Reading Notes: April 7th, 2022

“The cone of light rays which pass through the pupil of the eye forms an image on its rearward surface, the retina….Although the retinal image is inverted and the order within the image is therefore reversed, it nevertheless corresponds to the physical world as a projection. The assumption here (and throughout this book) is that for certain purposes we may treat the retinal image as if it were a two-dimensional pinhole image. It is important to note that this is not the kind of image defined by physical optics and used in the design of optical instruments, for this latter is three-dimensional. The formation of an image on the retina can be observed directly. If the excised eye of an albino rabbit is fixed into a hole in a card and pointed toward a scene, by holding it in front of one’s eye, one can actually see the inverted image on the curved rearward surface, looking something like a miniature photographic transparency. It is this demonstration which has led to the theory that the retinal image is a “picture.” The surface of the retina on which the image is projected is composed principally of extreme minute cells which contain photosensitive substances. Like the substances used in photographic emulsions, these are capable of reacting differently to the energy and wavelength of light. They are superior to any photographic emulsion, however, since they are self-renewing and capable therefore of registering the image continuously. Although the television camera can register an image continuously and in this respect is more like the retina, its mechanism is quite different. The cells of the retina are of two types, rods and cones.” (Gibson, The Perception of the Visual World, 46-48) 

“The rodlike and conelike cells of the retina, when they are stimulated by light, initiate nerve impulses in the neurons which make up the sheaf of fibers, a quarter of a million or so in number, which we call the optic nerve. So far as the evidence goes, these nerve impulses are excited independently of one another and travel their paths separately….The anatomical connections of the nerve fibers can, it is true, be traced….By far the largest part…connect with an area on the surface of the occipital lobes of the brain. The excitation of this cortical area is probably essential to all vision in man, for destruction of it produces blindness just as much as would injury to the eye or severing of the optic nerve. It has been assumed that the connection of the retina with this visual area was an exact point-to-point relationship and it was possible to infer that therefore the retinal image was projected on the brain in the same way that the physical world is projected on the retina. But the anatomical facts provide only a puzzling and very incomplete support for this assumption. Lateral connections exist among adjacent fibers both in the retina itself and at later stages in the tract between retina and brain. The amount of overlap is such that the “image” on the brain (if there were such a thing) would be very blurred. In all probability, it should not be thought of as an image, and even less, as a literal picture. It is an event composed not of light, but of nerve-cell discharges, and if a surgeon exposed the brain to view, there would be nothing to see.” (Gibson, The Perception of the Visual World, 49-50) 

“The image is an arrangement of focused light on a physical surface of two dimensions which is specific to an array of reflected light from physical objects and surfaces in three dimensions. Since the reflected light is specific to the objects and surfaces themselves, the image is also specific to them. Geometrically, we say that the image is a projection of the world. The conclusion is that the image is not a replica of the world. If taken seriously this conclusion has far-reaching implications. Unfortunately, the word “image” has more than one meaning. It may refer to an effigy or copy—the “graven image” of the Bible—or it may refer to the projected arrangement of light as just described—the image of physiological optics. The two meanings of the term are easily confused and there may be intermediate meanings between them. But the retinal image is unquestionably a projection rather than a facsimile. Everyone knows that objects themselves do not get into the eye. Neither do small replicas of things get into the eye…Nothing gets into the eye but radiant energy.  Only because it is focused is it specifically related to the object. The object therefore does not have a copy in the image but a correlate. The fact is that the optical image does not have to be like its object to make vision possible.” (Gibson, The Perception of the Visual World, 53-54) 

“The chief source of misunderstanding here is the assumption that the retinal image is a picture. It might be argued that even if the image is not a replica of the environment it is at least a representation of it. The apparent simplicity of this pictorial analogy for vision makes us reluctant to give it up, scientists as much as anybody else. But a picture as a representation of something is nothing if it is not presented to an eye. An unseen picture is only an arrangement of pigment spots, if it is a painting, or an arrangement of metallic grains of silver, if it is a photograph. It is simply a part of the material world which has to be seen, like anything else. If the retinal image were really a picture, there would have to be another eye behind the eye with which to see it. The notion that we see our retinal images is based on some such idea as a little seer sitting in the brain and looking at them. The question which arises is how he can see. The retinal image should not be thought of as a picture or a representation but as a physical arrangement on a two-dimensional surface. The correspondence between the world and the optical image need not be that between a thing and its copy; it need only be that between a material quality and its correlate.” (Gibson, The Perception of the Visual World, 54)