Whenever you think, what do you do? Your hand automatically goes at this place. When you make a mistake, your hand goes at the head. When you think or deeply ponder over something or some problem comes then too you keep your hand like this. If you go to any country or any corner of the world, you find that everywhere people think like this. This is the seat of wisdom. It is the peetha (seat) of knowledge. That is why chandan (sandal paste), kumkum or sindoor (vermillion powder) is applied there. It is symbolic.