The ability to form and bring about a mental image, one not directly traceable to a perceptual source, is habitually thought of as imaginative, or imagination proper.
It can be an evocation of a true or phoney remembrance, even a mix of both, hence its potential for innovative newness. An out of the blueness that qualifies the resulting image as such, imagined.
The imagination of the visual kind is usually stronger than the auditory, tactual, kinaesthetic, olfactory, and gustatory.
The etymology of the word imagination itself points to an action of mental picturing.
Notwithstanding, one can conjure not only sensorial images but also conceptual, symbolic ones.
Aristotle, the primaeval functionalist, cut to the chase reducing it all to a fairly straightforward process: sensations bring us perceptions of the world, memory allows us to store them, and imagination enables us to recreate mental images from memory corresponding to said perceptions, from the piling up of which we’re also able to derive other, more general ideas.
Gilbert Ryle found the concept of picturing, or visualising, a proper and useful one, as long as it doesn’t entail the existence of pictures that we might contemplate, or the existence of a gallery in which such pictures are ephemerally suspended. To him, imagining does occur, mentally, but one does not actually see or hear anything.
If you’re an explorer of the depths of the psyche like Freud, imagination obeys the pleasure principle, not the reality principle. Since there are only these two principles of mental function it makes sense the reality principle is the bummer on the way to one’s realisation — of such pleasures, yes. So, in the explorer’s own words:
“With the introduction of the reality principle one species of thought-activity was split off, it was kept free from reality-testing and remained subordinated to the pleasure principle alone. This activity is phantasying, which begins already in children’s play, and later, continued as day dreaming, abandons dependence on real objects.”
Back at the surface, Kosslyn’s island experiment is meant to support the hypothesis that the fruits of our imagination are actually images. It’s an imaginary island alright, but once you picture it and travel along its shores, your measured mental reaction time is proportional to the distance travelled, a finding also corroborated by fMRI (functional Magnetic Resonance Imaging) just by looking at the lightning going off in your brain’s visual cortex.
Not to Jerry Fodor. He’s got a LOTH (Language Of Thought Hypothesis) you see. Stuff that goes into our minds is coded and stored in a sort of logical, propositional, symbolic language, which when executed by the appropriate brain module, like the visual cortex, epiphenomenal imaging is displayed as output. If such code happens to be dispatched to Broca’s booth, a boringly elaborate, verbal description will come out instead.
Now what? Luckily we have Allan Paivio, a psychologist who also practised bodybuilding, to set the record straight, otherwise…
Allan’s Dual Coding Approach postulates that abstract stuff — devoid of images as such — is codified in language, symbolic, logic, whatever; and the concrete stuff, like a fork or your cat, is simultaneously codified as description and as an image. This hypothesis was also corroborated by fMRI scans. For instance, when thinking about a chair both areas of the brain light up, the one with the image of it, along with the one concerned with its function. Even more so if you think about a toilet; you’ll get pretty crazy flashing all around.
Can you imagine?
 Hunt, Morton (1993). The Story of Psychology. New York. Anchor Books.
 Ryle, Gilbert (1967). “Imagination” in Gustafson, Donald A. Essays in Philosophical Psychology. London. Macmillan.
 Freud, S. (1911). “Formulations on the two principles of mental functioning” in (2005) The Essentials of Psyco-Analysis. London. Vintage Books.
 Kosslyn, S. M., Ball, T. M., & Reiser, B. J. (1978). Visual images preserve metric spatial information: Evidence from studies of image scanning. Journal of Experimental Psychology: Human Perception and Performance, 4(1), 47–60