Chapter 12: Focal Neurobehavioral Syndromes

INTRODUCTION

One of the fundamental working hypotheses of modern neuroscience is that the functioning of the brain permits and shapes the expression of the mind. Cognitive, perceptual, affective, and behavioral capacities and limits are determined by neural structures and the dynamic flow of information within them. The shifting contents of momentary experience are the product of precisely coordinated, ever-changing combinations of electrical activity within richly connected neural networks. Put another way, neural states encode mental experiences. These complex neurobiological processes are scaffolded by evolution via inherited factors, animated by environmental inputs, and chiseled by experience and learning. The system learns about itself and the workings of its environment through continuous prediction and feedback, self-generated actions and environmental inputs shape this dynamic neural circuitry. Brain-behavior relationships are complex but patterned: specific neural activities support specific mental states and localized brain dysfunction leads to focal neurobehavioral changes. This schematic framework relating brain and mind is embedded in the clinical fields of behavioral neurology and neuropsychiatry and serves as this chapter’s main axiom.

There are many schemas for how to understand the brain’s role in supporting mental functioning. Highlighted below are several working hypotheses about brain organization that help to frame and add explanatory depth to the discussion of focal neurobehavioral syndromes. These ideas are introduced to stimulate the creative clinician to connect bedside observations not only to neuroanatomy, but to more foundational principles of cognitive neuroscience. Clinicians have a front-line opportunity to test, challenge, and improve these models to better understand and care for our patients. If past is prologue, clinician-driven ideas about brain-behavior relationships also promise to change our basic understanding of human cognition, affect, and behavior.

The brain is composed of specialized neurocomputational units whose activity is determined by local neurobiological factors

The brain is a collection of modular, often anatomically localized, units each designed to perform a specialized type of neural computation. The coordinated activity of these neural transformations form networks that encode the complex neural representations supporting cognition, affect, and behavior. Individual units receive information, perform a specific transformation, and share the result with other units. The dynamic integration of information flowing across multiple neurocomputational units supports the incredible diversity of cognitive functions. The particular transform performed by an individual unit is determined primarily by the neurobiological properties of its local brain tissue (e.g., cytoarchitecture, cell types, etc.).¹ A given unit is capable of receiving, transforming, and transmitting information from many different units/areas² via white matter pathways. It can contribute to a wide variety of functions depending on its connections with other units, enhancing efficiency. In contrast, a single function may be accomplished by different combinations of parts/units, which provides redundancy in case of injury. The redundancy idea—that there are multiple ways to perform a task—makes it difficult to precisely pinpoint the disrupted units/parts for any observed change in behavior. The flexible employment of these units and ...