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Signal The Brain: The function of the Detector is to remove the modulation from the intermediate-frequency signal after it has been amplified. (As indicated previously, the desired modulation information originally superimposed on the carrier wave was transferred to a lower radio-frequency signal—the intermediate frequency signal.) This demodulation is accomplished by using a rectifier and a filter. A diode commonly is used to produce a rectified i-f signal, which is filtered to obtain an audio-frequency signal. This audio signal is the output of the detector.
Chemical signals also occur between plants and insects. In Douglas fir trees damaged by storms, volatile terpenes are released that attract female beetles (Dendroctonus). Later, these females secrete a pheromone that attracts additional males and females. In this case, the signal provides the species with its ecological niche and the trees become infested.
In the laboratory, certain polyphemus moths were found to'mate only in the presence of a volatile emanation from oak leaves. The substance (frans-2-hexenal) was isolated and found to act on the female antennae, which in turn signal the brain.
Among the most interesting developments in brain studies was a finding that corrected the long-held conviction that glucose was the only fuel the brain was capable of using. George Cahill studied brain metabolism in persons who fasted for more than 30 days as part of a weight reduction program. By sampling blood entering and leaving the head, he was able to show that during fasting the brain will readily metabolize fatty acids, the breakdown products from deposits of body fat. Careful intelligence tests before and after the fast failed to show any mental impairment during the period of fasting. Cahill said that in fact the subjects were at least as sharp after fasting, and maybe a bit sharper. |
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