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P. Ramana Vani, R. Nagarathna, H. R. Nagendra and and Shirley Telles*

Vivekananda Kendra Yoga Research Foundation
No.9, Appaiappa Agrahara, Chamarajpet, Bangalore 560 018

(Received on August 1, 1996)

Abstract: The critical flicker fusion frequency (CFF) is the frequency at which a flickering stimulus is perceived to be steady, with higher values suggesting greater perceptual accuracy. The CFF was measured in two age-matched groups of healthy male volunteers whose ages ranged from 25 to 39 years, with 18 subjects in each group. After baseline assessments one group (yoga group) received yoga training, while the other group (control group) carried on with their routine activities. Yoga practices included asanas, pranayamas, kriyas, meditation, devotional sessions and lectures on the theory of yoga. After 10 days neither group showed a change in CFF. However, at 20 and at 30 days the yoga group showed significant increases in CFF by 11.1% and 14.9%, respectively (two factor ANOVA, Tukey multiple comparison test). The control group showed no change at the day 20 and day 30 followup. 

Key words: critical flicker; fusion frequency; visual perception; Yoga.


Meditation has been reported to cause significant changes in perception, attention and cognition (1). The increased sensitivity following meditation has been experimentally proved with different assessment criteria. The Rorschach test was used as a perceptual test and meditators were more sensitive to subtle aspects of colour and shading on the inkblots than they had been before meditation (2). This perceptual sensitivity was not restricted to subtle aspects of the stimuli, as detection of a high frequency flickering stimulus or the critical flicker fusion frequency (CFF) was found to improve (3). The CFF indicates the frequency at which a flickering light is perceived to be steady. Two possible physiological explanations for the "fusion" of a flickering stimulus, involving the peripheral visual pathway are as follows, viz (i) the frequency at which the optic tract discharges may limit the ability to perceive high frequency light stimuli and (ii) ganglion cells of the "on-off" variety, which discharge when illumination comes on and goes off produce a response which is indistinguishable from their discharge under steady illumination, at a frequency of about 35 flashes per second (4). In contrast to these explanations based on properties of the peripheral visual system being responsible for the phenomenon of flicker fusion, electrical recording at various levels of the visual pathway in both animal and human subjects have shown that the eye itself may respond at higher frequencies than the value of the CFF obtained by behavioural or psychophysical techniques, and is hence not the limiting factor in determining the CFF. This lead to the conclusion that temporal resolution of the flickering stimulus is often limited by the brain rather than the eye (5).

In the study cited above (3), 10 days of yoga training increased CFF, whereas a group which did not practice yoga but was retested at the same time showed no change. In attempting to explain this effect of yoga practice, it may be recalled that the effects of yoga in reducing physiological signs of stress, such as a decrease in heart and breath rate, and in oxygen consumption are known (6). This may be relevant because the CFF was found to be lower during specific stressors, such as food and water deprivation (7). The mechanisms for this have not been worked out.

With this background, the present study was conducted with the following aims and objectives: (1) To assess the effects of yoga training on the critical flicker fusion frequency (CFF), and compare these effects with retest without any intervention (control group) (2). To assess the effects of different durations (i.e. 10, 20 and 30 days of yoga training) on the CFF.

*Corresponding Author

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