Natural Vision Testing
Viewed with Natural Vision
The Frisby Stereotest presents ‘real depth’ objects, viewed naturally. This means they do not use stereograms to create depth effects, and they do not require wearing special glasses.
This avoids the disadvantages of stereograms for some patient groups, for example babies and young children, who may refuse to wear red/green or polaroid spectacles, or become distracted by them and not concentrate on the test. In addition, some orthoptists believe that such spectacles can be dissociating if the patient has fragile binocular single vision, and thus they can give a misleading result.
A Random Dot Test
The Frisby Stereotest is a random dot test. This means the test figure has the same random dot pattern as the background and so cannot be distinguished monocularly. It is a common misunderstanding to think that all random dot tests must have a fine dot texture. However the Frisby Stereotest is designed to include some coarse elements so that it will reveal stereopsis even for patients with ambylopia.
The Frisby Stereotest is not a stereogram test.
How Do Stereogram Stereotests Differ?
Stereogram-based stereotests assess stereopsis by presenting separate images to each eye, which the brain must combine to perceive depth. These tests rely on binocular disparity—the small differences between what each eye sees—to determine whether a person can experience stereopsis. They use specially designed images, often in the form of random dot patterns, which contain hidden shapes that can only be seen in depth when both eyes work together. However, to ensure each eye sees the correct image, these tests require special viewing aids like red-green glasses or polarised glasses. The brain fuses these two images, revealing the hidden 3D shape if stereovision is present.
While these stereograms tests are widely used, they are not always suitable for young children or individuals who struggle to wear the special glasses.
They also do not allow for repeat testing without the patient learning the hidden image and where it is in the stereogram.
Advantageous Features of the Frisby Stereotest
The Frisby Stereotest offers the key advantage that it creates a real depth target without the need for special eyewear, making testing easier and more natural.
It also allows for multiple testing over time of the same patient. This is because the test can be rotated and turned to present a new random position as many times as is required, and the patient cannot learn to find the target by memory or other monocular clues.
