Recent physiological and psychophysical research has challenged the traditional view that motion and form information are processed in distinct, parallel pathways in the visual system. Rapid movement creates 'motion-streaks' parallel to the motion trajectory, which facilitate motion detection. Some motion-selective neurons in striate and extrastriate cortex are sensitive to motion parallel to their preferred orientation, a possible neural substrate of motion-streak effects. As a psychophysical test of the cortical site of motion-form interactions, four experiments measured the duration and direction of the motion after-effect (MAE) generated by drifting dot fields in the presence of either vertical, horizontal or oblique counter-phase pedestal gratings. In Experiment 1 a single, horizontally drifting dot field was used; motion streak interactions predict stronger after-effects for horizontal gratings. Experiment 2 employed two transparently drifting dot fields (obliquely upwards and downwards), which produce a horizontal MAE. If motion-form interactions depend only on individual dot field trajectory, there should be no effect of grating orientation on MAEs after bi-directional adaptation. MAEs from both uni-directional and bi-directional adaptation were stronger using horizontal gratings than using vertical gratings. Experiments 3 and 4 found that an oblique pedestal did not alter the apparent direction of the MAE from bi-directional motion, despite the fact that it reduced MAE duration compared to a parallel pedestal. These results provide new evidence that the strength of adaptation to motion is affected by simultaneously presented orientation signals, and implicate motion integrating receptive fields in extrastriate cortex as the likely neural locus of this motion-form interaction.
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