• Three inbred mouse strains (BALB/c, C57BL/6, DBA/2) responded on fixed-ratio schedules of reinforcement. • The effects of two qualitatively different reinforcers on response rate were assessed. • Response rates were higher for milk compared to sucrose pellets on moderate ratios. • The responding of inbred mice is sensitive to reinforcer type.
Previous studies of inbred mouse strains have shown reinforcer–strain interactions that may potentially mask differences among strains in memory performance. The present research examined the effects of two qualitatively different reinforcers (heterogeneous mix of flavored pellets and sweetened-condensed milk) on responding maintained by fixed-ratio schedules of reinforcement in three inbred strains of mice (BALB/c, C57BL/6, and DBA/2). Responses rates for all strains were a bitonic (inverted U) function of the size of the fixed-ratio schedule and were generally higher when responding was maintained by milk. For the DBA/2 and C57BL/6 and to a lesser extent the BALB/c, milk primarily increased response rates at moderate fixed ratios, but not at the largest fixed ratios tested. A formal model of ratio-schedule performance, Mathematical Principles of Reinforcement (MPR), was applied to the response rate functions of individual mice. According to MPR, the differences in response rates maintained by pellets and milk were mostly due to changes in motoric processes as indicated by changes in the minimum response time (δ) produced by each reinforcer type and not specific activation (a), a model term that represents value and is correlated with reinforcer magnitude and the break point obtained under progressive ratio schedules. MPR also revealed that, although affected by reinforcer type, a parameter interpreted as the rate of saturation of working memory (λ), differed among the strains.