Arousal Indices

The most common index of arousal is the cortical electroencephalogram (EEG), which can be recorded from the scalps of most mammals. The electrical activity recorded at the scalp represents the summed voltages produced by extracellular neuronal activity beneath the recording electrode. Neuronal electrochemical currents are filtered by the skull and surrounding tissues as the currents spread passively to the recording device. Consequently, the exact location of a particular voltage change is difficult to determine.

The electrical voltage of the EEG ranges from a few microvolts in the aroused state to several hundred microvolts in deep, slow-wave sleep. Therefore, EEG voltage is inversely correlated with behavioral arousal. EEG frequency varies directly with arousal, with the highest values (50-60 Hz) associated with the highly aroused state and the lowest values (2 Hz) seen in comatose states. Both overt behavior and EEG characteristics tend to change gradually during the day. One exception to this is during sleep, when abrupt transitions between slow-wave sleep and rapid eye movement (REM) sleep occur. The EEG characteristics of REM sleep are nearly identical to those of the alert state and reflect episodes of dreaming.

The EEG can also be used to measure alerting and vigilance by measuring changes in electrical activity immediately following an environmental stimulus, a shift of attention, or a change in the task at hand. Although arousal state may be recognizable from a single, 3- to 5-sec epoch of the EEG, it is usually necessary to average tens or even hundreds of short (e.g., 300 msec) segments of the EEG following the external stimulus to measure the change caused by alerting.

Many other measures have been used extensively, including electrodermal responses, changes in blood chemistry, and even early gene expression. A discussion of these issues is beyond the scope of the current review, but excellent treatments may be found in texts on psychophysiology.

There are pitfalls in relying on any single measure of arousal. For example, alerting caused by fear-evoking stimuli causes an increase in heart rate and other autonomic indices. In contrast, phasic alerting caused by orienting toward a nonthreatening stimulus causes a slowing of the heart and other internal organs. Thus, a complete understanding of the arousal response requires a consideration of the pattern of responses and the context in which they occur.

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