THE VISUAL SYSTEM

Vision scientists are particularly concerned with how well the eye's retina transforms a visual image into neural code. That is, how our eyes work with our brain to translate images into visual perception.

The Optics of the Eye and the Retina/Brain Physiology :: The visual system is made up the following major subsystems: the optics of the eye and the retina/brain physiology. (In references, see: Ginsburg AP. Spatial filtering and visual form perception.)

The optical system of the eye consists of the cornea, pupil, and lens. Light from an image enters the eye through the cornea and pupil and passes through the disk-shaped lens. The lens focuses the image onto the back of the eye, or the retina.

The retina is made up of light sensitive vision cells called rods and cones, which process the images into neural code for further processing by the visual cortex in the brain. The central part of the retina, or fovea, contains the highest concentration of cones and provides the greatest detail, or sharpness, in our vision. The cells surrounding the fovea are composed of more rods than cones -- the farther from the fovea the higher the concentration of rods. The rods are more sensitive to light than cones are, therefore, the rods are used for seeing light sensitive objects rather than detail in objects.

The neural code that is generated by the vision cells in the retina is transmitted via the optic nerve and geniculate body to the visual cortex in the brain. The visual cortex processes and further refines this neural code into information to determine size, contrast, shape, detail, color, etc. The brain combines all of this information to produce visual perception.

Vision Cells Are Selective Filters :: Different vision cells filter different aspects of seeing. For example, our color perception uses three different cones in our retina which filter red, green and blue colors. The retina/brain physiology uses the outputs of those color filters to create the rich range of colors we see in the world. The retina/brain system also filters the image into different sizes and levels of contrast. Just as the red filters do not pass any information about the green and blue parts of the image, the small size filters do not pass any information about larger sizes in the image. In our everyday life, we see the simultaneous output of all the channels -- see the Channel Model for more information. (In references, see: Ginsburg AP. Spatial filtering and vision: implications for normal and abnormal vision.)

Many properties come into play at the cortical level that impact the final processing of the visual information. These include attention, expectancy, memory, identification and other cognitive perceptual properties. When examining the complexity of our visual system, it is easy to see how the quality of input can impact the quality of our visual experience.

 

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