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Functional Imaging Reflects the Metabolic Demand of Neural Activity
Functional Imaging Is Used to Probe Cognitive Processes
Imaging Perception with and Without Consciousness
Imaging Memory with and Without Consciousnes
Imaging Attentional Modulation of Conscious Perception
Functional Imaging Has Limitations
An Overall View
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The ability of neuro-imaging to observe areas of the human brain that are active during cognitive processes has helped to stimulate the current interest in the biological underpinnings of cognitive functioning. Because invasive experiments cannot be done ethically on humans, research on the biological basis of cognitive function was until quite recently confined to laboratory animals and clinical studies of patients with cognitive disorders.
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The development of techniques such as functional magnetic resonance imaging (fMRI) has made it possible to study human subjects, affording unprecedented views of the complexities of the intact working brain. Imaging of the living brain allows us to explore the behavioral significance of local neural circuits, such as cortical columns, as well as observe large-scale systems of interconnected brain regions concerned with specific mental processes such as seeing, hearing, feeling, moving, talking, and thinking.
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Functional Imaging Reflects the Metabolic Demand of Neural Activity
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Functional Imaging Emerged from Studies of Blood Flow
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Functional imaging evolved out of seminal studies in the late 1940s by Seymour Kety and Carl F. Schmidt, who succeeded in measuring blood flow in the living brain. Although Charles S. Roy and Charles S. Sherrington earlier had found a relationship between blood flow and brain metabolism, Kety and Schmidt were the first to quantify cerebral blood flow noninvasively.
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To accomplish this task, Kety and Schmidt measured the rate of cerebral blood flow by having subjects inhale nitrous oxide, a metabolically inert gas, and measuring its outflow concentration from the jugular vein (Box 20–1). In a series of landmark studies they evaluated how blood flow from the intact brain varied in different metabolic states, such as sleep and wakefulness, and in so doing they laid the foundations for modern functional imaging.
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Box 20–1 Application of the Fick Principle to Brain Metabolism
Devised as a technique for measuring cardiac output by Adolf Eugen Fick, the Fick principle states that an organ must receive blood at a rate that is equal to the rate at which the organ metabolizes a constituent of blood, divided by the concentration of that constituent.
The essence of the Fick principle is that blood flow to an organ can be calculated using a marker substance. The principle may be applied in many ways. For example, if blood flow to an organ is known, together with the arterial and venous concentrations of the marker substance, then the uptake or metabolism by the organ may be calculated.
Seymour Kety and Carl F. Schmidt adapted the Fick ...