The influence of retinal image motion on the perceptual grouping of temporally asynchronous stimuli.

Briefly presented stimuli can reveal the lower limit of retinal-based perceptual stabilization mechanisms. This is demonstrated in perceptual grouping of temporally asynchronous stimuli, in which alternate row or column elements of a regular grid are presented over two successive display frames with an imperceptible temporal offset. The grouping phenomenon results from a subtle shift between alternate grid elements due to incomplete compensation of small, fixational eye movements occurring between the two presentation frames. This suggests that larger retinal shifts should amplify the introduced shifts between alternate grid elements and improve grouping performance. However, large shifts are necessarily absent in small eye movements. Furthermore, shifts follow a random walk, making the relationship between shift magnitude and performance difficult to explore systematically. Here, we established a systematic relationship between retinal image motion and perceptual grouping by presenting alternate grid elements (untracked) during smooth pursuit of known velocities. Our results show grouping performance to improve in direct proportion to pursuit velocity. Any potential compensation by extraretinal signals (e.g., efference copy) does not seem to occur.

Can Home Monitoring Allow Earlier Detection of Rapid Visual Field Progression in Glaucoma?

PURPOSE: Recent developments in electronic technology are making it possible to home monitor the sensitivity of the central visual field using portable devices. We used simulations to investigate whether the higher test frequency afforded by home monitoring improves the early detection of rapid visual field loss in glaucoma and how any benefits might be affected by imperfect compliance or increased variability in the home-monitoring test. DESIGN: Computer simulation, with parameter selection confirmed with a cohort study. PARTICIPANTS: A total of 43 patients with treated glaucoma (both open-angle and closed-angle), ocular hypertension or glaucoma suspects (mean age, 71 years; range, 37-89 years), were followed in the cohort study. METHODS: We simulated series (n = 100 000) of visual fields for patients with stable glaucoma and patients with progressing glaucoma for 2 in-clinic (yearly and 6-monthly) and 3 home-monitoring (monthly, fortnightly, and weekly) schedules, each running over a 5-year period. Various percentages of home-monitored fields were omitted at random to simulate reduced compliance, and the variability of the home monitored fields also was manipulated. We used previously published variability characteristics for perimetry and confirmed their appropriateness for a home-monitoring device by measuring the device's retest variability at 2 months in a cohort of 43 patients. The criterion for flagging progression in our simulation was a significant slope of the ordinary least squares regression of a simulated patient's mean deviation (MD) data. MAIN OUTCOME MEASURES: The sensitivity for identifying rapid visual field loss (-2 decibels [dB]/year loss of MD). RESULTS: Although a sensitivity of 0.8 for rapid field loss was achieved after 2.5 years of 6-monthly testing in the clinic, weekly home monitoring achieved this by 0.9 years despite moderate test compliance of 63%. The improved performance of weekly home monitoring over 6-monthly clinical testing was retained even when home monitoring was assumed to produce more variable test results or be associated with low patient compliance. CONCLUSIONS: Detecting rapid visual field progression may be improved using a home-monitoring strategy, even when compliance is imperfect. The cost-benefit of such an approach is yet to be demonstrated, however.

Adaptive optics imaging in inherited retinal diseases: A scoping review of the clinical literature.

Adaptive optics (AO) imaging enables direct, objective assessments of retinal cells. Applications of AO show great promise in advancing our understanding of the etiology of inherited retinal disease (IRDs) and discovering new imaging biomarkers. This scoping review systematically identifies and summarizes clinical studies evaluating AO imaging in IRDs. Ovid MEDLINE and EMBASE were searched on February 6, 2023. Studies describing AO imaging in monogenic IRDs were included. Study screening and data extraction were performed by 2 reviewers independently. This review presents (1) a broad overview of the dominant areas of research; (2) a summary of IRD characteristics revealed by AO imaging; and (3) a discussion of methodological considerations relating to AO imaging in IRDs. From 140 studies with AO outcomes, including 2 following subretinal gene therapy treatments, 75% included fewer than 10 participants with AO imaging data. Of 100 studies that included participants' genetic diagnoses, the most common IRD genes with AO outcomes are CNGA3, CNGB3, CHM, USH2A, and ABCA4. Confocal reflectance AO scanning laser ophthalmoscopy was the most reported imaging modality, followed by flood-illuminated AO and split-detector AO. The most common outcome was cone density, reported quantitatively in 56% of studies. Future research areas include guidelines to reduce variability in the reporting of AO methodology and a focus on functional AO techniques to guide the development of therapeutic interventions.

The influence of perceptual stabilisation on perceptual grouping of temporally asynchronous stimuli.

Even during fixation, our eyes constantly make small, involuntary eye movements that cause the retinal image to be swept across our retinae. Despite this, our world appears completely stable, due to powerful perceptual stabilisation mechanisms. Whether these mechanisms are of functional consequence for visual performance remains largely unexplored, however. We directly tested this by using a perceptual grouping task, where physically aligned alternate grid elements were presented with an imperceptible temporal offset. Observers' abilities to reliably group the grid into rows (or columns) is posited to arise from the failure in compensation of retinal slip arising from the small eye movements that occur during the temporal offset, effectively introducing a spatial shift in the arrangement of grid elements. We incorporated this perceptual grouping task within the on-line jitter illusion, which temporarily disables perceptual stabilisation mechanisms through a 10 Hz flickering annulus of random noise (Vision Res 43 (2003) 957-969). Observers' abilities to correctly group the grid stimulus were measured with and without perceptual stabilisation mechanisms engaged (i.e. non-flickering vs. flickering annulus). Grouping performance was better when eye movements were perceived, suggesting that the influence of retinal slip is increased when perceptual stabilisation mechanisms are disabled. We therefore find that perceptual stabilisation can measurably influence visual function, in addition to its perceptual effects.

Reactivity in the human retinal microvasculature measured during acute gas breathing provocations.

Although changes in vessel diameter following gas perturbation have been documented in retinal arterioles and venules, these responses have yet to be quantified in the smallest vessels of the human retina. Here, using in vivo adaptive optics, we imaged 3-25 µm diameter vessels of the human inner retinal circulation and monitored the effects of altered gas-breathing conditions. During isocapnic hyperoxia, definite constrictions were seen in 51% of vessel segments (mean ± SD for pre-capillary arterioles -9.5 ± 3.0%; capillaries -11.8 ± 3.3%; post-capillary venules -6.3 ± 2.8%); these are comparable with responses previously reported in larger vessels. During isoxic hypercapnia, definite dilations were seen in 47% of vessel segments (mean ± SD for pre-capillary arterioles +9.8 ± 1.5%; capillaries +13.7 ± 3.8%; post-capillary venules +7.5 ± 4.2%); these are proportionally greater than responses previously reported in larger vessels. The magnitude of these proportional changes implies that the capillary beds themselves play an important role in the retinal response to changes in carbon dioxide levels. Interestingly, the distribution of microvascular responses shown here differs from our previously reported responses to flicker stimulation, suggesting differences in the way blood supply is coordinated following gas perturbation and altered neural activity.

Masking of random-walk motion by flicker, and its role in the allocation of motion in the on-line jitter illusion.

Typically, perceptual stabilization mechanisms make us unaware of the retinal image motion produced by the small, involuntary eye movements our eyes constantly make during fixation. The breakdown of perceptual stability is demonstrated by the on-line jitter illusion, in which a circular static pattern appears to jitter coherently when surrounded by a flickering annular pattern. Although both regions of the stimulus are subject to retinal motion from eye movements, the visual system attributes this motion to the central static region in the form of visual jitter, while the surrounding flickering region remains perceptually stable. We investigated factors influencing this allocation of motion and reference frame in the on-line jitter illusion. The flickering of the surround was found to impair the detection of simultaneous random-walk motion in this area, giving a detection reliability of around 80% for motion approximating that from fixational eye movements. Changes to spatial texture and location of flicker (centre vs. surrounding annulus) had little effect on the final percept. However, use of a nonconcentric stimulus resulted in a marked reduction in apparent jitter in all subjects. Our results suggest for the on-line jitter illusion, allocation of motion and reference frame is influenced by the general principle that, if one region surrounds another, the surrounding region tends to be allocated as the frame of reference. When this factor is controlled for, spatial textures, location of flicker, and the masking of motion by flicker have a smaller but measurable influence on the final percept.

Evidence of Flicker-Induced Functional Hyperaemia in the Smallest Vessels of the Human Retinal Blood Supply.

Regional changes in blood flow are initiated within neural tissue to help fuel local differences in neural activity. Classically, this response was thought to arise only in larger arterioles and venules. However, recently, it has been proposed that a) the smallest vessels of the circulation make a comparable contribution, and b) the response should be localised intermittently along such vessels, due to the known distribution of contractile mural cells. To assess these hypotheses in human neural tissue in vivo, we imaged the retinal microvasculature (diameters 3-28 µm) non-invasively, using adaptive optics, before and after delivery of focal (360 µm) patches of flickering visible light. Our results demonstrated a definite average response in 35% of all vessel segments analysed. In these responding vessels, the magnitude of proportional dilation (mean ± SEM for pre-capillary arterioles 13 ± 5%, capillaries 31 ± 8%, and post-capillary venules 10 ± 3%) is generally far greater than the magnitudes we and others have measured in the larger retinal vessels, supporting proposition a) above. The dilations observed in venules were unexpected based on previous animal work, and may be attributed either to differences in stimulus or species. Response heterogeneity across the network was high; responses were also heterogeneous along individual vessels (45% of vessel segments showed demonstrable locality in their response). These observations support proposition b) above. We also observed a definite average constriction across 7% of vessel segments (mean ± SEM constriction for capillaries -16 ± 3.2%, and post-capillary venules -18 ± 12%), which paints a picture of dynamic redistribution of flow throughout the smallest vessel networks in the retina in response to local, stimulus-driven metabolic demand.

Quantitative spatial and temporal analysis of fluorescein angiography dynamics in the eye.

PURPOSE: We describe a novel approach to analyze fluorescein angiography to investigate fluorescein flow dynamics in the rat posterior retina as well as identify abnormal areas following laser photocoagulation. METHODS: Experiments were undertaken in adult Long Evans rats. Using a rodent retinal camera, videos were acquired at 30 frames per second for 30 seconds following intravenous introduction of sodium fluorescein in a group of control animals (n = 14). Videos were image registered and analyzed using principle components analysis across all pixels in the field. This returns fluorescence intensity profiles from which, the half-rise (time to 50% brightness), half-fall (time for 50% decay) back to an offset (plateau level of fluorescence). We applied this analysis to video fluorescein angiography data collected 30 minutes following laser photocoagulation in a separate group of rats (n = 7). RESULTS: Pixel-by-pixel analysis of video angiography clearly delineates differences in the temporal profiles of arteries, veins and capillaries in the posterior retina. We find no difference in half-rise, half-fall or offset amongst the four quadrants (inferior, nasal, superior, temporal). We also found little difference with eccentricity. By expressing the parameters at each pixel as a function of the number of standard deviation from the average of the entire field, we could clearly identify the spatial extent of the laser injury. CONCLUSIONS: This simple registration and analysis provides a way to monitor the size of vascular injury, to highlight areas of subtle vascular leakage and to quantify vascular dynamics not possible using current fluorescein angiography approaches. This can be applied in both laboratory and clinical settings for in vivo dynamic fluorescent imaging of vasculature.