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The Human Visual System - The changing visual percept, Adaptation, Visual Consciousness

light dark changes occurs

KRYSTEL R. HUXLIN, PH.D.
University of Rochester Eye Institute

The human visual system has often been compared to a photographic camera, an analogy that fails quickly upon serious examination. Indeed, visual perception, with its colored, emotionally charged and contextually modulated, three-dimensional view of the world differs markedly from the rapid flow of two-dimensional information captured by the eye, or for that matter, a photographic camera. This sensory system allows humans to form the cognitive, emotional, and creative insight that drives them to take photographs in the first place. It also provides a biological means to make sense of the results. How is this possible? What makes the human visual system so different from a machine? This question will be explored by first delving into visual system structure and organization, before examining some of the more important aspects of visual processing.

The changing visual percept

One major difference between the human (or any other living) visual system and a camera is that the biological perception of the visual world is never static. It constantly changes based on prior experience, stored visual memories, and planned actions. This plasticity of visual perception is currently also a hot topic of research, both at the cellular and behavioral levels. It is hoped that knowledge gained will one day provide a usable substrate for machine learning and the development of “smart,” computerized the vision algorithms. At the cellular level, it is becoming increasingly clear that visual learning as a result of experience and intensive practice, both during development and throughout adult life, occurs as a result of molecular and structural changes in visual neurons at every level of the visual system. New, more efficient connections are formed, both within and between different visual centers. These structural and molecular changes alter the electrophysiological properties and visual processing abilities of visual neurons and lead to improved visual performance that is largely restricted to the trained visual function. Practice makes perfect, even in the visual system.

Adaptation

Visual adaptation is a process of adjustment of the visual system to its environment—one particular form of visual plasticity has great implications for photographers and photography. The dynamics of the visual system are such that it attempts to de-emphasize stable stimuli of long duration to preserve sensitivity to potential changes. This principle applies to a wide variety of stimulus attributes including lightness, color, size, motion, orientation, pattern, and sharpness. Brightness/lightness adaptation enables a person to see the environment with enormous variations in light level, such as from sunlight to starlight, which represents an illuminance ratio of about a billion to one. The increase in sensitivity that occurs with decreased light levels is a gradual process, requiring about 40 minutes to reach maximum dark adaptation. Dilation of the iris can increase the amount of light admitted to the eye by only about 16 times. Most of the increase in sensitivity that occurs during dark adaptation is the result of changes in the pigments in the retinal receptors and in neural processing. In contrast to dark adaptation, light adaptation occurs within a few minutes. Photographers who need dark adaptation to see clearly in low light-level situations, such as for certain darkroom operations and night photography, can avoid its quick dispersion by using dark eyeglasses when exposure to higher light levels is unavoidable. A fairly intense red light can be used in darkrooms without affecting dark adaptation because of the insensitivity of rod photoreceptors (which contribute most to vision at these light levels) to red light. Because of the change in sensitivity of the visual system during light and dark adaptation, the eye is a poor measuring device for absolute light levels. This is typical of most other human sensory systems as well. In visual environments containing a variety of tones, the adaptation level tends to be adjusted to an intermediate value that is dependent upon the size, luminance, and distribution of the tonal areas. This local adaptation enables a person to see detail over a larger luminance range, but is not so great so as to interfere with the judgment of lighting ratios on objects such as portrait models, where experienced photographers are able to judge 1:2, 1:3, and 1:4 lighting ratios, for example, with considerable accuracy.

Chromatic adaptation occurs primarily because of bleaching of cone pigments in the retina. Upon exposure to short-wavelength or blue light, for example, the pigment in the S cones is bleached, rendering them less capable of absorbing photons of that wavelength. The net effect is that the bluesensitive cones become less sensitive to blue light, which causes neutral colors that are viewed immediately following exposure to the blue light to appear yellowish (explained by blue-yellow color opponency—see the above section Perception of Color).

Adaptation to orientation and other, more complex spatial and temporal characteristics of our environment have also been reported and are mediated to a large extent by cortical mechanisms. For example, if a person wears prism eyeglasses that make everything appear upside down for several days, they will eventually perceive the world as normal again. If the glasses are then removed, the world again appears to be inverted until several days pass and it resumes a “correct,” perceived orientation.

Visual Consciousness

It seems appropriate to conclude this exploration of human vision with some thoughts on perhaps the least understood of visual processes—visual consciousness. It is certainly paradoxical that it should be least understood since it is the most fundamental property of the human brain that (still) separates humans from machines. According to Christof Koch and Francis Crick, the primary function of visual consciousness is first, to produce the best interpretation of the visual world around us, taking into account previous experiences and maybe, our goals. A second role for consciousness is to make this information available to those brain regions involved in the planning and execution of motor outputs. Much current research is devoted to finding the neural substrates of conscious visual perception, largely by contrasting them with neural substrates of unconscious vision. To date, medicine has not discovered a single brain area where this function resides or mechanism by which it arises. Instead, many, perhaps all, parts of the visual system appear to contribute to its existence. Like the prototypical “smart” robot in science fiction movies, with brightly colored electrical signals zooming across it central processing unit, it is possible that visual consciousness arises from the global, simultaneous activity of the entire visual system. Devising experiments able to capture this phenomenon mechanistically remains one of the greatest challenges faced by visual neuroscience this century.

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