Effects of differing response-force requirements on food-maintained responding in CD-1 mice

A review of the behavioral effects of response force: Perspectives on measurement and functions

Response force is a fundamental dimension of behavior. Yet, little is known about its functional significance for learning. The present review examines the behavioral effects of force across several domains. Along the way, advantages of different measurement strategies that have been used to study force are evaluated. The behavioral functions of force are also considered in light of two commonly expressed notions about behavior. First, the Law of Least Effort predicts that animals and humans will act in ways that minimize the costs of behaving. Second, it is widely held that work requirements and effortful responding are aversive. A review of the literature, especially regarding behavioral adaptations to force, is consistent with the Law of Least Effort on many points. Empirical data are less clear on the aversive properties of force requirements. For much of the literature, there is little in coherent findings. Many disagreements and inconsistencies pertain to measurement strategies that fail to record the full range of response variants. Consistent support for aversive functions of force requirements have been obtained only when studied as a negative reinforcer.

The isolation of motivational, motoric, and schedule effects on operant performance: a modeling approach

Dissociating motoric and motivational effects of pharmacological manipulations on operant behavior is a substantial challenge. To address this problem, we applied a response-bout analysis to data from rats trained to lever press for sucrose on variable-interval (VI) schedules of reinforcement. Motoric, motivational, and schedule factors (effort requirement, deprivation level, and schedule requirements, respectively) were manipulated. Bout analysis found that interresponse times (IRTs) were described by a mixture of two exponential distributions, one characterizing IRTs within response bouts, another characterizing intervals between bouts. Increasing effort requirement lengthened the shortest IRT (the refractory period between responses). Adding a ratio requirement increased the length and density of response bouts. Both manipulations also decreased the bout-initiation rate. In contrast, food deprivation only increased the bout-initiation rate. Changes in the distribution of IRTs over time showed that responses during extinction were also emitted in bouts, and that the decrease in response rate was primarily due to progressively longer intervals between bouts. Taken together, these results suggest that changes in the refractory period indicate motoric effects, whereas selective alterations in bout initiation rate indicate incentive-motivational effects. These findings support the use of response-bout analyses to identify the influence of pharmacological manipulations on processes underlying operant performance.

Effects of differing response-force requirements on food-maintained responding in C57Bl/6J mice

The effect of force requirements on response effort was examined using inbred C57BL/6J mice trained to press a disk with their snout. Lateral peak forces greater than 2 g were defined as responses (i.e., all responses above the measurement threshold). Different, higher force requirements were used to define criterion responses (a subclass of all responses) that exceeded the requirement and produced a reinforcer. The reinforcer was sweetened, condensed milk, delivered upon response termination. All mice were exposed to two ascending series of criterion force requirements (2, 4, 8, 16, and 32 g). Increasing the force requirement initially decreased criterion response rates, but criterion response rates recovered with continued exposure, except at the 32-g requirement. Response rates for all measured responses initially increased with increasing force requirements, but then decreased with continued exposure. The second exposure series produced more stable response rate changes than the first series. The time-integral of force (area under the force-time curve for individual responses, which is proportional to energy expenditure for each response) increased with the increase in the force requirement. The C57BL/6J inbred strain generated average force output similar to CD-1 outbred stock mice trained on the same force requirements. C57BL/6J inbred strain mice differed from CD-1 mice in initial response rates (for all responses above threshold) and had lower response rates at the 16 and 32 g requirements resulting in lower total force output. These data show for both mice types that increased force requirements resulted in increased overall responding (all measured responses), which contradicts a punishment interpretation of criterion response decrements. C57BL\6 inbred mice showed individual differences comparable to the outbred CD-1 stock. C57BL/6 mice did not maintain responding as well at the higher force requirements, which may be due to their small body size and weight, compared to the larger and heavier CD-1 mice.