The limits of parallel processing in the visual discrimination of orientation and magnification

We present the results of an experimental study on visual pattern discrimination by human subjects. The stimuli consisted of a single target element embedded in a multi-element reference field and response times, T1/2, for 50% probability of detection of the target element were measured as a function of the number of reference elements, N. Two response patterns were observed, one in which T1/2 is effectively independent of N, and the other in which it is approximately proportional to N. We argue that these two response classes reflect two different modes of discrimination, the former mediated by parallel processing, which does not require scanning of the elements in order to discriminate the target, whereas the other involves sequential scanning of the elements, in order that the target can be discriminated under foveal examination. We show that a target element differentiated from the reference elements by virtue of orientation or magnification can be discriminated under parallel processing, except when the value of the distinguishing parameter is too small, or when the individual elements are insufficiently well defined. We also establish that targets differentiated from the reference elements by (partial) charges in symmetry can also be discriminated by parallel processing.