"Short-stops" in rats with fimbria-fornix lesions: evidence for change in the mobility gradient.

Rats with damage to the hippocampal formation and allied structures are hyperactive in many test situations but the cause of this hyperactivity is not known. Here the activity of control rats and rats with fimbria-fornix lesions is documented in tests of overnight activity. Details of activity are then characterized from video recordings of behavior in an open field. Rats with fimbria-fornix lesions make significantly more stops of shorter duration and thus more individual trips than control rats but they do not differ in the distance traveled on individual trips or in travel speed. It is suggested that the main difference between fimbria-fornix rats and control rats is that when fimbria-fornix rats stop they remain "still" for shorter durations than do control rats. This finding is discussed in relation to a theory of locomotor/exploratory behavior, and in relation to its implications with respect to the performance of fimbria-fornix rats in studies of learning and memory.