Category learning in the brain

Abstract

The ability to group items and events into functional categories is a fundamental characteristic of sophisticated thought. It is subserved by plasticity in many neural systems, including neocortical regions (sensory, prefrontal, parietal, and motor cortex), the medial temporal lobe, the basal ganglia, and midbrain dopaminergic systems. These systems interact during category learning. Corticostriatal loops may mediate recursive, bootstrapping interactions between fast reward-gated plasticity in the basal ganglia and slow reward-shaded plasticity in the cortex. This can provide a balance between acquisition of details of experiences and generalization across them. Interactions between the corticostriatal loops can integrate perceptual, response, and feedback-related aspects of the task and mediate the shift from novice to skilled performance. The basal ganglia and medial temporal lobe interact competitively or cooperatively, depending on the demands of the learning task.

Figure 1

Categorization tasks. (a) Dot pattern prototype learning. A prototypical stimulus is selected (left), and category exemplars (right) are formed by randomly moving dots. Large amounts of movement (bottom) result in high distortion stimuli; smaller amounts of movement (top) result in low distortion stimuli. (b) Information integration task. Stimuli are formed by varying two incommensurate features: angle from vertical and width of the bars. Illustrated is a diagonal decision bound between categories; to learn the categorization successfully, subjects must integrate the knowledge of angle and width. (c) Cat-dog categorization task. Stimuli are formed as continuous morphs along each of the lines between prototype stimuli. The categorical decision bound arbitrarily divides the continuous perceptual space into two or three domains, or categories. (d) Arbitrary categorization task. Each stimulus is individually probabilistically associated with the categories; stimuli within a category do not share identifying common features. (e) “Same - different” rule task. Monkeys responded on the basis of whether novel pairs of images matched or did not match, depending on which rule was in effect.

Figure 2

Corticostriatal loops. The motor loop (blue) connecting the motor cortex with the posterior putamen. Executive loop (green) connects the prefrontal cortex and the parietal cortex with the anterior caudate nucleus. The motivational loop (red ) connects the ventral striatum with the orbitofrontal cortex. The visual loop (orange) connects extrastriate and inferotemporal cortices with the posterior caudate nucleus.