Learning of Relative Distance between Discrete Visual Landmarks by Pigeons (Columba livia)

In an open-field search task devoid of orienting cues and informative geometry, pigeons were trained to find a goal located at the midpoint of the hypothetical line connecting two discrete landmarks positioned in a linear array. Pigeons searched in substrate for an initially visible but eventually invisible goal location containing food. The distance between the landmarks (interlandmark distance) was fixed throughout training. Pigeons learned to locate the goal and continued to search at this location in the absence of food. After reaching training criteria, food-absent trials were conducted in which the interlandmark distance either remained the same as in training or was manipulated by contraction or expansion. Search error and location on novel interlandmark distances were identical to those obtained on training trials. Results suggest pigeons learned to search at a relative distance between the landmarks. Implications of a stable frame of reference as critical in spatial learning are discussed.