Academic journal article Cognitive, Affective and Behavioral Neuroscience

Distinct Mechanisms in Visual Category Learning

Academic journal article Cognitive, Affective and Behavioral Neuroscience

Distinct Mechanisms in Visual Category Learning

Article excerpt

The ways in which visual categories are learned, and in which well-established categories are represented and retrieved, are fundamental issues of cognitive neuroscience. Researchers have typically studied these issues separately, and the transition from the initial phase of category learning to expertise is poorly characterized The acquisition of novel categories has been shown to depend on the striatum, hippocampus, and prefrontal cortex, whereas visual category expertise has been shown to involve changes in inferior temporal cortex. The goal of the present experiment is to understand the respective roles of these brain regions in the transition from initial learning to expertise when category judgments are being made. Subjects were explicitly trained, over 2 days, to classify realistic faces. Subjects then performed the categorization task during fMRI scanning, as well as a perceptual matching task, in order to characterize how brain regions respond to these faces when not explicitly categorizing them. We found that, during face categorization, face-selective inferotemporal cortex, lateral prefrontal cortex, and dorsal striatum are more responsive to faces near the category boundary, which are most difficult to categorize. In contrast, the hippocampus and left superior frontal sulcus responded most to faces farthest from the category boundary. These dissociable effects suggest that there are several distinct neural mechanisms involved in categorization, and provide a framework for understanding the contribution of each of these brain regions in categorization.

A fundamental function of cognition is to interpret a wide range of sensory inputs in terms of a limited number of meaningful, abstracted categories. These categories greatly reduce the amount of information we have to process and are the foundation for perception and memory. Studies have focused on the initial learning of visual categories (Ashby & Maddox, 2005; Seger & Cincotta, 2005a) and the retrieval and representation of well-established categories (Gauthier, Skudlarski, Gore, & Anderson, 2000; Gauthier, Tarr, Anderson, Skudlarski, & Gore, 1999). However, the neural basis of the transition from initial learning to category expertise is poorly understood. In the present study, we sought to better characterize this transition by studying how recently learned visual categories are represented and retrieved.

Categories can be learned explicitly or in a procedural manner, and the neural mechanisms involved in category learning depend on the strategy utilized (Aizenstein et al., 2000; Reber, Gitelman, Parrish, & Mesulam, 2003; Seger & Cincotta, 2005a, 2005b). According to Ashby's (2006) COVIS (competition between verbal and implicit systems) model, when the rule that separates categories can be verbalized, brain regions supporting working memory that include prefrontal cortex (PFC) and the head of the caudate nucleus are implicated. In contrast, when the category rule is not verbalizable, a procedural system that depends on the tail of the caudate nucleus is involved. Neuroimaging studies have supported this model, demonstrating greater PFC activity during explicit than during implicit category learning (Aizenstein et al., 2000) and during retrieval of explicitly learned categories (Reber et al., 2003). In addition, the striatum has proven to be important for both implicit and explicit category learning (Seger & Cincotta, 2005a, 2005b). In particular, the body and tail of the caudate and putamen are associated with successful learning; the head of the caudate nucleus is important for executive functions in explicit learning tasks and for feedback processing in implicit learning tasks.

In addition to PFC and striatum, the hippocampus has been shown to be important for category learning and applying category rules. Hippocampal damage can impair initial category learning (Hopkins, Myers, Shohamy, Grossman, & Gluck, 2004), although some category learning tasks can be performed without an intact hippocampus (Knowlton, Squire, & Gluck, 1994). …

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