Categorization, Neural Basis

Within the cognitive sciences, categorization is defined as the act of responding differently to objects or events in separate classes or categories. It is a vitally important skill that allows us to approach friends and escape foes, to find food and avoid toxins. Not surprisingly, the scientific study of categorization has a long history. For most of this time, the focus was on the cognitive processes that mediate categorization. Within the past decade, however, the new tools of cognitive neuroscience have been used to investigate the neurobiology of categorization processes. As discussed in this entry, the consensus from this work is that all of the major memory systems probably contribute to category learning and that the neural circuits that mediate initial category learning are different from the circuits that enable us to respond automatically to highly learned categories.

Multiple Category Learning Systems

One recent discovery, which is due in part to this new emphasis on neuroscience, is that humans have multiple category-learning systems. An obvious hypothesis, which quickly followed this discovery, is that all major memory systems are capable of some form of category learning. Memory researchers have identified a number of unique human memory systems, which are commonly divided into two classes. Declarative memories are those that are accessible to conscious awareness, such as short-term or working memory and the memory of past episodes (episodic memory). Nondeclarative memories are those for which we have little conscious awareness. Included in this set are procedural memory (e.g., muscle memories, such as the exact actions performed when knotting a tie) and perceptual memories that result from repeated exposure to the same stimulus (i.e., the perceptual representation memory system). Different memory systems have different properties; therefore, each should be ideally suited to learning about unique types of category structures. As a result, changing the nature of the categories might change which brain areas mediate the learning.

Many brain areas have been implicated in category learning, but perhaps the two most important are the prefrontal cortex (PFC) and the striatum. The PFC, shown in Figure 1, is the anterior portion of cortex that lies behind the forehead, and the striatum (which includes the caudate nucleus and the putamen), shown in Figure 2, is a major input structure within a large collection of subcortical nuclei called the basal ganglia.

Figure 1 A human brain with the prefrontal cortex in a darker shade (the front of the brain is on the right)

Figure 2 The human striatum