Contrast Sensitivity and Efficiency at Search and Detection Tasks

Abstract

Abstract: : Purpose: Threshold contrast sensitivity can be a useful predictor of functional abilities. Measuring the speed of responses to low contrast stimuli may provide better insights into functional capabilities. Methods: Two new computer tests have developed to measure response times for search and detection tasks as the contrast of the stimulus is varied. In the first test, large numbers (1-8) 4-cm high appear on the screen in sequentially reducing contrast. The subject quickly touches the numbers in order and time is recorded. After a series of 6 display screens, there have been two presentations at each of 24 contrast levels. In the second test, a large (4-cm) solid square appears in one of four quadrants on the display screen. As soon as the subject touches a square, it disappears and a new square appears. There are 6 sweeps of the contrast range that can give two presentations at up to 24 contrast levels. The squares may be blinking, jumping or stationary. Both the number-search and square-detection tests provide measures of threshold CS, as well as response times as a function of contrast. Each test takes less than 2 minutes to complete. In a mixed group of 62 low vision clinic patients, we performed the number-search test and the detection test for both blinking and jumping squares. Visual acuity and Pelli Robson contrast sensitivity were also measured. Results: On average, there was marginally better (0.08 log unit) contrast sensitivity and better repeatability of response times for the blinking and jumping squares than for number-search tests. The PR CS thresholds correlate best with the thresholds for the number search task. (r= 0.70) For each subject, for each test, we can identify a critical contrast level at which the subject can still achieve their own best response time. Differences between critical and threshold contrasts tend to be more pronounced in glaucoma. There is large diversity in both CS and response time patterns amongst individuals with AMD and DR. Subjects with constricted fields show poorer contrast thresholds and repeatability on the computer screen tests, but this might be a relevant to predicting functional disabilities. Conclusions: New practical contrast sensitivity tests have been developed to provide useful measures of contrast sensitivity and response efficiency to low contrast targets.

Keywords: contrast sensitivity • low vision • vision and action