Wednesday 31 August 2011

Siamese Cat Binocular Vision

The Siamese cat is well known for a squint and a kinked tail. Both have been bred out by breeders over the years as they are considered "defects" but they are nonetheless part of this cat breed's heritage. The kinked tail in cat show competition is reason to disqualify the cat as I recall.

There are colorful legends regarding the squint that are usually about Siamese cats who defended temples and valuables. The task was arduous and it caused the cats to develop a squint!

The question on some people's minds is, "does the Siamese cat squint affect normal vision and particularly binocular vision?" Let's first say that it isn't just Siamese cats that have squints but they have a genetic predisposition to acquiring the squint. The cat below is not a Siamese but has a clear squint. Perhaps she is a Siamese mix (lynx point)

Not Siamese but clear squint - Photo by fazen (Flickr)

Cats have two eyes to allow them to judge distance and depth. Two eyes give animals a form of three dimensional vision. This is important to cats in making judgements on tracking objects, jumping and hunting etc.

It transpires that the squint is apparently a compensation for defective wiring of the nerves that go from the eyes to the brain. In normal cats half the optical nerves cross over to the side of the brain opposite to the position of the eye. This provides binocular vision.

In Siamese cats the nerve fibres that were not meant to cross over, do in fact cross over. This causes "the faulty positioning of the retinal map on the tectum". The tectum is a region of the brain, specifically the dorsal part of the mesencephalon (midbrain). The squint cancels out the effects of the faulty positioning on the tectum by "altering the positioning of the retina".

So there you have it. The squint develops over the first six to eight weeks of the kitten's life to make this compensation.

Siamese cat binocular vision is maintained by the famous Siamese cat squint. That's how nature has compensated for a genetically inherited neurological defect. Incidentally, the Siamese and Persian cats have the most genetically inherited diseases and are two of the most long standing purebred cats.

The references are from the New Scientist Aug 17, 1972. Thanks to Google Books.

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