Sensory discrimination: decision process

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Abstract
The processes that intervene between a relatively peripheral array of neural activity and a subject''s decision in a discrimination task are split into 2 sections: the ascending sensory processes that provide the final patterns of neural activity on which discrimination is based, and a process that yields decisions of the type required by the experimental design used in the psychophysical study. A theory of the decision process was developed. The decision theory deals with a class of experimental designs in which a [human] subject is required to make a decision about 2 stimuli s1 and s2 (e.g., s1 is larger than s2, s2 is the same as s1, s2 was the modified stimulus and so on). A mathematical representation for experimental designs of this type is developed. The decision process is analyzed in 2 forms: a multivariate form in which the discrimination decision results directly from multidimensional neural representations of the 2 stimuli and a bivariate form in which the final representation of each stimulus is a unidimensional variable. Conditions required for equivalence of these formulations are examined. The theory includes as explicit variables the experimental design, the subject''s discrimination strategy, bias, memory variance, bias variance, variance in the final neural representations of the stimuli and their functional dependence on the stimuli that they represent. Formulas are developed for the expected values of commonly used psychophysical measures such as the classical psychometric function, receiver operating characteristic (ROC) functions, discriminatory separation index (d''), and the difference limen. Optimum discrimination behavior is analyzed.