Psychometric function

A psychometric function describes the relationship between a parameter of a physical stimulus and the responses of a person who has to decide about a certain aspect of that stimulus. The psychometric function usually resembles a sigmoid function with the percentage of correct responses (or a similar value) displayed on the ordinate and the physical parameter on the abscissa. If the stimulus parameter is very far on one side of its possible range, the person will always be able to respond correctly. At the other end of the range, the person never perceives the stimulus properly and therefore the responses are at chance level. Inbetween, there is a transition range where the subject has an above-chance rate of correct responses, but does not always respond correctly. The inflection point of the sigmoid function or the point at which the function reaches the middle between the chance level and 100% is usually taken as sensory threshold.

A common example of such a procedure is visual acuity testing with an eye chart. The person sees symbols of different sizes (the size is the physical stimulus parameter) and has to decide which symbol it is.