Sunday, November 18, 2012

finally a post about those eye crank lines

has anyone ever tested basic visual psychophysics as a function of gaze direction? i don't think so. would it be interesting or important to do so? i think so.

1. when i crank my eyes out as far as i can, i see weird phosphene patterns around my foveae (below). nobody has given me a good explanation for what these phosphenes are, except that they are probably produced by some sort of tension or torsion on the optic nerve. this isn't much of an explanation, because the phosphenes are so local and fine that if it was torsion i would expect them to be everywhere. it could be the correct explanation, but then i need an explanation for why they aren't everywhere, or what is special about foveal optic nerve fibers etc etc in their placement in the optic nerve. the sort of thing i guess i could figure out from reading.
whatever the cause of this effect, it means that in the extreme, direction of gaze has an effect on low-level perception, i.e. i am seeing spatial phosphenes - which, really, look like band-pass patterns - and not hallucinating faces or whatever. so, it stands to reason that less extreme directions might also have effects that are more subtle.

anyways, i hope i am not tearing apart my optic nerves by doing this experiment. i try not to do it too often, but it's like thinking about reciting pi. when you think about reciting pi, you have to recite as many digits as you can remember. you can't stop. give me a second.

2. if e.g. contrast sensitivity is entirely determined by retinotopically coordinated visual mechanisms - i.e. retina, LGN, V1, striate cortex - direction of gaze shouldn't make any difference, because these areas don't know anything about direction of gaze. but visual areas in the parietal cortex do know about direction of gaze - areas like LIP and VIP combine input from the visual system, of such quality that it is used to plan eye movements, with proprioceptive, vestibular, motor, and other inputs.

it's implicit in the theory of psychophysics - the theory that physical stimuli are translatable into perceptual states, which are then behaviorally accessible - that the last stage of vision is motor, since no psychophysics can be done without motor responses. this is one reason why neuroimaging is not psychophysics.

so, if vision interacts with non-visual inputs, and if these same inputs mediate behavioral measurement of visual ability - i.e. psychophysics - then is it reasonable to suppose that direction of gaze should affect basic visual abilities? a good hypothetical mechanism for producing an effect would be the internal noise source. no one should suppose that the noise limiting performance is entirely visual, because this assumes that the rest of the system is deterministic, which it is not. since the rest of the system is not deterministic, the portion of the random variation that is contributed by the parietal cortex might well vary with the tonic motor state of the system; the part of the brain that is guiding or maintaining the motor aspects of the system, and mediating the responses of the system according to the experiment design, might be better adapted or learned in one gaze state than in others.

3. visual neglect. i guess this is a higher-level thing, but from what i've heard, it's independent of basic sensitivity; how could this have been confirmed? how can basic testing be carried out with the same quality in the neglect region as in the unaffected region? this sounds like something that's been tried over and over, and that i could go read about. a quick survey of some titles, abstracts, and a couple of the most relevant-sounding papers suggests that when such sensitivity has been measured, its in the non-neglect areas, but that the researchers are nonetheless looking for a connection. there's a paper where they suggest there's no difference in contrast sensitivity or s.f. discrimination between two groups of stroke patients, some with neglect symptoms, some without; that could mean that even a stroke big enough to cause neglect, while sparing early visual cortex, won't bother basic sensitivity, or that any serious enough stroke will impair sensitivity on basic tasks. hm...

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