Generally speaking, it is a term used to denote a state of heightened physiological activity in which there is an increased responsiveness to sensory stimulation or excitability. The relationship between such a state and behavior was encapsulated in the so-called Yerkes-Dodson law of arousal: there is an inverted-U relationship between performance and arousal such that an optimal performance on a task occurs at an intermediate level of arousal, and poorer performances at lower and higher levels (see figure below). The problem with such a formulation is that it is difficult to pin down exactly the meaning of arousal as it has so many different connotations. Is it a single physiological state or some collection of distinct, but interacting, states? One answer to this question is to hold that there is some generalized arousal state within which more specific types of arousal (e.g., fear, hunger, sex) interact in, as yet, some unknown ways. Whatever the answer, the application of the term ‘arousal’ to behavioural states in the developing infant, such that such states are depicted as taking up some position on a continuum of arousal, seems inappropriate.
