The New Science of Consciousness: Exploring the Complexity of Brain, Mind, and Self by Paul L. Nunez

Author:Paul L. Nunez
Language: eng
Format: epub
ISBN: 9781633882201
Publisher: Prometheus Books
Published: 2016-10-04T04:00:00+00:00

6.9 SUMMARY

The brain produces many kinds of rhythms—some are recorded on the human scalp with EEG; others are observed only with intracranial electrodes. The provocative label “rhythms” is appropriate for an orchestra, the brain metaphor proposed in this chapter. Brain rhythms are closely related to a broad range of mental functions, including information transfer, perception, muscle control, memory, and general states of consciousness. The structures creating these rhythms can range from single neurons to nested neural networks of different sizes, and even to globally generated phenomena; thus brain rhythms are expected to be scale-sensitive in experimental work.

Earlier chapters employed the football-fan metaphor to demonstrate states of functional localization versus states of global coherence. The magical-Christmas-lights metaphor indicated how distinct dynamic patterns of brain activity are measured at different spatial scales. Our fanciful orchestra is yet another system producing activity at multiple scales—a system of vibrating musical instruments with no conductor or conscious musicians. Orchestra rhythms are generated at several levels of organization—small-scale rhythms originate with violin strings, which stimulate a different mix of overtones produced by the violin body. The orchestra's string section provides an even higher organizational level. Similarly, brain rhythms are generated at several or perhaps many organizational levels, ranging from single neurons to the entire cerebral cortex and perhaps even the entire brain.

The frequencies at which objects like musical instruments tend to vibrate are the object's natural frequencies. The violin has four strings; each string has its own set of natural frequencies, consisting of the fundamental and its harmonics. The strings themselves produce very little sound; most of the sound is produced by the violin body, which responds selectively at its own natural frequencies, called the overtones. Resonance is this tendency to respond strongly only to certain frequencies of external stimulation—the violin body responds to oscillations of all four strings, but it responds most strongly to oscillations of the A string. Each string's set of natural frequencies, the fundamental and higher harmonics, may be adjusted by turning knobs at the end of the violin, thereby changing the tension in selected strings; tighter strings produce higher frequencies. In comparison, brain frequencies are controlled by various chemical neuromodulators.

EEG consists only of the large- to intermediate-scale brain rhythms that manage to reach the scalp; oscillations generated at smaller scales can be studied only with electrodes implanted inside the skull, either on the brain surface or inside the brain itself. One important type of oscillations consists of the alpha rhythms, a class of EEG signals with relatively large frequency components in the 8 to 13 Hz range when recorded from the human scalp. Alpha rhythms can provide a starting point for clinical EEG exams, posing questions about reaction to eyes opening, mental activity, hyperventilation, drowsiness, and so forth. Human alpha rhythms are part of a complex process, involving local networks as well as globally coherent phenomena.

Loss of normal consciousness occurs during sleep, consisting of progressively deeper stages 1 through 3, plus the rapid eye movement (REM) stage associated most strongly with dreaming.

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