The Relationship Between Geometric Shape and Slope for the Representation of a Goal Location in Pigeons (Columba livia)

The ability to use the geometric shape of the environment to orient in space and locate a goal has been shown in many vertebrate groups. Experimentally, however, spatial tasks are typically carried out on a horizontal surface. The present study explored the importance of the vertical dimension for representing a goal location and how solving a geometry task is affected by the presence of a vertical component in the environment. In a reference memory task, pigeons were trained to find a goal in an acute corner of an isosceles trapezoid arena, which could be placed on a flat or on an inclined surface. In Experiment 1, learning the task on a slope proceeded more rapidly than on a flat surface, presumably because of the additional kinesthetic, vestibular and visual information extractable from the inclined surface. Although the geometric shape of the arena was encoded, pigeons trained on a slope were almost exclusively relying on a goal representation based on the vertical and orthogonal axes of the slope to solve the task. In Experiment 2, pigeons learned the geometric shape of the arena at a similar pace whether training occurred on a slope or not, indicating a lack of cue competition between slope and shape geometry. In Experiment 3, pigeons were trained with three different orientations of the arena on the slope. Subjects encoded the shape of the environment; however, when tested in a novel arena orientation, pigeons did not generalize what they learned and did not choose the geometric correct corner. Surprisingly, however, they made systematic errors to the other acute, mirror image corner. It is hypothesized that this systematic error reflects the encoding of multiple orientation-specific slope-based goal representations during training. Overall, the present study showed, for the first time, that a slope gradient exerted stronger control over goal searching behavior than the geometric shape of the environment. Furthermore, it indicated that pairing the more salient slope cues with the less salient geometric cues did not overshadow geometric learning, but produced a peculiar effect, such as the failure to generalize geometric cues in a novel orientation