This is true mainly because when you sit for hours concentrating on any given object -- such as the breath or the space between your eyebrows -- verbal thinking and all other forms of mental processes are suppressed. This allows your brain to allocate more computational resources to the part of the nervous system that processes visual detail.
Scientific studies have discovered that as a child matures, the clarity of visual experience is reduced because vision actually becomes modified by the presence of reflective thought. This implies that if you can successfully suppress the movement of your mind even for just a short period of time, you may be able to see things in a totally new way.
INTRODUCTION TO ZEN MEDITATION: THE STILL POINT