Following an injury or stroke that damages the primary vision center in the brain, a person will lose some or all of their ability to see. In this kind of blindness the eyes still work to bring information into the brain; it is just that the brain center responsible for object recognition fails. People who suffer this kind of "cortical" blindness often display an interesting capacity to make judgments about objects that they truly cannot see. This phenomenon is termed blindsight and it has fascinated psychologists since it was first documented in the 1960s.
Imagine having a person with cortical blindness as a subject. You could hold a red ball in front of her open eyes and ask if she can see it. She would reply no, which is consistent with the fact that she is blind. Now you ask her to point to the red ball (which she has just denied seeing). What happens? She points directly to the red ball even though she does not have the ability to see it!
Blindsight is taken as evidence of the unconscious. Here one part of the mind knows about something that another part of the mind does not know about. There are many demonstrations of people with blindsight. For example, when an object is placed in front of a person with blindsight—that is, a person who does not know for sure whether it is there or not—that person can guess the color of that object at levels much better than merely by chance. In other words, such a condition illustrates that information that is unconscious (whether an object is or is not in front of the person) is actually being processed somewhere in the mind (because they know the color of objects that are presented).
An explanation for such "unconscious" perception has been offered in terms of nerve pathways from the eyes into the brain. The optic nerve carries information from the eye into the brain, and the majority of this information is transferred to the primary visual center in the striate cortex. However, pathways split off of the optic nerve before getting to the visual center and carry some of this visual information to other parts of the brain. These other centers may be involved in movement recognition or color recognition or even emotional evaluation. If the vision center were completely destroyed, the person would not recognize what the object was, but they might know if it was moving or how they felt about it.
One of the most interesting and robust examples of blindsight concerns the perception of the emotional significance of something that one does not see. In one study, a person with blind-sight underwent a conditioning procedure, where a visual cue which they could not see (a picture of a circle) was accompanied by an unpleasant shock whereas other visual cues (pictures of squares, rectangles, etc.) were not paired with shock. Following a period of conditioning, the stimuli shapes were later "shown" to the blind subject, and the subject exhibited a fear response to the circle but not the squares or rectangles (Hamm, Weike, Schupp, Treig, Dres-sel, & Kessler, 2003). These researchers argue that emotional conditioning does not require a conscious representation in the mind of the subject. Other studies of people with cortical blindness demonstrate that, when "shown" pictures of facial expressions, they can "guess" the emotions expressed in the faces even when they cannot see the faces being
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