The reference frame of visual motion priming depends on underlying motion mechanisms

Several different types of motion mechanisms function in the human visual system. The purpose of this study was to clarify the type of reference frame, such as retinotopic and spatiotopic frames of reference, at which those different motion mechanisms function. To achieve this, we used a phenomenon called visual motion priming, in which the perceived direction of a directionally ambiguous test stimulus is influenced by the moving direction of a preceding stimulus. Previous studies have indicated that negative motion priming is induced by a low-level motion mechanism, such as a first-order motion sensor, whereas positive motion priming is induced by a high-level motion mechanism, such as a feature-tracking system. In the experiments, subjects made a saccade after the termination of a smoothly drifting priming stimulus and judged the perceived direction of a 180° phase-shifted sine-wave grating presented subsequently in retinotopic or screen-based spatiotopic coordinates. By manipulating the stimulus parameters, such as primer duration, velocity, and contrast, both positive and negative priming were observed. We found that positive priming was observed in spatiotopic coordinates, whereas negative priming was observed in retinotopic coordinates. Prominent positive priming in spatiotopic coordinates was observed only when the interval between the priming and test stimuli was longer than around 600 ms. This delayed priming effect was not caused by saccadic eye movements. These results suggest that a low-level motion mechanism functions in retinotopic coordinates, whereas a high-level motion mechanism functions in spatiotopic coordinates, in which the representation builds up slowly.