Habituation can be defined as a process or as a procedure. As a process it is defined as a decrease in an elicited behavior resulting from the repeated presentation of an eliciting stimulus (a simple form of learning). As a procedure it is defined as the repeated presentation of an eliciting stimulus that may result in the decline of the elicited behavior (the process of habituation), an increase of the elicited behavior (the process of sensitization), or an initial increase followed by a decline of the elicited behavior (a sensitization process followed by a habituation process). A progressive decline of a behavior in a habituation procedure may also reflect nonspecific effects such as fatigue which must be ruled out when the interest is in habituation as a learning process.