402 Comparative Animal Physiology 



It is difficult to say which of the six aforementioned clues are used by 

 any given animal. Side-to-side movements of the head are used by some 

 animals as a means of improving parallax, either with monocular or with 

 binocular vision. Some birds may cock their heads several times at a morsel 

 of food before pecking it with complete assurance of its position; many shore 

 birds use vertical bobbing of the head in a similar manner. 



Movement Perception. The perception of movement and the factors which 

 influence it are decidedly complex. Under most conditions a small object 

 moving on an otherwise motionless field can be detected more easily than 

 the same object if not moving. The movement of an image on the retina 

 causes a change in the stimulation pattern which is more easily detected 

 than the details of the same pattern if it is unchanging. This is one reason 

 why flicker fusion curves offer a valuable index of vision. An object moving 

 very fast may be seen only as a blur; the same object moving very slowly may 

 not be seen at all. At intermediate speeds, however, the oudine may be 

 clearly seen. At speeds which result only in a blur, the details can some- 

 times be seen if the observer blinks rapidly, e.g., as when viewing a rotating 

 wheel or when a nearby picket fence is seen from a rapidly moving auto- 

 mobile. One possible explanation of this fact is that because of the persist- 

 ence of visual images the continuous series of images formed by the moving 

 object overlaps to form the blur. By blinking rapidly these may be cut into 

 a discontinuous series of distinct images. Under certain conditions, nystag- 

 mus, a fluttering movement of the eveball, may also seem to "stop" linear mo- 

 tion and thereby remove the blur. 



One mechanism for the production of discontinuous images for localized 

 areas of the retina of birds is the pecten. Of the thirty or more theories con- 

 cerning the function of the pecten it seems that the correct one is that of 

 Menner,'-"" who pointed out that the pecten serves as a grating which casts 

 a shadow on the retina. An object which is moving across the field of vision 

 forms an image behind the pecten which is discontinuous for localized areas 

 of the retina. The fusion frequency is higher for light of a given flickering 

 intensity for birds with a pecten than for animals lacking a pecten, particu- 

 larly when the light is on for a high proportion of a cycle. '^^ 



ADAPTATIONS TO MEDIA AND SUBSTRATE: Aquatic Vision. The focusing 

 mechanism of fishes is somewhat different from that of non-aquatic forms in 

 that the cornea has almost the same refractive index as the water and is al- 

 most useless as a focusing device. Therefore the curvature may be changed 

 to fit into the streamlining of the animal, and in some fishes the eyes are fur- 

 ther streamlined by the addition of an "adipose lid." 



One of the most striking differences between aerial and aquatic vision is 

 in the field of view. This is shown in Figure 113. Because of the total in- 

 ternal reflection of light which strikes a water-air interface at an angle of 

 less than 48.8 degrees (fresh water), the only view a fish has with an aerial 

 pathway is through a surface "window" which subtends a 97.6 degree angle. 

 The diameter of the window varies with depth of the fish. Because of refrac- 

 tion at the surface of the water the field of view spreads as it passes through 

 the surface, and distant objects (e.g., the shore and tree in the figure) are 

 compressed around its periphery. Objects on the bottom of the water (e.g., 

 the rock) may appear in the visual image close to objects which are on dry 



