Perceptual learning in primary and secondary motion vision

Specific improvements of perceptual capabilities with practise ape thought to give some clues about cortical plasticity and the localisation of cortic al processing. In the present study, perceptual learning is used as a parad igm to separate mechanisms underlying the perception of different classes o f 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 compositi on (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 pe rception of both motion classes. In a large group of subjects, coherence th resholds for direction discrimination in random dot kinematograms of Phi-, mu-, and theta-motion were repeatedly measured in a staircase paradigm. Tra ining effects were found on different timescales, within short sessions con taining multiple staircases and over training periods of several months. Th ey were fairly stable over long breaks without testing. When subjects were trained with two different motion stimuli in a sequence, an asymmetry in th e transfer of perceptual learning was revealed: sensitivity increases achie ved during practise of theta-motion are largely transferred to Phi-motion, but theta-motion perception does not profit from prior exposure to Phi-moti on. This finding supports the view derived from modelling of motion process ing that there must be at least partially separate systems. A primary motio n detection mechanism falls short of discriminating direction in secondary motion stimuli, whereas a mechanism able to extract secondary motion will b e inherently sensitive to primary motion. (C) 1999 Elsevier Science Ltd. Al l rights reserved.