We have constructed a general model for computing image velocities that is capable of representing multiple velocities occurring at occlusion boundaries and in transparently combined imagery. The behavior of the model is illustrated in the figure below. On the left are two transparently overlaid fields of random dots, moving in different directions. On the right is the output of the model, a bimodal distribution over velocity. The brightness at each point is proportional to the response of a velocity-tuned mechanism.
The computation is performed in two stages. The first stage computes normalized spatio-temporal energies, analogous to complex cells. The second stage uses the same operations (linear combination followed by normalization) to construct a velocity-tuned response, analogous to a hypothetical velocity cell as has been postulated to exist in area MT. We demonstrate the behavior of the model on a set of stimuli, indicating its consistency with human motion transparency phenomenology.
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