178 /
THE SENSES CONSIDERED AS PERCEPTUAL SYSTEMSSuperior Oblique Superior Rectus
Lateral Rectus
Medial Rectus
Figure 9.13 The human binocular equipment. This is the oculomotor system of man as seen from above in a dissected head. The flexible cables of the optic nerves are shown, together with the partial crossing-over of their fibers at the optic chiasma. The pairs of opposing eye muscles are labeled. On the left, a portion of the
superior oblique has been cut away to reveal the inferior oblique.
On the right, the superior rectus has been removed to reveal the
inferior rectus under the optic nerve. Note how each oblique muscle
operates by means of a tendon running through a hole or pulley in the side of the bony orbit. (After G. L. Walls, The Vertebrate Eye, Cranbrook Institute of Science, Bulletin No. 19, 1942.)
eye. As Figure 9.14 shows, there will always be disparity of pattern out- ward from the center if the world slants away. One array is skewed relative to the other by a slight perspective transformation. And by in- exorable laws of perspective the distribution of disparity in the two over- lapping arrays specifies the layout of the surfaces from which the light is reflected. If a binocular system can register the disparity, it can regis- ter information about the environmental layout.
In human observers binocular disparity can be studied experimentally with a stereoscope, and the resulting perception of pictorial depth is taken to demonstrate stereoscopic vision or, more properly,
binocular
stereo- scopic vision. Primates other than man have not yet been tested with./-- ---... /
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Disparity of the Two Arrays when Fixating the Horizon .
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Disparity of the Two Arrays when Fixating a Near Point.
Figure 9.14 The disparity betweeu the optic array admitted to one eye and that admitted to the other. The observer is look- ing down to the horizon. F is the point of fixation. The left eye is closer to the left-hand side of the road; the right eye to the right- hand side of the road. The array in one eye is skewed relative to that in the other, and the disparity increases from the horizon down to the locus of the observer's own body, as shown. When the ob- server's eyes converge and fixate on the road 30° downward from the horizon, the disparity above this point changes in sign but the skew relations are not altered. Note that these diagrams are cross- sections of the light sampled by each eye, not retinal images. (After Gibson, 1950.)
180 / THE SENSES CONSIDERED AS PERCEPTUAL SYSTEMS
stereoscopes (it is hard to get an ape to look into one, or to know what he would see if he did! ), but there is every reason to believe that they can use disparity information for getting about in the trees and for the con- trol of manipulation. The
simultaneous
pickup of two perspectives on the environment is a valuable supplement to thesuccessive
pickup of different perspectives, which will be described in the next chapter (see also, Gibson, 1950, Ch. 7). If the foregoing account is correct, animals other than primates must rely on the perspective of motion for unequiv- ocal spatial information, whereas primates can rely on both the perspec- tive of motion and the perspective of disparity. Squirrels and birds and flying insects seem to have excellent depth perception by means of the information resulting from motion parallax, but the depth perception of monkeys, apes, and man is doubly insured by the use of information resulting from binocular parallax. Historically, these have been called "cues" for depth perception.Compulsory convergence of the eyes with conjugation of their shifting fixations is
necessary
for the pickup of disparity information. It is alsodependent
on the pickup of binocular mismatch, as I have tried to sug- gest. The adjustments depend on the detection and the detection depends on the adjustments. But the calibration of the system is fragile and fre- quently imperfect. The whole thing is a sort of perceptual luxury. A good many persons have some degree of imbalance of the eye muscles and accordingly do not perceive the extra depth in a stereoscopic picture that is geometrically afforded. In these persons the visual system fails to register the extra information. They do not really need it for perceiving depth in the environment, since they have motion parallax.Compulsory convergence of the eyes, as contrasted with the merely optional convergence found in other animals, seems to entail a complete loss of the ability to perceive two different objects at the same time with two eyes, or two scenes at the same time. When each eye is given a different pattern, by optical trickery such as that of a haploscope, or by putting quite different pictures on the two sides of a stereoscope, the result is binocular rivalry in perception, one pattern supplanting or in- hibiting the other. Non-primates, certainly animals with lateral eyes, should have no difficulty in seeing one pattern with the center of one eye and another pattern with the center of the other eye. Surely the rabbit who sees half the world with one eye and the other half with the other does not go blind in one eye when the other is in use .
• THE ADVANTAGES OF A NIGHT RETINA, A DAY RETINA, AND A DUPLEX RETINA. Little has been said so far about the photosensitive cells in a retina, the microscopic pigmented rods and cones that are packed into it at one of its many layers. They are the energy receivers as distinguished from the information receivers. More is known about the transduction