Visual cortex

“ Nor is it at all difficult to understand how this general condition comes to be associated with the eye, and how the particular visual sensations come to be associated with something distant from the eye: and further, how this association of the condition with one thing, and of the sensations with another thing, (an association established by the touch and not by the sight,) dissolves the primary synthesis of seeing and colours. ”

“ Whether we press the retina, or prick, cut, pinch, or galvanize the living optic nerve, the Subject always feels flashes of light, since the ultimate result of our operations is to stimulate the cortex of his occipital region. Our habitual ways of feeling outer things thus depend on which convolutions happen to be connected with the particular end-organs which those things impress. We see the sunshine and the fire, simply because the only peripheral end-organ susceptible of taking up the ether-waves which these objects radiate excites those particular fibres which run to the centres of sight. ”

“ All those operations, therefore, which proceed from direct irritation of the nervous part of the visual organ, without the medium of the eye and of light, under the term subjective vision, are opposed to those phenomena produced by the media of eye and light, and known as objective vision. ”

“ Expressed physiologically, this means, that to have begun the general brain-building process with the aid of the eye insures some further self-development of the visual centre, and thus makes possible a kind of mental possession of which those born blind are inevitably deprived. ”

“ The End-organs and Their Response to Stimuli.—Thus the radiations of ether from the sun, our chief source of light, are so rapid that billions of them enter the eye in a second of time, and the retina is of such a nature that its nerve cells are thrown into activity by these waves; the impulse is carried over the optic nerve to the occipital lobe of the cortex, and the sensation of sight is the result. ”

“ For example, the very important optic nerve, which is necessary to vision, comes from each eye separately in a large bunch of fibres, and divides at the base of the brain, so that each eye sends impulses directly to the visual centres of both hemispheres. ”

“ In visual perception the sweep of the glance, that is the adjustment of the muscles of the inner eye, the outer eye and of the head, is susceptible of being either interrupted or continuous like any other muscular process; and its continuity is what unites the mere successive sensations of colour and light into a unity of extension, so that the same successive colour-and-light-sensations can be experienced either as one extension, or as two or more, according as the glance is continuous or interrupted ”

“ If we assume, on the other hand, that the movements and their feelings are alone possessed of the extensive quality, we make an unjustified hypothesis, for the phenomena compel us, it is true, to accord an influence to movement, but give us no right to call the retinal sensations indifferent, for there are no visual ideas without retinal sensations. ”

“ The complex figures have more features about them, all of which by way of the retina and optic nerve are represented in the cortex and thus more cortical cells are involved, which in turn produce a stronger stimulation of the muscles which move the eye in the proper way to see the figure, and thus the eye is held more strongly by the complex than by the simple figure. ”

“ The fundamental character of this spontaneous self-evolving procession of imagery is indicated by the significant fact that it tends to take place whenever the more retinal and peripheral part of the visual apparatus is affected by the exhaustion of undue stimulation, or even when the organism generally is disturbed or run down, as in neurasthenic conditions. ”

“ It now does not seem improbable that, when by the power of thought an image is evoked, a distinct reflex action, no matter how weak, is exerted upon certain ends of the visual nerves, and therefore upon the retina. ”

“ But how is it that the molecular modifications in the cerebral cells coincide with the modifications of the consciousness; how, for instance, do luminous vibrations falling upon the retina excite the modification of consciousness called visual sensation? These are problems we cannot solve. We may succeed in determining the exact nature of the molecular changes which take place in the cerebral cells when a sensation is felt, but this will not bring us an inch nearer to the explanation of the fundamental nature of sensation. ”

“ Sight.—Vision is a distance sense; we can see afar off. The stimulus is chemical in its action; this means that the ether waves, on striking the retina, cause a chemical change which sets up the nerve current responsible for the sensation.
The eye, whose general structure is sufficiently described in all standard physiologies, consists of a visual apparatus designed to bring the images of objects to a92 clear focus on the retina at the fovea, or area of clearest vision, near the point of entrance of the optic nerve. ”

“ Whatever its origin, the growth of civilization has served to develop this eye-mindedness of the race, and to increase and diversify the modes of its cultivation.
The eye, thus constantly stimulated in waking life, and attracting to its sensations the focus of attention, possessing, as it does, in the retinal fovea a special and [338] unique aid to concentrative attention, does not yield up its supremacy in the world of dreams. The visual centres subside but slowly from their day's stimulation ”

“ It stimulates the peripheral sense organ, the eye, the retina, the physiological processes aroused in the rods and cones are transmitted by the optic nerve and by the optic tracts to the visual centres of the occipital lobes, the functioning of which is accompanied by sensations of sight. ”

“ When thought is normal - that is, not attended by any excitement or strain - the circulation in the brain is normal, the supply of blood to the optic nerve and the visual centers is normal, and the vision is perfect. When thought is abnormal the circulation is disturbed, the supply of blood to the optic nerve and visual centers is altered, and the vision lowered. We can consciously think thoughts which disturb the circulation and lower the visual power ”

“ As there is nothing strange in the arrangement of our aesthetic paths, so there is nothing fundamentally peculiar in their mode of work; in the manner in which they respond to whatever stimulations their definite arrangements bring to them. We have said it is the fundamental function of every sensory state to tend to some act. Suppose a child to open his eyes for the first time, that they fall on a red blanket, and that they then close. We are not to think that his retinal stimulations would run up to the cortex and stop there at the first cells reached. ”

“ So, on the authority of repeated experience, whatever is unsuitable is gradually excluded, and only those eye movements are retained which further the proper convergence of the two retinal images.”104 Of course, the power of vision is by no means completely developed at five months, though the technique of the function, so to speak, is by that time essentially perfected. Now begin the real tasks of visual practice: acquiring familiarity with external objects, imprinting the visual images on the mind, and widening the scope of association. ”

“ We regard the organ and its dispositions merely as a general condition regulating the apprehension of the visual sensations, and no more.
Thus, by attending to the two associations that occur,—the association (in place) of visual sensations with tangible bodies that are not the eye; and the association (in place) of vision with a small tangible body that is the eye—the eye regarded as the condition on which the apprehension of these sensations depends; by attending to these, we can understand how a protensive interval comes to be recognised between the organ and its objects. ”

“ The 'explosion' of the cortex cells which, fired by the retinal currents, give us the color, may overflow to other regions and cells, which, consequently but secondarily, give us the 'like' or 'dislike.' That is, these aesthetics may be associated phenomena such as we are at present to neglect. ”

“ We continue our experiments until our test happens to be applied to one particular phenomenon which lies, if not directly, yet virtually, within the sphere of vision. We remove or veil this small visual phenomenon, and a totally different effect is produced from those that took place when any of the other visual phenomena were removed or veiled. ”

“ The retina catches the light-disturbance, and the optic nerve conveys it to the brain, and the brain-cells are agitated by it; but where does sight come in? At some point, doubtless, the agitations of the brain-cells or of their internal molecules are seen and interpreted; but the being that sees and interprets them may (we had almost said must) be capable of directly seeing and interpreting similar agitations in the outer world—that is, it may or must by its nature be capable of seeing the events of the outer world without the mediation of the end-organs or the brain. ”

“ The seat of vision is generally supposed to be the retina, though some philosophers regard the choroid coat as the sensitive tablet upon which the impression is made. We may trace the phenomena of vision up to this point, but no further. We know that a distinct image is formed upon one or other of the delicate coats of the eye, but the manner in which the sensation is conveyed to the brain is an inscrutable mystery. ”

“ Contrast is occasioned, not by a false idea resulting from unconscious conclusions, but by the fact that the excitation of any portion of the retina—and the consequent sensation—depends not only on its own illumination, but on that of the rest of the retina as well.
"If this psycho-physical process is aroused, as usually happens, by light-rays impinging on the retina, its nature depends not only on the nature of these rays, but also on the constitution of the entire nervous apparatus which is connected with the organ of vision, and on the state in which it finds itself." ”

“ Perhaps a friend calls our attention to the fact that a balloon is sailing through the air, or some fine morning, hearing in the fields the blithe song of the sky-lark, we look up and vainly try to bring the small image upon the place of distinct vision. Now, if an image which falls upon any other part of the retina is perceived, even indistinctly, an instant suffices to direct the eyes into the exact position requisite for clear vision—an example of the marvellous precision with which impressions are put in relation to each other by the unconscious action of the brain. ”

“ The retina is sensitive to the light. The light produces chemical changes on the retina. These changes set up an excitation in the optic nerves, which is conducted to a certain place in the brain, causing an excitation in the brain. Now the important point is that when this excitation is going on in the brain, we are conscious, we see the picture.
As far as science can determine, we do not see, nor hear, nor taste, nor smell, nor have any other sensation unless a sense organ is excited and produces the excitation in the brain. There can be no doubt about our primary, sensory experience. ”

“ Visual attention consists sometimes in moving the eyes to and fro, at other times, in fixing them in one immoveable attitude. We have seen that so far as the optical sensibility is concerned, there is at each instant an effective impression of a wide area, although of very unequal distinctness. The impressions derived from the movements of the eye are much more limited. At the same absolute instant of time, we can scan only a very small portion; say the outline of some isolated form, or the trace of an isolated movement. ”

“ When the visual plane is depressed and the eyes move to the right they rotate to the left; when they move to the left they rotate to the right. Through experience we unconsciously evaluate the muscular stresses, efforts, and movements accompanying the motion of the eyes and thereby interpret much through visual perception in regard to such aspects of the external world as size, shape, and distance of objects. Even this brief glimpse of the principal movements of the eyes indicates a complexity which suggests the intricacy of the explanations of certain visual phenomena. ”

“ Similarly, if the stimulus is sound, it must fall on the ear. A sense-organ, like a photographic plate, is responsive to stimuli of a certain sort: light falling on the eye has effects which are different for different wave-lengths, intensities, and directions. When the events in the eye due to incident light have taken place, they are followed by events in the optic nerve, leading at last to some occurrence in the brain—an occurrence which varies with the stimulus. The occurrence in the brain must be different for different stimuli in all cases where we can perceive differences. ”

“ The retina, unlike any other of the so-called sense organs, is a part of the cortex. The cortex is pre-eminently a centre of memories, conscious and unconscious. Rising from the classical discussions of the space problem are reasons to believe that our perceptions of space, figure, and perspective are peculiarly dependent upon developed memories. If it should happen that certain parts of the retina acting as cortex areas were the physical basis of certain memories fundamental to the workings of our eyes as space organs, we ought not to class these memories as sensations. ”