Measuring Visual Function with Paediatric Populations

Abstract

From birth, the human visual system undergoes several anatomical and functional changes before showing a functional response similar to adults. The Metropsis system can help you to monitor the development of core visual functions from age 5 years, as well as detect departures from the normal visual maturity caused, for example, by neuro-developmental disorders like amblyopia. Metropsis can also help you to monitor visual changes that take place as a consequence of treatment or rehabilitation.

Keywords: stereovision, stereo acuity, amblyopia, visual development, paediatric tests, pediatric tests, visual acuity, random-dot stereograms, RDS, depth perception.

What is amblyopia?

Amblyopia is a neuro-degenerative condition most often caused by an abnormal visual experience early in life (Levi et al. 2015). Its onset typically occurs within the first 3 years of life, and it is thought to reflect alterations in the properties of neurons in the early cortical areas, V1 and V2 (Levi, 2015). As a consequence, visual acuity, stereopsis and contrast sensitivity at high spatial frequencies (Levi, 2006) are compromised. 

Metropsis offers a variety of psychophysical tests, which allow the investigator to characterise changes in the above functions from age 5 years. Metropsis can also help the investigator to monitor visual changes that take place as a consequence of treatment or rehabilitation.

How can I measure stereo acuity with Metropsis?

The most common deficit associated with amblyopia is poor stereoscopic depth perception, which is often assessed by using psychophysical tests like Visual Acuity with Crowding Bars or stereo acuity tests. Metropsis offers two types of visual acuity tests with crowding bars.

Metropsis also offers a fast and simple test to measure stereo acuity based on dynamic Random Dot Stereograms (RDS). During this test, two vertically separated dynamic RDS are displayed on the screen. The upper RDS serves as a zero-disparity reference, while the lower RDS serves as the test of varying disparities. The patient is instructed to discriminate whether the upper or lower RDS is closer to them by pressing the top or bottom key on a response box. The RDS are generated such that the left and right eye are independently stimulated. This is achieved by using an LCD panel with a Film Pattern Retarded (FPR) layer that allows dichoptic stimuli to be viewed with passive polarized glasses.

FPR uses a spatial interleaving technique: dichoptic stimuli are encoded on alternate video lines so both eyes are independently stimulated on every video frame, but by different video lines. This means that, when viewed through a pair of passive polarised glasses, one eye will see even video lines and the other eye will see odd video lines. 

The Metropsis Stereo acuity test allows the investigator to measure fine thresholds in stereoscopic depth perception. This makes it ideal to assess the effect of rehabilitation programs aiming to restore visual acuity and stereopsis.

Why use Metropsis Stereo Acuity test?

1. It is difficult to assess stereoscopic vision in clinic, as some commercially available tests can be deceived by using monocular or binocular non-stereoscopic cues. Our Stereo Acuity test is free from either of those cues, and thus it can successfully be used to assess stereoscopic depth perception.

2. Our Stereo Acuity test takes on average two minutes and the task is very easy to perform even by 5-year-old children.

3. Our test can measure very fine stereo acuity thresholds, and thus it is an ideal test to assess the effect of rehabilitation programs aiming to restore visual acuity and stereopsis.

References

Levi DM, Knill DC, Bavelier D. Stereopsis and amblyopia: A mini-review. Vision Res. 2015;114:17–30.

Levi DM, Visual processing in amblyopia: human studies. Strabismus. 2006, 14(1):11-9.