An animal that has stepped over an obstacle with its
forelimbs uses a
memory of the obstacle to guide the
hind limbs so that they also clear the obstacle, even in situations in which long pauses are introduced between
forelimb and
hind limb stepping. To further clarify the features of
hind limb obstacle clearance
memory, the present study examined
hind limb obstacle clearance in the horse. A rider guided horses over obstacles and paused the horse over obstacles in tests that examined the relationship between
forelimb and
hind limb stepping, with the following results. First, the horses displayed
memory for an obstacle as measured by
hind limb lifting over the obstacle for durations lasting as long as 15 min. The response was not dependent upon ongoing visualization of the obstacle, as limb lifting was unaffected by
visual occlusion with blinders, a
blindfold, or by removing the obstacle during the pause. Second, previous experience of stepping over an obstacle
led to pause-related
hind limb lifting at the object's previous location even on trials for which there was no obstacle and so no preceding
forelimb lifting. Third, whereas a horse would lift its
hind limbs to clear two successively presented obstacles, replacing an obstacle before the horse after the
forelimbs had cleared the obstacle prevented subsequent
hind limb lifting at the obstacle's previous location. Taken together the results show that
hind limb obstacle clearance is guided by a place-object
memory. The results are discussed in relation to the differential sensory and memonic control of
forelimb and
hind limb stepping with the suggestion that place-object
memory can guide hind stepping as well as overshadow
working memory from front
leg stepping.
