Different methods of doing mental pranayama

You can do mental pranayama by different methods. Let us take a glance at the following techniques:
Ø When you do pranayama, start chanting OM mentally at each round (Chakra) of the cycle. You can also recite Bija mantras (mantras of deities) while doing it.
Ø Try to see an imaginary white hollow tube that runs from Mooladhara to Sahasrara. Also, try to visualize all the seven chakras on it, namely Mooladhara, Swadhistana, Manipura, Anahata, Vishudhi, Ajna and Sahasrara. Now while doing pranayama, try to touch all the seven chakras along the length of the white hollow tube.
Ø Look at the candle flame that goes up and down along the length of the white hollow tube. Try to trace the entire path mentally.
Ø See in your mind's eye the bright Sun that goes up and down along the length of the white hollow tube. Again, try to trace the entire path mentally.