From The Spirit Artist - Rev. Rita Berkowitz:
Some mediums see spirits as physical forms, but it is necessary to know that it is not the physical eye that is the center of psychic vision. It is within the forehead referred to as the third eye and usually (but not necessarily) seen when the physical eyes are closed.
A clairvoyant vision is generally discerned when ordinary consciousness is at rest. Sometimes it will be seen as a complete figure or the face of the spirit or it may be a nebulous form that seems to be hidden in a cloudy substance. Many times it is beautiful scenery and intensely colored symbols that seem to be swirling in the head of the medium. This experience of inward consciousness may seem to be merely imaginary, but when the sitter acknowledges it, the medium knows that he/she has experienced spirit communication. As the medium continues to work, the subtleties between imagination and clairvoyant communication are recognized.