ASYMMETRIES IN THE SENSITIVITY TO MOTION IN-DEPTH - A CENTRIPETAL BIAS

It is reasonable to ask whether observers are more sensitive to the pa ttern of image motion caused by forward locomotion through the environ ment than to the pattern caused by backward locomotion. The distributi on of sensitivities of cells in MT does show such a bias, although thi s bias is minimal at small eccentricities. Additionally, both locomoti on-induced stimulation and the sensitivities of MT cells suggest great er sensitivity should be obtained in the lower visual field. Previous research on this issue has usually employed frontoparallel motion in p atterns presented to one side of the fixation point. Both centrifugal and centripetal biases have been obtained. In this study the stimuli p resent motion signals that travel radially from (or towards) the fixat ion point. These stimuli, which produce a strong percept of motion in depth, are an adaptation of the global-dot-motion stimulus employed by Newsome and Pare. With these stimuli we find that sensitivity to moti on in depth is greater in the lower visual field than in the upper vis ual field, and that sensitivity is greater to centripetal motion than to either centrifugal or frontoparallel motion. This centrifugal bias in sensitivity decreases with eccentricity. The last two findings cont radict the notion that the bias is produced by the visual experience i nduced by normal forward locomotion and also that the detection of mot ion in depth is subserved by MT.