Landmark stability: further studies pointing to a role in spatial learning.

Two experiments were conducted to investigate the possible role of landmark stability in spatial learning. Rats were trained to search in a large arena for food hidden at a consistent distance and direction from either a single radially symmetric landmark or an array of two landmarks. We varied the relative degree to which the landmark array and/or the cues of the training context predicted the location of food, without varying the conditional probability of food being available given either cue. Experiment 1 used vestibular disorientation to ensure control of search location by experimenter-controlled cues. The results showed that making either a single landmark or a cluster of two adjacent landmarks the sole spatial predictor of reward location reduced the accuracy of search compared to a condition where both the landmark array and context cues were reliable spatial predictors. Varying global landmark stability had no effect when training was conducted using an array of two landmarks located some distance from each other. Context cues, when tested alone, triggered very little searching in appropriate locations, and the absolute magnitude of control over search was insufficient to account for the superiority of stable landmarks. The better learning with a stable landmark, and the dependence of this effect on the geometrical arrangement of landmarks, points to the conditions of spatial learning involving additional principles to those of simple associative conditioning. Experiment 2 examined landmark stability using a single landmark and fixed directional cues in the absence of vestibular disorientation. This also revealed a relative advantage of landmark stability, but animals with a landmark that moved from trial to trial did show some evidence of learning. Context cues when tested alone had minimal influence. Parametric manipulation of landmark stability offers a novel way of influencing spatial learning and thus understanding better the process through which egocentric representations of perceived space are transformed into allocentric representations of the real world.