Superior short-wavelength contrast sensitivity in asthenopics during reflexive readjustments of ocular accommodation.
The purpose of this work was to characterize contrast sensitivity (CS) under short-wavelength illumination in 20 symptom-free subjects and eight asthenopics: all had normal unaided or corrected visual acuity and no sign of oculomotor disease. Threshold CS was assessed using the von Békésy tracking method from a viewing distance of 2.4 m (0.40 D). Three counterbalanced tasks required central fixation of black-and-white square-wave gratings (1, 5, 10, 14 and 17 c/deg) presented through a low-pass filter blue lens and (1) a +1.50 D lens; (2) a -1.50 D lens and (3) a 0 D lens, while attempting accommodation to minimize blur. Baseline increases in eye strain, which approached high levels at the end of the experiment, did not differentiate between the two groups of volunteers. All the subjects made evident appropriate accommodation during the low blur condition (0 D); the CS curve exhibited the expected characteristics. When the minus lens was placed before the eyes of the observers the distant square-wave gratings were still seen clearly, the eyes presumably had accommodated by an amount equal to the power of the negative lens. Compared with symptom-free subjects, asthenopics exhibited greater CS at the intermediate spatial frequencies both during the low blur and the minus blur conditions. Asthenopics may exhibit an individualized sensory tendency to react more strongly to shorter wavelengths of light and may therefore reflexively 'drive' their accommodative system harder than symptom-free subjects. This would explain the presence of their asthenopia in the first place. Blue light may, in addition, induce more arousal and higher alertness in this category of participants. This would boost the oculomotor aspects of their performance. These findings add to the current understanding of individual variability in the level of oculomotor loads following strenuous near work.
Richter
H O
HO
Centre for Musculoskeletal Research, Department of Education and Psychology, University of Gävle, SE-801 76 Gävle, Sweden. hrr@hig.se
Knez
I
I
D000065:Accommodation, Ocular / Q000502:physiology*; D000221:Adaptation, Ocular / Q000502:physiology*; D000328:Adult; D001248:Asthenopia / Q000503:physiopathology*; D015350:Contrast Sensitivity / Q000502:physiology*; D005260:Female; D006801:Humans; D008027:Light; D008297:Male; D013997:Time Factors