Saturday, April 21, 2007

Visual perception and skiing

Bill Bialek gave a wonderful Pinkel Lecture at Penn yesterday. Much of the lecture was on simple models of visual categorization and tracking, including a very intriguing use of undirected graphical models (in CS jargon) to explain the joint distribution of spikes in groups of retinal neurons. Earlier in his talk, Bill mentioned in passing that low contrast leads poor velocity estimation by the neural circuitry responsible for estimating velocity from visual stimuli (Bill's well-known experiments were on flies). This is pretty obvious in hindsight, and there's in fact a long literature on it, but I never realized how it provides an obvious explanation for the difficulty of skiing in poor visibility. Falling snow, fog, or low light all reduce contrast. A typical problem for me in those conditions is overestimating or underestimating my speed. For example, in our tour on Ralston Peak last Saturday, I double-ejected from my bindings when I hit a patch of dense windpack, even though my bindings are set higher than the standard to avert such problems in the backcountry. I was just going faster than I thought. Everyone in the group fell on similar patches. The common factor was very low contrast from fog and wind-driven snow.

Another visual perception factor in skiing comes up in the advice for skiing in trees: look at the gaps, not at the trees. William Warren's models of optical flow as a control signal for object avoidance seem to fit the observations: by looking at the gaps, the average difference in optical flow vectors from the left and right visual fields indicates how fast you are moving away from the safe middle of the gap, since you tend to ski towards what you are looking at. Looking at the tree, the optical flow on the two sides is similar, and you have no lateral obstacle-avoindance signal.

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