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This chapter describes the connections of the orbital and medial prefrontal cortex. Intrinsic connections indicate the presence of distinct orbital and medial networks, with the orbital network concentrated in central OFC subregions, and the medial network involving medial structures, and some areas on the medial and lateral orbital surface. The medial and orbital networks show strikingly differential patterns of input and output. The orbital network receives substantial input from sensory systems including olfaction, taste, visceral, visual, and somatosensory systems, and may serve to assess sensory objects, especially food. The medial network has little direct sensory input, but provides output to the hypothalamus and periaqueductal gray, and is heavily connected with limbic regions such as the amygdala, entorhinal cortex, and hippocampus. The medial and orbital networks are also connected to distinct parts of the striatum and mediodorsal thalamic nucleus.

This chapter discusses how reprogramming the brain, by inducing visual inputs to innervate the auditory pathway, can reveal the relative influence of intrinsic and extrinsic factors in determining the function and organization of sensory cortex and thalamic nuclei. It describes its effect on retinal innervation, its physiological and behavioral consequences, and its potential influence on cortical circuitry.

This chapter begins with a historical account of the study of the saggital stratum (SS). It then discusses the results of the investigation of the SS of rhesus monkey brains. The SS is a major corticosubcortical white matter bundle that conveys fibers from the parietal, occipital, cingulate, and temporal regions to subcortical destinations in the thalamus, the nuclei of the basis pontis, and other brainstem structures. It also conveys afferents principally from the thalamus to the cortex. It may therefore be viewed as equivalent to the internal capsule in that it is a major subcortical fiber system and not exclusively a fiber tract linking the lateral geniculate nucleus with the calcarine cortex.

This chapter discusses thalamocortical interactions, an understanding of which is central to understanding perception and cognition. The visual thalamus consists of three main nuclei: the lateral geniculate nucleus (LGN), the thalamic reticular nucleus (TRN), and the pulvinar. The LGN is the thalamic station in the retinocortical projection and has traditionally been viewed as the gateway to the visual cortex. The TRN forms a thin shell of neurons surrounding the thalamus and providing an interface between the thalamus and cortex in that thalamocortical and (layer 6) corticothalamic projections also have collateral branches to the TRN. The pulvinar is a large nucleus located in the dorsal thalamus. It contains several visual maps that are reciprocally connected to striate and extrastriate cortex, in addition to being substantially interconnected with frontal and parietal cortex.

Jones' work on the structure and function of the central nervous system is distinguished by enormous breadth and scope, both intellectually and technically. He has made seminal contributions to understanding the circuitry, cellular properties, and basic organizational plans of the cerebral cortex and thalamus, their development, functional interrelationships, plasticity, and pathology. His early work laid the foundations for the understanding of cortical connectivity, and he was the first to attempt to unravel the intrinsic circuitry of the cerebral cortex using electron microscopy. His was the first modern systematic classification of cortical neurons, and in subsequent work his studies of their chemical characteristics form a basis for all subsequent studies. His book, The Thalamus, is one of the most cited publications in neuroscience. He is also a distinguished historian of Neuroscience.

Recovery of consciousness following severe brain injuries typically evolves through several stages marked by considerable behavioral variability. This chapter considers the role of ‘circuit-level’ mechanisms in the forebrain in the generation of behavioral variability and specifically emphasizes the contributions of the central thalamus to altered arousal regulation in neurological disorders of consciousness. Neurons within the central thalamus play a key role in forebrain arousal regulation, acting as a nexus for the influence of ascending brainstem/basal forebrain neuronal populations (‘arousal systems’) and control signals descending from frontal cortical systems. Clinical distinctions among neurological disorders of consciousness and some observations of wide fluctuations in behavioral responsiveness in severely brain-injured patients can be organized by considering the possible role of circuit-level alterations of function involving the central thalamus, striatum and frontal cortical systems.

All sensory information, except for olfaction, passes through the synaptic network of the thalamus just before reaching the cerebral cortex, suggesting that the thalamus may exert a powerful influence on sensory and motor processing. However, the connection is not unidirectional: layer VI of the cerebral cortex contributes up to 50% of the synapses on thalamic relay neurons, indicating that the thalamus and cerebral cortex are intimately associated in some form of reciprocal loop. Many previous studies have shown the thalamus to be critically involved in a wide variety of phenomena, including the generation of the electroencephalogram (EEG), the blocking of transmission of sensory information during slow-wave sleep, and the generation of generalized seizures. This chapter examines, at the subcellular, cellular, and network levels, the biophysical mechanisms for all three of these and related phenomena.

Each area of the cerebral cortex receives most of its afferent fibres from one specific projection nucleus of the thalamus. The topographical organization of this thalamocortical projection is such that a discrete volume in the thalamus sends its fibres to a circumscribed region in the cortex and contiguous volumes in the thalamus project to contiguous fields in the cortex. In this sense, a point-to-point projection could be considered, but this over-simplification does not take into account the mutual overlap of thalamic fibres within their ‘modular’ cortical arborization areas (0.5–1 mm). The topographical representation of the thalamic projection nuclei across the cerebral cortex is a topological problem because the thalamus is a three dimensional body, whereas the cerebral cortex constitutes a two-dimensional surface. In addition, the thalamic neurones which project to a small region in the cortex are not arranged in spherical volumes in the thalamus, but as elongated rods with their axes oriented either anteroposteriorly or more vertically.

This chapter discusses the results of experiments on neuroimaging of thirst by positron emission tomography conducted at the Research Imaging Center in San Antonio, Texas. The experiments focused on specific parts of the brain including the parahippocampal gyrus, the anterior and posterior cingulated gyri, the insula, and the thalamus. This chapter describes the changes in these brain areas during the first emergence of consciousness of thirst and after satiation of thirst.

This chapter focuses on one nucleus of the limbic thalamus: the nucleus reuniens. It is important as a way station between the medial prefrontal cortex and the hippocampus. Its midline location and hypothalamic connections suggest that it could contribute to our subconscious memory functions.

This chapter reviews the ability of neglect patients to navigate in three-dimensional space. It specifically addresses two questions about different navigational systems and about the nature of neglect. The first can be formulated as follows: since hemispatial neglect is a multimodal disorder, does it also produce an anisometry in environmental navigation when using either visual or nonvisual updating? The second question refers to the integration of visual and nonvisual information. Evidence shows that the integration of nonvisual information in processing a navigational space involves brain structures different from those involved in the interaction between multimodal information and the subjects' segmental responses. The studies reviewed so far suggest that brain-damaged patients, including patients with unilateral neglect, can process spatial information as well as normal controls when the entire body is moved. In addition, the lesion may be located either in the parietal lobe, disconnecting the pathway indirectly connecting the striate cortex with the parieto-insular vestibular cortex (PIVC) area, or in areas involved in the connection of the superior colliculus to the PIVC via the pulvinar and the thalamus.

This chapter attempts to illustrate that parietal lobe, frontal lobes, cingulate gyrus, striatum and thalamus constitute a large-scale network subserving spatial attention, that hemispatial neglect is a syndrome of the network as a whole, and that the complexity of this network is commensurate with the clinical heterogeneity of the neglect syndrome. It is noted that components of the ascending reticular activating system, such as the intralaminar thalamic nuclei, the brainstem raphe nuclei, the nucleus locus ceruleus, the ventral tegmental area-substantia nigra, and the nucleus basalis project to each cortical component of the attentional network. The reviewed data has resulted in the hypothesis that spatial attention is organized at the level of a distributed large-scale network revolving around three cortical components, each of which supports a slightly different neural representation of the extrapersonal space. Each of these components serves a dual purpose: it provides a local network for regional neural computations and also a nodal point for the linkage of distributed information. Moreover, the components of the attentional network can collectively specify whether and how an event in extrapersonal space will attract covert attentional shifts, orientation, foveation, manual grasp, and overt search behaviors.

This chapter discusses recent efforts at developing mechanisms for capturing the effects of fatigue on human performance. It describes a computational cognitive model, developed in ACT-R (adaptive control of thought-rational), that performs a sustained attentional task called the psychomotor vigilance task. It uses neurobehavioral evidence from research on sleep deprivation, in addition to previous research from within the ACT-R community, to select and to evaluate a mechanism for producing fatigue effects in the model. Fatigue is represented by decrementing a parameter associated with arousal in ACT-R, while also reducing a threshold value in the architecture to capture attempts at compensating for the negative effects of decreased arousal. These parameters are associated with the production utility computation in ACT-R, which controls the selection/execution cycle to determine which production (if any) to execute on each cognitive cycle. In ACT-R, this mechanism is linked to the basal ganglia and the thalamus. In turn, portions of the thalamus show heightened activation in attentional tasks under conditions of sleep deprivation.

This chapter describes the use of intraoperative microelectrode recordings and microstimulation in thalamus to study the effects of high frequency stimulation on the neural activity of thalamic neurons. In many neurons a high-frequency stimulus train (100-333Hz) was found to be followed by a long period of inhibition, typically longer than 2s. Interestingly, the long-duration inhibition was seen primarily in bursting cells (81% in bursting cells compared to 24% in nonbursting cells). In contrast, at low stimulation rates no inhibition was seen and instead an excitatory response was observed in some neurons. These findings are discussed in terms of possible mechanisms and their relevance to explaining the therapeutic effects of thalamic DBS.

This chapter aims to describe the structure and organization of cortex and thalamus. Thalamus and cortex are two closely interconnected structures and are a characteristic development of the mammalian brain. They form the focus of this book. The cortex is widely regarded as having six layers. In spite of local differences, the complex multilaminar structure distinguishes the neocortex from the phylogenetically older paleocortex and archicortex, which have a simpler organization with fewer distinguishable layers. Several different cell types are distinguishable in the cortex and this chapter explains these cell types.

This chapter considers a new classification, starting with glutamatergic inputs, and explore what insights this provides concerning circuits in thalamus and cortex. The chapter begins this classification with a distinction between drivers and modulators; this classification was originally based on thalamic relays for which the properties of the synapses and their function in the relay of information were known.

This chapter finds that wherever appropriate methods have been used to demonstrate the branched origin of a driver input to the thalamus, it has been found. However, such branching patterns have not yet been studied for many cortical areas. Further evidence about the branching patterns of these axons for species including and extending beyond mouse, rat, cat, and monkey, and for many driver afferents to higher order thalamic relays, is needed. This would test whether all driver afferents to thalamus, to first order as well as higher order relays, are branches of axons that innervate circuits concerned with the control of movements. The chapter shows that many of the axons carrying messages to thalamus for relay to cortex have branches that innervate motor centers so that the cortex receives a great many messages that have a dual significance, representing events in the body or the world on the one hand and instructions for upcoming movements on the other.

This chapter and the next explore the pathways that link the thalamus and cortex to each other and to the rest of the brain, considering each in terms of the major points about the neural connections raised in earlier chapters. These connections provide the essential links between cortex, lower centers, action, and perception. Understanding the messages and knowing the functional nature of the relays is basic to understanding how the organism relates to the world. It is important to recognize that, although we raise the issue in a few instances only, this possible duality is relevant for all of the pathways that carry messages to the thalamus for relay to cortex.