Figure 4: The ratio of the magnitudes of the responses of the model's first- and second-order channels is plotted as a function of stimulus displacement for each of the six types of stimulus. Solid symbols are used for dense stimuli, open symbols for sparse. The magnitude of a channel's output is calculated as the sum of its motion energy responses in the forward and reverse directions. The higher the response ratio, the more significant the first-order channel is in determining the perceived direction of motion.