Sight, the Sense of Vision

Generalities.

    The sight is usually the sense for which we attach the greatest importance. It is the sense that allows us to appreciate the appearance of the world around us. Still, what we see is very limited compared to what the world really is. For example, snakes have a good vision for the infrared spectrum, they distinguish their environment by the heat map around them. Our vision is limited to the visible spectrum, we can not see infrared or ultraviolet, and even less for radio waves, microwaves or gamma rays.

    To better understand eye functioning, you should have some knowledge in optics. The eye works like a camera; as for the camera, the light must pass through an iris that controls the amount of light entering the 'recording chamber', then through a lens system (cornea, lens and the liquid inside the eye) to reach a light sensitive surface, the retina. Both the eye and the camera has the ability to store those formed pictures. The camera will store either onto a film made of light-sensitive chemicals or, in modern days, onto a digital medium that could remain intact as long as it is copied. Photo-sensitive cells of the retina get excited and send electrical signals to our brain where memories are formed for a few second or for a life time, and could only be copied, transmitted or told about, with distortion. Although our cameras now have nice electronic features, they cannot, yet, associate senses; like the smell of the roses coming back every time I remember that Valentine's day she was, dressed like a princess, so touched by those red roses.

The Lens.

    The lens is the malleable lens of the eye. In addition to the convexity of the cornea and the light diffraction produced by ocular fluid, the lens can change shape allowing re-focusing when we look from near to far distances. This is necessary for the formation of a clear image on the retina.

    The cornea and the ocular fluid have optical properties that are fixed. The lens, however is not solid, just a bit gelatinous, and can elongate. It is made of liquid, encapsulated in a mesh of transparent, elastic and soluble proteins.

    The lens is surrounded , at its equator, by circular smooth muscles. Upon contraction, these muscles make the lens stretch and flatten its curvature allowing near and far focus. The smooth muscles are mostly innervated by parasympathetic nerve fibers. As we get older, our lens' proteins begin to denature and become more rigid and less translucent. This loss of malleability means that with aging we experience more difficulty to focus on close objects.

    Finally, because of its biconvex shape, the lens forms an inverted image on the retina. But the brain interprets these images and straightens them anyway. Even if you were to wear glasses that reverse the image, your brain will adapt to them in less than a day and you will see the images upside-up.

Inversion of the image by the lens.

The iris.

    The iris is a pigmented fibrous structure located just in front of the lens. It is its pigmentation that produces the color of our eyes; some people have blue eyes, and others have green, brown, black, or even when peers.

    The opening, the hole in the middle of the iris is named the pupil. It is through the pupil that the light enters the eye, and its diameter determines the amount of light that can enter. By contracting and relaxing a network of smooth muscles, the iris can adapt to the amount of ambient light, and consequently adjust the aperture of the pupil. In the dark, the iris dilates completely to leave an opening up to 8 mm in diameter, to get as much light as possible. In bright light, the iris contracts to the point of leaving an opening of only 1.5 mm in diameter, allowing only a small amount of light to enter the eye. Since the quantity of light entering the eye is a function of the surface area (radius squared) of this opening, this gives an adjustment factor of light intensity of about 30 times.

    As for the camera, the depth of focus is greater when the aperture of the pupil is small. So, the more light, the better our vision will be defined, sharp.

The Retina.

    The retina is like the film (or the photo sensors) in the camera, it is light sensitive.

    The retina is constituted by several layers of different cell types. Not intuitively, these layers seam arranged in the reverse order. Indeed, the light sensitive cells are practically the most remote, and the ganglion cells (those whose axons form the optic nerve) are located on the inner surface of the retina. Between these layers, there is a succession of cells which are used to refine the visual information, surrounded by supporting cells. The last layer is a pigmented layer of melanin (black substance) containing cells which absorb the rays of light that have not been picked up by sensitive cells, and prevent reflection into the eyeball.

    The cell layer which is sensitive to light contains the rod and cone cells. Cones are less sensitive to light than rods, but they are of three types, sensitive to primary colors (blue, green and red). Located at the macula (central point of the optical axis; macula fovea), there is a high concentration of these cones, which gives a particular acuity at the center of our visual field. Rods, on the other hand, are very sensitive to light but do not distinguish colors, this is why our night vision is mostly made of black and white.

Membrane stack in a rod (left) and a cone (right).

    Inside the rods, the chemical that reacts to light is a rhodopsin. Chemicals sensitive to light in the cones are collectively named the photopsines and differ only very little from rhodopsin. When the light energy is absorbed by the photosensitive substance, the latter breaks down and causes a series of chemical reactions reducing the sodium currents (Na +) in the cell, hence producing a negative current.

    Photoreceptors (rods and cones) are the only sensory cells that respond by generating a negative current when excited. Others, such as the skin receptors, respond by generating a positive current. The negative signal generated by the rods and cones, is simultaneously transmitted to bipolar cells and horizontal cells.

    The bipolar cells convert the negative signal into an excitation of ganglion cells whose axons form the optic nerve. Horizontal cells, excited by the negative signal of the rods and cones, and project laterally to inhibit neighboring bipolar cells. This increases the contrast between brightly lit areas and those that are less so.

    Amacrine cells are less known, they are activated only for a split second change in light intensity, and could participate to rapid transition of brightness.

    Ganglion cells are the cells whose axons form the optic nerve. Most of these cells, in the absence of any stimulus, have a constant discharge rate of 5 action potentials per second. They can be stimulated following light exposure, or inhibited by lateral inhibition from surrounding light. They are the cells that transmit light information to the brain.

Central Pathways.

    It is in the brain that all these little pixel of light will be analyzed and interpreted in a coherent series of images. Shapes, contours, distances and movements become a make that makes sense only in terms of experienced concepts. For example, if we give someone a pair of inverting glasses, our brain would work hard to flip back the image to make sense of the learned reality.

    An example that we interpret the images in the context of learned experience: in the figure on the left you probably see the face of a funny man. But if I told you that it is the image of a young woman kneeling down with one knee in arm. Now you will recognize that young woman anytime, as well as that bizarre the man's head. With this example, I wanted to you that given the ambiguity, our brain chose a first, the easiest, interpretation and then seek for other meaning if necessary. The figure bellow is another example of your brain trying to make sense of things.

What hides among those spots?

Response at the bottom of the page !

    To reach the brain, the visual information passes along the optic nerves. These nerves partially cross at the base of the brain (optic chiasm), in such a way that our right brain receives information from the left visual field and the left brain receives those from the right visual field. In the brain, the crude visual information reaches a first relay, the lateral geniculate nucleus. At this level, the information begins to refine; different cells transmit black & white or color information, the contours are more refined, and many cells respond to movements.

    Then the information is relayed to the primary visual cortex. The cortical neurons do not form a map of pixels, some neurons will respond to contours, direction and length, like a map of vectors. It is the same about colors, our primary visual cortex perceive shapes, contrasts and shades of colors. Subsequently, the information goes to other cortical areas (secondary, tertiary visual areas, etc.) where the information will be interpreted.

    The movement, the perspective, and the eye movements will all enrich the quality of our visual interpretation, taking into account the position and movement of our bodies in this environment. What a wonderful tool!