MONOCULAR STEREOPSIS 341 



Substitutes for Binocular Stereopsis — It would be rather hard to 

 say which of the possible monocular cues to distance (pp. 313-4) a given 

 animal can and does use. But one of these cues, the production of par- 

 allax by head movements, is also valuable for throwing objects into 

 'relief; and when an animal habitually employs this process the fact is 

 quite evident. A number of lateral-eyed vertebrates, whose binocular 

 fields are so narrow as to be practically useless, obtain a perception of 

 solidity and relief — a sort of monocular stereopsis — by invoking parallax 

 in one way or another. 



When, as children, we dropped a prized penny upon a brown rug on 

 which it became invisible, we located it by getting down on the rug and 

 placing an eye close to its surface, so as to see the profile of the coin in 

 relief. A few years ago, Joseph Grinnell called attention to the fact that 

 there are many birds which do something quite comparable. Birds either 

 eat moving food, pursuing it or waiting for it to come along; or they 

 seek motionless food, such as seeds. Birds in this latter category perform 

 what Grinnell called 'rapid peering' : they cock the head this way and 

 that several times before pecking at a seed or berry, thus placing it in 

 relief against its background from several different angles in quick suc- 

 cession, and identifying and localizing it with precision before pecking 

 it with assurance. 



The shadow cast by a solid object gives it relief, for when seen from 

 more than one angle either simultaneously (as in binocular vision) or 

 successively (as with rapid peering), different amounts of the shadow 

 are visible and the prominence of the object can then be evaluated. 

 Benner has recently shown that in the pecking of grains by chicks, the 

 shadow is of great importance. If the kernels were so illuminated that 

 their shadows were eliminated or displaced, the chicks ignored them. 

 Painted representations of shaded kernels deceived them, though Benner 

 says that they seemed aware that they were being fooled. One-eyed chicks 

 were as well able to peck accurately as two-eyed ones, for both used only 

 monocular parallax for ascertaining distances. Apart from experiments, 

 we have abundant evidence of the importance of shade and shadow to 

 animals for their perception of relief, in the form of the many dermal 

 camouflaging devices adopted (particularly by insects, fishes, and rep- 

 tiles) for obliterating shadows or for creating 'false relief through the 

 use of color spots graded in tone. The interested reader should consult 

 the work of Cott listed in the bibliography. 



