This essay is inspired by an article in the April 2010 issue of Scientific American: “Faulty Circuits,” by Thomas R. Insel, pp. 44-51. His main thesis is that “Neuroscience is revealing the malfunctioning connections underlying psychological disorders . . . Many illnesses previously defined as ‘mental’ are now recognized to have a biological cause: in autism, for example, it is an abnormality in the connections between neurons, often attributable to genetic mutations; schizophrenia is now viewed and treated as a developmental brain disorder. . . . certain other mental disorders such as depression, obsessive–compulsive disorder or post-traumatic stress disorder could also be physiological disorders of the brain.”
In binocular rivalry we have a circuit that, at first, seems to be faulty. More serious investigation reveals, however, that the circuit serves well toward “survival of the fittest” in certain dangerous circumstances.
Consider Fig. 1(a). Hold this figure about 20 cm (8 inches) from the eyes, and look at the central dot. (I am assuming that the reader has normal vision.) Now relax your gaze as if you are looking off into the distance. As you do so, the image seen by the left eye moves to the right so that the × lands to the right, as in Fig. 1(b), while the + ends up in the center above the (b). At the same time, the image seen by the right eye moves to the left, so that the + lands to the left, as in Fig. 1(b), while the × ends up in the center above the (b), superimposed upon the left eye’s +.
Fig. 1- Construction used to initiate binocular rivalry. (a) The drawing that should be held around 20 cm from the eyes. (b) Visual result when the eyes gaze off into the distance so that the central squares fuse.
When you print this figure, try to get, very approximately, 5 cm (2 inches) between squares, as shown. Also, part (b) should be deleted because it is a distraction.
When the square surrounding the + and × fuse together, you will get a quasi-stable image. (Note: Some people have difficulty getting this optical effect.)
The reason that the eye images shift laterally when we go from 20 cm to a distant location, of course, is that the vergence angle, the angle the eyes make with each other, has to change. This convergence operation is performed rapidly and automatically, not under our conscious control, via a built-in feedback system. (As the distance to the object of interest changes, the shape of the eye lens is also changed, if possible, by accommodation muscles, so as to maintain focus. This accommodation is also rapidly executed by a feedback circuit that is not under our control.)
Returning to Fig. 1(b), where the + and × are superimposed in the center: Almost immediately, you will lose the +, or the ×, or bits and pieces of the + or ×. The reason is that there is a conflict between the left eye’s + and the right eye’s ×. According to Wikipedia, “Binocular rivalry was discovered by Porta (1593, as cited in Wade, 1996). Porta put one book in front of one eye, and another in front of the other. He reported that he could read from one book at a time and that changing from one to the other required withdrawing the ‘visual virtue’ from one eye and moving it to the other. . . . [Binocular rivalry] interests people who see it as a key to finding a neural correlate of consciousness. . . . When one image is presented to one eye and a very different image is presented to the other, instead of the two images being seen superimposed, one image is seen for a few moments, then the other, then the first, and so on, randomly for as long as one cares to look.”
Where, in the visual system, does this blocking-out occur?
From the eyes, the optic nerves lead to two structures called the lateral geniculate nuclei (LGNs). If we follow the optic nerves carefully, we see a remarkable bifurcation: Approximately half of the axons from each eye cross over, so that one of the LGNs ends up with fibers from the left half of the visual field, and the other receives fibers from the right half. From the LGNs, the signal proceeds to the primary visual cortex at the back of the head. In other words, the visual field is split in half, and the two halves are separately processed without our being aware of the bifurcation. An object that moves from right to left in the visual field instantaneously “hops” across the gap between the left and right hemispheres. The neural networks are obviously continuous although they are physically separated. It is “natural” for the left hemisphere to handle the right half of the visual field, and vice versa, because rays of light cross over in passing through the lens of the eye.
The LGNs are layered structures. It is obvious that the blocking-out of various parts of the + and ×, in binocular rivalry, must take place in one or more layers of the LGNs.
Although the blocking-out may be an entertaining effect, it must be seriously preserved and fostered by millions of years of evolution. Consider, then, the dire consequences if the left- and right-eye superimpositions went unchallenged:
I happen to have macular degeneration. The central portion of my right eye’s visual field is distorted (horizontal and vertical lines are bent), reading is impossible, and the area is darker than normal because of many dead retinal cells (so explains my eye doctor). (The peripheral vision remains undistorted. I can detect a sneak attack from the right or left.) Nevertheless, I have absolutely no problem in reading, writing, or driving a car because that distorted central right-eye contribution is blocked out of conscious experience!
It is difficult for me to thread a needle, with one good eye, but this is hardly a drawback in “survival of the fittest.”
I believe the binocular rivalry effect is strongly maintained by evolution because injury to one eye must be fairly common amongst “wild” animals in their young “childbearing days” or beyond, when they contribute to the care of offspring. In other words, the ability to block out “bits and pieces of the + or ×” makes it possible for an animal to maintain good vision using the available surviving visual elements. Slightly different is the case when a person with good vision uses Fig. 1(a) to get binocular rivalry; here, both eyes are competent, but their images are in conflict. The LGNs apparently look for conflict but, instead of war, they decide that might is right, after all. (The jingoistic slogan, “Might Is Right,” has been featured by the military.)
Looking back to the beginning of this essay, we conclude that the binocular rivalry effect is not due to a “faulty circuit” as defined by Thomas R. Insel in his Scientific American article.