (left tilt: -3.9 ± 0.6° and right tilt: +2.55 ± 1.0°). Neither the SVV drift nor the SVV aftereffect were correlated with the changes in ocular torsion. Using the Bayesian spatial-perception model we show that the pattern of SVV drift and aftereffect in our results could be explained by a drift and an adaptation in sensory inputs that encode head orientation. The fact that ocular torsion (mainly driven by the otoliths) could not account for the perceptual changes suggests that neck proprioception could be the primary source of drift in upright perception during head tilt, and subsequently the aftereffect in upright position.

[Normal values of otolith function tests and age-related changes]

Objective:To explore the normal values of otolith function tests and agerelated changes in health volunteers.Method: One hundred and seventyone health volunteers were distributed to seven age groups, all subjects accepted otolith function tests, including fundus photography, static subjective visual vertical(SSVV), cervical vestibular evoked myogenic potential(cVEMP)and ocular vestibular evoked myogenic potential(oVEMP), the fundus photographs was used to measure the discfovea angle(DFA). DFA, SSVV and VEMPs were analyzed and compared among groups. Result: For DFA and SSVV,there were no significant differences either between different ages or between the two eyes in one individual（P>0.05）. For cVEMP, the detection rate declined with age over 60 years old（P<0.01）; the cVEMP threshold increased with every 20 years old(P<0.05); and the cVEMP amplitude decreased with every 10 years old(P<0.05),however, there was an exception that no significant difference was found between 41-50 years old and 51-60 years old groups（P=0.93）;the cVEMP P1 latency prolonged with age over 70 years old（P<0.01）.For oVEMP, the detection rate also declined with age over 60 years old（P<0.01）; the oVEMP threshold was lowest at the age less than 30 years old and the largest threshold was found at the age over 70 years old（P<0.01）; consistently, the oVEMP amplitude was found largest at the age less than 20 years old and lowest at the age over 60 years old(P<0.05).The P1 and N1 latencies were found significantly longer in the group of over 70 years old than other groups（P<0.01）.No significant difference was found between both sides in one individual for oVEMP threshold,amplitude or latency (P>0.05). Conclusion: In health volunteers,there were no obvious aged related changes in DFA and SSVV. However,the detection rate, threshold, amplitude and latency of cVEMP and oVEMP greatly changed with age.

Vestibular and oculomotor influences on visual dependency

Participants made verticality judgments using the rod-and-disk test, a test of visual dependence, and then repeated after caloric irrigation. If the combination of rotating disk and caloric increased the slow-phase velocity of the torsional nystagmus the tilt in subjective verticality increased, whereas reductions in eye velocity were associated with reduced tilt. Thus visual dependency measures are not only modulated by perceptual style but can also reflect local vestibulo-ocular function, specifically torsional eye movements.

The degree to which a person relies on visual stimuli for spatial orientation is termed visual dependency (VD). VD is considered a perceptual trait or cognitive style influenced by psychological factors and mediated by central reweighting of the sensory inputs involved in spatial orientation. VD is often measured with the rod-and-disk test, in which participants align a central rod to the subjective visual vertical (SVV) in the presence of a background that is either stationary or rotating around the line of sight—dynamic SVV. Although this task has been employed to assess VD in health and vestibular disease, what effect torsional nystagmic eye movements may have on individual performance is unknown. Using caloric ear irrigation, 3D video-oculography, and the rod-and-disk test, we show that caloric torsional nystagmus modulates measures of VD and demonstrate that increases in tilt after irrigation are positively correlated with changes in ocular torsional eye movements. When the direction of the slow phase of the torsional eye movement induced by the caloric is congruent with that induced by the rotating visual stimulus, there is a significant increase in tilt. When these two torsional components are in opposition, there is a decrease. These findings show that measures of VD can be influenced by oculomotor responses induced by caloric stimulation. The findings are of significance for clinical studies, as they indicate that VD, which often increases in vestibular disorders, is modulated not only by changes in cognitive style but also by eye movements, in particular nystagmus.

The distinctive vertical heterophoria of dyslexics

In this study, we looked for the presence of vertical heterophoria (VH) in 42 dyslexic children (22 males and 20 females) aged 118.5±12.9 months who were compared with a control group of 22 nondyslexic children (eleven males and eleven females) aged 112±9.8 months. Dyslexics presented a low-level (always <1 prism diopter) VH combined with torsion. This oculomotor feature clearly separates the dyslexic group from the normal readers group. It is independent of the type of dyslexia. The essential feature of this VH is a lability that appears during specific stimulation of sensory receptors involved in postural regulation. This lability is demonstrated using a vertical Maddox test conducted under very specific conditions in which postural sensors are successively stimulated in a predetermined order. A quantitative variation in this VH may be seen during the Bielchowsky Head Tilt Test, which reveals hypertonia of the lower or upper oblique muscles. Vertical orthophoria can be achieved by placing low-power prisms asymmetrically within the direction of action of the superior or inferior oblique muscles. The selection of power and axis is not only guided by elements of the eye examination but also from observation of postural muscle tone. All these elements suggest that the VH could be of postural origin and somehow related to the vertical action of the oblique muscles. VH and torsion are not harmful per se. There is no statistical relationship between their level and the various parameters used to assess the reading skills of dyslexic children. VH and torsion could be a clinical marker of global proprioceptive dysfunction responsible for high-level multisensory disturbances secondary to poor spatial localization of visual and auditory information. This dysfunction might also explain the motor disorders concomitant to dyslexia.

The effects of lateral head tilt on ocular astigmatic axis

Background:

Compensatory ocular counter-torsion (COCT) is supposed to maintain the eyes aligned with the visual environment following head tilt. Because of some recent controversies the functional capacity of this phenomenon was defined according to the extent of induced astigmatic axis error following head tilt.

Materials and Methods:

Objective autorefractometry was performed on 70 eyes with a regular astigmatism of ≥2D at vertical, right head tilt and left head tilt positions of 5°, 10°, 15°, 20° and 25°. Astigmatic axis error was calculated according to the difference between the defined axis at each tilted head position and the defined axis at the vertical head position. A tiltometer was used for this purpose to show the angle of head tilt without disturbing the process of refractometry.

Results:

The mean astigmatic axis error was 3.2° ± 1.5° and 18.4° ± 4.2° at the head tilt angles of 5° and 25° respectively. The mean percentage of tilt angle compensation by COCT was 36% and 26% at the head tilt angles of 5° and 25° respectively. There was a direct relation between the head tilt angle and the induced astigmatic axis error (ANOVA, P < 0.001, 95% of confidence interval [CI]). Astigmatic axis error values at right head tilt were significantly lower than their corresponding values at left head tilt (ANOVA, P = 0.04 95% CI).

Conclusion:

Any minimal angle of head tilt may cause erroneous measurement of astigmatic axis and should be avoided during refraction. One cannot rely on the compensatory function of ocular counter-torsion during the refraction.

Human ocular torsion and perceived roll responses to linear acceleration

We investigated if human ocular torsion (OT) and perceived roll (PR) are elicited in response to either dynamic interaural linear acceleration or dynamic roll tilt of the gravito-inertial force (GIF). We expanded a previous study [26] that measured only OT across a limited frequency-range (from 0.35 Hz to 1 Hz) by simultaneously measuring OT and PR at three very low (0.01, 0.02 and 0.05 Hz) and one high (1 Hz) frequencies. Three experimental conditions were investigated: (1) Y-Upright with acceleration along the interaural (Y) axis while upright, (2) Y-Supine with acceleration along the Y-axis while supine, and (3) Z-RED with acceleration along the rostro-caudal Z) axis with right-ear-down (RED). OT was measured by video-oculography, while PR was measured by use of a somatosensory bar. OT and PR were qualitatively different. Large OT responses were measured for Y-Upright and Y-Supine, while large perceived roll responses were observed for Y-Upright and Z-RED. OT for Z-RED was small, and PR for Y-Supine was absent. In conclusion, OT and PR appear governed by qualitatively different neural mechanisms. OT appears mostly influenced by central low-pass filtering of interaural graviceptor cues, while PR appears mostly influenced by roll tilt of the GIF.