Visual Perception and Mental Imagery

Two fascinating articles relating to visual perception involving color appeared in the March, 1986, issue of "Scientific American". The first article,"Mental Images and the Visual System," authored by Ronald A. Fink, professor of psychology at the State University of New York in Stony Brook, states that visual perception an mental imagery share some of the same neural processes in the human visual system.

Dr. Finke points out that people can often form mental images of an object that resemble the object's actual appearance and that the act of constructing such images often produces actual visual sensations that seem quite realistic.

He asks the reader to imagine that he or she may be looking at an elephant. "Does it have a curved trunk? What color are the tusks? How big are the eyes?" He states that most people attempt to answer those questions by inspecting a mental image in much the same way they would inspect a real elephant.

As an example, the article contains an illustration of a landscape originally done in watercolors by a woman, age 42, who has been blind since the age of 21. She paints from her mental images by lining up 24 watercolor jars in memorized order. She moves from one section of the paper to the next, determining what she has just finished by detecting the moisture with her fingertips. Each watercolor is typically composed of six layers of paint.

The second article by Jearl Walker is found in the section called "The Amateur Scientist" Walker writes about the "McCullough effect" which is named after Celeste McCulllough of Oberlin College who first reported this intriguing phenomenon in which the human visual system imposes color on a black and white grating.

In order to experience the effect, it is necessary to study a grating of black stripes interspaced with a striping of another color and then later look at a black and white grating that is identical in spacing and orientation to the first black and colored grating. The white stripes of the grating will appear to be tinged with the complementary opposite of the color in the first grating (accompanying diagrams in the article used complementary vivid green and magenta pink as examples).

The interesting aspect of this visual phenomenon is that, even though the color generated by the grating may grow fainter in time, it is reported to appear even after the observer delays viewing the second grating for hours, days, or even weeks. However, the color strength after a delay depends on the sleeping and eating habits of the observer.

For more info on both of the above articles, see "Scientific American", March 1986, Vol.254, No. 3, Finke: pp, 88-95; Walker; pp.112-118