Cortical Electroencephalogram

An alternative method to assess alertness is to measure neural activity using an electroencephalogram (EEG). The EEG is the summed electrical activity (postsynap-tic potentials) of many thousands of neurons. It is recorded noninvasively using scalp electrodes. Because the site of recording is on the scalp, much of the recorded signal emanates from cortical neurons. When neurons fire in synchrony, the EEG has a pattern of high amplitude and low frequency. Desynchrony, when the neurons fire independently, is reflected as low amplitude and high frequency in the EEG. The amplitude and frequency of the neural activity correlate with sleeping and waking states. Four different patterns of neural activity may be identified: alpha, beta, theta, and delta activity. Theta waves are highamplitude, low-frequency (4-7 cycles per second) waves that characterize the period of drowsiness before the delta waves (fewer than 3 cycles per second) of deep, so-called "slow-wave" sleep are seen. When subjects are awake, two patterns of neural activity are recorded in the EEG—alpha waves and beta waves. In quiet resting, particularly when the eyes are closed, alpha waves are recorded with a frequency of about 10 cycles per second. The higher amplitude, lower frequency pattern that defines alpha waves reflects neural synchrony. When subjects are alert, low-amplitude, high-frequency beta waves (15-30 cycles per second) dominate the EEG: This is the state of desynchrony. Desynchrony in the EEG can be elicited by stimulating the subject with a loud noise or by engaging the subject in a mental activity such as problem solving. These different patterns of neural activity recorded in the EEG represent a continuum of neural states rather than absolute qualitative differences. Thus, in the EEG trace, it is possible to identify periods of alpha activity or synchrony and beta activity or desynchrony, but it is not necessarily possible to determine when one type of activity gives way to another. The presence and proportion of beta activity in the EEG are correlated with a subjective sense and behavioral evidence of alertness in the subject.

The EEG also has features that may be related to vigilance and motor readiness; therefore, this technique might be a useful method to distinguish alertness from vigilance and motor readiness. The noise in the EEG signal is such that it is not easy to identify changes in the raw electrical signal evoked by stimuli. However, if the signals are averaged over many occurrences of an event, it is possible to extract an event-related potential from the background noise. In vigilance tasks, sustained negativity in the EEG reliably follows a warning signal that precedes a stimulus event. This particular scalp potential, with increasing negativity in the period preceding the expected target, is referred to as the N1. It is also called the expectancy wave or contingent negative variation (CNV) because it is elicited by neither the warning signal nor the stimulus event alone but by the expectancy of the target following the warning signal due to the contingency between them. It is not possible to conclude with certainty that the CNV does indeed reflect a single psychological process (by implication, the warning-target contingency) rather than the totality of multiple processes, which might include orientation to the warning signal and expectancy of the target stimulus. Nevertheless, the CNV appears to be a correlate of vigilance and the issue of whether the CNV reflects a single process informs the debate about whether vigilance is a single process. Another negativity in the EEG, the N2, is recorded over secondary motor cortex, preceding the onset of movement. This is called the readiness (or Bereitschafts) potential. This negative potential reflects movement preparation or motor readiness. The "vigilance decrement'' is reflected in the overall amplitude of the CNV and the motor readiness potentials; therefore, there is support for the contention that these negative brain potentials reflect the operation of effortful attentional vigilance.

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