Associate Professor Sally McFadden

© 2017 The Authors. PURPOSE. It has been proposed that the peripheral retina, responding to local optical defocus, contributes to myopia and associated altered eye growth in humans. To test this hypothesis, we measured the changes in central (on-axis) and peripheral ocular dimensions in guinea pigs wearing a concentric bifocal spectacle lens design with power restricted to the periphery. METHODS. Five groups of guinea pigs (n = 83) wore either a unifocal (UF) spectacle lens (-4, 0, or +4 Diopters [D]), or a peripheral defocus (PF) spectacle lens that had a plano center (diameter of 5 mm) with either -4 or +4 D in the surround (-4/0 or +4/0 D). The overall optical diameter of all lenses was 12 mm. Lenses were worn over one eye from 8 to 18 days of age for negative and plano lenses, or from 8 to 22 days of age for positive lenses. Refractive error was measured centrally and 30° off-axis in the temporal and nasal retina. The shape of the eye was analyzed from images of sectioned eyes. RESULTS. Lenses of -4 D UF induced myopia, reflecting enhanced ocular elongation, which was centered on the optic nerve head and included the surrounding peripapillary zone (PPZ, 188 in diameter). Some ocular expansion, including within the PPZ, also was recorded with -4/0 and +4/0 D PF lenses while the +4 D UF lens inhibited rather than enhanced elongation, centrally and peripherally. CONCLUSIONS. Peripheral defocus-induced ocular expansion encompasses the PPZ, irrespective of the sign of the inducing defocus. Understanding the underlying mechanism potentially has important implications for designing multifocal lenses for controlling myopia in humans and also potentially for understanding the link between myopia and glaucoma.

© 2017 Optical Society of America. Custom Spectral Optical Coherence Tomography (SOCT) provided with automatic quantification and distortion correction algorithms was used to measure the 3-D morphology in guinea pig eyes (n = 8, 30 days; n = 5, 40 days). Animals were measured awake in vivo under cyclopegia. Measurements showed low intraocular variability ( < 4% in corneal and anterior lens radii and < 8% in the posterior lens radii, < 1% interocular distances). The repeatability of the surface elevation was less than 2 µm. Surface astigmatism was the individual dominant term in all surfaces. Higher-order RMS surface elevation was largest in the posterior lens. Individual surface elevation Zernike terms correlated significantly across corneal and anterior lens surfaces. Higher-order-aberrations (except spherical aberration) were comparable with those predicted by OCT-based eye models.

© 2014 The Association for Research in Vision and Ophthalmology, Inc. METHODS. To induce accelerated growth and myopia, guinea pigs wore a -5 diopter (D) lens over one eye from 4 to 11 days of age. To induce inhibited growth, the lens was removed after 7 days of -5 D lens wear, and the eye allowed to recover from myopia for 3 days. Ocular parameters and Egr-1 mRNA levels were subsequently asse ssed, and compared to untreated fellow eyes and eyes from untreated littermates. Possible circadian changes in Egr-1 mRNA levels were also determined in 18 additional animals by taking measures every 4 hours during a 24-hour cycle. PURPOSE. The immediate early gene Egr-1 is thought to form part of the pathway that mediates abnormal ocular growth. This study investigated whether the mRNA expression levels of Egr- 1 in a mammalian retina are modulated differentially, depending on the direction of ocular growth. RESULTS. Ocular compensation to a -5 D lens occurred after 7 days (D ¿ -4.8 D, D ¿ + 147 lm growth, N = 20). In 5 highly myopic eyes (¿ -7.4 D), Egr-1 mRNA levels in the retina were significantly downregulated relative to contralateral control (51%) and age-matched untreated (47%) eyes. Three days after the -5 D lens was removed, eyes had recovered from the myopia (¿ -0.5 D, relative change of + 2.9 D, N = 4) and Egr-1 mRNA levels were significantly elevated relative to contralateral (212%) and untreated (234%) eyes, respectively. Normal Egr- 1 mRNA expression was higher in the middle of the day than in the middle of the night. Immunolabeling showed strong Egr-1 reactivity in cell bodies in the inner nuclear and ganglion cell layers. CONCLUSIONS. Egr-1 mRNA levels in a mammalian retina show a bi-directional persistent response to opposing ocular growth stimuli. This suggests retinal Egr-1 might act as a signal for the direction of ocular growth in different species.

Many lateral eyed birds are bifoveate or have dual retinal specialisations corresponding with their lateral and frontal viewing modes. For example, the eye design in the pigeon, which is typical of many granivorous birds, incorporates an optic axis for lateral distant viewing and a frontal area in the temporal retina associated with close binocular viewing. In this paper we ask how these two separate visual axis are used to focus on objects, not only at different eccentricities, but also at different absolute distances. Using simultaneous infrared (IR) photoretinoscopy and IR keratometry, we find that the pigeons optical system can indeed simultaneously support differential refractive states on the lateral and frontal axis. Maps of the refractive error (RE) and corneal power in cyclopleged and anaesthetized pigeons, reveal small but antagonistic gradients in different sectors of the visual field. In particular, we find that at rest, the frontal field is relatively myopic and yet the cornea has less power in this sector. We studied how these RE and corneal power maps actively vary during a reaching behaviour. We found that during the saccadic like peck response, the optic axis is not focussed on the grain and has only 30% of the required accommodation power. In contrast, we calculate that the frontal axis does have adequate power to focus on the grain. Furthermore, at the last stationary head fixation position (which occurs at an eye-grain distance of 67mm, SD=3.9mm) before the final descent when the eyes begin to close, accommodation on the frontal axis can be supported wholly by the cornea, which allows dramatic RE changes (-13D, SD=7.7) without unduly effecting the refractive error on the optic axis (-2D, SD=1.7). We propose that the shape of the cornea facilitates this local accommodation mechanism and that this dual visual system can be simultaneously focussed at disparate distances in space.

The amplitude of saccadic eye movements is adjusted by the oculomotor system. If a target is surreptitiously moved during saccades so that the eye lands beyond the target, a gradual decrease in the saccadic gain (saccade-size divided by target-displacement) ensues. What error signals guide this adaptation? We propose that visual attention helps keep track of the target across saccades, so that saccade accuracy can be determined by comparing the location of the fovea with the locus of attention. If attention plays this role, saccadic errors might be more apparent when the size of the attentional field is small than when it is large. We tested this by a conventional saccade adaptation paradigm in which the target stepped 8Ã 0 and then stepped back by 30% during each saccade, while the subject did one of two psychophysical tasks demanding either a small or large attentional field. Specifically, the saccade target consisted of a large outer (8.5°) and a small inner (0.8°) ring, each with several breaks, rotating in opposite directions. After each saccade, the number of breaks transiently changed. Subjects reported the number of breaks present during this period in the attended ring. Saccadic gain was assessed both during adaptation and by comparing blocks of saccades before and after 250 adaptation trials using a spot target that disappeared upon saccade onset. After adaptation, saccadic gain had decreased four times as much if subjects attended to the small ring than to the large (small ring: -0.11, SD, 0.031; large ring: -0.027, SD, 0.038, paired t-test, p < 0.01). Also, 4 of the 5 subjects had significantly negative gain slopes during the adaptation when attending to the small ring but none did when attending to the large ring. These findings argue that attention plays an important role in the adjustment of saccadic gain and suggest that the spatial scale of attention influences whether saccades are deemed to be accurate or in need of correction.

© OSA 2016.We quantified anterior-segment geometry in both control and lens-treated eyes of a guinea pig model in vivo, using custom-developed optical coherence tomography. Myopic eyes showed longer axial-lengths, thinner corneas, longer anterior-chamber-depth and steeper anterior lens.

© OSA 2016.Myopia is induced when growing eyes are exposed to hyperopic defocus or reduced vision. The associated changes and factors that influence the development of myopia in the guinea pig eye are described.