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18 BIOLOGICAL AND SOCIAL UNIVERSALSthat attenuated DA activity in mesolimbic regions has been linked to increasedmotivation for seeking out novelty, alcohol and other drugs, as well as otherpotentially rewarding stimuli (e.g., Gardner, 1999).The Adolescent AmygdalaThere are hints from research conducted both in laboratory animals and in humansthat the adolescent amygdala may be a particularly intriguing and importantarea for ontogenetic investigation. This region is of particular interest for studyduring the highly emotionally laden and peer-driven period of adolescence giventhe importance of the amygdala in processing of emotional stimuli (Baxter &Murray, 2002), modulating social behavior (Amaral et al., 2003), attributing affect,and establishing reward expectancies (Bechara et al., 1999; Holland &Gallagher, 2004). The amygdala also has bidirectional and functional connectivitywith the orbitofrontal cortex (OFC; Zald et al., 1998), a brain region likewisecritical for the expression of social behavior (Kolb et al., 2004), and reward circuitryrelated to addiction (Volkow & Fowler, 2000).Excitatory input from the amygdala (basolateral nucleus) to the PFC continuesto be elaborated through adolescence (Cunningham et al., 2002). The adolescentamygdala is more prone to induction of seizures by electrical stimuli than that ofyounger or older animals (Teresawa & Timiras, 1968) and exhibits a differentpattern of stress-induced activation of the immediate early gene c-Fos than seenin mature animals (Kellogg et al., 1998). Particularly intriguingly, the amygdalais one of the few forebrain regions where damage has been shown in animal studiesto markedly influence the timing of puberty, with reports of both precociousand delayed puberty following lesions involving this brain region (see Moltz, 1975,for review of this relatively old literature). These contrary findings may be easierto reconcile when it is recognized that the amygdala consists of numerous specificsubregions (nuclei) with different, and sometimes opposing, functional influences(e.g., Swanson & Petrovich, 1998).The emerging literature in human adolescents likewise is suggestive of developmentalalterations in amygdala function during adolescence, although thefindings are similarly mixed. A number of studies have used fMRI to examineamygdalar activation during exposure to emotional faces through childhood andadolescence and into adulthood, with activation patterns sometimes reported todecrease developmentally (Killgore et al., 2001) whereas other studies have reporteddevelopmental increases (Thomas et al., 2001) or no change in activationpatterns between adolescence and adulthood (Pine et al., 2001). Addingfurther to these inconsistencies are instances of ontogenetic differences inamygdalar activation that are sex-specific (e.g., McClure et al., 2004) or lateralized(e.g., Killgore & Yurgelun-Todd, 2004). Among the factors that maycontribute to these varying findings is the rapid habituation of this activation,