Motion-Acuity Test for Visual Field Acuity Measurement with Motion-Defined Shapes

Summary

A novel motion-based acuity test that allows the assessment of central and peripheral visual processing in low-vision and healthy individuals, along with goggles limiting peripheral vision compatible with MRI protocols, are described here. This method offers a comprehensive vision assessment for functional impairments and dysfunctions of the visual system.

Abstract

The standard visual acuity measurements rely on stationary stimuli, either letters (Snellen charts), vertical lines (vernier acuity) or grating charts, processed by those regions of the visual system most sensitive to the stationary stimulation, receiving visual input from the central part of the visual field. Here, an acuity measurement is proposed based on discrimination of simple shapes, that are defined by motion of the dots in the random dot kinematograms (RDK) processed by visual regions sensitive to motion stimulation and receiving input also from the peripheral visual field. In the motion-acuity test, participants are asked to distinguish between a circle and an ellipse, with matching surfaces, built from RDKs, and separated from the background RDK either by coherence, direction, or velocity of dots. The acuity measurement is based on ellipse detection, which with every correct response becomes more circular until reaching the acuity threshold. The motion-acuity test can be presented in negative contrast (black dots on white background) or in positive contrast (white dots on black background). The motion defined shapes are located centrally within 8 visual degrees and are surrounded by RDK background. To test the influence of visual peripheries on centrally measured acuity, a mechanical narrowing of the visual field to 10 degrees is proposed, using opaque goggles with centrally located holes. This easy and replicable narrowing system is suitable for MRI protocols, allowing further investigations of the functions of the peripheral visual input. Here, a simple measurement of shape and motion perception simultaneously is proposed. This straightforward test assesses vision impairments depending on the central and peripheral visual field inputs. The proposed motion-acuity test advances the capability of standard tests to reveal spare or even strengthened vision functions in patients with injured visual system, that until now remained undetected.

Introduction

Most of the available visual tests are directed to examine the features processed by central vision, relying on the input deriving from the central retina¹. The central retina has the densest cone-photoreceptor population for maximal visual acuity and lacks rod photoreceptors, which dominate the peripheral retina². The presence of densely packed photoreceptors is also reflected in an increased density of ganglion cells, which means a greater number of axons are directed to the optic nerve and, eventually, to the visual cortex. Outside the fovea towards the periphery, rods outnumber the cone photoreceptor³. With the rods' wider bodies and the sparser mosaic of photoreceptors, the peripheral retina is primarily responsive to night vision and motion awareness⁴.

Classically, it was believed that visual processing, depending on stimulation of the central part of the visual field, is devoted to the fine analysis of stationary objects, and its peripheral part is specialized in detecting motion and bringing objects to the central, foveal vision, where it is further analyzed^5,6. However, now we have emerging evidence showing that on the cortical level, the fine analysis of the stationary pathway is not fully separated from the motion-sensitive one^6,7,8. Testing form and motion perception simultaneously is classically performed using moving gratings⁹ and glass patterns¹⁰ and also concentric rings motion¹¹. Our goal is to introduce a test that is close to the normal life of visually disabled people, which can lower their frustrations and give hope by explicitly showing them that some features of their visual processing might still be preserved and even strengthened. The proposed motion-acuity test based on random dot kinematograms (RDKs) combines motion and shape perception analysis and simultaneously tests the functioning of the motion and shape perception. Within the motion-acuity test, there are many possibilities of psychophysical features to test, such as different velocities, directions, and contrasts of the RDKs. By changing the parameters, we can manipulate the strength of stimulation, either specific for the central processing or peripheral. For example, detecting fast-moving objects is a well-described feature specific to peripheral visual processing¹², whereas the processing of the darks on the bright background is preferentially processed by central vision¹³. This test was initially performed on patients with retinal degeneration of photoreceptors, either specifically located within the central or peripheral retina¹⁴. Retinitis pigmentosa (RP) is manifested with peripheral damage and prevails in ~1/5000 patients worldwide¹⁵. Stargardt disease (STGD), with a prevalence of ~1/10000, is the most common cause of juvenile macular degeneration (MD)¹⁶. Damage to the photoreceptors at the central retina, as in macular degeneration or as in retinitis pigmentosa at the peripheral retina, results in corresponding visual field losses. Those visual field losses are reflected in the impairments of the features specific to the given visual system regions¹⁷. Importantly, the regions of the visual system that receive inputs from unaffected parts of the retina are also affected. It was previously shown in animal models of macular degeneration¹⁸ that after binocular central retinal damage, not only is acuity aggravated, but motion perception, a feature characteristic of peripheral processing, is reinforced. The motion-acuity testing described here provides an important insight for planning visual rehabilitation procedures. A complete view of the interplay between central and peripheral parts of the visual field has a crucial role in understanding how lost functions might be taken over by the spare parts of the visual system and how this process can be supported by visual training rehabilitation procedures. Inline, the knowledge of how regional retinal degeneration affects visual processing, especially beyond its damaged parts, still remains incomplete. Optical tests are based on the measurements of the stationary shape features. For instance, the visual acuity measurements rely on stationary stimuli, either letters (Snellen charts), grating charts, or vernier acuity charts.

For the purpose of widening the insight into the dynamics between central and peripheral vision in healthy eyes and eyes that have impaired central/peripheral visual functions, a motion-based acuity test measuring the shape and motion perception simultaneously was introduced. The motion-acuity test is based on the detection of centrally located shapes in negative or positive contrast (dark or light dots), an ellipse and circle with matching surfaces, built from random dot kinematograms (RDK) and separated from the same RDK background by velocity, coherence, or direction. Acuity is measured as the minimal difference perceived between circle and ellipse dimensions, and the results are given in visual degrees at which the subject stops to perceive the difference. Additionally, to check if the luminance contrast influences the measured motion acuity, stimuli can be presented in negative (black dots on white background) or in positive contrast (white dots on black background). All the available information about positive contrast (ON type) and negative contrast (OFF type) processing in the visual system comes from the stationary stimulation of the central visual field^19,20. But how peripheral processing of motion signals depends on contrast remains fairly unknown^14,21. It was only established that sensitivity to high velocities is specific for peripheral processing, whereas central motion processing engages slow velocities at higher spatial frequencies presented in positive contrast (ON type)¹². The positive and negative contrast versions of the motion-acuity stimuli, as well as the ability to modify the velocity of dots, as well as the coherence or direction, is crucial for a more detailed description of the full visual field. Additionally, a mechanical narrowing of the visual field to central 10 degrees is proposed using goggles with lenses replaced with opaque ones with centrally located holes. This easily replicable narrowing system, suitable for fMRI and TMS protocols, allows further investigations of the functions of the peripheral visual input and how visual peripheries influence centrally measured acuity. A similar system was initially validated in previous studies¹⁴, in which it was found that motion acuity tests in negative contrast and in fast motion, strongly activating visual peripheries, are the most difficult for all participants. For patients with Stargardt disease, they were unmanageable. Importantly, attenuation of visual periphery stimulation, by diminishing the velocity of RDKs, improves acuity thresholds in all tested subjects. In conclusion, we propose the task with motion-acuity measurement based on simple shape discrimination. Therefore, the results are straightforward and easy to understand also for patients and their caregivers. The motion-acuity test presented here is also addressed for users outside academia. The task is easy to explain to a broad range of ages and patient groups.

Protocol

All procedures were performed following the relevant guidelines and regulations and were approved by the Ethical Committee, WUM (KB/157/2017). Written consent was obtained from all participants, ensuring that they understood the general aim of the experiment and that they understood the inclusion of their data for statistical analysis purposes. All presented visual stimuli are generated using a Java-based desktop application (Viscacha2) created for the purpose of these experiments.  1. Setu…

Representative Results

The motion acuity task generates, for each participant, one result file for each stimuli procedure. An exemplary log file for a test participant has been included in the repository inside the doc folder. From row 1 to row 31, various settings are reported, such as the name of the patient and the configuration settings. The task block starts from row 34 and reports important information necessary for further analysis: event time, event type, trial, duration, selection, correct, success, experimenter, external stimuli, rev…

Discussion

Here, a novel method is described for measuring visual motion acuity using a set of stimuli based on random dot kinematograms. The result is given as a minimal perceived difference between a circle and an ellipse, and it allows one to see when the subject stopped distinguishing shapes from each other. The smaller the difference achieved, the better acuity: it means that the subject can still detect where the circle is, even though it is almost identical to the ellipse. The motion-acuity test presented here brings the res…

Materials

Chinrest			custom-made
Computer			Windows 10 or higher
Display			1920 × 1080, 31 inches
EyeLink 1000 Plus	SR Research		desktop mount
USB Keyboard
USB mouse