Systems and methods for evaluation of visual function in a low-vision population

Abstract

To evaluate visual function in a low-vision population, a width of a first stripe to be displayed to a subject is determined based on a defined viewing distance and a first visual acuity value. A brightness characteristic width-wise across the first stripe is determined to provide isoluminance of the first stripe with respect to a background intensity of the screen. The first stripe is displayed in motion across the screen at a defined velocity, the first stripe. Tracking data is generated from movement of the subject's eyes using a camera. A tracking decision is determined characterizing whether the subject successfully tracked the first strip. A width is determined of a second stripe to be displayed in a further iteration of the test, or the test is ended, based on the tracking decision. A visual acuity threshold is determined based on a width of a last or penultimate displayed stripe.

Description

SYSTEMS AND METHODS FOR EVALUATION OF VISUAL FUNCTION IN A LOW-VISION POPULATIONCROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] The present application claims priority to U.S. Provisional Patent Application No. 63 / 643,700, filed May 7, 2024, which is hereby incorporated by reference in its entirety.TECHNICAL FIELD

[0002] The present disclosure relates to systems and methods for evaluation of visual function in a low-vision population and, in particular, performing such evaluation based on Optokinetic Nystagmus produced using a display device.BACKGROUND

[0003] Visual acuity is a common method of assessment in a clinical trial. The standard measurement of visual acuity is done with an eye chart, such as an ETDRS chart. In general, these types of eye charts designed for subjects with normal vision and are not useful and / or appropriate for subjects with very low vision, e.g., vision worse than 20 / 200.

[0004] Typically, for low vision subjects, vision is assessed using alternative techniques, such as, for example, using hand motion. In such a case, the clinician might wave their hand in front of the subject and ask whether they see anything and / or how many fingers the clinician is holding up. Such approaches provide very inaccurate assessments which are not usable in a clinical trial context where one is seeking to assess a treatment effect, as this requires precise measurements. For example, subjects in a clinical trial relating to inherited retinal disease may have vision that is very close to blindness. Conventionally, there are only limited ways toquantify visual acuity in such subjects. This is particularly true for child subjects, because they may not have good communications skills and, in some cases, may have had low vision their entire lives, which makes it difficult to explain to them what they are supposed to be seeing in the test.

[0005] Measuring visual acuity in low vision patients presents several challenges due to the nature of low vision and the limitations of standard visual acuity testing methods. Low vision can result from a wide range of conditions, affecting vision in various ways. Some patients may have central vision loss, while others may have peripheral vision loss or issues with contrast sensitivity. This variability makes it hard to find a one-size-fits-all method for assessing visual acuity. As noted above, conventional visual acuity tests, like the Snellen chart, are designed for detecting distance vision issues in people with relatively minor vision impairments. Low vision patients may not be able to see any of the letters on these charts, requiring alternative methods of testing that can accommodate their level of vision loss.

[0006] Other conventional approaches to measuring visual acuity in low vision subjects include the Berkeley Rudimentary Vision Test (BRVT), which is a chartbased visual acuity test designed to measure very low levels of vision, down to the range of techniques such as hand motion and finger counting. The BRVT includes several different levels, which are likely designed to test different degrees of visual impairment. The specific level used may depend on how poor a person’s vision is, with more rudimentary levels for those with very low vision. Chart-based tests like the BRVT rely on visual stimuli presented on a chart, often involving letters, shapes, or symbols of various sizes. The subject’s task is to identify or describe these stimuli, which requires a verbal response. This can be challenging for some subjects, andthe reliability of the test can depend on the subject’s understanding, cooperation, and communication skills.

[0007] Visual acuity measurements often do not capture the functional vision of low vision patients. For example, a person may have poor visual acuity but adapt to use their remaining vision effectively. Conversely, someone with relatively good acuity might struggle with daily activities due to poor contrast sensitivity or glare issues - many low vision patients have difficulties with contrast sensitivity and glare, which standard visual acuity tests do not measure. These aspects of vision are important for daily functioning but require specialized tests that are not always readily available or utilized in standard practice.

[0008] Low vision patients may adapt to the controlled environment of a testing setting, but this may not accurately reflect their visual capabilities in their everyday environments, which can be more variable and challenging. Visual acuity testing can be physically and mentally taxing for low vision patients, especially for those with severe impairments or additional disabilities. Ensuring patient cooperation and accurate responses can be more challenging, affecting the reliability of the test results.

[0009] Accurately measuring visual acuity in low vision patients often requires specialized equipment and tests, such as Early Treatment Diabetic Retinopathy Study (ETDRS) charts, low vision enhanced visual acuity tests, or tests for near vision and contrast sensitivity. Access to these specialized tools may be limited in some settings.

[0010] Conventional Optokinetic Nystagmus (OKN) approaches to measure visual acuity exploit the reflexive eye movement that occurs when an individual observes a large, continuously moving visual scene. An optokinetic test, often conducted using a spinning drum with gradings, may be used to assess visual response. The drum isspun slowly in front of a patient, and their eyes will naturally follow the lines on the drum in a jerky motion. This reflex, known as Optokinetic Nystagmus, consists of a slow phase, where the eyes follow the moving object, and a quick phase, where the eyes rapidly reset to their original position. The principle behind using OKN to measure visual acuity is based on the observation that the ability to elicit this reflex depends on the visibility of the moving pattern, which in turn depends on the visual acuity of the observer.

[0011] OKN testing can be a useful tool in certain contexts, especially for assessing visual function in individuals who cannot participate in more traditional forms of visual acuity testing. However, using conventional OKN approaches to measure visual acuity, especially in populations with low vision, has several shortcomings. OKN approaches measure visual acuity indirectly by assessing the ability to track moving patterns. This does not directly measure the sharpness of vision or the smallest detail that can be resolved, which are important components of visual acuity. The interpretation of OKN responses can be subjective, depending on the observer's experience and the clarity of the nystagmus response. This subjectivity can lead to variability in results across different testers or testing sessions. In some patient groups, such as infants, non-verbal individuals, or those with neurological conditions, it may be challenging to elicit a reliable OKN response or interpret it accurately. There is a lack of standardization in the stimuli used (e.g., size, speed, and pattern of movement), which can affect the comparability of results across different studies or clinical settings.SUMMARY

[0012] Disclosed embodiments relate to objective visual testing methods based on OKN to help track effect of treatment in retinal and optic nerve diseases in subjects with very low vision. Additionally, disclosed techniques provide an objective way of measuring visual acuity for non-verbal and pediatric subjects.

[0013] In one aspect, the disclosed embodiments provide methods, systems, and computer-readable media to evaluate visual function in a low-vision population. The method includes determining a width of a first stripe to be displayed to a subject on a screen based at least in part on a defined viewing distance and a first visual acuity value. The method further includes determining a brightness characteristic in a widthwise direction across the first stripe to provide isoluminance of the first stripe with respect to a background intensity of the screen. The method further includes displaying, to the subject of a test, the first stripe in motion across the screen of the display device at a defined velocity, the first stripe having the determined brightness characteristic in the width-wise direction. The method further includes generating tracking data from movement of the subject’s eyes using a camera to identify specified eye movements of the subject. The method further includes determining a tracking decision characterizing whether the subject successfully tracked the first stripe based at least in part on the tracking data. The method further includes determining a width of at least a second stripe to be displayed to the subject on the screen in a further iteration of the test or ending the test based at least in part on the tracking decision. The method further includes determining, upon ending the test, a visual acuity threshold based at least in part on a width of a last or penultimate displayed stripe.

[0014] Embodiments may include one or more of the following features, alone or in combination.

[0015] The visual acuity threshold may be determined based at least in part on the defined viewing distance and a visual angle subtended by the width of the last or penultimate displayed stripe. The screen may be the screen of a display device, and the method may further include positioning the display device to provide the defined viewing distance between the subject’s eyes and the screen. The method may further include setting a first contrast level of the first stripe of a defined set of contrast levels. The method may further include setting a second contrast level of the first stripe and repeating the test to determine a visual acuity threshold corresponding to each contrast level of the defined set of contrast levels.

[0016] In determining the width of the first stripe, the width of the first stripe may be calculated to correspond to a logMAR value between about 1 .0 and about 3.4. The defined viewing distance may be between about 30 cm and about 50 cm or between about 35 cm and about 45 cm. The defined velocity may be in a range of about 10 degrees / second to about 15 degrees / second or in a range of about 10 degrees / second to about 20 degrees / second. The specified eye movements may be indicative of Optokinetic Nystagmus (OKN). The display device may be a tablet computer. The display device may be an external monitor in communication with a computer or display device.

[0017] In one aspect, the disclosed embodiments provide a system for evaluating visual function in a low-vision population. The system includes a first display device comprising a screen and at least one processor and memory; and a camera. The memory stores code which when executed by the at least one processor causes the atleast one processor to perform methods disclosed herein. Embodiments may include one or more of the following features, alone or in combination. The system may include a second display device in communication with the first display device, the second display device comprising a screen, at least one processor and memory. The camera may be an infrared camera.

[0018] In one aspect, the disclosed embodiments provide a non-transitory, computer-readable medium storing instructions that, when executed by one or more processors of a display device, cause the one or more processors to perform methods disclosed herein.BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Fig. 1 depicts a system for evaluation of visual function in a low-vision population using a display device, according to disclosed embodiments;

[0020] Fig. 2 depicts a display screen for vision testing displaying a stripe having uniform brightness;

[0021] Fig. 3 depicts a display screen for vision testing displaying an isoluminant stripe with high contrast;

[0022] Fig. 4 depicts a display screen for vision testing displaying an isoluminant stripe with low contrast;

[0023] Fig. 5 shows a test set up for evaluation of visual function in a low-vision population;

[0024] Fig. 6 shows a subject and an operator positioned at a test set up for evaluation of visual function;

[0025] Fig. 7 is a plot of visual acuity test results for each eye of eleven normal subjects determined using Optokinetic Nystagmus - based visual testing with high and low contrast;

[0026] Fig. 8 is a plot of measured logMAR for low vision subjects determined using Optokinetic Nystagmus (OKN) testing for Test 1 vs. Best Corrected Visual Acuity (BCVA);

[0027] Fig. 9 is a plot of measured logMAR for low vision subjects determined using Optokinetic Nystagmus (OKN) testing for Test 2 vs. Best Corrected Visual Acuity (BCVA);

[0028] Fig. 10 is a plot of a pair-wise t-test comparison of OKN visual acuity Test 1 and Test 2.

[0029] Fig. 11 is a plot of measured logMAR for normal vision subjects determined using Optokinetic Nystagmus (OKN) testing with diffusers.

[0030] Fig. 12 is a table summarizing testing of four subjects with normal vision at each of three diffuser levels.

[0031] Fig. 13 depicts a method for evaluating visual function in a low-vision population.

[0032] Where considered appropriate, reference numerals may be repeated among the drawings to indicate corresponding or analogous elements. Moreover, some of the blocks depicted in the drawings may be combined into a single function.DETAILED DESCRIPTION

[0033] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments.However, it will be understood by those of ordinary skill in the art that embodimentsmay be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to obscure the present subject matter.

[0034] Disclosed embodiments are directed to the goal of providing a test that uniformly assesses vision, ranging from roughly 20 / 200 to light perception. This comprehensive test is meant to be objective, not requiring verbal responses, and accurately determines whether the subject can see the test stimuli.

[0035] Disclosed embodiments make use of Optokinetic Nystagmus (OKN), which is the eye's automatic movement when tracking a moving object (e.g., a rotating drum in conventional approaches). The eye's reflexive “flip” back to the other side, as a target comes back into view, indicates the optokinetic response. OKN is not a visual acuity test but, rather, it is a measurement of visual function, which is particularly useful for non-verbal subjects. However, conventional OKN testing does not, itself, quantify visual acuity.

[0036] Figure 1 depicts a system for evaluation of visual function in a low-vision population using a display device. The system 100 includes a display device for the subject, e.g., a tablet device, referred to as the subject display device 105, which includes a processor 110, memory 115, and a display screen 120, as well as other components typically found in computer or tablet, such as network and wireless communication interfaces (not shown). In embodiments, the subject display device 105 may be solely a display, in which case it would have a display screen but might not have a processor and / or memory, so the software would be running on the operator display device 140 (discussed below) which would be using the subject display device 120 as an external display.

[0037] The subject 125 is positioned in front of the subject display device 105 so as to provide a defined viewing distance between the subject 125 and the display screen 120. A camera 130, e.g., an infrared camera, is used to track the subject’s eye movements while images are displayed to the subject 125 on the subject display device 105. The system 100 may further include a second tablet device used by an operator 135, referred to as the operator display device 140, having components similar to those of the subject display device 105. The operator display device 140 may control various aspects of the visual test being performed.

[0038] In embodiments, the camera 130 is connected to the operator display device 140 and data from the test is accumulated by the operator display device 140. The software to execute the visual test may run on the operator display device 140 or there may be some portion which runs on the subject display device 105. In embodiments, the subject display device 105 merely acts as a second display for the operator display device 140, In embodiments, the subject display device 105 runs software which controls at least part of the test.

[0039] Another view of a test set up is shown in Fig. 5, which shows a test set up for evaluation of visual function in a low-vision population with examples of actual test hardware.

[0040] Disclosed embodiments establish a low-vision visual acuity test based on a strong link between visual acuity and the OKN response. Fig. 11 is a plot of measured logMAR for normal vision subjects determined using Optokinetic Nystagmus (OKN) testing with diffusers, i.e. , diffusion foil. This portable test was conducted on a tablet, placed a defined distance, e.g., 40 cm, away from the subject. The FDA part 11 compliant software displayed a secure stimulus, a single verticalstripe, for the subject to view. This stripe's brightness was fixed at a low luminance level. The stripe widths were determined based on logMAR values, which is a standard for measuring visual acuity. For instance, a logMAR of 1.0, which signifies a 20 / 200 vision, may be used to determine the stripe’s width for a particular iteration. The test covered a range of logMAR values from 1.0 (20 / 200 vision) to 3.4, indicating extremely low vision.

[0041] In this context, the size of the stripe was changed based on whether the subject could see it, i.e. , whether the subject could successfully track the moving stripe. If the subject could not see the stripe, its width was increased. Conversely, if the subject could see the stripe, its width was decreased. This process continued until the stripe was just barely visible to the subject, a point referred to as the threshold. Furthermore, the stripe was presented with varying levels of contrast, either high or low, depending on the stimulus. The stripe was specifically designed to be isoluminant, meaning its brightness matched the background, as discussed in further detail below.

[0042] The eye movements of the test subject were tracked using a camera, e.g., an infrared camera. The test was conducted by a technician sitting next to the subject with normal vision. In this regard, see Fig. 6, which shows a subject and an operator positioned at a test set up for evaluation of visual function. The aim was to assess the effectiveness of the test, ideally with subjects having low vision. However, since such subjects are difficult to find, a system of diffusion foil was used instead for initial testing. Diffusion foil is a type of medium that can be compared to a translucent window - one can see through it, but not clearly, thus imitating impaired vision. Thegoal was to determine the extent of the subject’s vision when looking through this diffusion foil.

[0043] In this experiment, a technician observed a subject as they viewed stimuli through different foils. The subject's responses were automatically recorded. The technician's role was to monitor the process. The experiment was first conducted with a group of normal subjects to establish a consistent pattern of behavior. The objective was to avoid significant fluctuations in the subjects' responses from one day to the next, e.g., to avoid a situation in which a subject would score 20 / 200 on the first day and 20 / 4000 on the second day. Responses in both of these ranges were examined.

[0044] Fig. 11 , discussed above, shows good consistency between test and retest scores. This was within the limits of the ranges being examined, which were based on a logMAR 2.3 reference. The upper dots on the graph represent what would be equivalent to a logMAR Snellen 20 score, i.e. , about 24 foils. This indicates quite low vision. The lower series of dots represents an error value of approximately 1 .3 or 20 / 400. These subjects would normally have 20 / 20 vision, so in this experiment, their vision was purposefully reduced. The subjects were shown a letter of a certain size through a filter and a determination was made whether they could see it. This is how it was confirmed that the foils were providing the desired level (i.e., the desired reduced level) of visual acuity.

[0045] Isoluminance is a factor when assessing visual acuity. When reading a standard eye chart, at first, the viewer can clearly see and identify the letters. As the viewer reads down the chart, there is a point at which the letters cannot be recognized, but the viewer can still detect that there is something there - this isreferred to as the level of detection. The level of resolution is when the viewer can identify the letter, for example, recognizing an 'E'. As vision deteriorates, one can only detect a blob-like shape on the screen. The challenge lies in ensuring that the point of detection and the point of recognition coincide. To achieve this, a stripe is used which has the property of isoluminance. The stripe has a bright center along its longitudinal direction and fades to black on the sides, i.e. , across the width of the stripe. The average brightness of the stripe matches the background. This means that the point at which the viewer loses sight of the stripe coincides with the point at which the viewer can no longer identify it. This phenomenon is known as vanishing optotypes. The letters seem to disappear instead of becoming blurry as the viewer’s ability to see them diminishes. This characteristic, arising from the property of isoluminance, helps to more accurately determine a viewer’s level of visual acuity.

[0046] In embodiments, a first phase of the test may involve a basic vision test that a subject takes in order to qualify for the experiment. The subject needs to have some level of vision; they cannot be completely blind. In this initial test, the screen alternates between black and white. This simple visual change is designed to attract the attention of the subject. The technician then observes whether or not the subject can discern this change, thereby determining if they have some level of vision. When the screen alternates from black to white, the subject is asked to indicate if they see a change, either verbally or through observable attention shifts for nonverbal subjects. The goal is to establish whether the subject can distinguish between black and white.

[0047] In embodiments, a second part of the test may involve a color change in one quadrant of the screen. The subject's response, either through eye movement orverbal confirmation, indicates whether they've noticed the change. In some embodiments, this part of the test may not be included. The first part of the test essentially checks if the subject's vision level is adequate enough to be quantified as visual acuity. The subject is then exposed to a moving stripe stimulus on a screen while their eye movements are tracked by an infrared camera. A technician monitors the subject's responses on a separate screen. When the subject's gaze or attention shift is detected by the software and eye tracker, an alert is given, indicating whether the subject's attention shift was detected.

[0048] In some experiments, an eye-tracking test was performed using a computer’s built-in camera instead of an infrared camera. Initially, the test automatically detects if there’s an eye movement that corresponds to the optokinetic response. If detected, the width of the stripe on the screen automatically adjusts until it reaches a threshold where the test terminates. The first part of the test involves flashing a black and white screen to capture the subject’s attention. This is considered to be a pre-qualification step for the subject.

[0049] In embodiments, as the test progresses, if the subject’s eye movements are registered, the size of the stripe is decreased. If the subject reports that they did not see the stripe or if they are not tracking it, the stripe width is increased. This process continues, adjusting the stripe size until it reaches a point where the subject cannot see it, which is considered to be the threshold. As explained in further detail herein, the stripe has a unique appearance, due to its isoluminescent properties, with a bright center portion running in a longitudinal direction and darker edges in the width-wise direction. This is designed to keep the average brightness of the stripeequal to that of the background, which is usually equal to zero in the case of a display device, e.g., a tablet device.

[0050] Regarding the use of high contrast versus low contrast strips in the vision test, it is explained above that the width of a stripe corresponds to a logMAR value and that the test continues by decreasing the width until the smallest observable width is determined, providing the threshold logMAR value. The use of high contrast strips is primarily for patients who have contrast deficiencies and struggle to see low contrast. In other words, these patients need a greater difference between the stripe and the background to see clearly, hence the use of high contrast strips.

[0051] Contrast is a significant variable in general visual acuity tests like eye charts. For instance, there are low contrast ETDRS (Early Treatment Diabetic Retinopathy Study) charts where the letters have a lower contrast relative to the background compared to a standard chart. Contrast is an important aspect of visual function, because it is our vision’s ability to distinguish an object from its background. Therefore, the flexibility to vary the contrast is desirable in these tests. The contrast levels used in particular tests are calibrated based on what the subjects can see and the visual acuity they can achieve at different contrast levels.

[0052] In embodiments, the vision test may not provide a separate score for contrast. Instead, the contrast levels are described as either high contrast logMAR or low contrast logMAR. The test typically starts with low contrast logMAR, and if the subject cannot see it, the test shifts to high contrast logMAR. Obtaining both high contrast and low contrast logMAR thresholds could provide more information than traditional vision tests, which only provide one threshold logMAR. For example, an ETDRS chart is set for maximum contrast. Therefore, comparing the subject'sresponses to both low and high contrast vision objects could potentially provide more comprehensive information about their visual function.

[0053] In traditional vision tests, the targets are typically stationary, such as letters on a chart. Determining visual acuity with a moving target, like the isoluminant stripe (or “strip” - these terms may be used interchangeably), can be more challenging. The goal is to have the subject perceive the stripe at a certain width. However, unless the stripe is isoluminant, meaning it has the same average level of brightness as the background, the subject might only perceive something moving, rather than discerning its specific width.

[0054] In embodiments, isoluminant stripes may be used as “targets” in a manner akin to the concept of "vanishing optotypes.” An optotype is a standardized symbol used in vision testing, like the letters on an eye chart. In the case of a single letter, such as the letter “E”, each part or bar of the letter needs to be resolved. The term “vanishing optotypes” refers to the Moorfields vision chart, which uses isoluminant letters instead of standard letters. The vanishing optotypes are designed to gradually reduce the possibility of detection, which makes it harder for subjects to guess at what they are seeing on the chart. The detection of stripes in the test can be considered to be resolution of a single target, akin to resolving the bars of a vanishing optotype letter. However, detection of a stripe involves detection of the presence of the stripe as an object (or “target”), as opposed to recognition of a letter or symbol.

[0055] Vision charts using vanishing optotypes are particularly useful for differentiating conditions like Age-related Macular Degeneration (AMD) from normal vision. Vanishing optotypes have been shown in some studies to be more effectiveat differentiating AMD from normal vision than a standard eye chart. This can be tied to the manner in which our eyes process contrast across a range of spatial frequencies. Spatial frequency refers to the size of the patterns that our eyes can distinguish, from broad patterns to very fine details. Visual acuity refers to our ability to see fine detail. However, depending on the type of visual impairment, such as AMD, there may also be a loss of ability to see low-detail patterns. Vanishing optotypes provide additional information about a subject's vision by testing their ability to see these different levels of detail, from high to low, providing a more comprehensive assessment of their visual function.

[0056] To create an isoluminant stripe, i.e. , a stripe with the same average brightness as its background, the luminance of the stripe is tapered or gradually reduced as it approaches the edges. When the luminance of the stripe is averaged from the edges of the stripe to the middle, i.e., across the width of the stripe, it equals the luminance level of the background of a display device, e.g., a tablet device.

[0057] In embodiments, the isoluminant property of the stripe may be achieved by defining the brightness of the stripe in the horizontal (i.e., width-wise) direction based on two Gaussian distributions. Isoluminance, in this context, refers to the condition where the average brightness of the stripe is equal to the display background, which means the stripe and the background have the same perceived luminance. Taking the difference of two Gaussian distributions (i.e., “Difference of Gaussians (DoG),” as it is called in the art) can be used to create a stripe that has a higher brightness in the middle and lower brightness at the edges, or vice versa. This can help give the stripe a more defined edge, or make it appear more “focused”. To define thebrightness across the width of the stripe in this way, one Gaussian distribution may be subtracted from another. A larger standard deviation might be used for one Gaussian to represent the overall stripe width, and a smaller standard deviation for the other to represent the brighter / darker core of the stripe.

[0058] To obtain isoluminance, the brightness scale factors of each of the two Gaussian distributions are adjusted so that the average brightness of the stripe equals the background brightness. This usually involves integrating the difference of the two Gaussian distribution functions over the width of the stripe and setting it equal to the product of the width and the background brightness and then solving for the scale factors.

[0059] In embodiments, an algorithm is used in the vision test which compares the speed and magnitude of the subject’s eye movement to the movement of the stripe displayed on the screen. The eye movement can be described as two phases of Optokinetic Nystagmus (OKN), a natural reflexive eye movement. The first phase, i.e. , the “slow” phase, involves the eyes tracking the moving stripe. The second phase, i.e., the “fast” phase, involves the eyes quickly moving back to their original position. This results in a “sawtooth” pattern of eye movement, which can be quantified in terms of frequency. An advantage of this algorithm lies in it being based on the timing of the display, i.e., the movement, of the stripe. This contrasts with traditional methods, such as the spinning drum, which use a variety of different strips without considering their timing.

[0060] It is important to note the difference in Optokinetic Nystagmus (OKN) responses between induction and suppression methods. OKN is a natural reflexive eye movement that occurs as a response to a moving visual field. Someconventional techniques rely on the use of suppression in the OKN response. In such approaches, a dot is placed in the middle of a field of moving stripes. The subject is asked to focus on the dot, which is gradually made so small that it cannot be seen and, therefore, the subject cannot fully suppress the OKN response. In other words, the goal of the test is to suppress the OKN response, rather than to induce it.

[0061] However, suppression approaches are not suitable for subjects with low vision. When observing the eye motion response from a field of moving stripes, such as an OKN drum, there are two phases: the slow phase where the eyes track the moving stripes, and the fast phase where the eyes quickly move back to their original position. In the suppression approaches, which use a field of stripes, it can be difficult to track the particular stripe being viewed during the fast phase when the subject has low vision and the subject’s focus may shift from one stripe to another during this phase. Therefore, there can be variations in the magnitude and speed of the subject’s eye movements. In the approaches described herein, there is less possibility for such variations, because there is only a single stripe to track and, therefore, the subject’s eyes more reliably make undergo a full OKN response.

[0062] Regarding the speed of movement of the stripe across the screen, this may be determined based at least in part on factors such as the desired test length (e.g., in the context of keeping clinical trial tests to a reasonable duration) and the characteristics of the display (e.g., refresh rate) and may be adjusted based on observation of subjects’ eye movements and other experimental information. For example, increasing the speed too much may result in decreased OKN response. In embodiments, the visual angle of the full screen width of the tablet may be about 38 degrees (for a viewing distance of about 40 cm). The stimulus motion may be setto cross the screen in about 3 seconds, which results in a velocity of the stimulus of approximately 12.7 degrees / second. In embodiments, a range of about 10 to about 15 degrees / second or a range of about 10 to about 20 degrees / second may be used.

[0063] The steps of a method for performing evaluation of visual function in a low- vision population may be summarized as follows:1 . Set up a tablet computer to display a series of striped stimuli. Ensure the subject is positioned at a defined distance, e.g., 40 cm, from the screen.2. An automated eye tracker and an infrared camera identify the Saccadic Optokinetic Nystagmus (OKN) eye movements of the subject.3. The system displays on a display device (e.g., a tablet device) moving stimuli, e.g., a stripe, with widths of the stimuli corresponding to acuity levels ranging, e.g., from logMAR 1.0 (20 / 200) to logMAR 3.4.4. The system varies the width of the stripe to determine a threshold (OKN Visual Acuity, or VA).5. The system presents stimuli at both reduced and high contrast to assess the subject’s ability to distinguish between different contrast levels (e.g., using two or three contast levels).6. Thresholds are determined at each level of contrast to ensure the accuracy and consistency of the results.

[0064] In Step 3, above, the method may start by presenting stripes on the tablet screen with widths corresponding to a logMAR 1 .0 (20 / 200) value. The stripe width may be gradually increased until it corresponds to a logMAR 3.4 value. Each stripe width represents a different level of visual acuity depending on its width.

[0065] In Step 4, above, after presenting each stripe width, the subject’s eye movements are observed using the automated eye tracker and infrared camera. If the subject is able to track the stripe successfully, the system decreases the width of the stripe (which corresponds to a decrease in the logMAR value, or an increase in visual acuity). This process is continued, with the system adjusting the stripe widthbased on the subject’s ability to track it, until a threshold is determined where the subject can no longer track the stripe. This is the Optokinetic Nystagmus Visual Acuity (OKN VA) threshold.

[0066] In Step 5, above, once the OKN VA threshold has been determined, the system changes the contrast of the stripe. The system presents the stripe at a reduced contrast and determines whether the subject can still track it. If so, the system decreases the contrast further until the subject can no longer track the stripe. This results in the OKN VA threshold at reduced contrast. The system repeats the process with a high contrast stripe to determine the OKN VA threshold at high contrast. This process tests the subject’s ability to distinguish between different contrast levels, which is an important aspect of visual function.

[0067] At step 6, above, at each contrast level of the striped stimuli, the system identifies a threshold for each contrast level. These thresholds are the stripe widths at which the eye’s Optokinetic Nystagmus (OKN) response changes noticeably. This change indicates a new level of visual acuity. The system repeats this process for both reduced and high contrast levels, resulting in a total of, e.g., six, thresholds. These thresholds help in determining the subject's visual acuity under different contrast conditions.

[0068] This following describes visual acuity experiments where the participants’ eye movements were tracked as they viewed a series of striped stimuli on a tablet computer. The striped stimuli were used to elicit Optokinetic Nystagmus (OKN), which is a type of eye movement where the eyes will make a quick shift (saccade) to follow a moving object and then slowly move back to their original position. Theexperiments involved subjects with normal vision and subjects with low vision and were conducted as a series of three pilot studies.

[0069] In the experiments, striped stimuli were presented on a display, e.g., the display of a tablet computer, to be viewed at a defined distance, e.g., 40 cm. The saccadic OKN eye movements were identified using an automated eye tracker and an infrared camera to objectively track and record the eye movements, without relying on the subjects’ subjective reports. The room lighting was turned off during the testing. Stripe widths of the stimuli were presented from logMAR 1 .0 to 3.4. The width of the stripe was varied to determine a threshold (OKN VA), i.e. , the smallest stripe width at which the subject’s eyes can still track the moving stripes, which indicates the limit of their visual acuity. Thus, the stimuli consisted of stripes of various widths, ranging from a logMAR (Logarithm of the Minimum Angle of Resolution) of 1.0 to 3.4. LogMAR is a common method of measuring visual acuity, with a higher logMAR indicating poorer vision. For example, a logMAR of 1 .0 corresponds to a Snellen fraction of 20 / 200. Stimuli were presented at both reduced and high contrast. Three thresholds were determined at each level.

[0070] In embodiments, the logMAR value for a particular stripe width may be determined based on the subtended visual angle of the stripe width for a specific viewing distance:LogMAR = log10(oc ° X 60) where: oc ° is the subtended angle (in degrees), w is the width of the stripe (in cm), d is the viewing distance (in cm).

[0071] For example, if the width of the stripe is 2 cm and the viewing distance is40 cm, the resulting subtended angle would be 2.86°, which results in a logMAR value of 2.24.

[0072] As explained above, the isolum inant property of the stripe may be achieved by defining the shape of the stripe in the horizontal direction based on a difference of two Gaussian distributions. In such a case, the shape of the curve (i.e. , the shape of the resulting distribution of brightness across the width of the stripe) approaches the background level gradually and does not have a clear boundary (see, e.g., Figs. 3 or 4), as in the case of a simple stripe (see, e.g., Fig. 2). At the center of the stripe, the intensity is highest. The intensity then drops below the background level and increases up to the background again toward the edges of the stripe. The way in which the stripe width is defined may be based on a subjective impression of the width. For example, if the minimum intensity level is lmin and the background level is IBG, the stripe width is defined at the point at which the intensity is increased to a defined fraction, e.g., 30%, of the difference between minimum and background. For example, if IBG = 80 and lmin = 67, then the width (w) is defined based on the point in the width-wise direction at which the intensity level, which may be referred to as the edge intensity level, reaches the defined threshold: ledge = 0.3 * (80 - 67) + 67 = 70.9.

[0073] The width of the stripe can range during the test from the widest at, e.g., logMAR 3.0 to the narrowest at, e.g., logMAR 1 .0. A modified binary search algorithm (see, e.g., A.J. Anderson, C.A. Johnson, “Comparison of the ASA, MOBS, and ZEST threshold methods,” Vision Research 46 (2006) 2403-2411 ) may be used to determine a threshold. As explained above, an infrared camera is used to trackthe subject's eye movement. When the stripe moves across the screen, and the subject’s eye makes a defined number, e.g., three, of confirming eye movements, known as OKN saccades, the subject is considered to have seen the stripe. If the subject sees the stripe, the width is reduced. If the subject does not see the stripe, the width is increased.

[0074] To evaluate OKN response in normal subjects, initial testing was performed on a group of 11 subjects with normal vision. Mean ETDRS BCVA ± SD was 0.02 ± 0.11 OD and 0.02 ± 0.07 OS. Mean age for the group was 60.5 ± 12.0 years. “ETDRS” means Early Treatment Diabetic Retinopathy Study, which is a standard chart used worldwide for measuring visual acuity, especially in research settings. “BCVA” means Best Corrected Visual Acuity, which is the sharpest, clearest vision a person is capable of achieving, with the aid of corrective lenses if necessary. “OD” is an abbreviation for “oculus dexter,” which is Latin for right eye, and “OS” stands for “oculus sinister,” Latin for left eye. Thus, the mean BCVA in the right eye was 0.02, with a standard deviation (SD) of ± 0.11 . For the left eye, the mean BCVA was also 0.02, with a standard deviation of ± 0.07.

[0075] Luminit light shaping diffusive media were used to reduce visual acuity to approximate visual impairment. Luminit (Torrance, California) is a company that makes light shaping diffusers, which are used to scatter light in specific patterns. Three levels of media were used: 10°x10°, 30°x30° and 60°x60°. These values refer to the diffusion angles of the media, meaning the angle at which light is scattered by the diffuser. A larger angle will scatter light more widely, simulating a more severe level of visual impairment. The 10°x10° diffuser was found to reduce visual acuity to 1 .8 logMAR. Similarly the 30°x30° diffuser was used to reduce visual acuity toapproximately 2.4 logMAR. A single Sloan letter was presented at the 40 cm distance sized to confirm impaired VA level. The Sloan letter set, which includes 10 specially designed letters, is often used in visual acuity tests. The letter was sized to confirm the impaired visual acuity level. This means the letter was presented at a size that the participant, with their artificially impaired vision, would be expected to barely be able to identify, confirming the effectiveness of the diffusive media in simulating visual impairment.

[0076] To evaluate OKN response in subjects with impaired vision, a group of 16 subjects with impaired vision was next tested. Mean age was 68.9 ± 7.93 years.Mean VA was 0.46 ± 0.77 logMAR (better eye) and 1 .59 ± 0.84 logMAR (worse eye) The test was fully automated with the eye tracker controlling the response and size of the subsequent bar.

[0077] The three pilots constituting the experiments are discussed in detail below.

[0078] The first, initial pilot involved testing normal subjects using tablet devices with diffusers. As shown in the table of Fig. 12, Initial testing included four subjects with normal vision at each of three diffuser levels, i.e. , at levels where visual acuity was artificially reduced using diffusive media at three different diffusion angles (10°, 30°, and 60°). The mean OKN VA for all subjects was measured at each diffuser level and expressed in terms of the logMAR scale for measuring visual acuity. As explained above, OKN VA stands for Optokinetic Nystagmus Visual Acuity, which is a measure of visual acuity based on the tracking movements of the eyes. Mean assessed OKN VA was 1.84 ± 0.019 logMAR at the 10° diffuser level, 2.31 ± 0.039 logMAR at the 30° level, and 2.56 ± 0.032 logMAR at the 60° level.

[0079] The determined thresholds, i.e. , the point at which a subject could no longer track the moving stripes, showed good agreement with visual confirmation using computer displayed optotypes (i.e., symbols used in chart-based vision testing) sized for 1 .8 and 2.4 logMAR. This means that when the subjects’ visual acuity was tested using the diffusers and then tested again using traditional optotypes, the results were consistent, demonstrating that the diffusers were effective at simulating the desired levels of visual impairment.

[0080] The second pilot involved testing normal subjects with variable stimulus contrast. Specifically, this phase of the experiments examined the effect of stimulus contrast on visual acuity. The testing was performed using only the 30° diffuser.Eleven subjects were tested at both high and low stimulus contrast levels. The term “stimulus contrast” refers to the difference in luminance between an object and its background. High contrast means there is a large difference between the maximum light intensity at the center of the stripe and the background, making the stripe more discernible. Low contrast means there is little difference in light intensity, making the stripe less discernible. OKN VA was calculated for both the high and low contrast conditions.

[0081] Fig. 7 is a plot of visual acuity test results for each eye of eleven normal subjects determined using Optokinetic Nystagmus - based visual testing with high and low contrast. The results showed that subjects had a mean OKN VA of 1 .22 ± 0.18 logMAR with high contrast and 2.11 ± 0.33 logMAR with low contrast for the right eye (OD). For the left eye (OS), the mean OKN VA was 1 .27 ± 0.26 logMAR for high contrast and 2.14 ± 0.35 logMAR for low contrast. Thus, as expected, visual acuity, as measured by OKN VA, was better (i.e., lower logMAR value) under highcontrast conditions and poorer (i.e. , higher logMAR value) under low contrast conditions for both eyes.

[0082] The third pilot involved testing low vision subjects. Sixteen subjects were tested in two sessions (“Test 1” and “Test 2”) using stimuli of lower contrast. Figs. 8 is a plot of measured logMAR for low vision subjects determined using Optokinetic Nystagmus (OKN) testing for Test 1 vs. Best Corrected Visual Acuity (BCVA). Fig. 9 is a plot of measured logMAR for low vision subjects determined using Optokinetic Nystagmus (OKN) testing for Test 2 vs. Best Corrected Visual Acuity (BCVA);

[0083] The eye with worse vision was tested unless the fellow eye had identical vision or light perception only. Four of the subjects had visual acuity so low that they limited to finger counting, and three subjects could only detect hand movements. Approximate visual acuity was estimated for these subjects for analysis and presentation.

[0084] Fig. 10 is a plot of a pair-wise t-test was conducted to compare the Optokinetic Nystagmus Visual Acuity (OKN VA) measurements taken at Test 1 and Test 2. A pair-wise t-test was conducted because the two groups are the same subjects tested at two different times in Test 1 and Test 2. Significant correlations were found between the Optokinetic Nystagmus Visual Acuity (OKN VA) measurements taken at Test 1 and Test 2 (R = 0.81 , p=0.000015), and between the OKN VA at Test 2 and the estimated VA (R = 0.56, p = 0.0076). A pair-wise t-test comparison of OKN VA Test 1 and Test 2 showed no significant difference, meaning the subjects' performance was relatively stable across the two tests. However, the differences between OKN VA Test 1 and estimated VA, as well as between OKN VATest 2 and estimated VA, were found to be statistically significant (p< 0.002).

[0085] Fig. 13 depicts a method (900) for evaluating visual function in a low-vision population. The method includes determining a width of a first stripe to be displayed to a subject on a screen based at least in part on a defined viewing distance and a first visual acuity value (910). The method further includes determining a brightness characteristic in a width-wise direction across the first stripe to provide isoluminance of the first stripe with respect to a background intensity of the screen (920). The method further includes displaying, to the subject of a test, the first stripe in motion across the screen of the display device at a defined velocity, the first stripe having the determined brightness characteristic in the width-wise direction (930). The method further includes generating tracking data from movement of the subject’s eyes using a camera to identify specified eye movements of the subject (940). The method further includes determining a tracking decision characterizing whether the subject successfully tracked the first stripe based at least in part on the tracking data (950). Based at least in part on the tracking decision, the method may branch to: (i) determining a width of at least a second stripe to be displayed to the subject on the screen in a further iteration of the test (960); or (ii) ending the test. Upon ending the test, a visual acuity threshold is determined based at least in part on a width of a last or penultimate displayed stripe (970).

[0086] Aspects of the disclosed embodiments may be embodied in the form of a system, a computer program product, or a method. Similarly, aspects of the disclosed embodiments may be embodied as hardware, software, or a combination of both. Aspects of the disclosed embodiments may be implemented as a computer program product saved on one or more computer-readable media in the form of computer-readable program code embodied thereon.

[0087] The computer-readable medium may be a computer-readable storage medium. A computer-readable storage medium may be, for example, an electronic, optical, magnetic, electromagnetic, infrared, or semiconductor system, apparatus, or device, or any combination thereof.

[0088] Computer program code in disclosed embodiments may be written in any suitable programming language. The program code may execute on a single computer, or on a plurality of computers. The computer may include a processing unit in communication with a computer-usable medium, where the computer-usable medium contains a set of instructions, and where the processing unit is designed to carry out the set of instructions.

[0089] The above discussion is meant to be illustrative of the principles and various embodiments. Numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated. It is intended that the following claims be interpreted to embrace all such variations and modifications.

Claims

CLAIMS1 . A method for evaluating visual function in a low-vision population, comprising: determining a width of a first stripe to be displayed to a subject on a screen based at least in part on a defined viewing distance and a first visual acuity value; determining a brightness characteristic in a width-wise direction across the first stripe to provide isoluminance of the first stripe with respect to a background intensity of the screen; displaying, to the subject of a test, the first stripe in motion across the screen of the display device at a defined velocity, the first stripe having the determined brightness characteristic in the width-wise direction; generating tracking data from movement of the subject’s eyes using a camera to identify specified eye movements of the subject; determining a tracking decision characterizing whether the subject successfully tracked the first stripe based at least in part on the tracking data; determining a width of at least a second stripe to be displayed to the subject on the screen in a further iteration of the test or ending the test based at least in part on the tracking decision; and determining, upon ending the test, a visual acuity threshold based at least in part on a width of a last or penultimate displayed stripe.

2. The method of claim 1 , wherein the visual acuity threshold is determined based at least in part on the defined viewing distance and a visual angle subtended by the width of the last or penultimate displayed stripe.

3. The method of any of claims 1 or 2, wherein the screen is the screen of a display device, the method further comprising positioning the display device to provide the defined viewing distance between the subject’s eyes and the screen.

4. The method of any of claims 1 to 3, further comprising setting a first contrast level of the first stripe of a defined set of contrast levels.

5. The method of claim 4, further comprising setting a second contrast level of the first stripe and repeating the test to determine a visual acuity threshold corresponding to each contrast level of the defined set of contrast levels.

6. The method of any of claim 1 to 5, wherein, in said determining the width of the first stripe, the width of the first stripe is calculated to correspond to a logMAR value between about 1 .0 and about 3.4.

7. The method of any of claims 1 to 6, wherein the defined viewing distance is between about 30 cm and about 50 cm.

8. The method of any of claims 1 to 6, wherein the defined viewing distance is between about 35 cm and about 45 cm.

9. The method of any of claims 1 to 8, wherein the defined velocity is in a range of about 10 degrees / second to about 15 degrees / second.

10. The method of any of claims 1 to 8, wherein the defined velocity is in a range of about 10 degrees / second to about 20 degrees / second.11 . The method of any of claims 1 to 10, wherein the specified eye movements are indicative of Optokinetic Nystagmus (OKN).

12. The method of any of claims 1 to 11 , wherein the display device is a tablet computer.

13. The method of any of claims 1 to 11 , wherein the display device is an external monitor in communication with a computer or display device.

14. A system for evaluating visual function in a low-vision population, comprising: a first display device comprising a screen and at least one processor and memory; and a camera, wherein the memory stores code which when executed by said at least one processor causes said at least one processor to perform the method of any one of claims 1 to 13.

15. The system of claim 14, further comprising a second display device in communication with the first display device, the second display device comprising a screen, at least one processor and memory.

16. The system of claim 14, wherein the camera is an infrared camera.

17. A non-transitory computer-readable medium storing instructions that, when executed by one or more processors of a display device, cause said one or more processors to perform the method of any of claims 1 to 13.

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