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Subjects were 8 male rats of the of the Sprague-Dawley strain, approximately 60 days old at the beginning of the experiment. Their weights were brought down to approximately 90% their ad lib weight, before habituation. Attempts were were made to maintain them at that weight throughout the experiment.

For half the rats, light-on served as SD, bar presses at this time were reinforced with food. Light-off served as Sd. Responses during this time were not reinforced. For the other half of the rats, the experimental conditions were reversed. The rats were only reinforced in the SD condition if the appropriate responses occurred.

Eight electronically controlled Skinner boxes were used. (Cloburn Instruments Model E10-10SF). The inside dimensions of the Skinner box were 30 x 24 x 27.5 cm. The floor of the box was 16 metal rods, 6 mm in diameter and 1 cm apart, parallel to the front panel. The front and back and top were of made metal. One side was a Plexiglas panel, and the fourth side was a Plexiglas door. The front panel contained the food cup, bar and 15 watt light bulb. The layout and dimensions of the front panel are presented in Figure 1.

Figure 1. Front panel of the Skinner box. All measurements are in centimeters.

The bar was 3.5 cm in length, 5 mm thick and protruded 2 cm into the box. A downward force on the bar delivered a Standard Bioserve Dustless Precision pellet weighing 45 mg into the food cup. The light, was a 15 watt light bulb, in a housing 1.5 cm in diameter, protruding 2 cm into the box. There was no control for light, sound or other visual stimulus outside of the Skinner box.

The rats were habituated to the experimental environment on the first day for 30 minutes. There was no food or light in the Skinner box. The operant level of bar presses was recorded during the habituation period.

This was followed by 6 days of shaping during 30 minute sessions, during which each depression of the bar was reinforced with a food pellet. When asymptote was reached, discriminant training began. Asymptote is defined as the average number of bar presses during the last 3 days of shaping.

The discrimination training was conducted for the next 7 days during 30 minute sessions. The light-on and light-off in the Skinner boxes were alternated at 1 minute intervals. A control session was run on the last day. The interval of the SD and S conditions was randomized for a total of 10 minutes each.

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