The Quantification of Behavior in the Presence of Compound Stimuli

Animals live in complex environments where multiple cues can provide consistent or conflicting information about how to behave most effectively. Previous research has described how animals combine information with qualitative combination rules; the goal of this article was to quantify the combination rule used by rats when 2 previously trained stimuli of separate modalities were presented simultaneously. Rats in a lever box were trained with 2 stimuli (light and tone) assigned given probabilities of food before they were tested in compound. Changes in the probability of food assigned to each stimulus produced linear changes in the rat's rate of responding to these stimuli, both when the stimuli were presented individually and in compound. Using a linear regression, 3 features of the stimuli (rate of reinforcement, probability of food, and rate of responding) were compared to see which accounted for behavior in the presence of the compound best; z-scores were used to account for between rat variability. The linear regression allows for direct comparisons to be made regarding what may be combined under these conditions; something many behavioral models cannot. Our analysis suggests that rats weigh each stimulus of the compound differentially. More specifically, they weigh the stimulus that had more frequent changes to the assigned probability of food more than the stimulus whose probability of food remained more consistent across phases of the experiment.