Vision and Blindsight
Implications Regarding Consciousness
Vision-- receiving and interpreting light signals from the environment in order to form an image in one's mind-- is an incredibly complex process. Somehow signals from photoreceptors located in the eye are converted into the conscious experience of sight. Of all the aspects of vision, perhaps the most difficult for us to comprehend scientifically is this notion of consciousness. Somehow the brain interprets light waves hitting the retina so that we are visually aware of our surroundings. While the mechanism of signal transduction from the photoreceptor through the visual cortex has been extensively elucidated, science has difficulty dealing with the phenomena of consciousness and awareness, especially on a reductionist level.
A recent neurobiological approach to understanding consciousness, at least on a perceptual level, has involved the study of the phenomenon of blindsight. Damage to areas of the visual cortex often result in complete or partial blindness. Although the eye itself is undamaged, patients report an inability to detect any light input in part of (or the entire) visual field. However, experiments regularly show that somehow, visual cues are processed. Visual inputs presented to the blind field affect the patient's response to stimulus in the normal visual field. Reaction times to stimuli are affected as well as the interpretation of the stimuli. A visual cues presented in the blind field may suggest a certain interpretation of an ambiguous stimuli. For example, the interpretation of the word "bank", presented as an auditory cue, differs depending on whether the word "river" or "money" is presented to the blind field, even though the patient does not report seeing these cues . Furthermore, patients are often able to respond directly to visual cues in their blind fields. Patients tend to correctly identify shape, color, and motion of inputs. In a forced choice experiment, where the subject is asked to identify certain features of a visual cue, the subject will perform much better than chance even though they feel as if they are randomly guessing (2).
While head trauma or tumors often induce the "psychic" blindness of these patients, a model has been developed in monkeys by removing all or part of the primary visual cortex. These monkeys are able to respond to visual inputs. They can be trained to touch illuminated bulbs rather than unlit ones and identify certain colors and patterns in order to obtain food. This phenomenon is believed to parallel human blindsight because when trained to respond differently according to whether there is a visual cue or not, these monkeys respond as if there were no cue when a visual input is presented to the blind field (1). It is therefore believed that these animals are able to respond to and identify features of a visual cue even though they do not report seeing it.
The phenomenon of blindsight has far reaching implications...