How does the brain form a full-color mental image of an object, person or event when the relevant object, event, or scene is not actually present to the senses?

By accessing the memory of that object.

For example, a recent experiment shows that certain parts of the brain are responsible for certain images.

When a person was asked to think about Jenifer Aniston (The actress) a certain part of their brain was active.

When they were shown a picture of Jenifer Aniston, the same part of the brain was active.

The same part of the brain that activates when you see an object is the same part which activates when you only imagine it.

 

have a look here http://mitworld.mit.edu/video/342

How does the brain form a full-color mental image of an object, person or event when the relevant object, event, or scene is not actually present to the senses?

 

Actually, the same way it sees it normally!