Book T of C
Chap T of C
This is the 2007 version. Click here for the 2017 chapter 04 table of contents.
Rods and cones differ in their placement within the eye. Cones are concentrated in the center of the retina in an area called the fovea (Latin for fireplace ). In the fovea centralis (center of the fovea) there are no rod receptors at all and 50,000 cones fit in an area the size of a dot on the letter i.
What is the fovea? What is the fovea centralis? How can you create a visual stimulus that just barely covers the fovea centralis?
When you look directly at something, the center part of image falls on the fovea. Only a small part falls on the fovea centralis. A thumbnail held at arm's length is sufficient to cover the fovea centralis.
Near the fovea, blood vessels and other structures are less numerous, so it is easier for the light to get through. This is shown in the figure, which shows a cross-section through the fovea.
Cross-section of the fovea
What is the periphery? Why can you see a dim star better in your periphery?
The area outside the fovea is called the periphery of the retina. A thing seen "out of the corner of the eye" is casting its image on the periphery. Because there are more rods in the periphery, and rods are more sensitive to light than cones, one can see a faint star in the night sky more clearly if one does not look directly at it. Expert astronomers are skilled with their peripheral vision. They can look sideways into a telescope, using rod vision, in order to detect a star.
Which receptors are more associated with high acuity? With movement detection?
As a rule, however, rod-based vision is not very good at picking out details. That is why night vision lacks detail: it is predominantly rod vision. Because cones are far more numerous in the fovea, acuity of vision is much greater in the center of the field of vision compared to the periphery. Cells in the periphery of the retina are very sensitive to movement. Movements in the periphery trigger an eye movement that centers the stimulus on the fovea where it can be given more detailed analysis.
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Copyright © 2007-2011 Russ Dewey