Why We Need to Pay Attention to Psychometric Function Slopes

One of the key tools of noninvasive visual assessment(\p)(p) is the measurement of visual thresholds in a variety of test targets, including letter acuity charts, perimetric visual fields, spatial and temporal sensitivity functions, color discrimination thresholds and so on through many variants. Implicit in the measurement of any threshold (of either the detection or discrimination type) is a psychometric function of percent correct versus stimulus strength. A neglected variable in the analysis of psychometric data is the steepness of the psychometric function, which is usually assumed to be fairly stable across conditions and observers and therefore to play little role in determining the measured sensitivity levels and their disturbances by eye disorders.