P3-18: Examining Neural Representation of Bi-Directional Motions with Directional Performance in Transparency Perception

When we look at two overlapping random-dot patterns moving toward different directions, we perceive two global motions simultaneously in the same region of a visual field; this perception is known as motion transparency. After Braddick and his colleagues' work on comparing perceptual performances in transparent and single motion stimuli (2002 Vision Research 42 1237–1248), it has been considered as one of the promising cues for revealing how superimposed motions are represented in the brain. The perceptual performance would reflect encoding property of overlapping motions, and it enables us to examine the encoding models quantitatively. In the present study, we carried out psychophysical experiments to measure the directional performances in motion transparency and examined if established models of MT responses, a simple weighted sum and a normalization model, were consistent with the performances obtained experimentally. In psychophysical experiments, we measured precisions, or standard deviations, of perceived angles between two overlapping motion directions. The result showed that the perceptual performance was getting worse as a directional difference between two motions increased, while the precision was improved when dot densities of two motions differed considerably. In computational analyses, we compared the experimental results with the encoding properties of MT population models by using Fisher information that told us the lower bounds of the variances of decoded directions. The analyses showed that there was a qualitative difference between the model properties and experimentally obtained performances. Our results suggest that conventional models of MT responses cannot interpret perceptual property of motion transparency