International Journal of Comparative Psychology

A model will be presented in which mirror image confusion is employed as an example of perception of shape orientation, occurring as a result of evolutionary change in vision and movement. In the most primitive condition, vertical and horizontal coordinates are absent and shapes are equivalent in terms of orientation. In this condition directionality in external space is not objectified and movement is reflexively toward or away from the visual target. In the second condition, only the horizontal axis is present. Changes in orientation from upward to downward are perceptually salient. Quadrupedal movement patterns and locomotion across land, dominated by the horizon, are associated with the evolution of a mammalian eye with enhanced acuity across the vertical axis. Vertical mirror image confusion ceases to exist. In the third condition, the vertical axis appears. Factors in primate evolution associated with the appearance of enhanced acuity along these visual axes are related to perception of lateral rotations. In the fourth condition, upright posture and development of lateral bias in eye movement are related to the human proclivity to differentiate right and left orientation of shapes while exhibiting increased difficulty in tasks that involve changes in the vertical orientation. Thus structural changes in evolution associated with posture and movement are demonstrated to account for differences in perceptual responses to orientation of shapes.