Module 16

OBJECTIVE: A complex cognitive behavior is the sum of its component cognitive processes.

Summary: A PET scan can show areas of the brain which are active during a cognitive process. When the cognitive process is a speech act, if we subtract stimulated areas on the PET scan known to be associated with motor activity, stress, etc., the remaining stimulated areas on the PET scan must be those stimulated by purely linguistic factors. This tells us which areas of the brain control these linguistic activities.

What is a PET scan and how does it work?

"PET" stands for "Positron Emission Tomography." Shortly before the scan, the subject receives radioactively labeled glucose. Glucose is the major source of energy for the brain. When the brain metabolizes its radioactively tagged glucose, the temporarily radioactive atoms remain inside the brain cells, emitting positively charged particles called "positrons." The positrons collide with electrons in the vicinity, giving rise to gamma radiation which can be mapped on a computer.

Thus, we can see where nerve cells in the brain are more active (a) during a particular task; (b) during a thought; (c) during an emotion; or (d) in the case of disease. The PETscan can also measure changes in local blood flow. Small blood vessels respond very quickly to the needs of brain cells, making it possible to measure brain activity as well in addition to measuring glucose metabolism. Cells having a higher metabolism (using more energy and having more blood flow) appear yellow and red ("hotter") in a PETscan. Areas that are blue or black show decreased or no activity. In addition to measuring glucose utilization and blood flow in vivo, in human subjects, the PET scan can also measure blood volume, oxygen consumption, tissue pH and activity of receptors in brain cells. It can measure any changes in metabolic functions; for instance, in the case where an alternation of structure produces a metabolic activation, such as in the case of a brain tumor, this can be detected with a PETscan.

In the image at the bottom from Dr. Raichle's Neuroimaging Lab at the Washington University School of Medicine in St. Louis, there are two different conditions. In the "Unpracticed" condition, an individual hears a text in order to learn a new language.

Watch the video below on Dr. Raichle's experiment.

WATCH THE VIDEO

Are certain cognitive functions localized in certain areas of the brain? If so, how can we find out which functions are localized where? Early phrenologists thought that bumps on the skull signaled specific human thought processes and emotions e.g., humorousness, generosity, etc. This is incorrect. However, it is true that certain networks of neurons located in localized areas perform certain thought processes. For, example, in the video above we saw that certain areas of the brain are responsible for language comprehension, hearing reception and motor control of the voice. We can use this information to determine which areas of the brain perform a certain cognitive function during a speech act.

In the video you just saw, Dr. Raichle was able to see which areas of the brain were stimulated by motor activity alone, by reading alone, by stress alone, etc. He subtracted out those areas from the stimulated areas on the PET scan when the subject has to generate a verb to each noun presented. What was left on the PET scan after the subtraction must be those stimulated by the purely linguistic factor of having to generate a verb to a noun.