From Nerve Cells to Cognition: The Internal Cellular Representation Required for Perception and Action

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 has been particularly good when we understand the anatomy and the connections of the 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. Analyses of vision, the sensory modality most thoroughly studied at the cellular level, show that information arrives in the brain from the retina in separate, parallel pathways, each dedicated to analyzing a different aspect of the visual image (form, movement, or color), and that these separate inputs are integrated into coherent images according to the brain's own rules. Different modalities of perception—an object seen, a face touched, or a melody heard—are processed similarly P.382 by 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 an image—it responds with a distinct pattern of firing that represents certain properties of the image. Each sensory system obtains information from the stimulus in this way and transmits this information along a pathway of cells leading to a specific region of cerebral cortex. In the cortex different unimodal regions representing different sensory modalities communicate through specific intracortical pathways with multimodal association areas, which select and combine signals 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 doe