* Arousal started out referring to an increased activity in the Neocortex at the same time as the Hippocampus expressed slow rhythmical oscillations.
* Then Arousal meant processes leading to increased levels of behaviour [in general].
* After that it simple meant that the individual was not sleeping.
* Lately Arousal has referred to an increased activity in the Hypothalamus - pituitary gland - adrenal cortex axis (the HPA-axis) which increased level of activity subsequently released steroids like cortisol, corticosterone, progesterone and testosterone - more about that later.
* Today Arousal has the meaning to refer to an unspecific general response towards all situations, which the individual can perceive as threatening. It is believed that both too high- and too low arousal can be harmful.
A straightforward thought was then that stereotypies might have a function coupled with Arousal. However, the problem with Arousal is that different behavioural- and physiological parameters do not always correlate.