Chapter 3: Functional Neurocircuitry of Cognition and Cognitive Syndromes

INTRODUCTION

To maintain our survival and well-being, the nervous systems must carry out a number of critical operations that comprise cognition. We need to process information about the external environment (sensory-perceptual representations and visuospatial cognition) and be able to prepare and execute actions (motor planning and output). We rely on information that allows us to communicate with others (language), and encode, store, and retrieve information (learning and memory). Importantly, we must be able to select, control, and monitor these complex operations as internal and external demands shift (executive control functions). This chapter will explore the neural circuitry underlying these varied cognitive processes. In our companion chapter (Chapter 12), we will illustrate how disruption of these operations can impact patients, and outline an approach to localization providing a series of clinical cases as examples. Of note, behavioral neurology and cognitive neuroscience are dynamically changing fields. We present a brief snapshot of the state of our discipline, and fully acknowledge the ongoing debate about which proposed frameworks provide the best account of the relevant data. In some of the “boxes” included throughout the chapter, we briefly highlight differences between classical and emerging theories, and touch upon some unresolved controversies. Other boxes will provide a summary of the neural circuitry subserving each of the major cognitive realms discussed.

Virtually all aspects of cognition arise from the dynamic interactions of distributed brain regions conceptualized as large-scale neural networks.^1,2 Brain networks fall into two different categories: structural networks, which represent anatomical wiring, and functional networks, which are derived from estimates of correlated neural activity among brain regions that are connected by underlying mono- or polysynaptic neuronal connections.³ Functional networks are interconnected ensembles of cortical and subcortical neurons that are coactivated to mediate a cognitive or behavioral function. A wealth of evidence from anatomical, neuroimaging, and physiological studies supports the idea that individual brain regions are functionally specialized and make specific contributions to cognition, and further, that these functionally specific regions comprise networks that mediate cognitive and behavioral functions. Some investigators have described a network’s functional connectivity as represented by temporal coherence in activity across different neurons or neuronal ensembles.⁴ Such work is increasingly informed by network or graph theoretical approaches that define nodes and edges (or paired relationships) between nodes.^2,5 Functional network connectivity can reflect both direct and indirect underlying anatomical connections via other cortical or subcortical regions, and the dynamic modulation of specialized brain networks are critical for the specialized cognitive functions of memory, language, attention, visual/spatial functioning, and executive control.⁶ Cognition and behavior are considered broadly to be emergent phenomena of large-scale neural networks and their subcomponent networks, while critical network centers, or nodes, are thought to constitute relay or integration centers.⁷ Lesions to the circuit can lead to the inability to transfer information from one node to another.^8,9 Recent work has focused on identifying the breakdown in dynamic connectivity within and between ...