Peripheral Acuity Research Articles

Do current standards of primary care of diabetes meet with guideline recommendations in Trinidad, West Indies?

Aims Primary care management of diabetes was examined using the Caribbean Health Research Council (CHRC) guidelines. Methods We retrospectively examined a cross-section of 646 type 2 people with diabetics over 12 months with 1st visit between 1997 and 2005. Results There were more women (65.8%) than men (34.2%) with age range between 29 and 89 years. Blood pressure and weight were evaluated in >95% of patients at each centre. Waist circumference and BMI were not measured at any time and HbA 1c was infrequently measured (1.6–7%) over the 12 months. Information on family history (87.5%), smoking and alcohol (78.1%), exercise (21.4%), socioeconomic status (19.4%) and education (0.3%), and fasting blood sugar (97.2%), lipid profile (51.8%) and serum creatinine (37.9%) were assessed at the 1st visit. At follow-up patients were advised on treatment compliance (47.2%), diet (34.2%), exercise (18.5%) and rarely on home monitoring of blood glucose (0.3%). Peripheral sensations, pedal pulses (6%), visual acuity (3.3%), fundoscopy (12.1%) and ECG (3.9%) were scarcely examined at the annual visit. Conclusions Current management of diabetes in primary care in Trinidad falls short of Caribbean guideline recommendations. The CHRC and Ministry of Health should jointly educate caregivers of diabetes to implement the guidelines, with annual audits to identify shortfalls in management.

Spatial and temporal limits of motion perception across variations in speed, eccentricity, and low vision

We evaluated spatial displacement and temporal duration thresholds for discriminating the motion direction of gratings for a broad range of speeds (0.06 degrees/s to 30 degrees/s) in fovea and at +/-30 degrees eccentricity. In general, increased speed yielded lower duration thresholds but higher displacement thresholds. In most conditions, these effects of speed were comparable in fovea and periphery, yielding relatively similar thresholds not correlated with decreased peripheral acuity. The noteworthy exceptions were interactive effects at slow speeds: (1) Displacement thresholds for peripheral motion were affected by acuity limits for speeds below 0.5 degrees/s. (2) Low-vision observers with congenital nystagmus had elevated thresholds for peripheral motion and slow foveal motion but resembled typically sighted observers for foveal motions at speeds above 1 degree/s. (3) Suppressive center-surround interactions were absent below 0.5 degrees/s and their strength increased with speed. Overall, these results indicate qualitatively different sensitivities to slow and fast motions. Thresholds for very slow motion are limited by spatial resolution, while thresholds for fast motion are probably limited by temporal resolution.

The effect of retinal image slip on peripheral visual acuity

Retinal image slip promoted by fixational eye movements prevents image fading in central vision. However, in the periphery a higher amount of movement is necessary to prevent this fading. We assessed the effect of different levels of retinal image slip in peripheral vision by measuring peripheral visual acuity (VA), with and without crowding, while modulating retinal image slip by using gaze-linked stimuli. Measurements were carried out at four isoeccentric positions at 5 and at 10 degrees eccentricity. Gaze position was monitored throughout using an infrared eyetracker. The target was presented for up to 500 msec, either with no retinal image slip, with reduced retinal slip, or with increased retinal image slip. Without crowding, peripheral visual acuity improved with increased retinal image slip compared with the other two conditions. In contrast to the previous result, under crowded conditions, peripheral visual acuity decreased markedly with increased retinal image slip. Therefore, the effects of increased retinal image slip are different for simple (noncrowded) and more complex (crowded) visual tasks. These results provide further evidence for the importance of fixation stability on complex visual tasks when using the peripheral retina.

New equally readable charts based on anisotropy of peripheral visual acuity1

Abstract: Anstis’ equally readable chart for visual acuity has been widely quoted in textbooks on visual perception. However, this chart does not reflect the anisotropy of peripheral visual acuity that has been reported by previous studies. Here, the authors reexamined peripheral visual acuity by measuring resolution thresholds for Landolt rings and recognition thresholds for hiragana letters as a function of retinal eccentricity across two principal retinal meridians: horizontal and vertical. Observers were required to identify the orientation of a gap in the Landolt ring or recognize a hiragana letter while the stimulus was moved slowly from the periphery toward the fovea across each of four meridians: nasal, temporal, superior, and inferior. The results revealed the horizontal‐vertical anisotropy of visual acuity in accordance with the results of previous studies. The mean thresholds obtained in the vertical meridian were approximately 1.51‐fold (for Landolt rings) and 1.42‐fold (for hiragana letters) higher than those obtained in the horizontal meridian at the same retinal eccentricity. The authors propose new equally readable charts for Landolt rings and hiragana letters.

Foveal and extra-foveal orientation discrimination

Performance can often be made equal across the visual field by scaling peripherally presented stimuli according to F = 1 + E/E (2) where E (2) is the eccentricity at which stimulus size must double to maintain foveal performance levels. Sally and Gurnsey (Vision Res 43:1375-1385, 2003 and Vision Res 44:2719-2727, 2004) have previously shown that estimates of E (2) for orientation discrimination are significantly larger (i.e., less spatial scaling is required) at stimulus contrasts near detection threshold than at contrasts well above detection threshold. To examine the nature of this effect parametrically we measured orientation discrimination thresholds at 0 degrees and 10 degrees eccentricity for three levels of Michelson contrast (3, 12 and 48%) and three stimulus length-to-width aspect ratios (36.4, 9.1 and 2.3) for a range of line sizes (0.19 degrees -36 degrees visual angle). On average, E (2) values decreased as stimulus contrast decreased, consistent with the previous results of Sally and Gurnsey (Vision Res 43:1375-1385, 2003 and Vision Res 44:2719-2727, 2004). It is proposed that contrast reductions have a proportionally larger effect on small orientation-selective units than large ones and thus produce a greater rightward shift of acuity functions (orientation threshold vs. size) at the fovea than in the periphery. This explains why less spatial scaling is required to equate foveal and peripheral acuity functions at low contrasts than at high contrasts.

Visualizing visual impairments

We developed and evaluated methods to visualize the limitations of visually impaired persons. Three types of simulation were developed: (i) fixation independent, showing which elements remain visible after ample inspection, related to object recognition, (ii) fixation dependent, showing which elements remain visible in the entire visual field while fixating one part of the image, related to search, and (iii) the effect of glare, an important factor of low vision in e.g. cataract patients. In experiments with impaired and unimpaired subjects we determined the extent to which a natural image can be blurred before it can be discriminated from the original, and how this is linked to Landolt-C visual acuity. The relationship between blur threshold and minimum angle of resolution was found to be independent of the cause of reduced vision: visual impairment, reduced contrast, or eccentric viewing. Based on this finding we propose a fixation independent simulation in which the local blur varies with local contrast, linked by the subject's contrast sensitivity function. The fixation dependent simulation type is derived from the peripheral acuity using the relationship between blur threshold and acuity, and perimeter data describing visual field defects. The glare simulation is based on a validated CIE model describing the veiling luminance due to a light source, treating all pixels as independent light sources. These visualizations should give unimpaired persons insight into the problems faced by persons with low vision. The fact that the visualizations are validated by human observer experiments make them suitable for evaluating and adapting designs of architecture, public (road) infrastructure, and products to the needs of the low vision community.

Can retinal eccentricity predict visual acuity and contrast sensitivity at the PRL in AMD patients?

Most patients with age-related macular degeneration (AMD) and bilateral central scotomas adopt a preferred retinal locus (PRL) for eccentric viewing. It is postulated that the optimal PRL position is inferior visual space as it provides a larger uninterrupted visual span for reading and mobility. However, the majority of patients place their PRL to the left of their scotoma in visual field space. The purpose of this study was to investigate which factors determine PRL location and specifically, whether the PRL develops in the area of peripheral retina with best visual acuity (VA) and contrast sensitivity (CS). 24 patients were recruited. They were tested on a scanning laser ophthalmoscope to identify the PRL in their better eye. An eyetracker was used to assess peripheral VA and CS. In 19 patients, the PRL was located near the area of best VA. In only 11 patients, the PRL was located near the area of best CS. Patients had worse VA and CS at their PRLs compared to normal vision subjects. VA was best near the PRL, but this was not the case for CS. We cannot distinguish whether the PRL develops at the area of best VA or if it improves at the PRL through practice.

Bodily Illusions Modulate Tactile Perception

Touch differs from other exteroceptive senses in that the body itself forms part of the tactile percept. Interactions between proprioception and touch provide a powerful way to investigate the implicit body representation underlying touch. Here, we demonstrate that an intrinsic primary quality of a tactile object, for example its size, is directly affected by the perceived size of the body part touching it. We elicited proprioceptive illusions that the left index finger was either elongating or shrinking by vibrating the biceps or triceps tendon of the right arm while subjects grasped the tip of their left index finger. Subjects estimated the distance between two simultaneous tactile contacts on the left finger during tendon vibration. We found that tactile distances feel bigger when the touched body part feels elongated. Control tests showed that the modulation of touch was linked to the perceived index-finger size induced by tendon vibration. Vibrations that did not produce proprioceptive illusion had no effect on touch. Our results show that the perception of tactile objects is referenced to an implicit body representation and that proprioception contributes to this body representation. We also provide, for the first time, a quantitative, implicit measure of distortions of body size.

Retinal stretching limits peripheral visual acuity in myopia

Axial elongation of the myopic eye has the potential to stretch the retina, thereby reducing the sampling density of retinal neurons. Resolution acuity in the peripheral field of normal eyes is known to be sampling-limited, which suggests that retinal stretching in the myopic eye should have a direct effect on resolution acuity everywhere in the visual field except perhaps the fovea, which is usually optically limited. We tested this prediction that neural sampling density is reduced in myopic eyes by measuring resolution acuity for sinusoidal gratings in the fovea plus five peripheral locations in 60 myopic subjects exhibiting a wide range of refractive errors. Control experiments using a detection paradigm to provoke spatial aliasing verified that peripheral resolution was sampling limited. Retinal spatial frequencies of the grating stimulus were computed assuming Knapps’ Law of visual optics, which ensures that retinal image size (in mm) is independent of refractive error when axial myopia is corrected by a spectacle lens located in the anterior focal plane of the eye. Results obtained at every retinal locus showed that resolution acuity declined linearly with magnitude of refractive error. Regression of the population data indicated that approximately 15D of refractive error doubles the spacing between retinal neurons, thereby halving peripheral resolution acuity relative to the emmetropic eye. Several subjects also demonstrated sampling-limited performance in the fovea, which indicated that optical filtering by the eye’s optical system failed to protect the fovea from aliasing artifacts of neural undersampling in these eyes. We conclude that stretching of the retina is a primary cause of reduced spatial resolution of the peripheral field, and occasionally of the fovea, in myopic eyes. Stretching appears to be locally uniform over the central ±15° of visual field but is globally non-uniform since the foveal region appears to stretch more than the globe itself.

"A moment's monument": the central vision of Italian sculptor Medardo Rosso (1858-1928).

One of the basic limitations on visual perception is that it is impossible, in any given moment, to see the world sharply and full of colors beyond the central area of the visual field. This fact was popularized and brought to the attention of artists in the nineteenth century. To accurately represent the 'impression', or vision of a single glance, an artistic work should contain only a central area in focus surrounded by a progressively greater blur. The work of the Italian sculptor Medardo Rosso (1858-1928) may be the first artistic representation of differences in central and peripheral acuity. Despite using the medium of sculpture, typically three-dimensional, Rosso conceived of his art as two-dimensional because in a given moment it is possible to view a scene from only one viewpoint. The analysis of Rosso's photographs of his own sculptures emphasizes the areas of detail and relative blur, allowing a reconstruction of his point de vue unique--where the observer should stand when viewing that specific sculpture. We argue that the role of central and peripheral vision in subjective perception is critical to understanding the work of Rosso, aptly defined by critics as monument d'un instant.

Is peripheral visual acuity susceptible to perceptual learning in the adult?

While it is generally accepted that foveal visual acuity in the adult has reached an optimal value, claims for improvement of peripheral acuity with training in the adult persist in the literature. Practice effects in peripheral hyperacuity have been amply documented. A carefully controlled test is here reported to examine the influence of training on the resolution thresholds for two lines and on Landolt C acuity measurements in the retinal periphery in eight normal adults. It involved 11–30 daily sessions of 300 responses with feedback. In some observers the first day's results were somewhat poorer, but otherwise the threshold curves were essential flat. Yet in the same location vernier acuity could be improved by 50% in six training sessions. Sustained and lasting neural modifications in peripheral vision can take place in stereoscopic, orientation, vernier, bisection and time discriminations, but not in resolution and Landolt C acuities.

Contour interaction in fovea and periphery.

It has been known for some time that both foveal and peripheral visual acuity are higher for single letters than for letters in a row. Early work showed that this was due to the destructive interaction of adjacent contours (termed contour interaction). It has been assumed to have a neural basis, and a number of competing explanations have been advanced that implicate either high-level or low-level stages of visual processing. Our previous results for foveal vision suggested a much simpler explanation, one determined primarily by the physics of the stimulus rather than the physiology of the visual system. We show that, under conditions of contour interaction or crowding, the most relevant physical spatial-frequency band of the letter is displaced to higher spatial frequencies and that foveal vision tracks this change in spatial scale. In the periphery, however, beyond 5 degrees, the physical explanation is not sufficient. Here we show that there are genuine physiological lateral spatial interactions, which are due to changes in the spatial scale of analysis.

The foveal ‘crowding’ effect: physics or physiology?

It has been known for some time that both foveal and peripheral visual acuity is higher for single letters than for letters in a row. Early work showed that this was due to the destructive interaction of adjacent contours (termed ‘crowding’ or contour interaction). It has been assumed to have a neural basis and a number of competing explanations have been advanced which implicate either high-level or low-level stages of visual processing. Our results suggest a much simpler explanation, one primarily determined by the physics of the stimulus rather than the physiology of the visual system. We show that, under conditions of contour interaction or ‘crowding’, the most relevant physical spatial frequency band of the letter is displaced to higher spatial frequencies and that foveal vision tracks this change in spatial scale.

Picturing peripheral acuity.

The grain of the retina becomes progressively coarser from the fovea to the periphery. This is caused by the decreasing number of retinal receptive fields and decreasing amount of cortex devoted to each degree of visual field (= cortical magnification factor) as one goes into the periphery. We simulate this with a picture that is progressively blurred towards its edges; when strictly fixated at its centre looks equally sharp all over.

The effects of diffusion blur on Snellen and grating acuity and foveal function in amblyopia.

In order to verify that the effects of diffusion blur on Snellen and grating acuity in amblyopic eyes resemble those obtained from the peripheral or central retina in normal controls, we conducted the following experiment using a liquid crystal window (Edmund Scientific Co.) to produce diffusion blur on Snellen and grating acuity. Spatial frequencies used for a Snellen chart and Teller acuity card were 3.2, 6.5, 13.0, 26.0 cyc/cm at a working distance of 55 cm. The values of diffusive blur on central and peripheral visual acuity obtained from 20 normal healthy control eyes were compared with those values of central visual acuity in 26 amblyopic eyes. The diffusion blur had a strong negative effect on both Snellen and grating acuity in amblyopic eyes, but it had more potent effects on grating acuity (p < 0.05). The diffusion blur values obtained from the central amblyopic retina were more compatible with those of the central retina than with those of the peripheral retina in the control group (p > 0.05). Snellen acuity obtained from diffusion blur overestimated grating acuity in the normal central acuity group and amblyopic central acuity group. The result of this investigation demonstrated that the liquid crystal diffusion blur had a strong negative effect on both Snellen and grating acuity and suggested that the visual function of an amblyopic retina resembled that of a normal central retina.

Measuring Human Detection Performance in Aircraft Visual Inspection

In airworthiness assurance, while there is a long tradition of measuring inspection reliability for machine-aided Non-Destructive Inspection (NDI), the more common visual inspection has received little attention. Yet inspection reliability measurements are needed if we are to set appropriate inspection intervals for airframe components. Visual inspection of aircraft is characterized as using multiple senses (despite its name) and having to inspect for multiple fault types, in contrast to NDI which is used for single specific fault types. The study here used 12 professional inspectors to perform nine visual inspection tasks on a long-service Boeing 737 aircraft. Each inspector worked over two days. Measures were taken of performance, strategy and individual differences. Only a fraction of the results are presented here, with a major finding that aircraft visual inspection has approximately the same reliability as industrial inspection. Individual differences were found, as well as correlations between certain aspects of performance and individual characteristics such as Field Independence and Peripheral Visual Acuity. However, there was little correlation between an individual inspector's performance on the different tasks, showing the difficulty of designing selection and placement procedures for such a wide-ranging job.

Foveal acuity, peripheral acuity and search performance: A review

Some of the more important results from 30 years of research on foveal and peripheral acuity, visual search performance and the relationships between them are summarised in this paper. The visual lobe area is a useful concept for representing peripheral acuity and has proven to be important in predicting performance for visual search tasks. Different approaches to measuring lobe area and experimental results relating peripheral acuity and visual search performance are summarised. Methods of measurement include full field mapping, eight-axis visual field mapping, four-, two- and one-axis lobe length measurement, and a card-sorting task. A brief review is also made of the various measures of static and dynamic foveal acuities using different types of tasks, their applications and factors affecting acuity values.

Development of Grating Acuity and Contrast Sensitivity in the Central and Peripheral Visual Field of the Human Infant

Central and peripheral visual functions were measured simultaneously in 39 infants from 10 to 39 weeks old using a dual-frequency VEP technique. Central acuity and contrast sensitivity over a 4 deg circular field were measured at 6 or 8 Hz. Peripheral acuity and contrast sensitivity were measured simultaneously at the other rate with a semi-circular stimulus extending from 8 to 16 deg. The EEG was analyzed at 12 and 16 Hz to determine the separate responses for the central and peripheral fields. Both central and peripheral VEP acuity developed over the age range tested. Central acuity improved by about a factor of 2.6 over the age range tested, while peripheral acuity improved by about a factor of 2.2. Central acuity was always higher by an average factor of about 2.3. Contrast sensitivity showed similar development for the central and peripheral fields with an early rapid rise in sensitivity. Copyright © 1996 Elsevier Science Ltd.

Off-axis optical quality and retinal sampling in the human eye

Using the double pass procedure, Navarro et al. (1993; Journal of the Optical Society of America A, 10, 201–212) measured the monochromatic modulation transfer function (MTF) of the human eye as a function of retinal eccentricity. They chose conditions as similar as possible to those encountered in natural viewing. We report new measurements obtained with conditions chosen instead to optimize retinal image quality: we paralyzed accommodation, used a 3 mm pupil, and corrected defocus and oblique astigmatism at each retinal location. MTFs were estimated at the tangential focus, circle of least confusion, and sagittal focus produced by oblique astigmatism. Though optical blur is well-known to have little effect on peripheral visual acuity, it can nonetheless substantially reduce aliasing by receptoral and post-receptoral spatial sampling.

Infant peripheral vision: The development of monocular visual acuity in the first 3 months of postnatal life

Quantitative data on the early morphological development of the human retina show that the peripheral region is relatively more mature than the central region. These results have stimulated researchers to compare the development of visual functions in the central and peripheral regions of the visual field. Here, we used preferential looking to evaluate 1-, 2- and 3-month-old infants' central and peripheral (10° and 30°) monocular visual acuity. There were three findings: (i) both central and peripheral acuities were poor at 1 month, improved over the age range tested, but were still about 3 octaves worse than adults' acuity; (ii) at all ages monocular acuity decreased with increasing eccentricity; (iii) 2- and 3-month-olds showed higher acuity for gratings in the temporal than in the nasal visual field at 30°. The implications of these results for issues in visual development are discussed.