Failure to find positive key-press contrast for milk reinforcers using a within-session procedure

Rats pressed keys for sweetened condensed milk delivered by multiple schedules. Component duration varied from five seconds to 16 minutes. Positive contrast did not occur for any duration. That is, the rate of responding emitted during a variable interval component of a multiple schedule was not greater when the other component was extinction than when it was an identical variable interval schedule. The function that related key pressing to component duration also differed from that reported by past studies of pigeons pecking keys. Finding such functional differences suggests that behavioral contrast may be mediated by different mechanisms for different responses. It also implies that neither differences in the rates of responding, nor differences in the parts of the body utilized by the response, produced past functional differences in the observation of contrast for different responses.