Separation in the visual field has divergent effects on discriminating the speed and the direction of motion.
Local motion in a visual scene allows the detection of prey or predator and predicts their future positions. Relative motion segregates objects and reveals their 3D relationships. 'Optic flow' - the motion of texture across the field - guides locomotion and balance. Given these several uses of visually perceived motion, it is unsurprising that many species have evolved hard-wired neural mechanisms to extract motion as a primitive feature of the visual world . In the cortex (e.g. [2-4]), and even the retina , of primates, cells are found that respond selectively according to direction of motion. In visual areas V1 and MT, some directionally selective cells are also tuned for the second attribute of motion, speed . It might be thought that the brain derives a single velocity signal from the activity in this population of neurons - since speed and direction must often be combined to predict an object's future position or to derive a 3D structure. However, we report here a striking difference in discrimination of the two attributes: Thresholds for direction, but not those for speed, increase with the spatial separation of the stimuli.