Motion-from-Form perception in Scotopic and in Photopic light conditions

Human visual perception is globally worsened under dim light condition. The aim of this study was to investigate how rod-dominated vision affects motion perception in a motion-from-form (MFF) and motion coherence (MC) task. Thirty healthy subjects (11 males, mean age 21.3 years) participated in the experiment. MFF and MC were tested both in scotopic and photopic light conditions. Both the MFF and MC stimuli consisted of nine empty circles moving coherently at a constant velocity of 10.7°/sec in one of eight directions (cardinal and oblique). Five noise-levels were used: starting from zero, the noise increased by 6 units in each following level. In the MFF test 169 empty white squares acted as stationary background. Nine target-stimuli were drawn in place of the same number of squares. To make the shape appear to move, in the subsequent frame each target-stimulus reverted to square while the next square changed to circle, following a specific direction. The noise consisted of circle positioned randomly in the space, irrespectively to their previous position. In the MC test the target-stimuli moved coherently on a black background and the noise was obtained by stimuli moving in a Brownian manner. The subjects had to identify the directions of the coherent motion in both tests. The accuracy was evaluated for each condition. ANOVA showed a significant global effect of light condition, evidencing disrupted performances in rod-dominated vision, significant differences between the types of stimuli (lower scores in MFF as compared to MC) and a significant interaction stimuli type by light condition. In conclusion, the results confirm that in scotopic light levels motion perception is not significantly altered using MC, while it is degraded in higher order motion stimuli, such as MFF.