LETTER TOTHE EDITOR Differential functions of ventral and dorsal striatum

Sir, The case study by Vo and colleagues (2014) aims to address the differential roles of ventral versus dorsal striatum in learning, specifically, whether they are essential for learning or simply involved in it. The authors reported a dissociation between action-value (based on the outcomes, values will be assigned to actions) and stimulus-value learning (values will be associated with the stimuli), and how impairment of the dorsal striatum will affect each of these processes. To achieve this, Vo and colleagues tested a patient (known as XG) who has bilateral damage to the dorsal striatum, while the ventral striatum including the nucleus accumbens is spared. To compare XG’s performance in different tasks with a healthy population statistically, the researchers tested 11 matched control subjects. Among the seven reinforcement learning tasks employed, three could be solved only by learning stimulus-values, one only by learning action-values, and the remaining tasks, with either strategy. Surprisingly, they found that Patient XG was able to learn all the tasks involving action-value learning and his performance resembled those of healthy controls. However, he was impaired at learning the tasks which could only be learned using stimulus values; his performance in those tasks was significantly poorer than controls and was no different from random. Vo et al. also analysed learning and performance processes using computational models. Regression modelling of current choice based on the previous choice and the history of rewards revealed the following: in action-value learning tasks, both Patient XG and healthy controls demonstrated normal patterns of reinforcement learning (i.e. exponential decline in weight of past trials). In contrast, Patient XG (compared to controls) did not show this pattern in stimulus-value tasks. In fact, the regression weights were not significantly different from zero, which indicated lack of learning. Consistent with this, a three-parameter reinforcement learning model explained choice behaviour better than the null model for controls in both tasks. For Patient XG, the model explained choice behaviour better for the action-value tasks, but could not explain the behaviour in stimulus-value tasks. These results address two different aspects of reinforcement learning: (i) the anatomical dissociation of the ventral striatum and dorsal striatum in learning; and (ii) the behavioural dissociation between stimulus-value versus actionvalue learning. These two aspects are addressed separately hereafter. Studies investigating functional segregation of the striatum suggest that the ventral striatum is more involved in goal-directed learning, whereas the dorsal striatum is more involved in action related habitual learning (Balleine et al., 2007; Balleine and O’Doherty, 2010; Redgrave et al., doi:10.1093/brain/awv084 BRAIN 2015: 138; 1–3 | e381