Nonlinearities in the visual processing of motion and form

Hoyle (1957) showed that, for a rigid sphere moving along the line of sight at constant speed, time to collision is approximately equal to the ratio θ/θ, where θ is the sphere´s angular subtense at the eye. This visual cue is available monocularly. The authors have shown that a sphere´s direction of motion in depth can be quantified in terms of the distance by which the sphere will miss the first nodal point of the eye. If one expresses this distance as ns (where s is the sphere´s radius, and n is a scaling factor), then n≈φ/θ, where φ is the translational velocity of the retinal image. The authors report experimental evidence that the human visual pathway contains neural mechanisms separately sensitive to the ratios θ/θ, φ/θ and θ. The authors have measured the ability to detect and recognise texture-defined letters whose visibility was progressively degraded by visual noise. These data were used to test three models, each of which consisted of a sequence of physiologically-plausible stages. In all three models, stage 1 was an array of parallel oriented spatial filters and stage 2 was rectification. The second nonlinear stage was multiplicative local inhibition or subtractive local inhibition or lateral facilitation. Agreement with letter recognition data was close for the two inhibition models and agreement with letter detection data was close for all three models. Agreement with letter recognition data was poor when the second nonlinear stage was omitted