The orientation dependence of the motion streak aftereffect reveals interactions between form and motion neurons

Abstract

The extended integration time of visual neurons leads to fast-moving objects producing the neural equivalent of an orientation cue along the axis of motion. The dominant model [Geisler, W.S. (1999). Motion streaks provide a spatial code for motion direction. Nature, 400(6739), 65–69] proposes that these ‘motion streaks’ resolve the inherent directional uncertainty arising from the small size of receptive fields in V1, by combining spatial orientation with motion signals in V1. This model was tested using visual aftereffects, where adapting to a static grating causes the perceived direction of a subsequently-presented fast motion stimulus to be repelled away from the adapting orientation. Using a similar adaptation method, we measured the angular dependence of this effect and found in each human observer that a much broader range of adapting orientations (mean of 38.82º instead of 21.72°) produced aftereffects than predicted by the current model of motion streaks. This suggests that motion streaks influence motion perception at a later stage than V1. We also found that varying the spatial frequency of the adaptor by approximately two octaves changed the aftereffect from repulsive to attractive for motion, but not form stimuli. Finally, manipulations of V1 excitability, using transcranial direct current stimulation, reduced the aftereffect, suggesting that the orientation cue is dependent upon V1. These results can be accounted for if the orientation information from the motion streak, gathered in V1, enters the motion system at a later stage of motion processing, most likely V5. A new computational model of motion direction is presented incorporating gain modifications of broadly-tuned motion-selective neurons, most likely in V5, by narrowly-tuned orientation-selective cells in V1, which successfully accounts for the data in the current study. These results stress that orientation places strong constraints on motion processing in a different way than the current models predict. Meeting abstract presented at VSS 2015