Global speed processing: evidence for local averaging within, but not across two speed ranges.

A primary task of the visual system is to extract the direction and speed of animate objects from the retinal image. We examined global speed processing by determining how local speeds are integrated and whether integration occurs across all speeds or within fixed speed ranges. The first experiment addressed how local motion signals are combined to determine the speed of an object in motion. Observers judged the speed of a moving cloud of dots that took a random walk in direction while the dots inside the cloud moved somewhat independently of the cloud itself. The apparent speed of the cloud of dots is found to change in proportion with the dot speed and is well predicted by calculating the average speed resulting from nearest neighbour matches across stimulus frames. The second experiment addressed whether local speeds are combined across all speeds or within fixed speed ranges for the detection of global motion. Global dot motion (GDM) stimuli that moved in a radial or rotational directions moving at a low speed of 1.2 degrees /s or a high speed of 9.6 degrees /s were used to measure the thresholds for detecting structured motion as a function of the speed of noise dots (0 degrees /s-10.8 degrees /s) added to the stimulus. With low-speed targets, only additional noise dots moving at low speeds interfered with signal detection. High-speed targets were only interfered with by dots moving at high speeds. This finding established the existence of at least two independent speed tuned systems in the range of speeds tested. Experiment 3 investigated how speed signals are combined within a system to determine the global speed. Using sectored radial GDM stimuli the perceived speed of the fastest dots was measured as a function of whether the speed of the dots in alternate sectors either activated the high or low-speed systems. Averaging only occurred when dots were all within the sensitivity range of the high-speed system, however, if alternate sectors activated separate speed systems, averaging did not occur. Thus local speeds are averaged, independent of direction, to derive a global speed estimate, but averaging only occurs within, and not across, speed tuned mechanisms.