What does it mean when the skin on my cats' backs twitches?

I've noticed this behavior on two of my cats, and I don't believe it is feline hyperesthesia (neither exhibit any of the other symptoms commonly associated with the condition, and the back twitching does not seem random).

Instead, I believe it is possibly a social/communication cue of some sort.

Typically, it seems that the two cats (our oldest, an alpha female, and our younger male) both will display the back-twitching behavior when I start talking to them.

What does this mean? Is it an invitation to pet them (both are quite affectionate)? Is there documented research that describes this behavior as a social cue among cats?

• It is perfectly normal for a cat's back to twitch from time to time. Cats have a sheet of muscle under the skin of their trunk called the cutaneus trunci muscle (CTM). There are involuntary responses to being touched on the skin, which result in twitching. This involuntary twitching may assist in flicking away insects that may land on the cats fur.

• This reflex can be stimulated by emotion in cats, fear and/or excitement. It is possible your cats are excited to see you, anticipating food, fun or affection (I would doubt they are fearful of you!).

• Cats tend to do this when patted in certain places, and for some, it can lead to agitation if they are being patted in a repeated fashion, like tapping, as opposed to stroking (this I learnt in a neuroscience lecture for humans, where tests were done on cats!).

• In terms of this being a communication tool between cats, I have not found any studies about this (there may be), but as with many features of animal body language, many of these twitches can be tells or signals to other animals that an animal is frightened, agitated or excited.

A decreased or absent reflex may show: ... excitement and fear.
An exaggerated reflex .... may simply be due to excitement and fear. (1)

• It is also normal for cats to have no twitching reflex.

The cutaneous trunci reflex is frequently absent in the cat. (2)

References:

• Descending pathways to the cutaneous trunci muscle motoneuronal cell group in the cat. Holstege G, Blok BF.

• Neurological Examination - Dog & Cat wikivet.net (1)

• Neurology - lecture notes Joane Parent DMV, MVetSc, ACVIM Neurology University of Guelph (2)