When we look at a scene, it often seems as if we perceive it all at once. Yet in fact, we are physically able to accurately view only a tiny portion of the scene at a time. Take a look at this image of a human retina (the back of your eyeball). The portion of the eye responsible for accurate vision is the miniscule fovea:
To get a sense of the portion of your visual field you are able to see in clear focus at one time, hold your thumb up at arm's length. The area of the fovea corresponds to the size of your thumbnail at that distance: less than 2 degrees of visual angle! To "see" a scene larger than your thumbnail, you must rapidly move your eye around the scene, and retain the out-of-focus elements in memory. As Greta pointed out in yesterday's post, that visual memory isn't always accurate.
This "boundary extension," one typical error we make in remembering scenes and first reported by Helene Intraub and Michael Richardson, can occur in a variety of circumstances, even when the observer is aware of the effect and attempts to compensate for it. Intraub, a psychologist at the University of Delaware, believes that boundary extension may be due to the active creation of a mental representation of a scene. Since our mind constructs a "scene" based on scant available information (the area visible by the fovea as the eye looks at different parts of the scene), it makes sense that it might also construct a representation of parts beyond the boundary of what is actually viewable. If we "fill in the blanks" inbetween places we've actively looked at, why not extend our representation beyond the boundaries of what we've seen as well?
But is this process unique to the visual system? Do we fill in the blanks and extend the boundaries of a scene that we perceive another way? Intraub developed an experiment to test another way of perceiving a scene: touch ("Anticipatory Spatial Representation of 3D Regions Explored by Sighted Observers and a Deaf-and-Blind-Observer," Cognition, 2004).
She showed participants six different "scenes" composed of real, physical objects (groups of ordinary things like toys, books, and toiletries), each demarcated by a "boundary" of black cloth. She then had an assistant remove the boundaries and asked the participants to mark where the boundaries had been. As expected, they placed the borders well beyond where they had been in the original scenes.
Next, she blindfolded another set of participants and showed them the same scenes, with an easily detectable three-inch-tall wooden "boundary" replacing the original cloth border. When they returned later to the same scenes with the border removed (and still blindfolded), they were asked to place wooden blocks where the borders had been.
Finally, she repeated the "blindfolded" condition with a volunteer who had been deaf and blind from early childhood. Intraub calls this participant (named only by her intials, KC) a "haptic expert" because she has spent her entire life negotiating the world by the sense of touch. At the age of 25, KC was a successful college student who could easily identify the objects in the experiment by touch (the only difficulty was reminding her to use the entire 30 seconds alotted for each scene, so as to match the blindfolded group).
KC's results were statistically indistinguishable from the blindfold group, and like the vision group, they, too, showed significant boundary extension. However, the vision group's boundary extension was greater in every case than the participants who had completed the task solely using touch. Intraub suspects that this difference between visual boundary extension and tactile boundary extension may be due to the different processes involved. There must be some limit to the amount a viewer extends the boundary of a scene: perhaps this limit derives from the way the viewer perceives the scene. Since we can easily see farther than we can reach, we extend boundaries farther when we view a scene with our eyes.
Intraub also points out that just because both vision and touch lead to boundary extension doesn't mean that boundary extension is caused by the same underlying mental process in both cases. Such a conclusion would require significant additional research.