Effect of target spatial frequency on accommodative response in myopes and emmetropes

This study investigates whether systematic differences exist between the accommodation response/stimulus curves of emmetropes and myopes when the targets are sinusoidal gratings, in particular whether myopic accommodation is relatively less effective when presented with targets of high spatial frequency due to increased tolerance to defocus blur. Ten emmetropes (overall mean sphere +0.19 D, range -0.37 to +1.37 D) and 10 myopes (overall mean sphere -2.89 D, range -1.13 to -6.63 D) viewed Gabor targets with dominant frequencies 1, 4, 8 and 16 c/deg. Maximal grating contrast was 80% and the full, green, stimulus field was 6 deg. Subjects were aged between 18 and 37 years. A further high-contrast 6/30 optotype target was included for comparison purposes. Viewing was monocular, the other eye being occluded. Stimulus demand was varied with trial lenses over the nominal range 0 to 6.0 D and the corresponding accommodation responses were recorded with an open-view, Shin-Nippon SRW-5000 auto-refractor. The resulting accommodation response/stimulus curves were characterized by their slopes over the stimulus range 1.5 to 6.0 D and by an "error index" indicating the extent to which the responses differed from the ideal 1:1 response/stimulus line. No significant differences were found between the mean accommodative behavior of the two refractive groups for any target. There were, however, substantial inter-subject differences. Some subjects in both groups showed more accurate responses with the higher spatial-frequency targets, while others showed optimal response at intermediate frequencies. Although it has been reported in the literature that, in comparison to emmetropes, myopes have reduced sensitivity to blur and response/stimulus curves of lower slope, the present study failed to demonstrate any reduction in their responses to gratings of relatively high spatial frequency. For each target the two refractive groups showed similar accommodative behavior.