Over the years that I’ve had cats, I’ve noticed that most of them have a certain spot on their back, where if you focus your petting in that area, the cat will kinda look up with sort of glazed eyes, and then start grooming itself almost uncontrollably.
Naturally, we’ve used this deviously over the years for our own personal amusement. 
We call it their “Spot”.
Has anyone else noticed this with their cats? And what’s the scientific reason for it?
I’ve always assumed that reaction was almost like a tickle they are trying to scratch, so I’m interested, too.
Yes, our cat had a point on her back, just above the tail, where, if you stimulated it (we often did it with our feet while she was lying on her side on the ground) she would begin making obviously involuntary grooming motions (rather jerky and uncoordinated) with her hind paw on her neck, chin area. I have no idea why cats are wired like this, though.
It did not seem to bother her very much. Sometimes there might be a slightly annoyed meow, but when we stopped tickling her with our toes she would relax again.
I’ve noticed this too with one cat, also in the back near the tail. If you offered her your arm or other part of the body she would groom that instead.
There are 2 such spots on cats’ backs.
One is lacated between the shoulders, at the base of the neck. That is about where mother cats pick up their kittens to move them. Because of this, massaging this spot often seems to relax comfort cats. It’s also the spot where you are instructed to put long-term flea medication. Because it’s a spot where the cat can’t groom themself, and thus remove (and ingest) the medication. And many cats enjoy being scratched or petted here – I think because of that, it’s a place they can’t scratch themself. (Horses have a similar ‘itch’ spot in an equivalent spot, at the base of the mane.)
The second spot is the one others identified, at the end of the back just before the tail. I believe some research indicates that the reaction here is a sex-related factor. And personally, I have noticed that they don’t seem to react so strongly here until the cat is sexually mature – kittens don’t seem to have that strong reaction here, for example.
My friend has 4 cats, two of them are mother and son long hairs. Both the long hairs have the “itchy” spot near their tails and will start grooming their own chest if you scratch. Didn’t notice the two short hairs doing this.
:eek: You don’t mean to tell me I’ve been inadvertently jerking them off, I hope.
My cat doesn’t react to being scratched by his tail. My mother’s cat, however, will start licking her lips involuntarily and will try to scratch you if you keep it up.
It’s not just cats. Most dogs also find it pleasurable if you scratch their back at the base of the spine(I think), where the tail is about to begin. My WAG is it’s an erogenous zone.