Each mantra has a deity (the deity it invokes), its own metre and its own seer (the seer who revealed it to the world). Mentioning the name of the rsi and touching our head with our hand have their own significance, that of holding his feet with our head. We first pay obeisance to the sages because it is from them that we received the mantras. We then mention the chandas or metre of the hymn and touch our nose with our hand. Chandas protects the sound of a mantra and is like its vital breath. So we place our hand on that part of our body with which we breathe. Without breath there is no life. While for all the Vedas taken as a whole Siksa is the nose and Chandas the foot, for the mantras proper Chandas is the nose.
When we commence to chant a mantra we must meditate on its adhi devata, or presiding deity, and feel his presence in our hearts. This is the reason why we touch our hearts as we mention the name of the deity.
The Vedapurusa stands on Chandas. " Chandah pado Vedasya": the Vedic mantras are supported by Chandas.
For a general background, please see here