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METHOD
Subjects
There were 2 groups of healthy male volunteers, i.e. yoga and control groups, with 18 subjects each. The yoga group was attending a one-month residential yoga training program. The age range was 25 to 39 years, with group average + SD, 33.9 ± 4.5 years.
The non-yoga (control) group was selected to exactly match the yoga group for age. These subjects were working and residing close to the yoga center. Hence both groups stayed in a quiet, rural area. The subjects of both groups also had comparable social and educational backgrounds. All subjects had normal vision without correction and none of them had colour blindness. The latter was measured in view of the fact that the colour of the stimulus has been reported to influence the CFF (8).
Design of the study
Both groups were assessed four times each, under similar conditions. Baseline assessments were made at the start of the study. After this the yoga group received training in yoga, while the control group carried on with their routine activities. Subsequent assessments for both groups were made after 10, 20 and 30 days.
Assessment procedure
Critical flicker fusion frequency was measured with a standard electronic apparatus (9) (Anand Agencies, Pune, India). This apparatus has a red light stimulus, 6 mm in diameter, with a luminous intensity of 50 mcd and flicker frequency which can be adjusted from 12 to 95 Hz. The stimulus was surrounded by a white background with illumination of 150 lux. The subject was seated at ease 50 cm in front of the apparatus (i.e. 80 cm from the stimulus). During the experiment all overhead lights were switched off except for a 40-w bulb fixed behind the subjects. The light: dark ratio was kept as 1 (50:50). The binocular response of all subjects was assessed in two types of trials: [i] ascending: the frequency of flicker was gradually increased, 0.5 Hz per step, from the minimum (12 Hz) till the subjects reported that it was "fused" or steady, this was the fusion threshold, and [ii] descending the frequency of the response was gradually reduced (0.5 Hz per step) from maximum (95 Hz) till the subjects reported that it was flickering, this was the flicker threshold. Each subject was assessed in 20 trials each, i.e. 10 ascending and 10 descending, given alternately. The values obtained in both types of trials were considered together and out of the 20 values obtained for each subject, the value to be used for statistical analysis was selected as follows: when at least 3 successive readings were the same or with not more than 0.5 Hz between them, this value was noted as the critical flicker fusion frequency or CFF (10).
Yoga training
The yoga group received training in physical postures (asanas, 90 minutes), cleansing practices (kriyas, 30 minutes), yoga voluntarily controlled breathing (pranayama, 60 minutes), meditation (60 minutes), devotional sessions (90 minutes) and lectures on the theory of yoga (60 minutes).
Data analysis
The CFF values of the two groups obtained with 4 assessments were compared for significant difference using a two factor ANOVA, where Factor A = yoga versus control group, and Factor B = the 4 assessments (baseline, day 10, day 20, day 30). The group mean values were compared for significant differences using the Tukey, multiple comparison test.
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