Friday, September 26, 2008

The Mysteries of Vision

One of the most misleading analogies in neuroscience is "eye is like a camera". It is nothing of the sort. The anatomy of the eye makes that plainly obvious. For example, look at the graphic above. The top most part is the Retinal Pigment Epithelium. It plays a vital role in re processing vitamin A, absorbing excessive light, providing nutrients for the photoreceptors, and eliminating waste from the retina. Some studies suggest that the failure of the RPE to process and remove waste products is a cardinal feature in Age Related Macular Degeneration. The photo-sensitive portion of the photoreceptors is directly underneath the RPE.

Here's the rub: light does not travel from the top but from the bottom of this graphic. For light to stimulate the photooreceptors it must travel through the ganglion cell layer right up to the tip of the photoreceptors near the RPE. Quite amazing that we see it at all and it explains why some bods refer to the retinal structure as being "back the front". Now find me a camera that works like that ... .

Over and above that consideration the brain also goes to tremendous lengths to process visual information. It has been estimated that up to 30% of all CNS tissue can be involved in visual processing. The signals from the retina first must travel to the lateral geniculate nucleus(LGN) a set of small nuclei in the thalamus, from there the journey continues to the back of the brain to the VI or striate cortex, which appears to involve rudimentary visual processing, and then must travel forwards again to the temporal lobe(for object identification) and the parietal cortex(for position in space determination). This is the simple explanation!

What is becoming intriguing in our the research into vision is the research indicating that when we see the world the image we have in our minds is the result of this visual processing that is combined with our visual memory storage. That is, to a certain extent at least, the world we see is contingent on what we have been seeing our whole lives. With this little introduction in mind, have a look at these interesting visual hallucinations.

Go to this link and then download the relevant files. Have fun!

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