Perceiving visual expansion without optic flow
Paul R Schrater ,
David C Knill ,
and
Eero P Simoncelli
Published in:
Nature, Vol 410, pp 816-819
12 April 2001.
© Macmillan Magazines Ltd.
When an observer moves forward in the environment, the image on his or
her retina expands. The rate of this expansion conveys information
about the observer's speed and time-to-collision. Psychophysical and
physiological studies have provided abundant evidence that these
expansionary motions are processed by specialized mechanisms in
mammalian visual systems. It is commonly assumed that the rate of
expansion is estimated from the divergence of the optic flow field
(the two-dimensional field of local translational velocities). But it
could also be estimated from changes in the size (or scale) of
image features.
To determine whether human vision uses such scale-change information,
we have synthesized stochastic texture stimuli in which the scale of
image elements increases gradually over time, while the optic flow
pattern is random. Using these stimuli, we show that observers can
estimate expansion rates from scale change information alone, and that
pure scale changes can produce motion after-effects. These two
findings suggest that the visual system contains mechanisms that are
explicitly sensitive to changes in scale.
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