Read this musing blog of a column, and realized I had never heard any explanation whatsoever for the phenomenon:
http://www.bayarea.com/mld/cctimes/living/columnists/gary_bogue/7511314.htm
So what DO cats see when they start staring intently at a blank wall? Ghosts? The future? Detached retinal floaters? Subsonic creaks from the house settling? Catnip-induced hallucinations?
huh?? you mean you can’t see it 
A broadcast from the mothership.
Serious answer: I’m thinking microscopic bugs, or nearly inaudible sounds.
Other serious answer: They’re cats. They’re trying to freak us out.
Dead people.
Seriously, I’ve noticed my cat jerking to see a flash of light or a shadow. He’ll then stare at the same spot for a while, apparently hoping to get another glimpse.
My cat goes bonkers over the movement of spots of light on the walls. So much so, in fact, that she will often stare at the shadows of birds on my windowsill rather than at the birds themselves.
Why she loves changing light and shadow so much, I’ll don’t know – but it does make for a lot of fun with her and a laser pointer.
IMHO, it’s just a response to an inaudible noise, or a brief flash of light or shadow. Cats will wait for hours, motionless, staring at a particular spot. My cat heard a mouse behind our stove. As soon as the mouse realized it had been discovered, it also froze. Kitty, though, can seemingly wait far longer. The mouse eventually figured the cat was gone, since there was a lack of movement or sound for so long, and comes out into the open. Kitty gets a meal. This is probably the same mechanism that is at work as they stare at the empty corner. They thought they heard something, or a moving shadow got their attention, and they’re damn detemined to make sure that there’s nothing to eat. While cats seem to have a terribly short attention span, that seems to be out the window when it comes to hunting.
Well, first of all, the author of the article is describing a Siamese.
Siamese are mentally… different… from other cats. I won’t pretend to have any insight into the mind of a Siamese, and I don’t particularly care to…
Secondly, the “chattering” behavior he describes is common for cats who are reacting to the sight of prey. My cat does it when he sees a fly buzzing around, and when squirrels are busy being squirrely outside the window. I haven’t read any definitive claims of what exactly is going on, but I have seen one suggestion that the cat is subconsciously preparing to deliver the “killing bite.” Another hallmark of this action is that the “cheek whiskers” fan forward.
As for what specifically they’re seeing? Who knows… the visual system of the brain contains many layers of processing to detect patterns and shapes and motion. Any one of those could be spontaneously triggering in the mentally, umm… different… cat, and eliciting a predatory reaction.
Cats are closer to wild than most other domestic animals… that’s why they’re so cool…
An often overlooked point about cats is that they are 3-dimensional protrusions into our world, and have a life in their own universe as well. Oh wait, that’s mice.
I concur with Eve’s “Other Serious Answer.”
MikeS, Back when I was doing projections for a coupla psychedelic bands, I used to work on making the animations with an LCD projector hooked up to my PC I’d leave it on, but only look up at the wall occasionally as adjusted animations were rendered & displayed, and then switch back to whatever software I was using and look back at the monitor.
Eventually, I was distracted by my cats literally crawling the walls, trying to stalk the mouse (heh) pointer as I fiddled with morph-anchors or some such thing. That was good for hours of entertainment-- $10,000 worth of electronics standing in for a length of weighted string.
As to the OP, it seems to me that they’re often tracking some invisible thing which is clearly defined in three-dimensional space. As much as I’d like to go all-over-spiritualist, I suspect that it’s just dust motes doing their convective dance.
Cats are somewhere around the order of ten times as sensitive to low levels of reflected light, as I understand it. Also, the tapetum (the reflective layer in the eye that makes kitty eyes shine so,) can shift short-frequency light from the blue area of the spectrum into a frequency that the retina is more sensitive to. A rough analog of what this might look like to a cat would be the way fluorescent colours seem to us to be self-illuminated under “black light” – light which is outside our visible spectrum is made visible by reflection. Based on that, my WAG is that some dust which is so “dark” as to be invisible to us has reflective properties which make it look like a tiny firefly to the kitten eye.
Caveat: this spectral speculation is just my intuition, based on dimly-remembered reading about why cats’ eyes have that spooky shine. Confirmation or denial of this hypothesis by someone with more familiarity about feline optical biology is anticipated.
I think sometimes they can see things we can barely see, or don’t notice.
[tiny bit off topic]
Years ago, I was hanging out with a friend of mine and her toddler daughter. Out of nowhere, the little girl starts pointing at nothing and going “Look! Look! Pretty!” Her mother and I are really intrigued, and start staring at nothing too. Eventually this turns to a discussion on how it’s been said that children can see ghosts and angels and stuff. Well, we’re a little bit freaked out at this possibility, and are getting kind of nervous.
After five or ten minutes, we realize that the little girl isn’t seeing anything amazing at all! She’s looking at the little dust particles in a sunbeam.:smack:
[/tiny bit off topic]
I think cats might do stuff like that too. Tiny particles of dust floating in the air might be pretty darned interesting for a cat.
Dust motes.
Hm.
Considering how stoned people react to tracers… and can stare at their hand, moving back and forth in front of their face, in utter fascination…
… I am inclined not to question the motives of a CAT.
My miniature Shi Tzu does the same thing. I’ll ask her when she comes out of the next trance.
“Beware of the Cog”
I like this explanation a lot. The cat that owns us will sit for hours in the front window, staring at this one bush as though he’s afraid if he takes his eyes off it for a second, it’ll come hurtling through the glass…
THEY SEE DEAD PEOPLE
Well, ok, lame joke, but couldn’t resist 
I wonder if a cat would be able to see Bruce_Daddy’s post, above?
*Said the Tom cat to the little Fuzz,
“Do you see what I see?
Way off in the dark, little Fuzz,
Do you see what I see?
A mouse, a mouse, dancing in the night
With a tail as big as a kite,
With a tail as big as a kite.”
Said the little Fuzz to the other kits,
“Do you hear what I hear?
Squeaking in the walls, other kits,
Do you hear what I hear?
The mice, the mice running everywhere
We must bring them a terrible scare
We must bring them a terrible scare.”
Said the other kits to the brood Queen,
“Do you know what we know?
In your cat bed-warm mighty Queen,
Do you know what we know?
The mice, the mice are running through the walls
We must grab those mice by the tail - this isn’t the Pit :eek:
We must grab those mice by the tail.”*
Mousely Catmas!
What really freaks me out is when all five of my cats’ heads turn, as if on synchronized swivels, to look at the same empty space. Then they stare intently for several minutes. They must be hearing some noise that is inaudible to me, but it always makes me wonder what fascinating/horrifying thing I’m missing. The few times I’ve ever taken the trouble to investigate, it’s turned out to be a bug. They can HEAR a BUG WALKING! Amazing.
Did you ever hear a bug, walking?
Well, I did!
Are you wondering what I’m stalking?
It’s well hid!
Have I ever really mystified you
pursuing the mi - nute?
Oh I know what is really there
But you just think I’m being cute.