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Chapter twelve: Visual and hydrodynamic flow perception
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Figure 12.1 Optic flow perception. (a) Optic flow simulation mimicking a forward movement
on a straight path through a cloud of particles. The motion vectors emanate from one point,
the focus of expansion (FOE; large white dot), that indicates the direction of movement. (b)
Performance of a harbor seal during optic flow testing. (Modified from Gläser, N. et  al., PloS
One, 9(7), e103555, 2014.) The seal’s performance is depicted in detection of deviations (delta
phi) between a cross superimposed on the simulation and the actual FOE (in %) as a function
of the angular distance between the cross and the FOE (delta phi; in deg). The seal was still
able to detect deviations of 0.6° between a cross and FOE (50% threshold). The inset shows the
optic flow projection used for testing optic flow perception in a harbor seal. The gray ring was
shading the region in which the FOE (white dot, not visible during experiments) could be programmed. A green cross was superimposed that could either match or deviate by an angular
distance (delta phi) from the FOE.