Dictionary of Biological Psychology
A general state of alertness. During wakefulness, arousal mechanisms have been said to increase or decrease the efficiency of information processing by the CEREBRAL CORTEX. The relationship between arousal and level of performance is not straightforward however, and is traditionally described by an inverted-U shaped function known as the YERKES-DODSON LAW. This indicates that increased arousal may not always be facilitatory to behaviour, which may be because very high levels of arousal are harder to control or coordinate in neural terms. In some circumstances, arousal is thought to provide added intensity to emotional or motivationally salient (see MOTIVATION) stimuli. In other situations, arousal level may determine not whether a response occurs, but what type of behaviour is performed. For instance, in studies with rats, feeding may occur at one level of arousal, while FIGHT-OR-FLIGHT occurs at another.
As a general rule, level of arousal increases throughout the day and can be modified by a number of different stimuli. Sensory input (particularly noise) increases arousal, as do incentives (see INCENTIVE), unsatisfied biological drives (such as HUNGER, THIRST and SEXUAL BEHAVIOUR), and stimulants (such as AMPHETAMINE, CAFFEINE and NICOTINE). For instance, there is enough caffeine in a few cups of coffee to produce an increased capacity for sustained intellectual effort and decreased reaction times. Sensory deprivation and SLEEP deprivation can also reduce levels of arousal. The interactions between these different modulators on level of arousal are complex and poorly understood.
In some cases it is clear that deterioration in performance due to a ‘dearousing’ stressor can be compensated for by the application of an ‘arousing’ stressor. However, it is also clear that in general there is no simple arithmetical interaction, since arousal is not a unitary process.
Measurement of arousal level is not straight-forward. Some indices, such as increased heart rate, also occur during exertion due to the need for additional OXYGEN in muscle tissue. A quantitative measure of arousal can be extrapolated from an analysis of the different frequencies of electrical activity present in the cortical ELECTROENCEPHALOGRAM (EEG): a fast ‘desynchronized’ EEG is typical of a state of high arousal. Although EEG desynchrony (see SYNCHRONY/DESYNCHRONY) is also characteristic of REM SLEEP (rapid eye movement sleep)—hence the term PARADOXICAL SLEEP—arousal is clearly dissociable from this by taking into account parallel changes in neck muscle tone (see ELECTROMYOGRAM), since this measure is high during waking and negligible during REM sleep.
From a physiological perspective, arousal is thought to occur as a result of increased activity within the ASCENDING RETICULAR ACTIVATING SYSTEM (ARAS) of the brain: interactions of the ARAS with many highly distinct cortical and subcortical systems are important for the coordination of arousal processes specific to external circumstances and internal needs. The complex projection systems of the different neurotransmitters within the ARAS imply that different stimuli will interact with specific neurochemical sub-components and modify arousal in unique ways. For instance, many centrally acting stimulants, including amphetamine and caffeine, have their principal arousing actions through the NORADRENERGIC sub-component of the ARAS at a number of different locations in the forebrain, notably the thalamus, hypothalamus and cortex. Nicotine, on the other hand, acts through the CHOLINERGIC sub-component of the ARAS, having potential arousing effects at nicotinic receptors in the THALAMUS, BASAL FOREBRAIN and cortex. TAIL-PINCH studies in rats have also demonstrated the powerful, but non-specific, role of DOPAMINE in arousal processes: if food is present, the rat will eat; if water is present, it drinks; presence of rat pups invokes maternal behaviour; and the presence of females induces sexual behaviour; three pinches a day in the presence of food can cause a rat to overeat until it is obese (see OBESITY). Such evidence, alongside the anatomical organisation of the DOPAMINERGIC projection system, suggests that arousal in dopaminergic terms is to intensify the impact of motivationally salient stimuli. However, despite the involvement of widespread brain nuclei in arousal processes, a focal point is clearly the thalamus, which modifies sensory, motor and motivational information en route to different specialized regions of the cortex. At the level of these thalamic synapses, there are clear similarities in physiological terms between attention and arousal functions.
WENDY L.INGLIS
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