Frequency content of the retinal stimulus during active fixation

Under natural viewing conditions, the retinal input depends not only on the external scene but also on the observer's behavior. During fixation, the luminance modulations caused by small, involuntary eye movements profoundly influence the spatiotemporal stimulus on the retina. In this study, we examined the frequency content of the retinal stimulus during the normal instability of visual fixation. Eye movements were recorded while subjects freely viewed grayscale natural images. On the basis of the recorded traces we reconstructed the spatiotemporal retinal input experienced by the observers, i.e. the movie resulting from scanning the image according to subjects' eye movements. We then selected periods of fixation and estimated the power spectrum of such input. The results of this analysis show that, outside of the zero temporal frequency plane, the spatiotemporal spectrum of the retinal stimulus during fixational instability is space-time separable. That is, the spatial distribution of power was similarly organized on every nonzero temporal frequency plane, while the total amount of power at any given temporal frequency was entirely determined by the power spectrum of eye movements. The luminance modulations caused by fixational instability had the effect of flattening the 1/f2 power spectrum of natural images. That is, at every nonzero temporal frequency, the amount of power present at different spatial frequencies was approximately constant. This effect occurred only when viewing images with a scale-invariant power spectrum, like natural images, and was lost when the trajectories of eye movements were artificially enlarged. It is often argued that the shapes of neuronal receptive fields in the early visual system act to reduce the redundancy of input signals. Our results show that, during normal fixation on natural stimuli, the visual input which drives neurons sensitive to temporal modulations is already decorrelated in space.