Brain scanning, also known as brain imaging, is potentially a major tool for providing physical insight into how a brain works. This appendix is an introduction to current brain scanning technologies.
Richard Caton (1842–1926) in Liverpool was first to notice electrical waves in the cerebral hemispheres of rabbits and monkeys in 1875. The first recording of human brain waves, that is, alternating potentials detected by electrodes on the scalp, was made in 1924 by Hans Berger (1873–1941). He named the resulting images electroencephalograms (EEGs). EEGs are noninvasive and have proved useful for sleep analysis, coma, and brain death verification. When assessing epilepsy, EEGs can detect seizures and can be accompanied by tests that stimulate a patient in order to study seizure triggers.
After preparing the scalp area by shaving and applying a conductive gel, EEG recordings are obtained by placing electrodes on the scalp. Each electrode feeds a differential amplifier with a voltage gain of 1000 to 100,000, that is, 60 to 100 dB. Scalp voltages are very low, with a maximum of about 100 μV. If measured invasively on the surface of the brain (by penetrating the skull), this goes up an order of magnitude, to about 1 mV. Differential amplifiers are employed because they resist the pickup of electrical noise. The EEG typically has millisecond resolution, depending on the bandwidth of the differential amplifier.
EEGs are able to detect evoked ...