As you start to make progress in concentration meditation, you may start noticing an unusual visual phenomenon that starts showing up in your visual field at times of heightened concentration. It can appear whether your eyes are open or closed. In Buddhist frames of reference, this phenomenon is known as the the sign or the nimitta. (I will just call it the sign.)
The sign freaked me out a bit when it first started showing up for me, but I soon relaxed as I realized that it didn’t seem at all harmful. For me, it has always dissipated quickly at the end of my formal meditation sessions. Sometimes it shows up outside of formal meditation at times when I’m especially mindful.
The sign looks different for different people. For me, it often shows up as a pulsating wave-like pattern of light and dark that pulsates at a frequency of about 1/4 Hz (once per 4 seconds). As my attention stabilizes, the sign often stabilizes too, into a pattern of symmetrical, concentric rings that close in on themselves toward the center of my field of vision with each pulse. I often find the sign to be captivating, beautiful, and quite psychedelic-looking. It sometimes looks a lot like what I imagine a will-o’-the-wisp might look.
The sign is one of many types of meditative experiences that you may experience during concentration meditation.