18: The Organization of Cognition

Introduction

• Functionally Related Areas of Cortex Lie Close Together

• Sensory Information Is Processed in the Cortex in Serial Pathways

• Parallel Pathways in Each Sensory Modality Lead to Dorsal and Ventral Association Areas

  • The Dorsal Visual Pathway Carries Spatial Information and Leads to Parietal Association Cortex

  • The Ventral Visual Pathway Processes Information About Form and Leads to Temporal Association Cortex

• Goal-Directed Motor Behavior Is Controlled in the Frontal Lobe

  • Prefrontal Cortex Is Important for the Executive Control of Behavior

  • Dorsolateral Prefrontal Cortex Contributes to Cognitive Control of Behavior

  • Orbital-Ventromedial Prefrontal Cortex Contributes to Emotional Control of Behavior

• Limbic Association Cortex Is a Gateway to the Hippocampal Memory System

• An Overall View

In the previous chapter we examined how the activity of single nerve cells can be related to the internal representations required for simple cognitive tasks. In this chapter we survey the anatomical and physiological organization of the cortex through which the activity of populations of neurons mediate complex aspects of cognition. For this purpose we draw on the insights that have emerged from studies of the neural mechanisms of cognition in monkeys and studies of cognitive impairment resulting from brain injury in humans. In the following chapter we extend this analysis to the premotor cortex and the control of voluntary movement. In Chapter 20 we discuss how neuroimaging studies of cognition in humans are consistent with the findings from these experimental and clinical studies.

Cognitive functions are mediated by specialized areas of neocortex distributed across the cerebral hemisphere in an orderly arrangement. This was already well established in 1962 when Alexander Luria published his landmark Higher Cortical Functions in Man. Neurologists of Luria's generation knew that lesions at neighboring sites on the cortical surface tend to produce related symptoms. For example, lesions of the occipital cortex give rise to lower-order visual deficits (cortical blindness), whereas lesions of adjacent temporal cortex result in higher-order visual deficits (object agnosia). Similarly, lesions in the posterior sector of the frontal lobe give rise to lower-order motor deficits (weakness and paralysis), whereas more anterior lesions give rise to higher-order deficits of executive control (the prefrontal syndrome). How can these observations be explained?

Luria proposed that sensory and motor cortex comprise multiple specialized subareas that are connected hierarchically. A primary sensory area lies next to a secondary sensory area that in turn borders a tertiary area. These areas have progressively more complex functions, culminating in the integration of multiple sensory modalities in the tertiary zones. Areas in the frontal lobe concerned with motor behavior are similarly organized. The primary motor cortex lies next to a secondary motor area (the premotor cortex) that in turn borders a tertiary area, the prefrontal cortex, concerned with the executive control of behavior. In Luria's scheme sensory information flows into the central nervous system through a series of synaptic relays from primary to secondary to tertiary sensory areas, whereas motor commands flow from ...