Opposing reward-epoch population coding in anterior insular cortex dissociates sign- and goal-tracking despite equivalent conditioned responding

Abstract

When Pavlovian lever cues predict reward, rats diverge in how they express learned associations: sign-tracking (ST) rats approach and engage the cue with greater persistence, while goal-tracking (GT) rats approach the reward site and are more consistently outcome-sensitive. The anterior insular cortex (aIC) and its interactions with amygdala and striatal regions are differentially recruited across phenotypes, implicating aIC as a node where associative strategy diverges at the circuit level. To characterize this neural divergence independent of differences in conditioned responding, we recorded single-unit aIC activity in ST and GT rats performing an odor-guided sucrose discrimination task with equivalent approach behavior. At the population level, GT aIC was dominated by task-excited neurons that drove sucrose discrimination, while ST aIC carried a greater proportion of task-inhibited neurons whose suppression carried the dominant discrimination signal, a dissociation specific to the reward epoch. During cue sampling, the balance of excited and inhibited neurons did not differ between phenotypes, but temporal structure diverged: ST cue-excited neurons showed transient CS-onset responses without sustained discrimination, while GT cue-excited neurons maintained sustained CS+/CS- discrimination into the reward epoch. ST cue-period firing predicted post-cue approach latency, with stronger responses associated with slower goal approach, a relationship absent in GT. The majority of optogenetically identified aIC projection neurons targeting basolateral amygdala and nucleus accumbens were cue-inhibited. These findings reveal that individual differences in associative strategy are encoded in the organizational structure of aIC population activity, positioning aIC as a site where motivational strategy is translated into distinct outputs.