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DISCUSSION
The present study showed a significant increase in critical flicker fusion frequency (CFF) of the yoga group after 20 and 30 days compared to the baseline. The CFF values of the control group did not change significantly on repeat assessment.
A previous study (3) had demonstrated a significant increase in CFF values after 10 days of yoga training (similar to the program in the present study). The increase was 5 Hz on an average. In the present study there was a non- significant increase at 10 days. The increase was 3.9 Hz on an average. The difference between the significant effect observed earlier, versus no change (or more accurately the non-significant, lower magnitude increase) in the present study could be related to the difference in age groups of the subjects of the two studies. Subjects of the earlier study were university students between 17 and 22 years of age, whereas the subjects of the present study were between 25 and 39 years, with an average of 33.9 years. It is established that the intra-individual variability of the CFF declines with age (11). This is true for the age difference between the earlier and present studies (average age 20 years versus 33.9 years, respectively). Hence any intervention (e.g. yoga) would be expected to cause smaller magnitude changes (i.e. less intra-individual variability) in the present subjects. The reason for the reduced variability is not clear, both peripheral (retinal) and central (sub-cortical, cortical) factors have been implicated. Hence the present subjects were less amenable to change in the CFF with yoga compared to the earlier group of younger subjects (3).
The absence of change in the control group showed that replications spaced 10 days apart do not increase CFF on retesting. Hence the increase in CFF after 20 and 30 days of yoga training can be attributed to yoga. While the mechanism underlying the change is not known, we may speculate that the increase in CFF may be due to the effects of yoga in reducing physiological signs of stress, such as a decrease in heart and breath rate, and in oxygen consumption (6). As already described (Introduction) this may be relevant because the CFF was found to be lower during specific stressors, such as food and water deprivation (7). Further studies are required to understand the mechanisms and identify whether a specific yoga practice is specially helpful or whether the effect depends on a combination of yoga practices.
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