Definition and Approach of Traditional Psychology
In his excellent book on the subject, Alan Richardson defines mental images as follows:
"Mental imagery refers to (1) all those quasi-sensory or quasi-perceptual experiences of which (2) we are self-consciously aware, and which (3) exist for us in the absence of those stimulus conditions that are known to produce their genuine sensory or perceptual counterparts, and which (4) may be expected to have different consequences from their sensory or perceptual counterparts." (Richardson, Alan, Mental Imagery, New York: Springer Publishing Company, Inc., 1969, pp. 2-3)
The first two conditions are understood to imply the existence of subjective experiences not directly measurable by empirical means. Although there is no direct method of measuring the quality of someone's experiences, we can ask people to describe their experiences. Richardson and others have proposed various imagery scales to give some indication of how vivid (percept-like) a particular experience might be. The last two conditions are generally measurable by objective, empirical means.
The issue of empirical measures, however, (in my view) is secondary to the more important features that differentiate this definition of mental imagery from the approach endorsed by some researchers in cognitive science. According to the traditional approach, mental images are
The traditional conception of mental images is much closer our ordinary or common sense view of what mental images are. There is an important difference, however. Many people will insist that their memory consists of a storehouse of (visual) mental images which they can call up and "look" at any time. These images in memory, therefore, would be conscious while we experience them, but unconscious when they are hidden away in the storehouse of memory. The "storehouse" view of visual memory has important parallels to Kosslyn's definition of mental imagery.More on the approach used by Richardson and traditional psychology.
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