Discrimination of brightness differences by rats with food or brain-stimulation reinforcement.

Rats were trained to respond to the brighter of two keys. Four animals were trained with food pellets and four with electrical brain stimulation. Each discrimination sequence was initiated when the animal broke a light beam at the rear of the chamber, turning on the key lights and starting a 30-sec reinforcement period. An initial response on the brighter key was immediately reinforced, and further responses on the brighter key were then intermittently reinforced. Any time the dimmer key was pressed, a 30-sec timeout was introduced. During timeout, no response had any programmed consequence. When the reinforcement period or the timeout ended, a new discrimination sequence could be initiated. Daily 1-hr training sessions were conducted, and after seven or eight sessions, all animals were at or near errorless performance levels. The luminance of the brighter key was then systematically reduced, in seven steps, with two 30-min test sessions at each step. Orderly psychometric functions were generated for individual animals. Initial acquisition, once position preferences were broken, was equally rapid for food and for brain-stimulation animals, and the two reinforcement procedures yielded comparable levels of brightness discriminability.