Behavior
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Measuring the Behavioral Effects of Intraocular Scatter
Chapters
Summary February 18th, 2021
In this protocol, we outline the conceptual design elements and structural development of a glare acuity apparatus. Additionally, the design of a device for measuring positive dysphotopsia (halos, spokes) and two-point light thresholds is described.
Transcript
The behavioral consequences of intraocular scatter are often severe and central to many common problems such as driving accidents. This study represents a novel method for measuring the effects of scatter on visual recognition. The main advantage of this methodology is its high degree of ecological validity while using relatively simple optics.
Cataract is the leading cause of blindness in the world. Prior to overt disease, the lens scatters light and degrades vision for decades. Proper measurement could motivate more timely treatment.
Working on an optical table, install a 1000 watt xenon arc lamp with the associated power supply at the posterior end of the bench. Install the first lens at a position that collimates the light from the source and introduce an optical element to remove heat within the optics generated by the intense light source. Introduce the next lens within the optical system to focus light to a small point on the 100 millimeter circular neutral density filter, which attenuates light over a linear range of about two log units of optical density.
Determine the nominal position of the filter using a digital readout coupled to a potentiometer. Install a diffuser behind the neutral density filter. Use a calibrated radiometer to determine the actual amount of light transmitted that corresponds to the circular filter's position and to periodically confirm that the overall energy within the system remains constant over the course of the experiment.
Use a mechanical shutter or a blocking filter and holder to occlude the stimulus between trials. Add the next lens to the system, a collimating lens, placed such that light expands to match the diameter of each letter aperture, fully illuminating the optotype. Construct the letter apertures or purchase them as metal stencils.
Place the letter apertures in a circular rotator with spring-loaded tabs and divots to lock each letter in place so there is no movement of the wheel during the experiment. Next, baffle the system such that subjects can only see the back-illuminated letter apertures. For instance, place the optics of the system in one room with the subject in an adjoining room.
Position a hole within the doorway adjoining the rooms and align it so that subjects cannot see the experimenter or stray light. To ensure that the position of the eye relative to the visual system is fairly precise, create some form of head and chin rest assembly mounted on a moveable cart. Add a mount behind the tube to allow for the use of trial lenses to correct for refractive error using standardized lenses without tinting.
Use a laser level to ensure alignment of the eyepiece with the optics. Before beginning the protocol, explain the nature of the experimental task by showing the subject super threshold stimuli. Let me know when you're able to see the letter and the letter you can see.
Use a random letter generator to organize the letters on the wheel into a unique, random order. Use the method of limits to get close to the threshold and then constant stimuli to obtain a precise value of the subject's glare recognition acuity threshold. Ensure that the subject is aware that the task is fairly simple.
Run enough trials to generate a psychometric function that allows for derivation of an accurate probabilistic threshold. The results indicated the variation in the number of letters seen at one relatively bright intensity level. A wide variation was present even when testing healthy young subjects.
Data from the halos and spokes were obtained from different samples of 23 young subjects. Both samples were recruited from the student population at the University of Georgia and all subjects had good acuity. The minimum distance required to resolve two points of light as distinct was also measured.
Despite the sample being so homogenous, there was wide variation in the behavioral measures of scatter, which standard clinical measures of visual function failed to quantify. It is important to remember to use a light source that effectively simulates white sunlight. A mounted laser level is useful to ensure overall alignment.
There are a number of known variables that co-vary with glare disability, such as age, covert anterior ocular disease, and others. The effects of such variables on recognition under glare conditions has yet to be evaluated. This method will allow such studies.
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