How is the pattern of light energy projected onto the light sensitive cells and transformed into a model of the external world?
When light (electromagnetic radiation of a certain energy) enters the eye and strikes the retina, the decomposition of pigments in the rods and cones results in electrical activity which is integrated in the cells (ganglion and others) comprised in some the layers of the ten layer system of the retina.
The ganglion cells feed the brain with information coded in chains of electrical impulses; its rate is proportional of the logarithm of the intensity of the original stimulation (Fechner's law). Other attributes of the light, such as color are determined by which cells are firing (the cone cells are differentially sensitive to red green and blue). These cells are intermixed in the fovea and their relative excitations provides the brain with information about the color of the objects being viewed.
The retina is divided vertically down the middle; the nerve fibers from the left half of each retina send information about the right half of the visual field to the striate cortex in the left occipital lobe of the brain.
|The nerve fibers from the eye, reaching the striate cortex, preserve the
topology and much of the geometry of the imaged scene information, the visual
projection area is in approximate one to one correspondence with the retina
Stimulation of the nerve cells in the visual projection area causes subjects to "see" elementary visual events and lesions in this area lead to blind spots in the visual field
The region of the striate cortex immediately surrounding the visual projection is called the visual association area
Stimulation of the visual association area give rise to complex recognizable visual hallucinations and lesions in this area leads to disturbance of the perception of complete visual complexes, the inability to recognize complex objects or their pictorial representations.
|Figure Nerve pathways from the retina to the visual cortex (adapted from
Fishler and Firschein, 1988)
There is very little we can say about the relationship between brain architecture and the performance of high level functions, but we know something about how some of the more elementary neural processing is accomplished. Some of these are so important that are used in visual systems of almost every species.
We will consider the following:
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Last modified on April 05, 1999, G. Scott Owen, firstname.lastname@example.org