PURPOSE. We have investigated the effects of contrast and print size on response time in a visual search task. METHOD. We have developed a contrast sensitivity test in which numbers 1 through 9, in graded contrast levels were presented simultaneously on a computer display screen. We measured subjects’ response times taken to correctly locate the target numbers using a computer mouse. Contrasts were varied systematically in log increments of 0.2 or 0.1 over the ranges of 0.0-1.6 and 1.4-2.2 log units, respectively. Our nine normally-sighted subjects were also tested with standard clinical tests, the Pelli-Robson Contrast Sensitivity Test and visual acuities with a Bailey-Lovie chart.
RESULTS. Curves showing response times versus contrast for 3 print sizes were determined by averaging values across subjects. With the 27 mm numbers, there was little difference in response time, until there was a rise in response time after log contrast sensitivity levels of 1.70. For the 11.3 mm print, there seems to be an earlier rise and increase in response time after a log contrast sensitivity of 1.50. For the 4.7 mm numbers, search was more difficult and most subjects were unable to detect targets above 1.5 log units (for the "thresholding" condition) and a dramatic increase in time was seen after a log contrast of 1.00.
CONCLUSIONS. We found systematic relationships between search efficiency, contrast and print size. Search times were slower for smaller print, even at high contrast. Response times were slower at lower contrast levels and this is more pronounced for smaller print sizes. Our test can be used to provide a systematic means of measuring contrast threshold and search efficiency with reduced contrast. With minor modification, it may become a useful clinical test for low vision patients.