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DISCUSSION
The present study showed that a month of yoga training caused a significant decrease in the degree of optical illusion perceived using the Muller-Lyer lines. For "out" trials the decrease was 86.4% and for "in"’ trials it was 86.0%, hence the average decrease was 86.2%. A previous report described how focusing the gaze reduced the optical illusion by 79% (3). In the present study subjects of the yoga group were trained for a month in yoga practices which involved’ actually focusing and defocusing the gaze on a specified object (as in trataka or meditation), or "focusing - defocusing" the attention on the breathing/body sensation's (as in asana and pranayama practice). Hence the combination of focusing followed by defocusing appears to be more effective in reducing the degree of illusion compared to focusing alone.
Retinal, cortical and cognitive-judgmental factors influence the perception of a geometric illusion (2). At the retinal level mutual inhibition of spatially adjacent neural units account for the perception of an illusion involving intersecting lines (4). A cortical "satiation" model has been proposed to explain the role of cortical factors in perceiving an illusion (5). It states that if a specific cortical locus is continuously stimulated the neural substrate becomes refractory and hence difficult to activate. This refractoriness spreads spatially to include, nearby neural units. If a test line is presented so that the activation caused by it overlaps the refractory region, the response will be inaccurate and may result in an illusory perception.
Where there is a decrease in the degree of illusion perceived over a short period it is generally understood that retinal or cortical factors would not contribute to the change which would be mainly due to cognitive- judgmental factors (6). Changes within a month fall into this category, as opposed to changes taking place over one or more years. Cognitive-judgmental factors involve the way in which the subject interprets incoming visual information based on experiences, hypotheses and strategies of judgment. Hence in the present study the training in focusing defocusing through yoga for a month may have influenced the cognitive-judgmental factors of the subjects, to significantly reduce the degree of illusion perceived.
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