A compensation for field expansion caused by moving forward

When an object increases in size and its retinal image expands, it is perceived to grow. But image expansion caused by one’s approaching an object of constant size does not result in perceived growth of the object. This is due, in part, to correct size perception which takes the distance of the object into account. But perceived growth may have another component, the perceived expanding motion of the object’s contours. Failure of growth to be perceived when the image expansion is caused by approaching an object may, in addition, be the result of a compensating process that prevents expandingmotion to be perceived when the image expansion occurs during a subject’s forward movement. That such a compensating process operates was demonstrated in an indirect manner. We made use of the fact that prolonged exposure of a retinal area to the same motion process leads to decrease in the speed of the resulting perceived motion and to motion aftereffect. When a compensating process operates, it might have an effect on these two consequences of motion perception, and such a result was obtained. Under conditions that would bring the compensation into effect, namely, when prolonged exposure to an expanding motion occurred only during the subject’s forward movements, subsequent speed decrease was significantly diminished and motion aftereffects occurred substantially less often than in the control conditions.