Physics seems to have done reasonably well in studying electrons, although there is not to this day a precise definition of this term. - Stephen Kosslyn

A mental image occurs when a representation of the type created during the initial phases of perception is present but the stimulus is not actually being perceived; such representations preserve the perceptible properties of the stimulus and ultimately give rise to the subjective experience of perception.

These organizational processes result in our perceptions being structured into units corresponding to objects and properties of objects. It is these larger units that may be stored and later assembled into images that are experienced as quasi-pictorial, spatial entities resembling those evoked during perception itself.... It is erroneous to equate image representations with mental photographs, since this would overlook the fact that images are composed from highly processed perceptual encodings.

Mr. Magoo Rule: Text and graphics must be easily distinguished and recognized. Don ́t make your audience members feel like the vision challenged Mr. Magoo—viewers of an electronic slideshow should not risk eyestrain!