When the visual delay is artificially lengthened, the image motion model begins to oscillate out of control. Note that the pursuit of Monkey Mo does not become unstable at long delays. If anything, the spontaneous oscillations are then actually better damped. For these reasons, Ringach argued that the image motion model was inadequate, and proposed the tachometer feedback model as a replacement. We have thusfar dealt with linear versions of the two models.
Real pursuit does, however exhibit some relevant non-linearities.
Eye acceleration is not quite a linear function of image velocity, but saturates slighty. If image acceleraton drives eye acceleration, as proposed by the image motion model, this relationship is likely not entirely linear either. Both these nonlinearities are include in the full versions of the two models, but are not included here as their effects on the qualitative behavior of the models is minimal. A more important nonlinearity is the potential dependence of the image acceleration signal upon image velocity. If the pursuit system is indeed sensitive to image acceleration, it may be most sensitive when image velocity is near zero.