2 MECHANISMS UNDERLIE PROCESSING OF STOCHASTIC MOTION STIMULI

We have constructed ''limited lifetime'' stochastic motion stimuli usi ng Gabor functions instead of dots, thereby controlling the local attr ibutes of spatial frequency and orientation. Human psychophysical data for direction discrimination using these stimuli reveal two qualitati vely distinct kinds of processing. For small displacements, direction discrimination performance as a function of displacement is scaled wit h spatial frequency in a manner consistent,vith a linear filtering mot ion mechanism. Motion perception for relatively large displacements is not directly related to the spatial frequency, and is consistent with a nonlinear process which signals motion of contrast envelopes. (C) 1 998 Elsevier Science Ltd. All rights reserved.