Gasper Tkacik
Wednesday 25th March 2015
Time: 4pm
Basement Seminar Room
Alexandra House, 17 Queen Square, London, WC1N 3AR
“Beyond sensory bottleneck: Efficient coding of elements of visual form”
It has long been appreciated that the statistical properties of natural
stimuli shape neural processing mechanisms in the sensory periphery, but
the extent to which such a principle can be formulated for and applied to
central processing is unclear. The periphery faces a transmission
bottleneck, so efficiency implies compression of signal components with a
predictably wider range. Cortex faces a different challenge – it must use
limited samples to make inferences to guide decisions. In this regime,
efficient coding predicts the opposite from the periphery: that greater
resources are allocated to the signal components with a wider range. To
test this hypothesis, we carry out two parallel studies. In one, we
measure the joint distribution of local two-, three-, and four-point
spatial correlations in an ensemble of natural images. In the other, we
measure human perceptual sensitivity to these correlations and their
combinations via psychophysical experiments that use synthetic visual
textures. We show that psychophysical performance, described by dozens of
independent parameters, can be predicted with surprising accuracy from the
distribution of spatial correlations found in the natural images. Thus,
the efficient coding principle extends beyond the sensory periphery to the
central nervous system, where it applies in a very different guise and
accounts for the sensitivity to higher-order elements of visual form.