Perceptual learning in primary and secondary motion vision.

Specific improvements of perceptual capabilities with practise are thought to give some clues about cortical plasticity and the localisation of cortical processing. In the present study, perceptual learning is used as a paradigm to separate mechanisms underlying the perception of different classes of motion stimuli. Primary motion stimuli (phi-motion), are characterised by displacements of the luminance distribution. However, for secondary motion stimuli the movement is not accompanied by a corresponding luminance shift. Instead, moving objects are defined by their temporal frequency composition (mu-motion) or by motion itself (theta-motion). On theoretical grounds, the perception of secondary motion requires a higher degree of nonlinearity in the processing stream than the perception of primary motion but debate continues as to whether there might be a unique mechanism underlying the perception of both motion classes. In a large group of subjects, coherence thresholds for direction discrimination in random dot kinematograms of phi-, mu-, and theta-motion were repeatedly measured in a staircase paradigm. Training effects were found on different timescales, within short sessions containing multiple staircases and over training periods of several months. They were fairly stable over long breaks without testing. When subjects were trained with two different motion stimuli in a sequence, an asymmetry in the transfer of perceptual learning was revealed: sensitivity increases achieved during practise of theta-motion are largely transferred to phi-motion, but theta-motion perception does not profit from prior exposure to phi-motion. This finding supports the view derived from modelling of motion processing that there must be at least partially separate systems. A primary motion detection mechanism falls short of discriminating direction in secondary motion stimuli, whereas a mechanism able to extract secondary motion will be inherently sensitive to primary motion.