Reduced visual acuity is mirrored in low vision imagery

Research has examined the nature of visual imagery in normally sighted and blind subjects, but not in those with low vision. Findings with normally sighted subjects suggest that imagery involves primary visual areas of the brain. Since the plasticity of visual cortex appears to be limited in adulthood, we might expect imagery of those with adult-onset low vision to be relatively unaffected by these losses. But if visual imagery is based on recent and current experience, we would expect images of those with low vision to share some properties of impaired visual perception. We examined key parameters of mental images reported by normally sighted subjects, compared to those with early- and late-onset low vision, and with a group of subjects with restricted visual fields using an imagery questionnaire. We found evidence that those with reduced visual acuity report the imagery distances of objects to be closer than those with normal acuity and also depict objects in imagery with lower resolution than those with normal visual acuity. We also found that all low vision groups, like the normally sighted, image objects at a substantially greater distance than when asked to place them at a distance that 'just fits' their imagery field (overflow distance). All low vision groups, like the normally sighted, showed evidence of a limited visual field for imagery, but our group with restricted visual fields did not differ from the other groups in this respect. We conclude that imagery of those with low vision is similar to that of those with normal vision in being dependent on the size of objects or features being imaged, but that it also reflects their reduced visual acuity. We found no evidence for a dependence on imagery of age of onset or number of years of vision impairment.

Self-Report in Functional Assessment of Low Vision

Previous research on the relationship between performance of complex tasks and low vision has offered few clear predictors of visual function. In some clinical, screening, and rehabilitation settings, access to measures of vision whose validity extends to the conditions of daily life is limited. The low vision individual may be uniquely qualified to report on such inaccessible performance situations. We have constructed a 57-item questionnaire, administered by interview, designed to predict performance in low vision patients with visual field defects, on four broad categories of visual function: finding, detecting, scanning, and tracking. The finding and detecting categories were further subdivided into subcategories of functional visual field (e.g., finding close, detecting close and above). Subjects rated their experienced degree of difficulty on a wide variety of common tasks which specifically demand these functions. Validity was assessed with 41 subjects. The present study suggests that self-report can be an effective predictor of function, especially in conjunction with clinical psychophysical techniques.

Vision-based Mobile Indoor Assistive Navigation Aid for Blind People

This paper presents a new holistic vision-based mobile assistive navigation system to help blind and visually impaired people with indoor independent travel. The system detects dynamic obstacles and adjusts path planning in real-time to improve navigation safety. First, we develop an indoor map editor to parse geometric information from architectural models and generate a semantic map consisting of a global 2D traversable grid map layer and context-aware layers. By leveraging the visual positioning service (VPS) within the Google Tango device, we design a map alignment algorithm to bridge the visual area description file (ADF) and semantic map to achieve semantic localization. Using the on-board RGB-D camera, we develop an efficient obstacle detection and avoidance approach based on a time-stamped map Kalman filter (TSM-KF) algorithm. A multi-modal human-machine interface (HMI) is designed with speech-audio interaction and robust haptic interaction through an electronic SmartCane. Finally, field experiments by blindfolded and blind subjects demonstrate that the proposed system provides an effective tool to help blind individuals with indoor navigation and wayfinding.

User Interface Preferences in the Design of a Camera-Based Navigation and Wayfinding Aid

Introduction

Development of a sensing device that can provide a sufficient perceptual substrate for persons with visual impairments to orient themselves and travel confidently has been a persistent rehabilitation technology goal, with the user interface posing a significant challenge. In the study presented here, we enlist the advice and ideas of individuals who are blind with respect to this challenge, for an envisioned camera-based aid to navigation and wayfinding.

Methods

We administered a short questionnaire about user preferences and needs for such a device to a sample of 10 well-educated, employed (or retired) visually impaired participants with light perception or less, who were familiar and comfortable with assistive technology. Generally, the items were rankings of relative priority.

Results

Participants preferred speech as a communications medium for navigating the environment; preferred controlling the auditory display by querying the system rather than interacting via a menu or receiving a stream of continuous speech; and preferred providing input to the system through a keypad rather than through a voice recognition system. Architectural features such as doors and stairs were ranked the most important environmental objects to be located with such a device (over furniture, persons, personal items, and even text signs).

Discussion

Our sample reported a desire for devices that can guide them to architectural features of their environment. They appear to prefer device interfaces that give them control, and would rather query a system than interact with a menu. They prefer unobtrusive input on a device via keypad rather than through voice recognition.

Implications for practitioners

Designers of camera-based navigation devices may wish to consider the preferences of our sample by incorporating a query-based interface with simple keypad input and speech output, and to include in their object recognition efforts the goal of identifying architectural features that are significant to users who are blind in navigation.

Rethinking ADA signage standards for low-vision accessibility

Americans With Disabilities Act (ADA) and International Code Council (ICC) standards for accessible buildings and facilities affect design and construction of all new and renovated buildings throughout the United States, and form the basis for compliance with the ADA. While these standards may result in acceptable accessibility for people who are fully blind, they fall far short of what they could and should accomplish for those with low vision. In this article I critique the standards, detailing their lack of evidence base and other shortcomings. I suggest that simply making existing requirements stricter (e.g., by mandating larger letter size or higher contrasts) will not ensure visual accessibility and therefore cannot act as a valid basis for compliance with the law. I propose two remedies. First, requirements for visual characteristics of signs intended to improve access for those with low vision should be expressed not in terms of physical features, such as character height and contrast, but rather in terms of the distance at which a sign can be read by someone with nominally normal (20/20) visual acuity under expected lighting conditions for the installed environment. This would give sign designers greater choice in design parameters but place on them the burden of ensuring legibility. Second, mounting of directional signs, which are critical for effective and efficient wayfinding, should be required to be in consistent and approachable locations so that those with reduced acuity may view them at close distance.

Improvements in vision-related quality of life in blind patients implanted with the Argus II Epiretinal Prosthesis

BACKGROUND

The purpose of this analysis is to report the change in quality of life (QoL) after treatment with the Argus II Epiretinal Prosthesis in patients with end-stage retinitis pigmentosa.

METHODS

The Vision and Quality of Life Index (VisQoL) was used to assess changes in QoL dimensions and overall utility score in a prospective 30-patient single-arm clinical study. VisQoL is a multi-attribute instrument consisting of six dimensions (injury, life, roles, assistance, activity and friendship) that may be affected by visual impairment. Within each dimension, patients were divided into two groups based on how much their QoL was affected by their blindness at baseline (moderate/severe or minimal). Outcomes were compared within each dimension sub-group between baseline and the combined follow-up periods using the Friedman test. In addition, data from the six dimensions were combined into a single utility score, with baseline data compared to the combined follow-up periods.

RESULTS

Overall, 80 per cent of the patients reported difficulty in one or more dimensions pre-implant. Composite VisQoL utility scores at follow-up showed no statistically significant change from baseline; however, in three of the six VisQoL dimensions (injury, life and roles), patients with baseline deficits showed significant and lasting improvement after implantation with Argus II. In two of the three remaining dimensions (assistance and activity), data trended toward an improvement. In the final VisQoL dimension (friendship), none of the patients reported baseline deficits, suggesting that patients had largely adjusted to this attribute.

CONCLUSION

Patients whose vision negatively affected them with respect to three VisQoL dimensions (that is, getting injured, coping with the demands of their life and fulfilling their life roles) reported significant improvement in QoL after implantation of the Argus II retinal prosthesis. Furthermore, the benefit did not deteriorate at any point during the 36-month follow-up, suggesting a long-term, durable improvement.

Performance of real-world functional vision tasks by blind subjects improves after implantation with the Argus® II retinal prosthesis system

BACKGROUND

The main objective of this study was to test Argus II subjects on three real-world functional vision tasks.

DESIGN

The study was designed to be randomized and prospective. Testing was conducted in a hospital/research laboratory setting at the various participating centres.

PARTICIPANTS

Twenty eight Argus II subjects, all profoundly blind, participated in this study.

METHODS

Subjects were tested on the three real-world functional vision tasks: Sock Sorting, Sidewalk Tracking and Walking Direction Discrimination task MAIN OUTCOME MEASURES: For the Sock Sorting task, percentage correct was computed based on how accurately subjects sorted the piles on a cloth-covered table and on a bare table. In the Sidewalk Tracking task, an 'out of bounds' count was recorded, signifying how often the subject veered away from the test course. During the Walking Direction Discrimination task, subjects were tested on the number of times they correctly identified the direction of testers walking across their field of view.

RESULTS

The mean percentage correct OFF versus ON for the Sock Sorting task was found to be significantly different for both testing conditions (t-test, P < 0.01). On the Sidewalk Tracking task, subjects performed significantly better with the system ON than they did with the system OFF (t-test, P < 0.05). Eighteen (18) of 27 subjects (67%) performed above chance with the system ON, and 6 (22%) did so with system OFF on the Walking Direction Discrimination task.

CONCLUSIONS

Argus II subjects performed better on all three tasks with their systems ON than they did with their systems OFF.

Five-Year Safety and Performance Results from the Argus II Retinal Prosthesis System Clinical Trial

PURPOSE

The Argus II Retinal Prosthesis System (Second Sight Medical Products, Inc, Sylmar, CA) was developed to restore some vision to patients blind as a result of retinitis pigmentosa (RP) or outer retinal degeneration. A clinical trial was initiated in 2006 to study the long-term safety and efficacy of the Argus II System in patients with bare or no light perception resulting from end-stage RP.

DESIGN

Prospective, multicenter, single-arm clinical trial. Within-patient controls included the nonimplanted fellow eye and patients' native residual vision compared with their vision with the Argus II.

PARTICIPANTS

Thirty participants in 10 centers in the United States and Europe.

METHODS

The worse-seeing eye of blind patients was implanted with the Argus II. Patients wore glasses mounted with a small camera and a video processor that converted images into stimulation patterns sent to the electrode array on the retina.

MAIN OUTCOME MEASURES

The primary outcome measures were safety (the number, seriousness, and relatedness of adverse events) and visual function, as measured by 3 computer-based, objective tests. Secondary measures included functional vision performance on objectively scored real-world tasks.

RESULTS

Twenty-four of 30 patients remained implanted with functioning Argus II Systems at 5 years after implantation. Only 1 additional serious adverse event was experienced after the 3-year time point. Patients performed significantly better with the Argus II on than off on all visual function tests and functional vision tasks.

CONCLUSIONS

The 5-year results of the Argus II trial support the long-term safety profile and benefit of the Argus II System for patients blind as a result of RP. The Argus II is the first and only retinal implant to have market approval in the European Economic Area, the United States, and Canada.

A trend analysis of the low-vision literature

An account is given of the design and compilation of a freely-available database, updated on a monthly schedule, listing journals, books, conference proceedings and online publications concerned with any and every aspect of low vision. Specific topic areas can be located by means of keywords and through other bibliographical details such as the author’s name, year of publication, etc. The authors present a summary historical analysis revealing that there has been an unprecedented rise over the last few decades in the number and diversity of journals and other literature sources addressing issues to do with low vision. Among the opportunities provided by the database are searches to ascertain not only what is currently being practiced and researched but also to identify the changes in the priorities being given to the multitude of topics of interest to optometrists, ophthalmologists, paediatricians, psychologists, educators, rehabilitation practitioners, epidemiologists, gerontologists and other professional workers.

An analysis of observer-rated functional vision in patients implanted with the Argus II Retinal Prosthesis System at three years

Objective

The purpose of this analysis was to compare observer‐rated tasks in patients implanted with the Argus II Retinal Prosthesis System, when the device is ON versus OFF.

Methods

The Functional Low‐Vision Observer Rated Assessment (FLORA) instrument was administered to 26 blind patients implanted with the Argus II Retinal Prosthesis System at a mean follow‐up of 36 months. FLORA is a multi‐component instrument that consists in part of observer‐rated assessment of 35 tasks completed with the device ON versus OFF. The ease with which a patient completes a task is scored using a four‐point scale, ranging from easy (score of 1) to impossible (score of 4). The tasks are evaluated individually and organised into four discrete domains, including ‘Visual orientation’, ‘Visual mobility’, ‘Daily life and ‘Interaction with others’.

Results

Twenty‐six patients completed each of the 35 tasks. Overall, 24 out of 35 tasks (69 per cent) were statistically significantly easier to achieve with the device ON versus OFF. In each of the four domains, patients’ performances were significantly better (p < 0.05) with the device ON versus OFF, ranging from 19 to 38 per cent improvement.

Conclusion

Patients with an Argus II Retinal Prosthesis implanted for 18 to 44 months (mean 36 months), demonstrated significantly improved completion of vision‐related tasks with the device ON versus OFF.

Long-Term Results from an Epiretinal Prosthesis to Restore Sight to the Blind

PURPOSE

Retinitis pigmentosa (RP) is a group of inherited retinal degenerations leading to blindness due to photoreceptor loss. Retinitis pigmentosa is a rare disease, affecting only approximately 100 000 people in the United States. There is no cure and no approved medical therapy to slow or reverse RP. The purpose of this clinical trial was to evaluate the safety, reliability, and benefit of the Argus II Retinal Prosthesis System (Second Sight Medical Products, Inc, Sylmar, CA) in restoring some visual function to subjects completely blind from RP. We report clinical trial results at 1 and 3 years after implantation.

DESIGN

The study is a multicenter, single-arm, prospective clinical trial.

PARTICIPANTS

There were 30 subjects in 10 centers in the United States and Europe. Subjects served as their own controls, that is, implanted eye versus fellow eye, and system on versus system off (native residual vision).

METHODS

The Argus II System was implanted on and in a single eye (typically the worse-seeing eye) of blind subjects. Subjects wore glasses mounted with a small camera and a video processor that converted images into stimulation patterns sent to the electrode array on the retina.

MAIN OUTCOME MEASURES

The primary outcome measures were safety (the number, seriousness, and relatedness of adverse events) and visual function, as measured by 3 computer-based, objective tests.

RESULTS

A total of 29 of 30 subjects had functioning Argus II Systems implants 3 years after implantation. Eleven subjects experienced a total of 23 serious device- or surgery-related adverse events. All were treated with standard ophthalmic care. As a group, subjects performed significantly better with the system on than off on all visual function tests and functional vision assessments.

CONCLUSIONS

The 3-year results of the Argus II trial support the long-term safety profile and benefit of the Argus II System for patients blind from RP. Earlier results from this trial were used to gain approval of the Argus II by the Food and Drug Administration and a CE mark in Europe. The Argus II System is the first and only retinal implant to have both approvals.

Toward a Computer Vision-based Wayfinding Aid for Blind Persons to Access Unfamiliar Indoor Environments

Independent travel is a well known challenge for blind and visually impaired persons. In this paper, we propose a proof-of-concept computer vision-based wayfinding aid for blind people to independently access unfamiliar indoor environments. In order to find different rooms (e.g. an office, a lab, or a bathroom) and other building amenities (e.g. an exit or an elevator), we incorporate object detection with text recognition. First we develop a robust and efficient algorithm to detect doors, elevators, and cabinets based on their general geometric shape, by combining edges and corners. The algorithm is general enough to handle large intra-class variations of objects with different appearances among different indoor environments, as well as small inter-class differences between different objects such as doors and door-like cabinets. Next, in order to distinguish intra-class objects (e.g. an office door from a bathroom door), we extract and recognize text information associated with the detected objects. For text recognition, we first extract text regions from signs with multiple colors and possibly complex backgrounds, and then apply character localization and topological analysis to filter out background interference. The extracted text is recognized using off-the-shelf optical character recognition (OCR) software products. The object type, orientation, location, and text information are presented to the blind traveler as speech.

Interim results from the international trial of Second Sight's visual prosthesis

PURPOSE

This study evaluated the Argus II Retinal Prosthesis System (Second Sight Medical Products, Inc., Sylmar, CA) in blind subjects with severe outer retinal degeneration.

DESIGN

Single-arm, prospective, multicenter clinical trial.

PARTICIPANTS

Thirty subjects were enrolled in the United States and Europe between June 6, 2007, and August 11, 2009. All subjects were followed up for a minimum of 6 months and up to 2.7 years.

METHODS

The electronic stimulator and antenna of the implant were sutured onto the sclera using an encircling silicone band. Next, a pars plana vitrectomy was performed, and the electrode array and cable were introduced into the eye via a pars plana sclerotomy. The microelectrode array then was tacked to the epiretinal surface.

MAIN OUTCOME MEASURES

The primary safety end points for the trial were the number, severity, and relation of adverse events. Principal performance end points were assessments of visual function as well as performance on orientation and mobility tasks.

RESULTS

Subjects performed statistically better with the system on versus off in the following tasks: object localization (96% of subjects), motion discrimination (57%), and discrimination of oriented gratings (23%). The best recorded visual acuity to date is 20/1260. Subjects' mean performance on orientation and mobility tasks was significantly better when the system was on versus off. Seventy percent of the patients did not have any serious adverse events (SAEs). The most common SAE reported was either conjunctival erosion or dehiscence over the extraocular implant and was treated successfully in all subjects except in one, who required explantation of the device without further complications.

CONCLUSIONS

The long-term safety results of Second Sight's retinal prosthesis system are acceptable, and most subjects with profound visual loss perform better on visual tasks with system than without it.

Clothing Matching for Visually Impaired Persons

Matching clothes is a challenging task for many blind people. In this paper, we present a proof of concept system to solve this problem. The system consists of 1) a camera connected to a computer to perform pattern and color matching process; 2) speech commands for system control and configuration; and 3) audio feedback to provide matching results for both color and patterns of clothes. This system can handle clothes in deficient color without any pattern, as well as clothing with multiple colors and complex patterns to aid both blind and color deficient people. Furthermore, our method is robust to variations of illumination, clothing rotation and wrinkling. To evaluate the proposed prototype, we collect two challenging databases including clothes without any pattern, or with multiple colors and different patterns under different conditions of lighting and rotation. Results reported here demonstrate the robustness and effectiveness of the proposed clothing matching system.

Letter case and text legibility in normal and low vision

It is thought by cognitive scientists and typographers alike, that lower-case text is more legible than upper-case. Yet lower-case letters are, on average, smaller in height and width than upper-case characters, which suggests an upper-case advantage. Using a single unaltered font and all upper-, all lower-, and mixed-case text, we assessed size thresholds for words and random strings, and reading speeds for text with normal and visually impaired participants. Lower-case thresholds were roughly 0.1 log unit higher than upper. Reading speeds were higher for upper- than for mixed-case text at sizes twice acuity size; at larger sizes, the upper-case advantage disappeared. Results suggest that upper-case is more legible than the other case styles, especially for visually-impaired readers, because smaller letter sizes can be used than with the other case styles, with no diminution of legibility.

Serifs and font legibility

Using lower-case fonts varying only in serif size (0%, 5%, and 10% cap height), we assessed legibility using size thresholds and reading speed. Five percentage serif fonts were slightly more legible than sans serif, but the average inter-letter spacing increase that serifs themselves impose, predicts greater enhancement than we observed. RSVP and continuous reading speeds showed no effect of serifs. When text is small or distant, serifs may, then, produce a tiny legibility increase due to the concomitant increase in spacing. However, our data exhibited no difference in legibility between typefaces that differ only in the presence or absence of serifs.

Lapse resistance in the verbal letter reporting task

Lapses, or misreporting errors, can affect accuracy of threshold measurements. Assumptions about lapse rate, especially in untrained observers, have consequently guided the design of at least one clinical psychophysical test. Lapse rate was assessed using a verbal letter identification paradigm like that used in visual acuity and letter contrast sensitivity testing. Subjects occasionally made slip-of-the tongue errors but spontaneously corrected them. Lapse rate (excluding such errors) was 0-3 errors per 1,536 (average rate of 0.0005). In this common clinical paradigm, in which observers set their reporting pace, and where opportunity to amend responses is available, lapse rate is negligible.

Improving the design of the letter contrast sensitivity test

PURPOSE

To develop a small-format letter contrast sensitivity test with improved accuracy, suitable for portable use and near testing, and having a simple and unambiguous scoring system that can be used with reference to existing norms.

METHODS

A near test, printed on resin-coated paper and mounted on plastic was developed by using Sloan letters, with 0.04 log unit contrast decrements between each letter and a simple scoring procedure. Monte Carlo methods and a Weibull function model of visual performance were used to assess test accuracy.

RESULTS

The new test has 28% lower score standard deviations than the Pelli-Robson Contrast Sensitivity Chart, over a wide range of low misreporting rates, while maintaining test scores that differ from the Pelli-Robson by 0.01 log unit, permitting use of norms collected with the Pelli-Robson test.

CONCLUSIONS

While maintaining comparability with Pelli-Robson norms, the new test has improved accuracy in comparison with the Pelli-Robson chart and several other advantages that result from its smaller size.

Adjustable typography: an approach to enhancing low vision text accessibility

Millions of people have low vision, a disability condition caused by uncorrectable or partially correctable disorders of the eye. The primary goal of low vision rehabilitation is increasing access to printed material. This paper describes how adjustable typography, a computer graphic approach to enhancing text accessibility, can play a role in this process, by allowing visually-impaired users to customize fonts to maximize legibility according to their own visual needs. Prototype software and initial testing of the concept is described. The results show that visually-impaired users tend to produce a variety of very distinct fonts, and that the adjustment process results in greatly enhanced legibility. But this initial testing has not yet demonstrated increases in legibility over and above the legibility of highly legible standard fonts such as Times New Roman.

How crowding affects letter confusion

BACKGROUND

Acuity for letter recognition is known to be worse when multiple letters are presented with narrow interletter spacings than with wide spacings. How would interletter spacing affect the kind of errors made by human subjects?

METHODS

Five-letter strings that were randomly drawn from the 26 uppercase letters of the English alphabet were presented foveally to the subjects. The interletter spacings were 1.0 and 0.1 letter height. Letter confusion matrices were constructed from the data collected using these spacing conditions.

RESULTS

Narrow- and wide-spacing letter strings produced different letter confusion matrices. Aside from the letter confusions that were shared by both wide- and narrow-spacing strings, narrow-spacing strings produced more random confusions and a set of unique letter confusions, which was not observed under the wide-spacing condition.

CONCLUSION

Increased random guessing and lateral interactions between features of neighboring letters can account for most of the acuity deterioration observed under the narrow-spacing condition.

Apparent string shortening concomitant with letter crowding

In our previous studies of the crowding effect, we have observed that human observers tend to underestimate the length of a letter string (the number of letters in the string) when the letters are close to visual acuity, and the interletter spacings are small. In this study, we asked our observers to identify letters in randomly presented four-letter and five-letter strings. We found that, when a priori knowledge of the lengths of letter strings was not available, the probability of underestimating string length increased with decreasing interletter spacing. The causes of underestimation errors appeared to be the omission of an interior letter and the merging of two neighboring letters. Since our experiments were conducted in the foveal region, neither spatial uncertainty nor split attention can explain the underestimation errors. The effect of the point spread function of the eye on closely packed letter strings is discussed.

Elicited sequential presentation for low vision reading

This paper reports on a variant of the rapid serial visual presentation (RSVP) technique for low vision reading called elicited sequential presentation (ESP). In both techniques, words are presented sequentially at a constant screen location, but with ESP, the reader elicits presentation of each new word by means of a button press, rather than (as with RSVP) being presented with it automatically at fixed intervals. An experiment comparing reading speeds using, ESP, RSVP and a conventional closed-circuit television (CCTV) reading aid showed that for 15 slow readers who were customary CCTV users with low vision, ESP is superior to RSVP and yields reading speeds averaging 47% faster than RSVP--about the same as CCTV reading speed. The log of the ratio of ESP to RSVP reading speeds was significantly negatively correlated with the log of RSVP reading speed, showing that slower readers benefit more than faster readers; regression predicted no benefit for readers who read with RSVP at 133 wpm or greater. Finally, word length and word presentation duration chosen by subjects reading with ESP were significantly correlated, suggesting that part of the benefit of ESP is due to reader's ability to allocate time based on word length and difficulty.

Detection and identification of mirror-image letter pairs in central and peripheral vision

Reading performance is poorer in the peripheral than in the central visual field, even after size-scaling to compensate for differences in visual acuity at the different eccentricities. Since several studies have indicated that the peripheral retina is deficient with respect to spatial phase discrimination, we compared the psychometric functions for detection (D) and identification (I) of size-scaled, mirror-symmetric letters (i.e. letters differing in the phase spectra of their odd symmetric components) at three inferior field eccentricities (0, 4, and 7.5 deg) using a two-alternative, temporal, forced-choice procedure and retinal image stabilization to control retinal locus. Each subject's data were fit with Weibull functions and tested for goodness-of-fit under several hypotheses. This analysis revealed that while the psychometric functions were of constant shape across eccentricity for the respective tasks, they showed statistically significant variations in the D/I threshold ratios. However, these variations were so small that poorer reading outside the fovea is unlikely to be due to reduced letter discriminability that might occur secondary to a loss of peripheral field phase sensitivity.

Binocular enhancement of visual acuity

Using a computerized test system, we compared binocular and monocular visual optotype acuity, varying both contrast and contrast disparity between the two eyes. When contrast was the same in the two eyes, binocular acuity was better than best monocular acuity by an average of 0.045 log minimum angle of resolution, or 11%. When contrast differed in the two eyes, binocular acuity in most but not all cases was still better than the monocular acuity of the eye that received the higher contrast. This binocular advantage became smaller but remained significant as contrast disparity became larger. These results are most simply explained by threshold contrast summation of high-spatial-frequency letter components.

On the statistical reliability of letter-chart visual acuity measurements

PURPOSE

To determine boundary values of test-retest reliability and sensitivity to acuity change that are unlikely to be exceeded in any clinical situation, for a popular visual acuity chart (Lighthouse/ETDRS) with three scoring methods; to discuss general methodological issues associated with statistical accuracy of optotype chart testing; and to link measures of test reliability to measures of sensitivity to change.

METHODS

Five highly practiced subjects were tested using a computer-controlled acuity testing system in a procedure designed to reduce measurement error. Subjects read the computerized chart 156 times, yielding a sample of 78 test-retest comparisons.

RESULTS

Under conditions likely to minimize variability, visual acuity may, with 95% confidence, be ascertained only within +/- 0.1 log units, using this chart with the recommended letter-by-letter scoring. Detecting a significant change in visual acuity requires about +/- 0.14 log units for the same degree of confidence.

CONCLUSIONS

These measurements may be viewed as approaching the upper limit of reliability of this letter chart. Reliability probably is considerably less in typical usage.

Modulation transfer functions and contrast sensitivity through low vision telescopes

Telescopes are coherently coupled to the eye. Because their wavefront aberrations may be altered by the optics of the eye, especially by accommodation, some researchers question the ability of their modulation transfer functions (MTF) to predict changes in contrast sensitivity functions (CSF) through them. We measured the CSFs of visually normal and aphakic subjects through telescopes. We found that MTF appears to be useful for ranking the telescopes, and accommodation appears to improve focus and partly balance wavefront errors. Our results suggest that cascading (multiplying the contrast of the instrumental MTF with the unaided CSF at each spatial frequency) is useful for predicting visually aided CSFs to within 4 dB.

An instrument for measuring the modulation transfer functions of low power telescopes and telemicroscopes

An instrument using an electro-optical Fourier method for measuring the modulation transfer function (MTF) of low power telescopes and telemicroscopes is described. Because these devices are afocal, or nearly so, relay optics are needed to form real images at the detection section of the apparatus. The system is capable of measuring the MTF in monochromatic and white light, at any target azimuth, across the field of view, and through focus. The target system contains 14 square-wave gratings with spatial frequencies that range from 2.5 to 156 cpd. Images of these gratings are scanned across a slit. The output data are fed to a first-order recursive digital Butterworth bandpass filter for MTF analysis. The apparatus is diffraction limited at f/31.4. Therefore, it negligibly affects the measurement of the MTF of telescopes and telemicroscopes tested with exit pupils of up to 6.4 mm.

Mental imagery and sensory experience in congenital blindness

Imagery of congenitally blind and normally sighted subjects was compared in two experiments. In the first, subjects were asked to estimate how far away objects appeared in images. The results showed that blind subjects imaged objects "within arms' reach", and with only a slight tendency to image larger objects farther from them. In contrast, sighted subjects tended to image larger objects as if they were farther away. In addition, unlike the sighted, blind subjects' images also failed to overflow an "image space" of fixed size. Finally, blind subjects were, with one exception, unable to mimic successfully the responses of a sighted person, when explicitly asked to do so. In the second experiment, subjects pointed to the left and right sides of three objects imaged at three distances. Sighted and blind groups both showed a decrease in pointing span with the size of the object, but only the sighted subjects showed a decrease with increased image distance, in accordance with the laws of perspective.

The adaptive significance of the location of the optic disk

Although the 'filling in' of each blind spot by healthy retina in the other eye has long been described as an adaptive property of the spatial arrangement of the optic disks, an explanation of why the disks are specifically located where they are has yet to be proposed. A rationale for their horizontal position in humans is offered that is based on the projections of the blind spots in visual space in relation to fixation distance and to the protrusion of the bony facial occlusion of the nose bridge.

Advanced early and late atrioventricular block in acute inferior wall myocardial infarction

Seventy-six patients with acute inferior acute myocardial infarction (AMI) and advanced atrioventricular (AV) block are described. According to pre-established ECG criteria and time of appearance of the advanced AV block, patients were divided into two groups. The early block group consisted of 31 patients who developed advanced AV block during the hyperacute ECG stage of AMI. Advanced AV block in these patients was characterized by early appearance, short duration, third-degree type block, poor response to atropine, and increased need for pacemaker therapy. The late block group consisted of 45 patients who developed advanced AV block during subsequent ECG stages of AMI. Advanced AV block in these patients was characterized by late appearance, longer duration, second-degree type block, positive response to atropine, and diminished need for pacemaker therapy. Morbidity and mortality also differed between both groups. Patients with early block had more syncope (32% vs 2%, p less than 0.0001), more left heart failure (36 vs 7%, p less than 0.005), and more cardiogenic shock (39% vs 2%, p less than 0.001) than patients with late block. The mortality rate in the early block group was high (23%) and similar to that reported in the literature, whereas the mortality rate in the late block group was low (7%, p less than 0.05) and similar to the mortality rate reported for acute inferior AMI without advanced AV block. These data identify a subgroup of patients with acute inferior AMI and advanced AV block, which accounts for the high mortality rate reported in this group of patients.

Multiform accelerated idioventricular rhythm in acute myocardial infarction: electrocardiographic characteristics and response to verapamil

Thirteen patients with acute myocardial infarction with multiform accelerated idioventricular rhythm (AIVR) occurring during the first 12 hours of monitoring in the coronary care unit are described. This arrhythmia, similar to the more common uniform AIVR, was intermittent, did not cause hemodynamic compromise, and was not related to more serious ventricular arrhythmias. There was no correlation between the bundle branch block pattern of the multiform AIVR and the electrocardiographic location of the myocardial infarction, but there was a perfect correlation between the frontal plane electrical axis of the multiform AIVR and the electrocardiographic location of the myocardial infarction. The presence of fusion beats between the different forms of AIVR suggests multifocality rather than multiformity. Intravenous verapamil (3 to 5 mg bolus) was administered to 6 patients with multiform AIVR in whom the arrhythmias were persistent enough to allow the evaluation of the effect of verapamil on the arrhythmia. Verapamil caused no change in the rate of AIVR in 1 patient, but in a second patient it decreased the rate by 20 beats/min. In 4 patients, verapamil abolished the arrhythmia: in 2 patients carotid sinus pressure (induced sinus slowing) allowed the emergence of the AIVR at a lower rate, and in the remaining 2 patients the arrhythmia was not observed.

The dependence of the induced effect on orientation and a hypothesis concerning disparity computations in general

The induced size effect is an apparent rotation about a vertical axis that results from binocularly viewing a target in which one half-image is vertically magnified. A previous paper (Arditi et al., 1981, Vision Res. 21, 755-764) described a theory of this effect in terms of horizontal disparities that exist between vertically magnified images of oblique features and their unmagnified counterparts. The present studies test two aspects of that theory: the requirement of oblique features in stimuli eliciting the induced effect, and the assumption that binocular associations (inputs to disparity computations) are made across horizontal meridians. The former aspect was confirmed in a stereo discrimination experiment in which the direction of rotation (tilt) for crossed line patterns of varying orientation was judged, for a fixed vertical magnification of one half-image. The latter aspect was rejected on the basis of the results of that experiment, and of two experiments in which observers matched the apparent tilt of the lines with a horizontal adjustment line which could be stereoscopically rotated in depth. The data and some associated demonstrations suggest that stereoacuity and apparent depth of oblique lines vertically magnified in one half-image are determined by the horizontal separation between binocular points which are nearest in a fixed binocular coordinate map, rather than by purely horizontal point-matchings. This "nearest neighbor hypothesis" seems to be operative in classic measures of stereoacuity as well as in the induced effect.

Ventricular fibrillation complicating temporary ventricular pacing in acute myocardial infarction: significance of right ventricular infarction

Five patients with acute myocardial infarction had ventricular fibrillation as a complication of indicated temporary pacing. All five patients had evidence of right ventricular infarction (three patients with postmortem confirmation). The presence of right ventricular infarction seems to be a contributing mechanism involved in the induction of ventricular fibrillation during temporary pacing for bradyarrhythmia complicating acute myocardial infarction.