Seeing Blood Vessels in the Eye
posted on 21 Mar 2014 by guy
last changed 3 Jun 2015
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ages: 6 to 99 yrs
budget: $0.00 to $1.00
prep time: 0 to 5 min
class time: 5 to 15 min
Describes a technique for seeing the shadows of blood vessels in your eye. Provides a great opportunity for discussing the anatomy of the eye. Requires a dark place.
For more examples of optical illusions caused by visual desensitization, check out our summary curriculum on After Images and Optical Illusions.
Fig. 1: Schematic of how the visual image is formed in the eye. Light rays from the object are focused by the cornea and the eye lens to project an inverted image on the retina at the back of the eye. The image information is carried by the optic nerve out of the eye through a hole in the retina.
Fig. 2: Schematic of layers in the retina. The back of the retina is at the top of the diagram. Blood vessels (not shown) overlay the front of the retina at the bottom of the diagram. Adapted from work made available by Peter Hartmann and Marc Gabriel Schmid at Wikimedia Commons.
This lesson concentrates on the blood system that feeds the eye, and a technique for seeing a dramatic image of blood vessels in the eye. For more general discussion of the physiology of the eye, please see our lesson on Anatomy of the Eye.
blood vessels in the eye
Light that enters the eye is focused onto the retina in back, where it produces an inverted image (figure 1). The retina contains photosensitive cells at the back which record the image and send the visual signal to the brain (figure 2).
The eye is fed by a network of blood vessels, which connect both at the back of the retina and in front through a thin network of capillaries. Many of the larger blood vessels lie in front of the photosensitive layer, casting shadows on the photosensitive cells. Surprisingly, we don't notice these shadows during the course of everyday life because
- most of the time, the view we see is too busy for us to notice the fine shadows of blood vessels, and more importantly,
- the shadows do not move.
The photocells desensitize to the presence of shadows, and the brain edits out the static image much like it ignores the blind spot in the eye. For more details about our surprising ability to ignore these visual images, see our lessons on Blind Spot, and Cheshire Cat Illusions.
have a look
The eye and brain do their best to hide the presence of blood vessels in the eye, but by careful arrangement, we can get glimpse of them. Sit in a dark room and shine a small flashlight just above your closed eyelid, a few millimeters away. You'll want to use the smallest flashlight you can find, one that runs off a single AAA battery, focused as tightly as possible. Jiggle the light back and forth as you aim it at your eye. After just a little while you should start to notice a pattern that looks like a river delta with many tributaries (figure 3). The "rivers" are the shadows of blood vessels lying just above your retina.
It's necessary to move the flashlight back and forth so that the image of the shadow moves. In that way, the photocells do not have a chance to desensitize, and the brain is stimulated by the changing image. Give yourself a minute to see the image, moving the flashlight faster and slower, and all over your field of vision. It may take a while for you to recognize and focus on the blood vessels, but when you do, they should really jump out at you.