Effects of the number and form of stimuli on visual search in the pigeon.

Pigeons' pecks at small luminous forms on a large display field were photoelectrically recorded. Pecks to target (S+) forms were reinforced with food; pecks to distractor (S-) forms produced a short time-out. The speed and accuracy of search for a target declined as the number of distractors increased to a maximum of 15. Search was further impaired when the distractors were quite similar to the target. However, search was unaffected by a change in the number of potential targets (memory set size) from one to two, nor was it clearly affected by the use of several different distractor forms instead of a single repeated form. Search was swift over a large (50 degrees) visual field, in the apparent absence of substantial head and eye movements.