Absence of a chromatic linear motion mechanism in human vision

We have investigated motion mechanisms in central and perifoveal vision usi ng two-frame random Gabor kinematograms with isoluminant red-green or lumin ance stimuli. In keeping with previous results, we find that performance do minated by a linear motion mechanism is obtained using high densities of mi cropatterns and small temporal intervals between frames, while nonlinear pe rformance is found with low densities and longer temporal intervals [Boulto n, J. C., & Baker, C. L. (1994) Proceedings of SPIE, computational vision b ased on neurobiology, 2054, 124-133]. We compare direction discrimination a nd detection thresholds in the presence of variable luminance and chromatic noise, Our results show that the linear motion response obtained from chro matic stimuli is selectively masked by luminance noise; the effect is selec tive for motion since luminance noise masks direction discrimination thresh olds but not stimulus detection. Furthermore, we find that chromatic noise has the reverse effect to luminance noise: detection thresholds for the lin ear chromatic stimulus are masked by chromatic noise but direction discrimi nation is relatively unaffected. We thus reveal a linear 'chromatic' mechan ism that is susceptible to luminance noise but relatively unaffected by col or noise. The nonlinear chromatic mechanism behaves differently since both detection and direction discrimination are unaffected by luminance noise bu t masked by chromatic noise. The double dissociation between the effects of chromatic and luminance noise on linear and nonlinear motion mechanisms is not based on stimulus speed or differences in the temporal presentations o f the stimuli. We conclude that: (1) 'chromatic' linear motion is solely ba sed on a luminance signal, probably arising from cone-based temporal phase shifts; (2) the nonlinear chromatic motion mechanism is purely chromatic; a nd (3) we find the same results for both perifoveal and foveal presentation s. (C) 2000 Elsevier Science Ltd. All rights reserved.