The fruition of karma is relative to the medium. Let us understand with the help of an illustration that in the case of some karmas, the vipaka is mental derangement. In that situation, the mati and the intellect act in the contrary manner. Only through the metamorphosis of the mati, the intellect and the emotion the karma yields its own fruit. The karma cannot yield the fruit except through a medium.
When there is fruition of the asat vedaniya karma, first of all there is an emission of the energy of the karmic body. That energy disturbs the energy of the taijas-sharira. The energy of the karma-sharira (coded record of one's past deeds, operating under the law of action and reaction)disturbs the body energy and the biological energy. This affects our nervous system, which in turn affects our glandular system and all functions of the body. This is the beginning of the disease. The asat vedaniya karma carries out its function. Whenever there has to be the fruition of karma, there has to be some advance preparation. This advance preparation is done through the body.
Karma (operation of spiritual energy) are of four kinds: of the body, of the mind, of the speech and of the karma. There is an incident mentioned in the Bhagvati Sutra: Gautama asked Mahavira, "Lord! How does a living being experience the sat vedaniya karma and the asat vedaniya karma?” Lord Mahavira replied, "The sat or the asat vedaniya karmas are experienced through the medium of these four kinds of karmas. They become the medium to experience the sat or asat vedaniya karma." One karana is the mind, through which the sat and the asat can be experienced. Similarly, the sat and the asat can be experienced through the body and the brain.