Aversive properties of negative incentive shifts in Fischer 344 and Lewis rats

Abstract

Research on incentive contrast highlights that reward value is not absolute but rather is based upon comparisons we make to rewards we have received and expect to receive. Both human and nonhuman studies on incentive contrast show that shifting from a larger more-valued reward to a smaller less-valued reward is associated with long periods of nonresponding - a negative contrast effect. In this investigation, we used two different genetic rat strains, Fischer 344 and Lewis rats that putatively differ in their sensitivity to aversive stimulation, to assess the aversive properties of large-to-small reward shifts (negative incentive shifts). Additionally, we examined the extent to which increasing cost (fixed-ratio requirements) modulates negative contrast effects. In the presence of a cue that signaled the upcoming reward magnitude, lever pressing was reinforced with one of two different magnitudes of food (large or small). This design created two contrast shifts (small-to-large, large-to-small) and two shifts used as control conditions (small-to-small, large-to-large). Results showed a significant interaction between rat strain and cost requirements only during the negative incentive shift with the emotionally reactive Fischer 344 rats exhibiting significantly longer response latencies with increasing cost, highlighting greater negative contrast. These findings are more consistent with emotionality accounts of negative contrast and results of neurophysiological research that suggests shifting from a large to a small reward is aversive. Findings also highlight how subjective reward value and motivation is a product of gene-environment interactions.

Keywords: Cost; Emotionality; Fischer 344 rats; Lewis rats; Negative contrast; Reward.

Figure 1

Response latencies for Fischer 344 and Lewis rats. Plots show time taken to respond after receiving a large/small reward in the presence of a cue signaling an upcoming large/small reward. Response latencies are plotted as a function of increasing fixed-ratio (FR) cost requirements ranging from 1 to 100. Cost was manipulated between sessions. (Bars represent 95% confidence intervals).

Figure 2

Response latency differences for Fischer 344 and Lewis rats. The left plot highlights negative contrast effects in the form of latency differences between the large-to-small and small-to-small shifts. A significant interaction was observed with Fischer 344 rats exhibiting longer response latencies with increasing cost. In contrast, the right plot shows positive contrast effects in the form of latency differences between the small-to-large and large-to-large shifts; no significant findings were found. Results suggest a larger negative contrast effect in Fisher 344 rats, lending support for the emotionality account of negative contrast. (Bars represent 95% confidence intervals).