The interaction between sensory and tonic factors in the perception of the vertical in pigeons

In two separate experiments, 21 pigeons were given 10 days of training to peck a key on which a white vertical line was exposed. Next, the birds were given generalization tests along the angularity dimension with the floor tilted 24° to the right or to the left. Reliable asymmetrical gradients were obtained with enhanced responding to the side toward which the floor was tilted. This posturally induced distortion in the visual perception of the vertical indicates that sensory-tonic interactions typically observed in humans have a parallel in this lower species.