Neural Response to Emotional Stimuli of Phylogenetic and Ontogenetic Significance

Copyright 2010 by Psi Chi, The International Honor Society in Psychology (Vol. 15, No. 1/ISSN 1089-4136) A n emotional response is multidimensional, with perceptual, expressive, and subjective components. These responses can be attributed both to factors that are developmental or “ontogenetic” as well as to factors that are innate or “phylogenetic” (Mühlberger, Wiedemann, Herrmann, & Pauli, 2006). Ontogenetic emotional reactions often stem from associative learning or evaluative conditioning, processes that lead to the formation of likes and dislikes. These learned reactions can arise from firsthand experience (De Houwer, Thomas, & Baeyens, 2001) or because a third party has instructed the learner about the importance of a stimulus (Phelps et al., 2001). Phylogenetic emotional reactions may instead be hard-wired in the brain or may result from learning mechanisms that have been preserved across the course of evolution. For instance, human infants and monkeys will quickly learn to startle at the sight of a snake (DeLoache & LoBue, 2008; Ohman & Mineka, 2001), and rats may be born with fight-or-flight mechanisms triggered by odors of predators that they have never before encountered (Kobayakawa et al., 2007). Although the behavioral manifestations of the responses elicited by ontogenetic and phylogenetic stimuli appear similar, there is some evidence to suggest that the neurobiological mechanisms yielding those responses may be different. For example, a rat’s freezing behavior in response to a conditioned fear was more reliant on prefrontal regions than was a rat’s ability to freeze in response to an evolutionarily-relevant predator (Canteras, 2003). Even within the same general structure, different subregions may be responsive to ontogenetic versus phylogenetic stimuli; within the mouse olfactory lobe, the glomerular structures that respond to unconditioned odors and lead to an innate fear response are not the same as those that respond to conditioned fear-evoking stimuli (Kobayakawa et al., 2007), and different nuclei of the amygdala have been implicated in innate versus learned fear responses in rats (Thompson, Sullivan, & Wilson, 2008). These studies suggest that, at least in nonhuman animals, fear responses elicited via phylogenetic means rely on different neural circuitry than those elicited via ontogenetic factors. Although there has not been research directly examining this topic in humans, there have been studies implicating the orbitofrontal cortex (OFC), anterior cingulate cortex (aCC), and other frontal regions with emotional conditioning and associative learning (e.g., Gottfried & Dolan, 2004, O’Doherty, 2007; Rolls & Grabenhorst, 2008). The hypothesis tested in the present study was that these regions would also be active when a person processed stimuli of ontogenetic relevance, even after the affective significance of those stimuli had been well learned. Thus, we expected to see greater aCC and OFC activity during the processing of ontogenetic versus phylogenetic stimuli. Because the aCC and OFC have been implicated in the processing of pleasure as well as fear (Kringelbach, 2005), we also hypothesized that this pattern of activity would hold for both threatening and pleasure-eliciting stimuli. Although our hypotheses were dominated by an interest in frontal-based activity, we used whole-brain imaging procedures that could Neural Response to Emotional Stimuli of Phylogenetic and Ontogenetic Significance