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Visual Displays > Human Visual System - 4 of 8
 
 

The Retina and Fovea

The retina, a ten-layered sheet of nerve cells, lines the inside of the eye. The retina receives images in the form of lightwaves. Cells of the retina perform the initial processing of images and transmit the resulting neural impulses to the brain via the optic nerve. The retina of each eye has approximately 126 million receptor cells: 120 million rods and 6 million cones. Rods are responsible for our night vision and cones for our day and color vision.

Directly opposite the lens, the retina contains a small, circular, yellow-colored area, the macula lutea. This is the area of the retina responsible for high acuity vision. When the eye is focused on any object, the object’s retinal image is always focused on the macula. The macula contains the fovea and foveal pit where visual acuity is highest. The foveal pit contains dense, highly packed red and green cones. Significantly, it contains no blue cones, and no rod photoreceptors. It is here that we experience our sharpest, most acute vision and color discrimination.

The fovea and foveal pit are where visual acuity and color vision are the best because of the high concentration of cones. The fovea is located directly opposite the eye's lens system for optimum focusing. (Image courtesy of the University of Utah, John A Moran Eye Center, WebVision at www.webvision.med.utah.edu)

The yellow pigment of the macula lutea absorbs short-wave length (blue) light. The lens absorbs short-wave length light as well. Even if short-wave light reaches the fovea, recall the foveal pit contains no blue cones. Therefore, we are effectively color blind for blue at the point of sharpest focus on the retina. The eye does not use blue to resolve details. Blue is better perceived in the periphery of the retina.

The ganglion nerve cells that collect and aggregate nerve impulses converge at the optic disk and form the optic nerve. At the optic disk there are no photoreceptor cells which creates a blind spot on the retina.

 
   
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