A new methodological approach to the study of habituation: the use of positive and negative behavioral indices of habituation.

This report details a new method to measure habituation in an open-field. In addition to the measurement of spontaneous locomotor activity, time spent per visit to the center zone (CZ) is also measured. Critically, a small object is placed in the CZ to modulate an animal's investigatory response. When an animal is first exposed to the open-field, the presence of the object does not affect the duration of its visits to the CZ but, if the animal is given one additional 10-min exposure to the open-field, then the presence of the object substantially increases the duration of its visits to the CZ. The presence of the object, however, has no effect on the rat's locomotor activity. Thus, habituation could be observed by two different measures: a decrease in locomotor activity and an increase in an animal's investigatory response to a stimulus object. A basic problem with a reliance solely upon a decrease in locomotor activity to measure habituation is that it represents a negative change in behavior. The present method circumvents this shortcoming by incorporating a positive behavioral measure of habituation in conjunction with the measurement of locomotor activity. This modification of the open-field test offers substantial utility for studies of neurotoxicology and memory because one can assess concurrently treatment effects on motor activity, attention to an object and memory.