Susana T.L. Chung, OD, PhD
Professor of Optometry & Vision Science
University of California, Berkeley - USA
Functional Consequences of Fixational Eye Movements in Normal Vision and in Macular Disease
A recent theory posits that fixational eye movements (FEMs) serve to reformat the visual input of natural images, so that the amplitude of the spatial frequencies of the input image is equalized across a range of frequencies. This “spectral whitening” effect is postulated to improve the processing of high-SF information. This theory requires normal FEMs. Given that people with macular disease exhibit abnormal FEM characteristics, do their FEMs also result in spectral whitening? We first replicated the whitening effects in a group of young and older adults with normal vision by examining the power spectra of “movies” created by translating natural scene images according to the FEMs of individual observers. Then we repeated the analyses for a group of observers with macular disease who lost their foveal vision. In general, observers with macular disease also demonstrate whitening, but the amount of whitening is less than that obtained in normal vision.
The finding that FEMs lead to whitening of visual input does not imply whether FEMs are beneficial, or detrimental, to fine spatial tasks. Therefore, we used a scanning laser ophthalmoscope (SLO) to measure the performance for identifying the orientation of a grating, either presented at the fovea for observers with normal vision, or at the preferred retina locus for observers with macular disease. The SLO allows us to present visual stimuli at precise retinal locations, and to stabilize stimuli on the retina such that we could manipulate the amount of retinal image motion. For observers with normal vision, the best performance occurred not at values of retinal image motion corresponding to the FEMs, but at a stabilization gain of 0.43, meaning that normal vision could benefit from more stable FEMs. People with macular disease showed more idiosyncrasies — performance was maximal with their FEMs in some cases; while in other cases, performance would benefit from either more, or less retinal image motion. Implications of these results will be discussed.