Piero Scaruffi(Copyright © 2013 Piero Scaruffi | Legal restrictions )
These are excerpts and elaborations from my book "The Nature of Consciousness"
"Mental imagery" is seeing something in the absence of any sensory signal, such as visualizing an object that is not actually present. The mystery is what is seen if in the brain there is no such image. When I stare at an object, i “see” the image that the visual system creates in the brain (whatever projection of dots through the retina to this or that region of the brain). But am i “seeing” when i am simply imagining a Ferrari?
Scientists have found no pictures or images in the brain, no internal eye to view pictures stored in memory and no means to manipulate them. Nevertheless, there is an obvious correspondence between a mental image of an object and the object.
The US psychologist Ronald Finke, for example, has identified five principles of equivalence between a mental image and the perceived object: the principle of implicit encoding (information about the properties of an object can be retrieved from its mental image), the principle of spatial equivalence (parts of a mental image are arranged in a way that corresponds to the way that the parts of the physical object are arranged), the principle of perceptual equivalence (similar processes are activated in the brain when the objects are imagined as when they are perceived), the principle of transformational equivalence (imagined transformations and physical transformations are governed by the same laws of motion), the principle of structural equivalence (the mental imagery exhibits structural features corresponding to those of the perceived object such that the relations between the object's parts can be both preserved and interpreted).
During the 1980s the debate became polarized around two main schools of thought: either (The US psychologist Stephen Kosslyn) the brain maintains mental pictures that somehow represent the real-world images, or (the US psychologist Zenon Pylyshyn) the brain represents images through a “non-imaginistic” system, namely language, i.e. all mental representations are descriptive and not pictorial (there are no picture-like representations in the brain).
Kosslyn put forth a representational theory of the mind of a "pictorial" type, as opposed to Jerry Fodor's propositional theory and related to Philip Johnson-Laird's mental models. Kosslyn thinks that the brain builds visual representations, which are coded in parts of the brain, and which reflect what they represent. Mental imagery involves scanning an internal picture-like entity. Mental images can be inspected and classified using pretty much the same processes used to inspect and classify visual perceptions. For example, they can be transformed (rotated, enlarged, reduced).
There exist two levels of visual representation: a "geometric" level, which allows one to mentally manipulate images, and an "algebraic" one, which allows one to "talk" about those images.
Kosslyn thinks that mental imagery achieves two goals: retrieve properties of objects, and predict what would happen if the body or the objects should move in a given way. Reasoning on shapes and dimensions is far faster when we employ mental images than concepts.
Kosslyn's is a theory of high-level vision in which perception and representation are inextricably linked. Visual perception (visual object identification) and visual mental imagery share common mechanisms.
Opposed to Kosslyn's “pictorialism” is Pylyshyn's "descriptionalism". Pylyshyn believes in a variant of Fodor's language of thought: to him images are simply the product of the manipulation of knowledge encoded in the form of propositions.
The “dual coding theory” of the Canadian psychologist Allan Paivio mediates these positions because it argues that the mind may use two different types of representation, a verbal one and a visual one, corresponding to the brain's two main perceptive systems. They both "encode" memories, but they do so in different ways (codes).
The neural processes that correspond to mental imagery in seeing people have also been detected in the brains of congenitally blind people. Thus mental imagery cannot possibly depend on forming a mental reconstruction of former visual sense experiences.
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