Background & Purpose: Cast shadow is a salient monocular depth cue. This is well demonstrated in a motion illusion called &quat;square-over-checkerboard". In the movie scene, there is a square with its shadow cast on the background at the right-bottom corner of the square. When the cast shadow moves toward and away from the square in the front plane, observers perceive a strong sensation that the square approaches and recedes from them, respectively, although the size and position of the square are constant. In the present study, we investigated whether macaque monkeys can also perceive the motion in depth, such as approaching and receding, by using cast shadow.
Methods: A female Japanese macaque was used. It wore a liquid crystal shutter for a three dimensional vision, and was trained in a motion discrimination task. A square was presented at the center of display with binocular disparity. When the square approached to the monkey, it was trained to release a lever immediately after the change of color of fixation target (GO response). On the other hand, when the square receded from it, it was required to release the lever after a holding period (delayed GO response). After monkey achieved 90 % accuracy, catch trials were intervened randomly at one-fifth of all trials. In the catch trial, a center square without disparity was presented with its cast shadow at the right-bottom corner. If monkey perceive motion in depth by cast shadow, it would show the same response in the catch trials as in the trials with binocular disparity. To avoid learning, monkey was rewarded in all catch trials.
Results: Initially, the monkey showed delayed GO response to both approaching and receding square in the catch trials. However, monkey became to respond to the approaching and receding square with GO and delayed GO response, respectively. The accuracy of the catch trials finally achieved more than 90 %. The responses were preserved even when the location and shape of the visual stimulus were changed.
Conclusion: The present results showed behaviorally that adult macaque monkey can extract information of depth from visual scene by using cast shadow.