Predicting the present direction of heading

Humans perceive heading accurately when they rotate their eyes. This is remarkable, because (1) the pursuit eye movement makes the retinal flow more complicated; and (2) the eye rotation causes a continuous change of the heading direction on the retina. The first problem prevents a simple association of the centre of flow on the retina with the heading direction. To solve it, the brain needs to take into account the flow associated with the eye's rotation. But even if this is done correctly, the resulting estimate of the heading is retino-centric and changing over time. Thus, the processing time to retrieve the heading from the flow field will cause a lag with respect to the actual heading direction. We investigated the latency for heading perception. We presented step wise changes of the centre of expanding flow to stationary and moving eyes. This mimics the movement of the heading direction across the retina, but avoids the complicating effects of rotational flow. For a stationary eye, we found a bias in perceived heading that corresponds to a latency of 300 ms or more. Yet, errors in heading perception are marginal normally, because we found an opposite bias for the moving eye, which counters the errors due to latency and a changing retino-centric heading direction. This suggests that the current heading direction is predicted from the extra-retinal signal and the delayed visual signals.