The Psychology of Learning and Motivation

Advances in Research and Theory
1. Auflage 2006
ISBN: 978-0-08-046363-6
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, Band Volume 46, 304 Seiten

Reihe: Psychology of Learning and Motivation

The Psychology of Learning and Motivation publishes empirical and theoretical contributions in cognitive and experimental psychology, ranging from classical and instrumental conditioning to complex learning and problem solving. Volume 46 contains chapters on category learning, prototypes, prospective memory, event memory, memory models, and musical prosody. - Discusses the concepts of category learning, prototypes, prospective memory, event memory, memory models, and musical prosody - Volume 46 of the highly regarded Psychology of Learning and Motivation series - An essential reference for researchers and academics in cognitive science

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The Role of the Basal Ganglia in Category Learning
F. Gregory Ashby; John M. Ennis Publisher Summary
This chapter discusses the role of the basal ganglia in category learning and provides an overview of the functional neuroanatomy of the basal ganglia, including its relatively unique neural plasticity. It reviews the behavioral neuroscience studies that originally called attention to this brain region as a possible important locus of category learning. The most important category-learning tasks that are used with human subjects are described. The rule-based and information-integration tasks are more dependent on basal ganglia function. In rule-based tasks, the categories can be learned via some explicit reasoning process. Whereas in information-integration category-learning tasks, accuracy is maximized only if information from two or more stimulus dimensions is integrated at some predecisional stage. The chapter also reviews the relevant neuropsychological patient data, with a focus on patients with basal ganglia disease. The neuroimaging data is also discussed and the COVIS theory is described along with some possible future extensions of the model. I Introduction
Categorization is 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 friend and escape foe, to find food and avoid toxin. Every organism must have some categorization ability. Even bacteria categorize. For example, the bacterium Escherichia coli tumbles randomly until it encounters a substance that it categorizes as a nutrient. It then suppresses its tumbling behavior and swims up the concentration gradient in search of the nutrient's source. The scientific study of human category learning has a long history. For most of this time, the focus was on the cognitive processes that mediate categorization. Within the past decade, however, considerable attention has shifted to the study of the neural basis of category learning. Categorization is an ancient skill, so we should expect to find some categorization abilities in phylogenetically older parts of the human brain. In fact, this research indicates that among the most important brain areas in human category learning are the basal ganglia, a prominent collection of subcortical structures that have been implicated in procedural learning. Squire (1992) was perhaps the first to propose that the basal ganglia might play some role in human categorization. The first theory of category learning that assigned a major role to the basal ganglia was COmpetition between Verbal and Implicit Systems (COVIS) (Ashby, Alfonso-Reese, Turken, & Waldron, 1998). Briefly, COVIS postulates that human category learning is mediated by multiple systems, with two hypothesized systems that assign a key role to the basal ganglia—a logical-reasoning system and a procedural-learning system. The past few years have seen many results that link the basal ganglia to category learning. These new data were collected using a wide variety of methodologies, including animal lesions, single-cell recording, functional neuroimaging, traditional cognitive testing, and a diverse set of research subjects, including animals, healthy humans, and various neuropsychological patient groups. This chapter reviews evidence that the basal ganglia play various important roles in category learning. We begin with an overview of the functional neuroanatomy of the basal ganglia, including its relatively unique neural plasticity. We then review the behavioral neuroscience studies that originally called attention to this brain region as a possible important locus of category learning. Next we describe the most important category-learning tasks that are used with human subjects. Section V reviews the relevant neuropsychological patient data, with a focus on patients with basal ganglia disease, while Section VI reviews the existing neuroimaging data. Section VII describes the COVIS theory in more detail, and Section VIII considers some tests of this theory. Section IX discusses some possible future extensions of the model, and in Section X we close with some general comments and observations. II Functional Neuroanatomy of the Basal Ganglia
This section reviews the functional neuroanatomy of the basal ganglia, with special emphasis on features that are relevant to category learning. For more details, see Gerfen and Wilson (1996). The basal ganglia, which are an important collection of subcortical structures, include input structures, output structures, and collections of cells that produce the neurotransmitter dopamine. A schematic illustrating the functional anatomy of the basal ganglia is shown in Fig. 1. Fig. 1 Schematic illustrating major structures and primary projections of the basal ganglia (GPi: internal segment of the globus pallidus, GPe: external segment of the globus pallidus, STN: subthalamic nucleus). The input structures include the caudate nucleus, the putamen, and the nucleus accumbens. The caudate nucleus and putamen together are often referred to as the neostriatum, and when the nucleus accumbens is added, the entire set is called the striatum. For category learning (at least with visual or auditory stimuli), the caudate nucleus is the most important of these three structures. The striatum receives numerous prominent inputs. For category learning, the most important of these are from cortex. In humans, all areas of cortex (except V1) send direct excitatory projections to the striatum. The putamen receives input from somatasensory and motor areas, the caudate receives input from visual and auditory association areas and frontal cortex, and the nucleus accumbens receives input from orbitofrontal cortex and anterior cingulate (Heimer, 1995). The projections from cortex to the striatum are characterized by massive convergence. In fact, it has been estimated that the convergence ratio from cortex to the striatum is approximately 10,000:1 (Wilson, 1995). Thus, the striatum is in a unique position in the human brain, since it receives direct but highly compressed input from virtually the entire cortex. Compared to cortex, the structure of the striatum is extremely simple. It contains a single layer composed of medium spiny cells. The dendrites of these cells receive input from the axons of cortical pyramidal cells, and the medium spiny cell axons project out of the striatum to the basal ganglia output structures—primarily the globus pallidus and substantia nigra pars reticulata. The medium spiny cells are gabaergic and hence inhibitory, with a low spontaneous firing rate. The output structures of the basal ganglia include the globus pallidus, the substantia nigra pars reticulata, and the subthalamic nucleus. There are two primary output pathways from the striatum to cortex, called the direct and indirect pathways. In this section, we focus on the direct pathway, which is the more relevant pathway to current theories of category learning. The indirect pathway is discussed in Section IX. In the direct pathway, the medium spiny cells project from the striatum to the internal segment of the globus pallidus or the substantia nigra pars reticulata.1 These gabaergic cells then project to the thalamus, which in turn, sends excitatory projections to cortex. Spontaneous activity in the globus pallidus is high (Wilson, 1995), and the globus pallidus tonically inhibits the thalamus. Cortical activation of the striatum, however, causes the striatal medium spiny cells to inhibit the pallidal cells, thereby releasing the thalamus from its tonic inhibition. Because of this functional architecture, the basal ganglia are frequently described as applying a brake on cortex because they tonically prevent the thalamus from stimulating cortex. Cortex can release the brake by stimulating the striatum. Dopamine-producing cells originate in the ventral tegmental area (VTA) and the substantia nigra pars compacta (SNPC). The VTA dopamine cells project to frontal cortex (primarily orbitofrontal, prefrontal, and anterior cingulate cortices) and structures in the limbic system (primarily amygdala and nucleus accumbens). These dopamine pathways constitute the mesocorticolimbic dopamine system. Dopamine cells in the SNPC project to all input and output structures of the basal ganglia and form the nigrostriatal dopamine system (Heimer, 1995). The axons of dopamine cells display many characteristic varicosities that each contains a high density of synaptic vesicles. Stimulation of a single dopamine cell therefore causes dopamine release at a wide number of target sites. For this reason, dopamine is often classified as a neuromodulator rather than as a neurotransmitter. Within the striatum, the varicosities tend to be localized in the vicinity of the dendritic spines that characterize the medium spiny cells. These same spines are the terminal projection sites of the axons of glutamatergic pyramidal cells from cortex (DiFiglia, Pasik, & Pasik, 1978; Freund, Powell, & Smith, 1984; Graybiel, 1990; Smiley, Levey, Ciliax, & Goldman-Rakic, 1994). There is good evidence that dopamine modulates the effects of presynaptic glutamate release into medium spiny cell synapses in two separate ways. First, it increases postsynaptic signal-to-noise ratio and second, it promotes long-term potentiation (LTP), which effectively strengthens the synapse. Both of these actions are thought to be dependent on exactly which...