According to Nature, scientists have performed experiments, in which they were able to monitor individual neurons in a human brain associated with specific visual memories.
The volunteers, having their brains implanted with electrodes, were able to control images on screen by changing firing rates in individual brain cells.

The results imply a neural mechanism in which the mental creation of images overrides visual input (see video).
You may read the full article in Nature here or access the full paper of the study here.

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