Limitations to visual detection of suprathreshold patterns

Measurements of detection thresholds for gratings in noise have revealed that human performance is limited by internal noise and by sampling inefficiency. Many common visual tasks involve detection of targets that are above threshold contrast, and we sought to determine the limits to visual detection in such conditions. Simple reaction time (RT) was measured for detecting a flashed Gabor patch in dynamic Gaussian white noise. The ideal observer in a simple RT task must form an estimate of the time of arrival of the signal and hit the button at that time. For an ideal observer, the variance of this time-of-arrival estimate increases linearly as the variance of the external noise increases. The human observer's RT variance behaves in a similar way, but humans have low sampling efficiency and add internal noise. As the signal contrast increases, the internal noise remains constant, but the sampling efficiency increases. Thus observers in everyday situations may detect visual patterns better than might be expected from extrapolation of threshold data. [Supported by a grant from the Engineering and Physical Sciences Research Council (UK) to WS.]