The effect of UnV in reducing LIH in a duration-dependent manner has been reported previously by Schmid et al.,
24 where hyperopic refraction decreased by 8.4%, 27.7%, and 42.2% on exposure to 3, 6, and 9 hours of UnV by D5, respectively. Correspondingly, exposure to UnV for 3, 6, and 9 hours per day increased AL elongation by 11.1%, 22.2%, and 44.4%, respectively.
24 In comparison, by day 8 we observed a 34.8%, 42.5%, and 62.6% decrease in hyperopic refraction and a 4.8%, 31.0%, and 81.0% increase in AL elongation on exposure to 2, 4, and 6 hours of UnV, respectively. The increased impact of UnV observed in our study potentially could be attributed to disparities in the experimental protocol, such as the age (visual maturation), strain of chickens, and duration of the experimental protocol, as well as background lighting, visuospatial surroundings during UnV and the timing of UnV (centered around noon for this study and spilling into the afternoon). In fact, 2 hours of myopic defocus (+10 D) applied at noon or in the evening is more effective at reducing ocular growth compared with continuous or morning-only myopic defocus.
36 Similarly, 2 hours of positive lens removal at noon and in the evening caused increase in ocular growth more than morning removal.
36 When it comes to the temporal dynamics of hyperopia induction, it has been proposed that temporal changes induced by compensation to positive lenses, although duration dependent, is nonlinear, because the rise and fall of the internal emmetropization signal is not directly proportional to the duration of lens wear, but rather based on the frequency of wear with short durations.
26 In addition, earlier studies investigating the impact of UnV on LIH reported that interrupted hyperopia (UnV = 2 hours of relief from +4 D) resulted in a myopic shift in refractive state compared with the constant hyperopic group in tree shrews.
37 These findings, along with ours, suggest that UnV pushes toward emmetropization based on the updated (i.e., the temporary hyperopic defocus created during UnV) state of image defocus. Conversely, using +5 D lens wear, Zhu et al.
38 showed that even 30 minutes of UnV twice a day can result in a 43% increase in hyperopia in marmosets. These findings, although contradictory to ours, suggest that the inherent emmetropization signal to low myopic defocus (+5 D) does not decay when the treatment period is long (4 weeks) accompanied by multiple visual stimulation (UnV/ LIH × twice a day).