Inferring action-dependent outcome representations depends on anterior but not posterior medial orbitofrontal cortex

Although studies examining orbitofrontal cortex (OFC) often treat it as though it were functionally homogeneous, recent evidence has questioned this assumption. Not only are the various subregions of OFC (lateral, ventral, and medial) hetereogeneous, but there is further evidence of heterogeneity within those subregions. For example, several studies in both humans and monkeys have revealed a functional subdivision along the anterior-posterior gradient of the medial OFC (mOFC). Given our previous findings suggesting that, in rats, the mOFC is responsible for inferring the likelihood of unobservable action outcomes (Bradfield, Dezfouli, van Holstein, Chieng, & Balleine, 2015), and given the anterior nature of the placements of our prior manipulations, we decided to assess whether the rat mOFC also differs in connection and function along its anteroposterior axis. We first used retrograde tracing to compare the density of efferents from mOFC to several structures known to contribute to goal-directed action: the mediodorsal thalamus, basolateral amygdala, posterior dorsomedial striatum, nucleus accumbens core and ventral tegmental area. We then compared the functional effects of anterior versus posterior mOFC excitotoxic lesions on tests of Pavlovian-instrumental transfer, instrumental outcome devaluation and outcome-specific reinstatement. We found evidence that the anterior mOFC had greater connectivity with the accumbens core and greater functional involvement in goal-directed action than the posterior mOFC. Consistent with previous findings across species, therefore, these results suggest that the anterior and posterior mOFC of the rat are indeed functionally distinct, and that it is the anterior mOFC that is particularly critical for inferring unobservable action outcomes.