Blindsight

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This article is about the neurological phenomenon called 'blindsight'. For the documentary Blindsight, see Blindsight (2006 film).

Blindsight is a phenomenon in which people cannot consciously see a certain portion of their visual field but still behave in some instances as if they can see it.

[edit] Technical details

Visual processing in the brain goes through a series of processing stages. Destruction of the first visual cortical area, primary visual cortex (or V1 or striate cortex) leads to blindness in the part of the visual field that corresponds to the damaged cortical representation. The area of blindness - known as a scotoma - is in the visual field opposite the damaged hemisphere and can vary from a small area up to the entire hemifield.

Although individuals with damage to V1 are not consciously aware of stimuli presented in their blind field, Lawrence Weiskrantz and colleagues showed in the early 1970s that if forced to guess about whether a stimulus is present in their blind field, some observers do better than chance. This ability to detect stimuli that the observer is not conscious of can extend to discrimination of the type of stimulus (for example, whether an 'X' or 'O' has been presented in the blind field). This general phenomenon has been dubbed blindsight.

It is unsurprising from a neurological viewpoint that damage to V1 leads to reports of blindness. Visual processing occurs in the brain in a hierarchical series of stages (with much crosstalk and feedback between areas). As V1 is the first cortical area in this hierarchy, any damage to V1 severely limits visual information passing from the retina, via the LGN and then V1, to higher cortical areas. However, the route from the retina through V1 is not the only visual pathway into the cortex, though it is by far the largest; it is commonly thought that the residual performance of people exhibiting blindsight is due to preserved pathways into the extrastriate cortex that bypass V1. What is surprising is that activity in these extrastriate areas is apparently insufficient to support visual awareness in the absence of V1.

An innovative research technique is providing insight into why some blind people are able to sense and describe objects they cannot see. The phenomenon of "blindsight" occurs in some people who suffer injuries to the primary visual cortex, the region of the brain considered essential for sight. Blindsight allows people to use visual information they get through their eyes even though they have no consciousness of the visual experience, said Christopher Mole, a postdoctoral fellow in philosophy at Washington University in St. Louis, Missouri. "But that of course is quite hard to show in the lab," he said. A team of psychologists at Rice University in Houston, Texas, may have found a way to directly study blindsight in the lab. They are using electromagnetic stimulation on the brains of people who can see to render them partially and temporarily blind. "The way it works is an electric current inducts into the brain via a magnetic pulse, and that causes a disruption of underlying neurons in the brain," said Tony Ro, a member of the Rice team. "What this technique allows us to do essentially is in a safe and noninvasive way shut down a portion of the brain temporarily," he added. Ro and colleagues report their technique and findings in the current issue of the journal Proceedings of the National Academy of Sciences. Mole said the Rice team reports "compelling proof" for blindsight. Unconscious Pathway Blindsight is most prevalent among people who suffer damage to the primary visual cortex, such as in some stroke victims, Mole explained. However this is never "clean" or specific damage—other parts of the brain are also impaired. Studies with these patients are therefore difficult, he said. To study blindsight directly, researchers often purposefully and permanently disrupt the primary visual cortex in monkeys and other mammals, a method that would be unethical to use on humans, Mole said. "What [Ro's team] has done is cleverly manage to interfere with the brain in a totally temporary way … It doesn't have any long-term lasting effects at all," he said. The technique devised by the Rice researchers induced blindness for a fraction of a second in people who ordinarily have good vision. During the state of temporary blindness, an object was flashed on a screen in front of the test subjects' eyes. In one experiment the object was either a vertical or horizontal bar, and the subjects were asked to guess the bar's orientation. In the second experiment the researchers flashed a colored disc, and subjects were asked to guess the color. In both experiments the blinded volunteers correctly guessed the characteristics of the objects at much higher levels than chance alone. This fits the definition of blindsight and raises the question of how it is possible. "What we believe is happening is people are able to discriminate orientation and color—as our experiments showed—by processing routes into the brain that aren't consciously accessible," Ro said. "We believe there are pathways that go from the eyes into the brain that bypass the normal routes tied to conscious processing of information." Ro added that the study supports the theory that these pathways go to a visual center in the brain that is more sophisticated than the visual centers common to all mammals. This suggests the pathways may be unique to higher-order species. The test results also show that volunteers were more accurate when they were more confident in their guesses. "It's unclear what that reflects, but what we think it reflects is that this unconscious processing system can contribute to feelings of certainty," Ro said. In follow-up experiments the team will test why people feel varying levels of confidence in their guesses. Perhaps the unconscious processing routes are stronger in some people than others, Ro said.

[edit] References

• Danckert, J. & Rossetti, Y. (2005). "Blindsight in action: what can the different sub-types of blindsight tell us about the control of visually guided actions?". Neurosci Biobehav Rev 29 (7): 1035–1046. 
• Stoerig, P. & Cowey, A. (1997). "Blindsight in man and monkey". Brain 120: 535–559. 
• Weiskrantz, L (1986). Blindsight: A case study and its implications. Oxford, Oxford University Press. ISBN 0-19-852192-8.