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N.V. Bilton, D. Casablanca, S.A. McFadden; Short Periods of Spatial Frequency Exposure Protects the Chick Eye From Myopia . Invest. Ophthalmol. Vis. Sci. 2005;46(13):1979.
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Vision guides emmetropisation but the nature of the cues remain elusive. Form deprivation induces myopia and degrades image contrast and spatial frequency (SF) (Bartmann & Schaeffel, 1994). Exposing chicks to complex visual stimuli through high powered lenses suggests a role for SF in eye growth (Wildsoet & Schmid, 2001). We exposed chicks to stimuli without lenses to see if SF might directly control eye growth.
Hatchling chicks (n=49) were reared communally on a 12:12 h light:dark cycle. At 2 days of age refractive error (streak retinoscopy) and ocular components (A Scan ultrasound) were measured. From 3 to 8 days of age, chicks watched a drifting grating at the end of a 1 m dark tunnel for 10 min every hour for 12 hours. At all other times chicks were in darkness. In the 10 min exposure periods, chicks were shown a high contrast sinusoidal grating of SF 0.25, 2, 6, or 10 cpd or a blank screen. Additionally, chicks were raised in constant darkness or left in their communal (C) environment during the 10 min light periods. At the end of the 6 days, refractive error and ocular parameters were measured again.
Ten minutes of normal vision per hour (C birds) was sufficient to induce the rate of eye elongation found in our chicks raised in a normal light cycle with 12 h of continuous light (86 µm vs 90 µm per day respectively). Dark reared chicks had excess ocular growth, some 1.6 times that of the C birds (139 µm per day) and were 5 D more myopic. Importantly, the eye is very sensitive to the SF content of the visual environment in this 10 min window. Maximum protection from myopia occurred if stimulated close to their acuity limit at 6 cpd. As the SF became lower, ocular elongation was accelerated, while SF’s above the cutoff induced myopia (Fig. 1).
Eye growth in the chick is modulated by image SF with the best protection from myopia being near the visual resolution limit of the eye, and short 10 min periods are sufficient to alter eye growth. If humans are like chicks, it might be that short periods of restricted spatial frequency viewing, even at distance, can create myogenic conditions, and that children should regularly view high contrast images close to their visual resolution limit.
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