Today it is very widely believed that much imagery functions as mental representations (or mental models), playing an important role in memory and thinking. Philosophers such as George Berkeley and David Hume, and early experimental psychologists such as Wilhelm Wundt and William James, understood ideas in general to be mental images. Due to the fundamentally introspective nature of the phenomenon, there is little to no evidence either for or against this view.
It has sometimes been assumed that, like humans, some types of animals are capable of experiencing mental images. However, the majority of philosophical and scientific investigations of the topic focus upon visual mental imagery. As contemporary researchers use the expression, mental images or imagery can comprise information from any source of sensory input one may experience auditory images, olfactory images, and so forth. The nature of these experiences, what makes them possible, and their function (if any) have long been subjects of research and controversy in philosophy, psychology, cognitive science, and, more recently, neuroscience. Mental imagery can sometimes produce the same effects as would be produced by the behavior or experience imagined. There are sometimes episodes, particularly on falling asleep ( hypnagogic imagery) and waking up ( hypnopompic), when the mental imagery, being of a rapid, phantasmagoric and involuntary character, defies perception, presenting a kaleidoscopic field, in which no distinct object can be discerned.
Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification.