The full-field stimulus threshold (FST) is a visual psychophysical test designed to assess a person’s light sensitivity threshold by identifying the lowest luminance flash that elicits a visual sensation. FST can be utilized to quantify low vision when tests such as full-field electroretinography (ERG), visual acuity, perimetry, and microperimetry are not sensitive enough to measure the patient’s vision.
Determine the light sensitivity threshold
Patient must first fully dark adapt, preferably in a quiet room. To begin the test, the patient should remain calm and alert, holding a button box. Their heads should be positioned in the Ganzfeld ColorDome. The test will deliver simultaneous auditory and visual stimuli. Each time the patient hears a “beep,” they should respond by pressing either “yes” or “no” based on whether they perceive the light. A threshold is determined within 1-2 minutes.
Differentiate rod and cone sensitives
The FST test functions by automatically adjusting flash intensities until the software accurately measures and reports the subject’s threshold to a flash of light. To differentiate between rod and cone sensitivities, tests can be performed using blue and red light. Rods are more sensitive to blue light than red, leading to noticeable differences in thresholds for these colored stimuli. White light stimuli an be used if rod/cone differentiation is not necessary.
Interpret FST threshold responses
Results are displayed by color and eye tested. Each threshold is graphed as an individual result, with the X-axis indicating flash intensity and the Y-axis indicating the probability of seeing the flash. The threshold is defined as the luminance which a patient can see 50% of the time. It is denoted by a square on the graph. The DiagnosysFST proprietary quality algorithm rates each response on a scale from 0 to 3, ranging from not acceptable to very good.
New ways to use DiagnosysFST
Variations of the DiagnosysFST now include mesopic and photopic light conditions, in addition to scotopic light conditions. Learn more about these light-adapted sensitivity thresholds. You can also do central field testing using the iMask with DiagnosysFST.
References
Jolly, J.K., Grigg, J.R., McKendrick, A.M. et al. ISCEV and IPS guideline for the full-field stimulus test (FST). Doc Ophthalmol 148, 3–14 (2024). https://doi.org/10.1007/s10633-023-09962-7