Testing the Perrone and Stone (1994) Model of Heading Estimation

Human observers cannot judge heading accurately in the presence of simulated gaze rotations under many conditions [Royden et al. (1994). Vision Research, 34, 3197-3214]. They make errors in the direction of rotation with magnitudes proportional to the rotation rate. Two hypotheses have been advanced to explain this phenomenon. The extra-retinal-signal hypothesis states that the observer's estimate of gaze rotation is always based on an extra-retinal signal such as an efference copy. In the absence of such a signal, the observer assumes that no rotation has taken place and responds accordingly. The retinal-image hypothesis states that visual input dominates when the extra-retinal signal is small or absent; under this hypothesis, errors with simulated rotations are the consequence of faulty visual mechanisms. Perrone and Stone [(1994). Vision Research, 34, 2917-2938] proposed a model that purports to account for these errors using retinal-image information (optic flow) alone; its assumptions make it inefficient under some conditions. The most important of these assumptions is that the fixated target is stationary with respect to the world (the gaze-stabilization constraint). I compared the model's performance to human data from two experiments of Royden et al. [(1994). Vision Research, 34, 3197-3214]. One experiment simulated translation while tracking a target attached to the scene (gaze-stabilized), while the other simulated translation while tracking a target that was not attached (gaze-unstabilized). The incorporation of the gaze-stabilization constraint leads to a predicted asymmetry for the errors in the gaze-unstabilized experiment that is not observed in human data. I conclude that the model as it stands is not consistent with human behavior. It is possible, however, that the predicted asymmetry is masked in human data by a counteracting asymmetry in a hypothetical processing stage subsequent to the heading estimation that extrapolates the observer's future path of self-motion.