Primary Gaze Position Research Articles

Human rather than ape-like orbital morphology allows much greater lateral visual field expansion with eye abduction.

While convergent, the human orbit differs from that of non-human apes in that its lateral orbital margin is significantly more rearward. This rearward position does not obstruct the additional visual field gained through eye motion. This additional visual field is therefore considered to be wider in humans than in non-human apes. A mathematical model was designed to quantify this difference. The mathematical model is based on published computed tomography data in the human neuro-ocular plane (NOP) and on additional anatomical data from 100 human skulls and 120 non-human ape skulls (30 gibbons; 30 chimpanzees / bonobos; 30 orangutans; 30 gorillas). It is used to calculate temporal visual field eccentricity values in the NOP first in the primary position of gaze then for any eyeball rotation value in abduction up to 45° and any lateral orbital margin position between 85° and 115° relative to the sagittal plane. By varying the lateral orbital margin position, the human orbit can be made “non-human ape-like”. In the Pan-like orbit, the orbital margin position (98.7°) was closest to the human orbit (107.1°). This modest 8.4° difference resulted in a large 21.1° difference in maximum lateral visual field eccentricity with eyeball abduction (Pan-like: 115°; human: 136.1°).

The surgery for congenital nystagmus with strabismus

Objective To investigate the efficacy of different surgeries for congenital nystagmus with strabismus.Methods The data of 24 cases who underment operation for congenital nystagmus with strabismus were retrospectively studied.The surgery was designed based on the location of intermediate zone,compensatory head posture,the degrees of strabismus and the dominant eye.The correction of compensatory head posture was performed in dominant eye and the correction of strabismus was performed in undominant eye.Results All patients were followed up for 3-12 (6.38 ±2.18) months.The compensatory head posture disappeared after surgery in 20 cases(83.33％) and was much improved in 3 cases(12.50％) with residual head tilt within 15° in the original direction.Overcorrection was observed in one case (4.17％).The eye position was normal in 18 cases (75.00％),slight undercorrection or overcorrection in 5 cases (20.83％),and apparent undercorrection in one case (4.17％) after surgery.Nystagmus in the primary position of gaze was reduced or disappeared in all cases.Binocular visual acuity in the primary position of gaze was improved over 2 lines compared with preoperative one in all cases.The binocular visual acuity and degree of head tilt were significantly improved after surgery (P ＜ 0.01).Conclusion Reasonable design of surgery for congenital nystagmus with strabismus can have good clinical efficacy,which should correct strabismus and compensatory head posture. Key words: Nystagmus, congenital; Strabismus; Operation

Extraocular muscle injury during endoscopic sinus surgery: a series of 10 cases at a single center.

Extraocular muscle (EOM) injury is a rare but serious complication of endoscopic sinus surgery (ESS). The aim of this study is to describe the clinical characteristics and course of EOM injury occurring during ESS. Retrospective case series. Medical records and CT images of patients who suffered from EOM injury after ESS between 2006 and 2012 were retrospectively reviewed. Patient demographics, endoscopic anatomy, type of surgery (primary or revision), predisposing risk factors, site and extent of injury on CT imaging, and associated complications were evaluated. In addition, data regarding ophthalmologic management and clinical outcomes were collected. Ten patients with EOM injuries after ESS were included in this study. One patient was undergoing revision ESS. All patients sustained medial rectus muscle injury and one patient suffered concurrent ipsilateral inferior rectus muscle injury. A microdebrider was used in nine cases. Right-sided injury (90% of patients) was more prevalent than left-sided injury, and 70% of injured medial rectus muscles were completely transected. After subsequent strabismus surgery, 8/9 patients regained binocular single vision in primary gaze despite residual diplopia in some gaze positions. Although proper ophthalmologic surgery after EOM injury may improve deviation in the primary gaze position, none of the patients regained normal EOM movement. Therefore, prevention of this complication through adequate surgical technique and precautions is important.

Webino syndrome caused by meningovascular syphilis. A rare entity with an unexpected cause

Case reportThe patient is a 57-year-old obese and hypertensive male. His chief complaints were double vision and dizziness, with mild exodeviation in both eyes in primary gaze position in the ocular motility examination, but more predominant in the left eye. The exotropia was noticeably more evident on the attempted upgaze. On horizontal gaze, the abducting eye deviated fully, but the adducting eye did not cross the midline. Nystagmus in the abducting eye and convergence impairment were found. Pupil size and testing were normal. Ataxia and areflexia were also present. Bilateral internuclear ophthalmoplegia was suspected and imaging and laboratory tests were performed. The CAT scan showed a right occipital hypo-attenuated lesion. In the MRI scan, a mesencephalic subacute ischemic lesion was found, involving the medial rectus subnuclei. Blood and cerebrospinal fluid test for syphilis were positive. DiscussionBilateral internuclear ophthalmoplegia is a very uncommon–and difficult to diagnose–condition. In the reported case the lesion involved the medial rectus subnuclei. This fact could explain the exotropia in the primary gaze position, and supports that it is not possible to exclude the involvement of the medial rectus subnuclei in the webino syndrome. The rapid identification of the pathology contributed to the better prognosis of the patient.

Bilateral Superior Oblique Palsy: Etiology and Therapeutic Options

To study the causes of bilateral superior oblique palsy (BSOP), treatment with botulinum toxin and/or surgery, and outcome of treatment. This was an 11-year retrospective study of patients with BSOP treated with injections of botulinum toxin (Botox), surgery, or both. Treatment was considered successful when anomalous head turn and diplopia in primary gaze position and downgaze resolved. Bilateral superior oblique palsy was diagnosed in 12 patients (8 male; mean age, 29.5 years). Palsy was secondary to a neoplasm in 3 cases and to head trauma in 2. In 2 cases, it was ischemic, in 2 it was congenital; the remaining cases were iatrogenic (hydrocephalus secondary to meningitis, 1), hemorrhagic (1), and idiopathic (1). The clinical manifestations recorded were diplopia (10), anomalous head posture (9), V pattern (12), subjective excyclotorsion (8), and objective excyclotorsion (6). Recovery was spontaneous in 1 case with neoplastic disease. Botox was injected in 8 cases (inferior oblique and/or inferior rectus muscles [successful in 2]), and subsequent surgery was required in 6. Two patients underwent surgery without prior injection of Botox. The most common surgical technique was recession of the inferior oblique muscle (6 patients), either as the only operation or associated with other procedures. The final result was good in 72.72% (8/11). Mean follow-up was 62.6 months (range 9-99 months). Causes of BSOP were varied (most frequently neoplastic). Botox was effective as the only treatment in 25% (2/8). Outcome was good in a high percentage of cases with Botox, surgery, or both.

A New Digital Image Analysis System for Measuring Blepharoptosis Patients’ Upper Eyelid and Eyebrow Positions

To accurately assess eyelid and eyebrow position, we have developed a new measurement method using a digital image analysis system. (1) Plotting the corneal limbus by transferring to the software digital images of the patient's eyes in the primary gaze position; (2) determining the corneal ring, center, and longitudinal diameter; and (3) measuring the upper eyelid and eyebrow position. The method was tested on 24 patients with senile blepharoptosis and 45 young healthy eyelids without ptosis. The upper eyelid position of patients with blepharoptosis was significantly lower, and their eyebrow position is significantly higher than those of healthy subjects. The upper eyelid and eyebrow position of the patients with blepharoptosis corelated, but no correlation was observed in the healthy subjects. Our measuring system is a simpler, easier, and more accurate way of measuring both upper eyelid and eyebrow position than are currently used techniques.

Exophthalmometric value and palpebral fissure dimension in an African population

Background: Normative data for exophthalmometric values and palpebral fissure dimensions are important for earlier diagnosis of orbital and palpebral fissure pathologies. Objective: To describe the normal exophthalmometric values and palpebral fissure dimensions among healthy adults of Black race. Design: A cross-sectional descriptive study. Setting: Population based (general community). Method: Population-based multistage random sampling was used to recruit 1020 subjects who had no orbital or eye lid diseases and were free from systemic disease like thyroid eye disease that may affect orbital volume. These healthy adult subjects had their exophthalmometric values and palpebral fissure dimensions (horizontal and vertical palpebral fissure, lateral and medial canthal distances, inter-outer canthal distance, inter-inner canthal distance, margin reflex distance, and lid crease) measured with a Hertel's exophthalmometer and a plain non-stretchable plastic ruler, respectively. Results: A total of 1020 subjects (515 females and 505 males) of age range 16-85 years were studied. The mean ± SD exophthalmometric values were 15.27 ± 2.5 and 15.31 ± 2.4 mm for the right and left eyes, respectively. In the right eye, the mean palpebral fissure height was 8.3 mm, mean palpebral fissure width was 32.1 mm, mean upper eyelid crease was 7.1 mm, mean margin reflex height was 3.2 mm, mean lateral canthal distance was 12.9 mm, and the mean medial canthal distance was 10 mm at primary position of gaze. Conclusions: Exophthalmometric values and palpebral fissure dimensions were obtained among healthy adult Nigerians. Knowledge of these normal parameters is valuable in the management of eyelids and orbital diseases, as well as in manufacturing spectacles.

Extraocular muscle injury during endoscopic sinus surgery: A series of 10 cases at a single center

Extraocular muscle (EOM) injury is a rare but serious complication of endoscopic sinus surgery (ESS). The aim of this study is to describe the clinical characteristics and course of EOM injury occurring during ESS. Retrospective case series. Medical records and CT images of patients who suffered from EOM injury after ESS between 2006 and 2012 were retrospectively reviewed. Patient demographics, endoscopic anatomy, type of surgery (primary or revision), predisposing risk factors, site and extent of injury on CT imaging, and associated complications were evaluated. In addition, data regarding ophthalmologic management and clinical outcomes were collected. Ten patients with EOM injuries after ESS were included in this study. One patient was undergoing revision ESS. All patients sustained medial rectus muscle injury and one patient suffered concurrent ipsilateral inferior rectus muscle injury. A microdebrider was used in nine cases. Right-sided injury (90% of patients) was more prevalent than left-sided injury, and 70% of injured medial rectus muscles were completely transected. After subsequent strabismus surgery, 8/9 patients regained binocular single vision in primary gaze despite residual diplopia in some gaze positions. Although proper ophthalmologic surgery after EOM injury may improve deviation in the primary gaze position, none of the patients regained normal EOM movement. Therefore, prevention of this complication through adequate surgical technique and precautions is important.

Sunglasses with Thick Temples and Frame Constrict Temporal Visual Field

Our aim was to compare the impact of two types of sunglasses on visual field and glare: one ("thick sunglasses") with a thick plastic frame and wide temples and one ("thin sunglasses") with a thin metal frame and thin temples. Using the Goldmann perimeter, visual field surface areas (cm²) were calculated as projections on a 30-cm virtual cupola. A V4 test object was used, from seen to unseen, in 15 healthy volunteers in the primary position of gaze ("base visual field"), then allowing eye motion ("eye motion visual field") without glasses, then with "thin sunglasses," followed by "thick sunglasses." Visual field surface area differences greater than the 14% reproducibility error of the method and having a p < 0.05 were considered significant. A glare test was done using a surgical lighting system pointed at the eye(s) at different incidence angles. No significant "base visual field" or "eye motion visual field" surface area variations were noted when comparing tests done without glasses and with the "thin sunglasses." In contrast, a 22% "eye motion visual field" surface area decrease (p < 0.001) was noted when comparing tests done without glasses and with "thick sunglasses." This decrease was most severe in the temporal quadrant (-33%; p < 0.001). All subjects reported less lateral glare with the "thick sunglasses" than with the "thin sunglasses" (p < 0.001). The better protection from lateral glare offered by "thick sunglasses" is offset by the much poorer ability to use lateral space exploration; this results in a loss of most, if not all, of the additional visual field gained through eye motion.

Unique morphology of the human orbit among the Hominoidea

AbstractPurpose Like many mammal predators, Humans have frontal (forward‐facing) orbits. This design allows a large overlap of monocular visual fields with good stereoscopic vision but is considered to harm lateral space perception. In Humans, on average, temporal visual field extends 95° in primary position of gaze but 128° with eye abduction. Which anatomical peculiarity may allow such a visual field expansion?Methods Comparative orbit osteology study in 100 human skulls and 120 Apes’ skulls (30 gibbons; 30 orang‐utans; 30 gorillas; 30 chimpanzees and bonobos). Orbit width and height were recorded. Using a protractor and laser levels two orbit angles were recorded: “convergence angle” (the lower this angle, the more frontal the inner orbital rim orientation) and “opening angle” (the higher this angle, the more backward the temporal orbital rim position).Results The largest orbit width/height orbit ratio is 1.19, in Humans (p &lt; 0.001). Humans have a higher “convergence angle” (98.1°) than all Apes except gibbons (99.2°; p &gt; 0.05). Humans have by far the largest “opening angle” (107.1°; p &lt; 0.001) and the largest difference between “opening angle” and “convergence angle” (9°; p &lt; 0.001).Conclusion The largest orbit width/height ratio found in Humans suggests a design that favours lateral vision. More specifically, human orbital rim is unique in that, while frontal, it has by far the most backward temporal orbital rim. This peculiarity ‐ likely and adaptation to terrestrial life with upright bipedal locomotion ‐ allows both good stereoscopic vision and large temporal visual field extent through eye motion.

Eye motion increases temporal visual field extent

To study the impact of eye motion on visual field extent. Visual fields were tested in 15 healthy volunteers with the Goldmann perimeter using a V4 test object, from seen to unseen, first in primary position of gaze, then allowing eye motion. Temporal points falling out of the cupola were tested again after a controlled nasal head rotation using a headband prototype fitted with a line-laser level having two orthogonal vial levels. Visual field surface areas (cm(2) ) were calculated as projections on a 30-cm virtual Goldmann cupola, whose extent would have been large enough to include all points. Reproducibility error of the method assessed by calculation of the relative difference between surface areas of 12 visual field tests and 12 visual field retests was estimated at 14%. Hertel exophthalmometry was recorded to study the influence of globe position on visual field extent. Binocular visual field surface area increased by 37% with eye motion (p-value=1.20·10(-9) ). This increase was highest (46%; p-value=1.2·10(-24) ) in the temporal quadrant. Median maximal visual field temporal eccentricity with eye motion was 128.3° (minimum: 109.5°; maximum: 137.7°) and more than 135° in four eyes of three subjects. Hertel exophthalmometry was positively linked to visual field temporal surface area with eye motion (p-value=0.013). Eye motion greatly expands the temporal visual field. This peculiarity is likely an adaptation to terrestrial life with upright bipedal locomotion and may save head movements through horizontal eyeball scanning.

Effect of Suturing the Silicone Rod to the Tarsal Plate and the Suture Material Used on Success of Frontalis Suspension Surgery

To evaluate the effect of suturing the silicone rod to the tarsal plate and the suture material used in suturing on surgical success of frontalis suspension for congenital ptosis. Eighty eyes of 56 consecutive patients who underwent frontalis suspension surgery using silicone rods for congenital ptosis between 2005 and 2011 were included in this retrospective study. The surgical results of eyes were compared, in which silicone had not been sutured to the tarsal plate (group 1), with those sutured with monofilament polypropylene suture (group 2), and those sutured with polybutylate-coated braided polyester suture (group 3). Surgical success indicated that the postoperative eyelid position was maintained within 1 mm of the normal eyelid position, that is, 1 mm below the superior limbus in bilateral cases and the same level with the other eyelid in unilateral cases, in the primary position of gaze. Postoperative eyelid level beyond these limits was defined as surgical failure. The mean follow up was 16.3 months (6-38 months) and the mean age was 5.48 years (1-24 years). Thirty were male and 26 were female patients. Surgical failure occurred in 9 of 18 (50%) cases in group 1, in 5 of 24 (20.8%) cases in group 2, and in 5 of 38 (13%) cases in group 3 during the follow-up period after adequate eyelid elevation had been achieved just after the operation. The difference between surgical success rates in group 1 and group 2 (p = 0.047) and group 1 and group 3 (p = 0.003) was statistically significant. However, the difference between group 2 and group 3 did not reach statistical significance. Suturing the silicone rod to the tarsal plate increases the surgical success rates of frontalis suspension surgery for congenital ptosis. Suturing with polybutylate-coated braided polyester, even though statistically insignificant, seems to be more effective than suturing with monofilament polypropylene.

The chameleon syndrome: acute convergence paralysis

A 78-year-old female patient without vascular risk factors presented with sudden left eye deviation at awakening. Neither diplopia nor other symptoms were reported. On admission, left eye exotropia at primary...

Síndrome de webino secundario a sífilis meningovascular. Una entidad excepcional con una etiología inesperada

Case reportThe patient is a 57-year-old obese and hypertensive male. His chief complaints were double vision and dizziness, with mild exodeviation in both eyes in primary gaze position in the ocular motility examination, but more predominant in the left eye. The exotropia was noticeably more evident on the attempted upgaze. On horizontal gaze, the abducting eye deviated fully, but the adducting eye did not cross the midline. Nystagmus in the abducting eye and convergence impairment were found. Pupil size and testing were normal. Ataxia and areflexia were also present. Bilateral internuclear ophthalmoplegia was suspected and imaging and laboratory tests were performed. The CAT scan showed a right occipital hypo-attenuated lesion. In the MRI scan, a mesencephalic subacute ischemic lesion was found, involving the medial rectus sub-nuclei. Blood and cerebrospinal fluid test for syphilis were positive. DiscussionBilateral internuclear ophthalmoplegia is a very uncommon -and difficult to diagnose- condition. In the reported case the lesion involved the medial rectus sub-nuclei. This fact could explain the exotropia in the primary gaze position, and supports that is not possible to exclude the involvement of the medial rectus sub-nuclei in the webino syndrome. The rapid identification of the pathology contributed to the better prognosis of the patient.

Comparison of EyeCam and anterior segment optical coherence tomography in detecting angle closure

To compare the diagnostic performance of EyeCam (Clarity Medical Systems, Pleasanton, CA, USA) and anterior segment optical coherence tomography (ASOCT, Visante; Carl Zeiss Meditec, Dublin, CA, USA) in detecting angle closure, using gonioscopy as the reference standard. Ninety-eight phakic patients, recruited from a glaucoma clinic, underwent gonioscopy by a single examiner, and EyeCam and ASOCT imaging by another examiner. Another observer, masked to gonioscopy findings, graded EyeCam and ASOCT images. For both gonioscopy and EyeCam, a closed angle in a particular quadrant was defined if the posterior trabecular meshwork was not visible. For ASOCT, angle closure was defined by any contact between the iris and angle anterior to the scleral spur. An eye was diagnosed as having angle closure if ≥2 quadrants were closed. Agreement and area under the receiver operating characteristic curves (AUC) were evaluated. The majority of subjects were Chinese (69/98, 70.4%) with a mean age of 60.6 years. Angle closure was diagnosed in 39/98 (39.8%) eyes with gonioscopy, 40/98 (40.8%) with EyeCam and 56/97 (57.7%) with ASOCT. The agreement (kappa statistic) for angle closure diagnosis for gonioscopy versus EyeCam was 0.89; gonioscopy versus ASOCT and EyeCam versus ASOCT were both 0.56. The AUC for detecting eyes with gonioscopic angle closure with EyeCam was 0.978 (95% CI: 0.93-1.0) and 0.847 (95% CI: 0.76-0.92, p < 0.01) for ASOCT. The diagnostic performance of EyeCam was better than ASOCT in detecting angle closure when gonioscopic grading was used as the reference standard. The agreement between the two imaging modalities was moderate.

Eye motion increases temporal visual field extent

AbstractPurpose To study the impact of eye motion on visual field extent.Methods Visual fields were tested in 15 healthy volunteers with the Goldmann perimeter using a V4 test‐object, from seen to unseen, first in primary position of gaze, then allowing eye motion. Temporal points falling out of the cupola were tested again after a controlled nasal head rotation using a headband prototype fitted with a line‐laser level having two orthogonal vial levels. Visual field surface areas (cm2) were calculated as projections on a 30 cm virtual Goldmann cupola whose extent would have been large enough to include all points. Reproducibility error of the method assessed by calculation of the relative difference between surface areas of 12 visual field tests and 12 visual field retests was estimated at 14%. Hertel exophthalmometry was recorded to study the influence of globe position on visual field extent.Results Binocular visual field surface area increased by 37% with eye motion (p‐value = 1.20.10‐9). This increase was highest (46%; p‐value = 1.2.10‐24) in the temporal quadrant. Median maximal visual field temporal excentration with eye motion was 128.3° (minimum: 109.5°; maximum: 137.7°) and more than 135° in 4 eyes of three subjects. Hertel exophthalmometry was positively linked to visual field temporal surface area with eye motion (p‐value = 0.013).Conclusion Eye motion greatly expands the temporal visual field. This peculiarity is likely an adaptation to terrestrial life with upright bipedal locomotion, and may save head movements through horizontal eyeball scanning.

Sunglasses with wide temples and thick frame severely constrict temporal visual field extent

AbstractPurpose To compare the impact of two types of sunglasses on visual field and glare: one (“thick sunglasses”) with a thick plastic frame and wide temples, and one (“thin sunglasses”) with a thin metal frame and thin temples.Methods Using the Goldmann perimeter, visual field surfaces (cm2) were calculated as projections on a 30 cm virtual cupola. A V4 test object was used, from seen to unseen, in 15 healthy volunteers in the primary position of gaze (“base visual field”), then allowing eye motion (“eye motion visual field”) without glasses, then with “thin sunglasses”, followed by “thick sunglasses”. Visual field surface area differences superior to the 14% reproducibility error of the method and having a p‐value inferior to 5% were considered significant. A glare test was done using a surgical lighting system pointed at the eye(s) at different incidence angles.Results No significant “base visual field” nor “eye motion visual field” surface area variation were noted comparing tests done without glasses and with the “thin sunglasses”. In contrast, a 22% “eye motion visual field” surface area decrease (p‐value = 2.7.10‐13) was noted comparing tests done without glasses and with “thick sunglasses”. This decrease was most severe in the temporal quadrant (‐33%; p‐value = 6.3.10‐20). All subjects reported less lateral glare with the “thick sunglasses” than with the “thin sunglasses” (p‐value = 6.10‐5).Conclusion The better protection from lateral glare offered by “thick sunglasses” is offset by a severe temporal “eye motion visual field” surface area constriction.

Unilateral rectus muscle recession in the treatment of Duane syndrome

To determine the outcome of unilateral single horizontal rectus recession surgery in patients with Duane retraction syndrome. A retrospective review was conducted of 27 medical records of patients with Duane syndrome who underwent unilateral medial rectus recession for esodeviations and lateral rectus recession for exodeviations. The criteria for evaluating surgical success included improvement of abnormal head position, reduction of strabismus deviation, and preservation or improvement of stereoacuity. Patients had an average recession of 6.3 mm and were followed for an average of 4 years. Of the 27 patients treated with unilateral horizontal rectus muscle recession, 93% had a postoperative head turn of <10° in primary position and 85% an ocular deviation of <10(Δ). An excellent outcome, defined as a combination of stereoacuity of <100 arcsec, head turn <10°, and strabismus of <10(Δ), was achieved by 63% of patients. Unilateral single horizontal rectus muscle recession improves or eliminates head turn, corrects strabismus in primary position of gaze, and maintains good stereoacuity in a majority of patients with Duane syndrome.

Induced prismatic effects due to poorly fitting spectacle frames

To provide patients with the best vision possible practitioners undertake comprehensive visual examinations and optical dispensing procedures. Ensuring proper frame alignment is an important part of the spectacle dispensing process. Ideally,for maximum optical benefit, patients should look through the optical centers of their lenses. Rays entering a lens at any point other than the optical center will be deviated as long as the lens has power, thus inducing prismatic effects. These prismatic effects may lead to visual complaints such as asthenopia,blurry vision and headaches. The aim of the study was to investigate the prevalence of induced prismatic effects due to poorly fitting spectacle frames. A sample of 100 spectacle wearing subjects was selected as participants. Questionnaires were completed and any symptoms experienced recorded. The habitual spectacle position in the primary position of gaze was noted, pupil and optical centers marked and where relevant, induced prism was calculated. Data was analysed using the ANOVA Kruskal-Wallis and Mann-Whitney tests. On initial observation of frame position, about45% of subjects were wearing their spectacles incorrectly. A comparison of the marked pupil centers in primary gaze and optical centers revealed that 100% of participants were found to not be looking through the optical centers of their lenses. Fifty one percent were within horizontal and 3.12% within vertical ANSI tolerances. Symp-toms were reported by 50% of subjects experiencing base-out, 0% base-in and 47% vertical induced prismatic effects. However, no correlation existed between those subjects experiencing induced prism and symptoms reported. Prism adaptation may account for some participants being asymptomatic. Sixty three percent of subjects were not aware of the importance of properly fitted spectacle frames. Proper optical dispensing with associated patient education is necessary to achieve optimal optical benefits of spectacles and careful attention should be given to this aspect by practitioners. It is recommended that patients return periodically to their optometrists to have the frame alignment assessed and the fit modified if necessary. (S Afr Optom 2011 70(4) 168-174)

Electrophysiological recordings in humans reveal reduced location-specific attentional-shift activity prior to recentering saccades

Being able to effectively explore the visual world is of fundamental importance, and it has been suggested that the straight-ahead gaze position within the egocentric reference frame ("primary position") might play a special role in this context. In the present study we employed human electroencephalography (EEG) to examine neural activity related to the spatial guidance of saccadic eye movements. Moreover, we sought to investigate whether such activity would be modulated by the spatial relation of saccade direction to the primary gaze position (recentering saccades). Participants executed endogenously cued saccades between five equidistant locations along the horizontal meridian. This design allowed for the comparison of isoamplitude saccades from the same starting position that were oriented either toward the primary position (centripetal) or further away from it (centrifugal). By back-averaging time-locked to the saccade onset on each trial, we identified a parietally distributed, negative-polarity EEG deflection contralateral to the direction of the upcoming saccade. Importantly, this contralateral presaccadic negativity, which appeared to reflect the location-specific attentional guidance of the eye movement, was attenuated for recentering saccades relative to isoamplitude centrifugal saccades. This differential electrophysiological signature was paralleled by faster saccadic reaction times and was substantially more apparent when time-locking the data to the onset of the saccade rather than to the onset of the cue, suggesting a tight temporal association with saccade initiation. The diminished level of this presaccadic component for recentering saccades may reflect the preferential coding of the straight-ahead gaze position, in which both the eye-centered and head-centered reference frames are perfectly aligned and from which the visual world can be effectively explored.