Motion segregation from speed differences: Evidence for nonlinear processing

This paper examines observers' ability to detect regions delimited by speed differences. Several types of translational motion stimuli, of varying task difficulty, were tested over a wide range of base speeds. The observer's task was to decide whether a region of dots moving at a different speed from the base speed was located to the left or right of the display's center. Both increments (dots within the test region moving faster than the base speed) and decrements (dots within the test region moving slower than the base speed) were examined. When the task was relatively difficult the following asymmetries were found: at slow base speeds, incremental thresholds were lower than decremental thresholds; at fast base speeds, the reverse pattern occurred. When the task was relatively easy, no consistent asymmetries were found. It is proposed that the visual system encodes speed through a sigmoidal nonlinear response function, and it is shown that this one nonlinearity can be used to explain results for both easy and difficult tasks.