While limited an emergent human literature corroborates the

While limited, an emergent human literature corroborates the importance of amygdalostriatal interactions in affective valuation and learning, though their precise nature and direction is less clear. For example, increased task-based functional connectivity between the amygdala and striatum is associated with better learning of reward predicting information when preceded by social cues (Watanabe et al., 2013). Conversely, fear conditioned stimuli inhibit both the ability to update the representation of that 5-aminosalicylic acid as a reward-predicting stimulus (i.e., reversal learning, which additionally involves the OFC (for exetnsive reviews see Kringelbach, 2005; Rolls, 2000)), which is reflected in the inhibition of activation in the striatum, among other regions (Wittmann, 2014). The implementation of computational approaches to fMRI––i.e., applying mathematically formulated models of psychological processes (Niv, 2009)––has facilitated more mechanistic and precise characterizations of amygdala and striatum function in valuation and learning which could inform thoughts regarding interactive functions. The ventral striatum is frequently implicated in the computation of prediction-error signaling in humans in a variety of contexts, both non-social and social, and in instrumental and pavlovian situations (Daw et al., 2011; Fareri et al., 2015a; Li et al., 2011; O\'Doherty et al., 2004; Schonberg et al., 2007). Computational processes subserved by the amygdala also support varied roles in affective and associative learning. The amygdala has been implicated in performing prediction-error related computations during instrumental learning and decision-making (Prévost et al., 2011; Rutledge et al., 2010), which may be dissociable depending on incentive context: the BLA encodes action-value and uncertainty signals during reward-learning, whereas the CeA does so during avoidance learning (Prévost et al., 2011). Further, during Pavlovian learning contexts, the amygdala supports a more general associability signal––i.e., the strength of the association between a stimulus and outcome, agnostic to expectations (Rescorla and Wagner, 1972)––as compared to a prediction error computation, which instead relies on the ventral striatum (Li et al., 2011). A complex interplay between the amygdala and striatum thus appear important with respect to affective valuation and learning, performing dissociable computations as a function of varied contexts; however, initial signals regarding stimulus/action values is generated in the amygdala which may then be used by the ventral striatum in terms of updating expectations for learning.
Amygdala development The early years of life require an ability to rapidly learn and assess environmental contingencies. Studies of structural and functional development of the amygdala indicate that early life is a time of dynamic change in the amygdala, which could facilitate affective learning (Tottenham and Sheridan, 2009). The rodent amygdala is largely structurally intact early in life (Bouwmeester et al., 2002; Chareyron et al., 2012), and extensive prenatal neurodevelopmental differentiation renders the human amygdala structurally present and developed by birth (Ulfig et al., 2003) as well, with rapid growth reported in the first year of life (Gilmore et al., 2012). While structural changes continue to occur within the amygdala into early adulthood (Giedd et al., 1996b; Goddings et al., 2013; Hu et al., 2013; Wierenga et al., 2014), evidence in non-human primates (Payne et al., 2010) and in humans (Hu et al., 2013; Ostby et al., 2009; Wierenga et al., 2014) indicates that the most rapid rates of structural change in the amygdala occur prior to adolescence. Massive changes in amygdala function emerge in these early years as well, impacting our ability to learn about the environment. During development, social and affective signals gleaned from the facial expressions of others provide a primary source of information about the world. As such, affective facial displays are typically employed as affective stimuli in studies of human amygdala development. For example, fearful faces provide cues as to an impending threat in the environment, while happy faces may signal social approval. The functional response profile of the amygdala to these important socio-affective signals significantly changes across the lifespan, though the nature of change tends to vary as a function of the question posed. Initial studies indicated that children and early adolescents (aged approximately 9–13) showed stronger amygdala reactivity to neutral compared to fearful faces, whereas adults showed the opposite pattern (Thomas et al., 2001 though see also Pagliaccio et al., 2013). Neutral expressions may thus be more ambiguous/threatening early in life, or alternatively, the amygdala may not be able to effectively evaluate aversive/threatening facial expressions early on. Peak amygdala reactivity has also been reported during adolescence when averaging activation across valence of affective facial expressions (Hare et al., 2008; Moore et al., 2012). However, isolating the response of the amygdala to threatening stimuli (i.e., fearful faces) in comparison to an implicit baseline condition reveals that threat-based amygdala reactivity is high early in life (i.e., childhood) and decreases throughout adolescence into adulthood (Gee et al., 2013b see also Vink et al., 2014). Similar linear age-related declines in amygdala reactivity emerge when: (1) considering faces of varied emotional expression (Swartz et al., 2014) and varied social value (i.e., mothers, strangers) compared to baseline (Tottenham et al., 2012) (2) examining responses to others in pain (Decety and Michalska, 2009; Decety et al., 2011); (3) evaluating positive (e.g., food) stimuli (Silvers et al., 2014); and (4) comparing only adolescents to adults (Guyer et al., 2008; Monk et al., 2003). The overall pattern of increased amygdala reactivity during early life suggests an enhanced ability to detect affective stimuli early in life.