Blindsight is an ability to perceive and respond to visual information without conscious awareness. People with blindsight are technically blind, meaning that they are unconscious of their surroundings and they can’t tell the light from the dark. Nevertheless, these people are able to use non-conscious knowledge to make decisions about their environment and act accordingly. Blindsight brings into question the actual process of “seeing.” If humans can see without conscious awareness, then what defines vision and how important is it for us to be aware of what we are looking at?
When discussing blindness, one usually assumes retinal blindness, which results from damage to the eye. Blindness can also follow damage to the visual cortex. The visual cortex is located in the occipital lobe and is divided into the striate (V1) and extrastriate regions (V2, V3, V4, V5). Blindsight typically occurs in patients with striate damage following head trauma, stroke, or removal of tissue. When both hemispheres of the visual cortex are damaged, patients lose their ability to see in both visual fields. If the damage is limited to one hemisphere, this condition is referred to as hemianopia: inability to perceive half of the visual field contralateral to the damaged hemisphere. Although only a small number of patients with occipital cortex damage have blindsight, existence of this phenomenon indicates that not all aspects of vision depend on the visual cortex.
In order to study blindsight researchers use “forced-choice” tasks. Patients are shown shapes of different contrasts, sizes, or colors, and they have to choose between two options (Was the shape moving up or down?). In some experiments patients have to guess, point, or manipulate objects (see Ramachandran video below). Although they deny any conscious awareness of what they see on the screen, the rate of correct responses is above chance, in some cases reaching 100%. Cortically blind patients can detect flickers, movement, discriminate between shapes and colors, and they adjust their grasp to match the shape and size of an object they claim not to see. Blindsight also helps patients navigate through space. An amazing example of blindsight comes from patient TN, who has bilateral damage to his primary visual cortex (V1). TN was able to walk through a corridor full of obstacles without running into them while claiming that he could see nothing (see Scientific American video).
The phenomenon of blindsight is not, however, accepted across all investigators. Some researchers have argued that blindsight may be an artifact of testing or serve as evidence that some of the primary visual cortex is still functioning. The latter argument is countered by evidence that patients with no primary visual cortex still perform well above chance on the forced-choice tasks. Also, those patients who retain parts of their primary visual cortex show no metabolic activity in the spared region when they respond correctly in forced-choice tasks. Another problem with blindsight is how one defines awareness. It is not clear whether blind patients are truly unaware of the visual stimuli or whether they have a higher threshold for awareness. It is possible that although they may consciously be aware of seeing something, they deny awareness because it’s not what it could be. Despite the criticisms, experimental evidence continues to support that vision is not entirely dependent on the visual cortex.
Why does blindsight happen? Scientists argue that people with visual cortex damage retain connections between the retina of the eye and the superior colliculus. The superior colliculi play a role in orienting visual attention towards sudden changes in the environment. For example, you can thank your superior colliculus the next time you duck out of the way of a Frisbee before you see it. Thus, people with blindsight retain the ability to respond to attention grabbing changes. However, new research suggests that the lateral geniculate nucleus (LGN) in the thalamus plays a more crucial role in blindsight. In a recent fMRI experiment, monkeys with intact LGN were able to discriminate between high contrast stimuli and they showed activity in extrastriate cortical regions, whereas those with damaged LGN were not. Although it is still not clear whether it is the superior colliculus or the LGN that are necessary for blindsight, both represent an evolutionary older visual pathway that humans share with lizards, birds, and rodents.
Blindsight can perhaps be viewed as an example of implicit knowledge. Implicit knowledge is the non-conscious information that humans use to navigate space, communicate with others, and make decisions. Both healthy and cortically blind people show evidence of affective blindsight: they respond to emotional information without being consciously aware of it. Imaging experiments with cortically blind adults show activation in the amygdala, the part of the brain associated with emotional processing, when they are exposed to happy or fearful faces. People with normal vision also show amygdala activation to emotional information that is flashed at them below the threshold of awareness. Thus, all of us possess blindsight to a degree.
The fact that implicit knowledge affects our decisions and behavior is hardly surprising considering that healthy people process and respond to a lot of information subliminally. As Dr. Ramachandran points out in the video, we spend most of our day on autopilot, unaware of our actions. For example, as you are reading, you might be snacking, tapping your foot, or fidgeting with a pencil. Although your consciousness is engaged with the reading, your brain is able to execute the necessary motor plans for you to perform extraneous tasks.
Is blindsight a window into the nature of consciousness? How does the phenomenon of blindsight relate to other instances of cognitive or emotional processing we are not aware of? Can you think of examples? How can blindsight inform our understanding of the organization of the human brain?
The New York Times: Blind, Yet Seeing: The Brain’s Subconscious Visual Sense
Blindsight: Seeing without knowing it
Scientific American movie on Patient TN
Ramachandran on how “blind-sight” gives us clues about the nature of consciousness
Cowey, A. (2010). The blindsight saga. Experimental Brain Research, 200(1), 3-24.
Cowey, A., & Stoerig, P. (1991). The neurobiology of blindsight. Trends in neurosciences, 14(4), 140-145.
Schmid, M. C., Mrowka, S. W., Turchi, J., Saunders, R. C., Wilke, M., Peters, A. J., et al. (2010). Blindsight depends on the lateral geniculate nucleus. Nature, 466(7304), 373-377.
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