Abstract
Purpose: :
It is well known that visual acuity is reduced under defocus but can slightly improve when defocus persists. It is assumed that selective increase in contrast sensitivity at higher spatial frequencies is responsible for the improvement (here referred to as contrast adaptation) but its time course and the effect of the sign of defocus on contrast adaptation have not been studied.
Methods: :
Student subjects, wearing their corrections if applicable, watched a movie on a computer screen at 1 m distance for 10 minutes. During this time, the right eye wore -4D (n=10), -2D (n=11), or +4D lens (n=25 subjects). Infrared photorefraction was used to track accommodation and gaze direction. After the 10 minute period, the lens was removed and two sine wave patterns of 3.22 cyc/deg at a contrast of 0.1, embedded in Garbor patches that subtended 2 deg, were presented on a gray screen. A shield in the saggital plane between both eyes ensured that each eye saw only the patch on the ipsilateral side. The subjects was asked to match the contrast that was perceived between both eyes (supra-threshold contrast matching), using the arrow keys of the key board. The procedure was repeated every minute for a period of 6 minutes.
Results: :
(1) During movie watching, the subjects' accommodation was appropriate for the left eyes, without lens. (2) Defocus imposed by the lenses had no effect on contrast thresholds in the right eyes. (3) With +4D lenses, the supra-threshold contrast sensitivity increased by about 30% (p<0.001). (4) Using the -4D lenses, no change in contrast sensitivity was induced. (5) Changes in contrast sensitivity persisted for about 3 minutes.
Conclusions: :
That the previous "history of visual experience" affects contrast sensitivity in humans for several minutes is in line with a hypothesis that contrast adaptation might respresent an error signal for eye growth control. A 2-3 minute time constant is interesting since Wallman et al have observed that at least 2 minutes of defocus are necessary to induce refractive errors in chicks. Based on the optics, it is unexpected that contrast adaptation is induced only by myopic, but not by hyperopic defocus. Apparently, the sign of defocus information, known to be generated in the retina at least in chicks, may influence contrast adaptation, although the mechanism is not clear.
Keywords: adaptation: blur • myopia • perception