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. 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. however, the majority of philosophical and scientific investigations of the topic focus upon "visual" mental imagery. it has sometimes been assumed that, like humans, some types of animals are capable of experiencing mental images. due to the fundamentally introspective nature of the phenomenon, there is little to no evidence either for or against this view.