There are two types of psychometric function: A theory of cue combination in the processing of complex stimuli with implications for categorical perception

Psychometric functions are customarily fitted using normal distribution fun ctions. However, categorical perception identification tasks give functions that are more sigmoid than the normal distribution (M. Treisman, A. Faulkn er, P. L. N. Naish, & B. S. Rosner, 1995). This can be explained by a theor y of discrimination, cue combination theory (CCT), presented here. This the ory assumes that complex stimuli (e.g., speech sounds) are composed of comp onents or cues that vary in informativeness. The decision mechanisms sample individual cue dimensions and combine the information from each. The model generates a normal psychometric function if all cues sampled are informati ve. If not, the function becomes more sigmoid. CCT offers accounts of discr imination (categorical and continuous) and sensory reaction times, adaptati on, sequential effects, contextual effects, trading relations, speed and ac curacy, perceptual learning, and the separable-integral distinction.