[Q] Visual detection of moving vs. stationary stimuli
mike at psych.ualberta.ca
Thu Mar 24 11:03:20 EST 1994
anderson at phage.cshl.org (John Anderson in Anderson Lab) writes:
>mike at psych.ualberta.ca (Mike Dawson) writes:
>>I do not know of any *normal* creature of this sort. However, there
>>is a very famous clinical case, the so-called "motion blind patient".
>>This woman has suffered damage to visual system MT from a stroke.
>>Her ability to see static objects is unaffected, as measured by a
>>large number of standard psychophysical techniques. However, her
>>ability to see movement has almost been totally erased.
>This has always struck me as very weird. What does she see instead of
>motion? Nothing? Or a series of "still images"? (But wouldn't this
>"look" like motion?) Or what?
The most detailed account of her own experience is in the Zihl paper
that I previously cited; if you are really interested, you should
look it up.
At any rate, she apparently sees a series of still images which,
importantly, are *not* linked by any movement. So, she has trouble
pouring coffee because she does not see it rising to the top of the
cup; she has trouble sitting in rooms with more than one person (many
people walking around her appears like people suddenly appearing in
one location, then another); she has trouble crossing streets because
she cannot predict future locations of moving cars.
Note that a series of static images only constitutes motion if
motion is computed between them; there are lots of difficult
computational problems solved by the visual system as it renders
identity links between frames of view, or as it generates some
sort of difference signal from one view to the next (for a good intro
to this, see Marr's (1982) book Vision, and look up the material on
directional selectivity and motion correspondence). If these problems
are not solved -- eg, because the mechanisms that solve these problems
have been damaged by a stroke -- motion will not be experienced,
because any continuity between static frames of view will not be
generated by the visual system.
Michael R.W. Dawson email: mike at psych.ualberta.ca
Biological Computation Project, Department of Psychology
University of Alberta, Edmonton, AB CANADA T6G 2E9
Tel: +1 403 492 5175 Fax: +1 403 492 1768
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