The role of corticostriatal loops in auditory category learning
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Sounds can signal danger (e.g., roar of a lion), pleasure, (e.g., music), or carry linguistic relevance (e.g. speech). For sounds to guide behavior, the complex soundscape must first be appropriately categorized (Bizley & Cohen, 2013; Nelken, Bizley, Shamma, & Wang, 2014). Currently, our understanding of the neural correlates of auditory categorization and learning is largely constrained to the cerebral cortex (Leech, Holt, Devlin, & Dick, 2009; Lim, Fiez, & Holt, 2014; F. Ohl, Scheich, & Freeman, 2001; F. W. Ohl & Scheich, 2005). Here, I focus on the striatum and its extensive connectivity between the cerebral cortex, referred to as corticostriatal loops (Parent & Hazrati, 1995). In vision, these loops have been purported to be involved in sensory, executive, motivational, and motor processing during acquisition of novel categories (Seger & Miller, 2010). An influential theory in visual category learning posits that the executive loop is critical in developing, testing, and using reflective rules to categorize percepts, whereas the motor loop is critical in reflexively learning categories (Ashby & Maddox, 2005, 2011). In this dissertation, I use a combination of structural and functional neuroimaging methods and behavioral training approach to examine the role of corticostriatal loops in auditory category learning. Structurally, I show that the connectivity between the auditory cortex and the caudate nucleus (sensory loop) relates to individual variability in speech category learning. Functionally, I show that successful categorization of speech sounds is associated with greater recruitment of the motor loop during stimulus, and a combination of executive, motivational, and motor loops during feedback processing. Finally, I present evidence that reflective learning of the auditory categories involves recruitment of the prefrontal cortex, whereas reflexive learning primarily involves the motor loop (Ashby & Maddox, 2005, 2011). Altogether, these results suggest that (1) multiple corticostriatal loops are engaged during auditory category learning; (2) successful categorization of a stimulus is contingent on recruitment of the prefrontal or motor cortex; and (3) feedback is integrated throughout training via executive and motivational loops.