Studies of psychophysical methods for measuring visual thresholds.

The comparative adequacy of various psychophysical procedures employed to measure visual thresholds has been investigated. Adequacy is defined by: (a) reliability; (b) inferred validity; and (c) sensory-determinacy Reliability refers to the extent to which repetitions of measurements made under presumably identical experimental conditions differ from each other. Inferred validity refers to the extent to which measurements depend upon variables which are generally conceded to be irrelevant to the visual functions of interest. Sensory-determinacy refers to the absolute magnitude of the threshold. The lower the threshold, the more sensory-determinate it is considered to be. Procedural variables which have been studied include the response utilized by the subject to indicate discrimination; the number, spacing, and order of light intensities presented in the measurement series; the general attitude which the subject adopts; and the extent to which the subject is given knowledge of the correctness of his responses. Psychophysical procedures have been found which appear to have optimum reliability, validity, and sensory-determinacy.