17: From Nerve Cells to Cognition: The Internal Representations of Space and Action

Introduction

• The Major Goal of Cognitive Neural Science Is to Understand Neural Representations of Mental Processes

• The Brain Has an Orderly Representation of Personal Space

  • The Cortex Has a Map of the Sensory Receptive Surface for Each Sensory Modality

  • Cortical Maps of the Body Are the Basis of Accurate Clinical Neurological Examinations

• The Internal Representation of Personal Space Can Be Modified by Experience

• Extrapersonal Space Is Represented in the Posterior Parietal Association Cortex

• Much of Mental Processing Is Unconscious

• Is Consciousness Accessible to Neurobiological Analysis?

  • Consciousness Poses Fundamental Problems for a Biological Theory of Mind

  • Neurobiological Research on Cognitive Processes Does Not Depend on a Specific Theory of Consciousness

  • Studies of Binocular Rivalry Have Identified Circuits That May Switch Unconscious to Conscious Visual Perception

  • Selective Attention to Visual Stimuli Can Be Studied on the Cellular Level in Nonhuman Primates

  • How Is Self-Awareness Encoded in the Brain?

• An Overall View

Neural scientists believe that a cellular approach is necessary to understand how the brain works. Considering that the brain has a hundred billion nerve cells, it is remarkable how much can be learned about mental activity by examining one nerve cell at a time. Progress is particularly good when we understand the anatomy and connections of functionally important pathways.

Cellular studies of the sensory systems, for example, provide important insight into how stimuli at the body's surface are translated by the brain into sensations and planned action. In the visual system, the sensory system most thoroughly studied at the cellular level, information arrives in the brain from the retina in parallel pathways dedicated to analyzing different aspects of the visual image—form, movement, and color. These separate inputs are eventually integrated into coherent images according to the brain's own rules, rules that are embodied in the circuitry of the visual system.

Different modalities of perception—an object seen, a face touched, or a melody heard—are processed similarly by the different sensory systems. Receptors in each system first analyze and deconstruct stimulus information. Receptors at the periphery of the body for each system are sensitive to a particular kind of physical event—light, pressure, tone, or chemical odorants. When a receptor is stimulated—when, for example, a receptor cell in the retina is excited by light—it responds with a distinct pattern of firing that represents certain properties of the stimulus. Each sensory system obtains information about the stimulus in this way and this information is conveyed along a pathway of cells leading to a specific (unimodal) region of cerebral cortex. In the cortex different unimodal regions representing different sensory modalities communicate with multimodal association areas through specific intracortical pathways, and in this network signals are selected and combined into an apparently seamless perception.

The brain thus produces an integrated perception because nerve cells are wired together in precise and orderly ways according to a general plan that does not vary greatly among normal individuals. Nevertheless, the connections ...