Visual Salience Controls the Speed of Evidence Accumulation in Value-Based Decisions by Rats

Studies of visual discrimination in rodents can confound the effects of cue salience with reward value making it difficult to determine which factor guides choice behavior. We examined this issue by testing how changes in relative salience affect decision dynamics in rats (nine females and five males) performing a two-alternative forced-choice task with visual cues associated with high or low sucrose rewards. After initial training with high- and low-luminance cues, we introduced a novel cue of intermediate luminance as a "luminance shift" test. The intermediate luminance cue substituted for either the brighter or dimmer cue and had the same reward value as the replaced cue. We found that while rats maintained a preference for the higher-value option, the introduction of a perceptually more similar cue reduced choice preference and eliminated latency differences compared with baseline. Using drift diffusion modeling, we found that the luminance shifts caused a reduction in the drift rate and had no effects on the decision threshold. This finding suggests that the relative salience of the options determines the efficiency of evidence accumulation in value-based decisions. Furthermore, this effect on the drift rate shows a dissociation from our previous work ( Palmer et al., 2024), where prefrontal cortex inactivation affected only the decision threshold. Our findings clarify the distinct contributions of sensory input and prefrontal function in the decision process.