Changing disparity is a possible cue for stereomotion perception. We propose the changing disparity energy model, a physiologically plausible model for neurons tuned to changing disparity. This model combines the disparity and motion energy models commonly used to model cortical neuron outputs. The model outputs are consistent with psychophysical experiments indicating that stereomotion speed discrimination thresholds for dynamic random dot stereograms are higher than for random dot stereograms. Thus, these experimental results are not necessarily strong evidence for the existence of an inter-ocular velocity difference cue. The model also predicts a relationship between the speed discrimination threshold ratio and the dot density.