On Being Human: Why Mind Matters by Jerome Kagan

Author:Jerome Kagan [Kagan, Jerome]
Language: eng
Format: epub
ISBN: 9780300220834
Publisher: Yale University Press
Published: 2016-03-22T06:00:00+00:00

The Meaning of Blood Flow

Most conclusions about the relations between brain activity and psychological outcomes in humans are based on only one measure of neuronal activity, called the BOLD signal, that reflects the pattern of blood flow in the brain of a person lying supine in a magnetic scanner. The BOLD signal is the product of the different rates of decay in the magnetic fields of oxygenated and deoxygenated hemoglobin following an event that generated a surge of blood to the brain. Because the BOLD signal is an indirect index of neuronal activity, it is useful to consider the many factors that influence this popular measure.

The brain state implied by blood flow to the event scientists present to their subjects, called state E, is influenced by the person’s usual brain state (state U), which varies with his or her mood, mental set, and brain physiology at the time. The latter property can vary with the time of year. Megan Miller and colleagues at the University of Pittsburgh discovered that the volume of the hippocampus is slightly larger in the summer, when there are many hours of sunlight, than the winter. The density of the receptors for the molecule serotonin, which influences neuronal excitability, also varies with the hours of sunlight. Male embryos conceived in early spring, when the hours of sunlight are increasing, experience a larger than normal surge of testosterone during the summer when the hours of daylight are the longest. One consequence of this fact is a greater slowing of the growth of the left hemisphere. Most neuroscientists do not worry about the effect on blood flow of a subject’s month of birth or the season of the year when they were assessed.

Participants who are unusually anxious over the prospect of having their brain measured are apt to secrete one or more molecules that affect the blood flow pattern. Esther Keulers and colleagues at Maastricht University found that adolescents who secreted cortisol prior to being placed in the scanner, perhaps because they were anxious, showed less blood flow to certain sites. The more than 100 trillion microbes that occupy the human gut secrete a variety of molecules that influence a person’s brain state. Future neuroscientists studying blood flow may decide to gather information on each person’s cortisol level and gut bacteria in order to better understand their results.

Each participant’s guess about the purpose of the study affects the brain response. For examples, subjects who assumed that the scientist was evaluating their intelligence by having them read words varying in emotionality would activate brain sites that might not correspond to the sites activated by subjects who thought the experimenter was interested in assessing their mental health.

State E, which incorporates state U, is also sensitive to the context of an incentive event. Every neuroscientist knows that the brain integrates information from a circuit that registers the identity of an object with information from another circuit that registers where in space the object is located. Nonetheless, scientists often present adults with

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