Dear mhlneuro,
Thank you for your questions. They are good ones. I'll give you answers to each question:
"Is there a reason that NSR uses 15 minutes vs 20 in TM and a 4 vs a 2 minute post meditation period?"
Yes. NSR and TM have a very big difference: TM is taught in person by a qualified and experienced teacher. NSR is learned on one's own from a manual and CD. While doing it yourself is fun and can be easier than travelling to a TM center several times, we don't get the interaction of personal instruction. And we don't have the ready access to our teacher and our meditation center to get support in the form of confirmation that we are practicing correctly, advanced lectures, as well as group meditation, picnics, and other center activities.
Consequently, in NSR we need to be a little conservative with our guidelines. 15 minutes instead of 20 means that the balance with activity will be better: the effects of stress release (see other postings here in the forum) will be more manageable. Four minutes rest at the end of each meditation session means that any residual stress release activity will have a bit more time to settle down so it isn't carried into activity as discomfort or irritability with others.
"Why was this particular mantra used for NSR?"
I assume you mean instead of one or more of the TM mantras.
When NSR was invented (around 1999), it was felt that using a different mantra would help to draw an additional distinction between NSR and TM, to help provide some legal protection. However, since then it has become clear that we have a legal right to offer instruction in transcending, including providing a genuine, effective Vedic mantra (which ours is). Feedback indicates that 100% of our clients either have a great experience with NSR and choose to continue meditating, or have some problem and choose not to contact us for help in resolving it. We know from this (and from our
scientific research) that our mantra is effective.
"Do different mantas have different effects on the nervous system?"
Yes. When we meditate, the mind settles down into a very delicate state of heightened functioning. in this quiet state any sound will have a greater effect. Many people find the sound of fingernails (or chalk) scraping a chalkboard to be very unpleasant. And this is with the mind in the active waking state of consciousness.
Vedic mantras are intended for particular life styles. The mantra "Om" or "Aum", for example, enhances a reclusive way of life. For an active person to use this mantra would be a mistake, since they would tend over the months and years to isolate themselves from others and withdraw from activities. Random sounds can have even worse effects. For safety, we only use sounds that have the benefit of thousands of years of use as vehicles of transcending.
David Spector
NSR Meditation/USA