The Pali term nimitta, means a “sign," i.e., the idea or image conceived, or an object perceived. The keeping in mind of a nimitta, and maining it is a sign of concentration. When you develop an anapanasati nimitta (i.e., that of the breath), and practice it further, then you need a good knowledge of teachings (suttas) for you to overcome obstacles that you face. The ‘nimitta’ is your main object of focus. When the hindrances are being overpowered by the jhana factors inwardly, on the side of the nimitta too certain changes are taking place. The original object of concentration, i.e., in the mindfulness of breathing the touch sensation of the breath, with the strengthening of concentration the breath gives rise to another object called the "learning sign" (uggaha-nimitta).
For the breath it will be a reflex image arisen from the touch sensation of the air currents moving around the nostrils. When the uggaha-nimitta appears, the meditator leaves the breath nimitta and fixes his attention on this new object. In due time still another object will emerge out of the uggaha-nimitta. This is called the "counterpart sign" (patibhaga-nimitta). It is a purified mental image many times brighter and clearer than the uggaha-nimitta.
These nimittas can be either beneficial or harmful, true or false, so we shouldn't place trust in them. That is why before getting into meditation, one should be very clear of the purpose for the meditation. It should be solely for the aim of attaining Nibbhana. Then you will not give into these distractions. When meditating alpha brainwave is produced in you when your body calms down and your mind is completely relaxed. Your brain's thinking is slower, your mind is clear and you may even feel slightly drowsy. If you are not firmly grounded on Anicca, Dukkha, Anatta you tend to cling in to these without contemplating the impermanency of these. If we're thoroughly mindful and alert, they can be beneficial, as you are firmly grounded on Anicca, Dukkha, Anatta. But if our powers of reference are weak or if we lack strength of mind, we're likely to follow the drift of whatever images appear, sometimes losing our bearings to the point where we latch on to the images as being real.
The uggaha-nimitta can be experienced in some peoples normal life too. The worldly knowledge is of two types – one that comes from studying books or ones that comes from thinking things through. This second kind of knowledge arises within the mind itself. People who are trained in the theoretical sciences work with their thinking. They think to the point where an idea appears as a picture in the mind, like an uggaha-nimitta (spontaneous image). When the picture appears in the mind, they may sketch it down on paper, and then experiment with physical objects to see if it works. If it doesn't work, they make adjustments, creating a new idea from their old idea — keeping at it until they find what works in line with their aims.
