Abstract
Purpose :
A recent theory posits that slow-drifts of fixational eye movements (FEMs) serve to reformat the visual input of natural images, so that the amplitude of the spatial frequencies (SFs) of the input image no longer decreases in proportion with 1/SF, but is equalized across a range of SFs. This "spectral whitening" effect is postulated to improve the processing of high-SF information. This theory requires normal FEMs with small-amplitude slow drifts that approximate Brownian motion. Given that people with macular disease exhibit abnormal FEM characteristics, with large amplitude of slow drifts and microsaccades, do their FEMs also result in spectral whitening?
Methods :
Retinal images of 16 observers with bilateral macular disease (age: 58–87, logMAR acuity: 0.48–1.32) and 14 age-matched adults (controls) with normal vision (age: 62–84, logMAR acuity: ≤0.0) were recorded using a Rodenstock scanning laser ophthalmoscope while observers monocularly viewed a 1° cross for trials of 30 s. FEMs were recovered from the recordings using a brute-force cross-correlation algorithm at a sampling rate of 540 Hz. Segments of FEMs of durations ranging from 50 to 500 ms, without (i.e. drift-only) or with intervening microsaccades, were extracted from the eye-position trace of each trial. Each of these segments was used to create a movie, simulating how a natural scene image moved on the retina due to FEMs. 48 images were used as input to recreate these movies. The spatio-temporal amplitude spectrum of each movie was computed using a 3D Fast Fourier Transform.
Results :
Across all conditions (different segment durations, drift-only or drift+microsaccades) and observers (macular disease vs. controls), the amplitude spectrum of the movies simulating the effect of FEMs shows a reduction for low SFs, such that the spectrum is virtually constant for SFs up to ~10 c/deg (“spectral whitening”). The extent of whitening changes minimally with duration, is slightly less (10%) when microsaccades are present, and ~20% less in people with macular disease than in controls.
Conclusions :
Our finding that spectral whitening is observed under a variety of conditions invalidates the assumption that whitening is a property of normal fixational drifts. More importantly, whitening is observed in people with macular disease, implying that the abnormal FEMs of these individuals may also benefit vision by improving the processing of higher-SF information.
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.