Analysis of locomotor activity in the rat: parallelism index, a new measure of locomotor exploratory pattern.

Several measures of locomotor activity in rats, including the distance covered, movement time, speed of progression, and sinuosity showed significant changes in extreme situations after prolonged preadaptation or after stimulation with I.P. amphetamine. The same measures, however, either did not change or poorly reflected the changes in locomotor activity consistent with progressive habituation in successive daily or weekly recordings. Other movement parameters, such as the average angle of turns, did not change or changed marginally, even in extreme situations. A new locomotor parameter, the parallelism index, has been proposed. It reflects the overall tendency to turn and the angular magnitude of turns. The parallelism index is not directly dependent on the distance covered by the animal, and seems to significantly reflect subtle changes in the pattern of locomotor activity that is characteristic of the exploration of an unfamiliar environment compared to the locomotor movement in frequently visited areas. A significant decrease of the parallelism index with time or repeated exposure to the cage indicates that rats perform more turns, and/or more turns under larger angles, in a more familiar or a more explored environment. It is postulated that the parallelism index decreases with the familiarity of the area being explored.