ACCOMMODATION AND THE 3RD SPATIAL HARMONIC

Purpose. Mathews and Kruger recently reported that accommodative track ing of sine wave grating targets is best between 1 and 7 cpd. Their st udy suggests that the spatial,third harmonic might not enhance the acc ommodative response if the fundamental is higher than roughly 2 cpd. T his study tested the effect of adding the third harmonic with targets that included 1, 3, and 5 cpd fundamental sine wave gratings, compound gratings with the third harmonic of each of these fundamentals added in either peaks-add or peaks-subtract phase, as well as each third har monic presented alone at one-third of the contrast of the fundamental gratings. Methods. Accommodation was monitored continuously with a dyn amic infrared optometer while the subject viewed the gratings in a Bad al stimulus system. Dioptric vergence changed sinusoidally over a 2.00 or 0.50 D range at 0.2 Hz. A Fast Fourier Transform extracted the amp litude and phase of each response at the temporal frequency of the sti mulus. Results and Conclusions. The third harmonic enhanced the accomm odative response only when the fundamental was 1 cpd.This was true at both amplitudes of target motion. This contradicts models that suggest that high spatial frequencies are used to refine the accommodative re sponse. Second, the responses to the peaks-add and peaks-subtract phas e conditions were essentially the same for each fundamental. This sugg ests that the effect of harmonics cannot be described by any simple li near relationship between the increased luminance gradient of the grat ing target and the accommodative response. Third, the accommodative re sponse to gratings with a 5 cpd fundamental were smaller than the resp onses to gratings with the 3 cpd fundamental. This contradicts the gra dient hypothesis of accommodative control inasmuch as the luminance gr adient is steeper in the gratings with the 5 cpd fundamental. Alternat ively, it seems that harmonics will enhance an accommodative response regardless of phase if they are effective accommodative stimuli on the ir own.