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.