Tuesday Apr 19, 2005

a nice illusion showing eye movements in action

so the other day i was talking about eye movements and how visual processing is non-uniform across the visual field. it's hard to see these in action without investing in some expensive equipment, or without staring at someone else's eyes for a long time. so to save on romantic misunderstandings, check out the following demo, or this one's even more infuriating. what you notice is the grey dots at the intersections appear and disappear the whole time. what's going on? you can read about why here too, but for our purposes the demo shows two things. first, your eyes really are moving the whole time (unless you make a concerted effort not to move them of course. even then, there's a slight jitter, due to an imbalance in the antagonistic muscles controlling the eye), since the dots and dot-free zone keep shifting, and second that processing in central vision is different, since no dots appear where you're looking directly.

Thursday Apr 14, 2005

HCI and vision research

I've done a bit of vision research work (not in Sun) and still do from time to time so i might blog about this a bit.

Human Computer Interaction is a research area that's always interested me peripherally, but I'm too lazy to investigate it in any depth. I've often wondered if any of the more recent vision research work has had a big impact, I imagine so. The upshot of all of this research can be summarized pretty succinctly - vision is nothing like what you think it is (and i don't mean this smugly - vision is nothing like what i thought it was either). I might blog about this a little, but i'm no expert, so forgive me if get some details wrong. And much of this is old news, but i think it's fascinating stuff, and can bear retelling, even if badly.

Try the following experiment - hold your thumb out at arms length, and look at the size of your fingernail. That's the size of the area in your central visual field within which you do vision well. By "well" i mean you can see fine detail clearly, perceive colour etc. Outside this, these abilities fall off rapidly. Having said this your peripheral vision - what you see "with the corner of your eye" - kicks in in low light conditions, and it's good at detecting movement.

So how come it appears i can see everything in front of me perfectly? If only a small part of the visual field is processed with "high fidelity", why don't i see a blurred, black-and-white mess elsewhere? Part of the answer - you move your eyes. About 3 times a second. There are a few different types of eye movement, but the exploratory type are known as saccades, and these are rapid jumps, followed by pauses - known as fixations - where you take in information (I seem to recall that the early experiments with eye movements were done by attaching a probe to your eyeball, which along with the Stanford Prison Experiment, goes to show that the 60s was a dangerous time to participate in a psychology study). So you engage in this "jump - fixate -jump" cycle the whole time, and actively construct a picture of the world. It's kind of shocking to realise that even the serene contemplation of a painting in an art gallery involves all this frantic movement (i defy you to move any part of your body 3 times a second without attracting concerned glances).

The next question is why does the visual system opt for this approach? Well, i seem to recall that to support "high-fidelity vision" throughout the visual field, it's been calculated that you would need a brain 100 (i think?) times the size. Speaking as the owner of a rather large head as it is, i'm somewhat glad Nature compromised on this one.

So what's the point of all this for HCI? Well, if visual processing is non-uniform across the visual field (and it is), it clearly matters where you look (and where you don't). maybe i'll get to this tomorrow :-)




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