Blind people have revolutionised our view on vision. Biology text books still teach us that vision functions roughly as light hitting the eyes where special cells – rods and cones – turn it into neural signals. These travel to the back of the head, the visual cortex, for brain processing leading to something we experience as ‘seeing’. Some blind people have offered a completely new picture. They see without visual cortex. They see without rods or cones. They see without experiencing ‘seeing’.
The visual cortex lies right at the back of the head and it is – as the name suggests – responsible for vision. If you lose it, you can’t see anymore. This happened to a partially blind patient only known by his initials DB, a man brought to scientific fame in 1974 by an article in the journal Brain. In it, Lawrence Weiskrantz and colleagues describe how DB is asked to say whether he is presented an X or an O in an area of his visual field where he is blind. DB performs more than 80% correct despite only guessing.
What happened when DB was told about his visual abilities? ‘[H]e expressed surprise and insisted several times that he thought he was just “guessing.” [H]e was openly astonished’ (p. 721). This phenomenon has been termed blind-sight and it is very unlike normal vision. It is usually much worse but there are exceptions. For example, DB is actually better at ‘blind-seeing’ very faint lines compared to his intact visual field or normal people’s vision (Trevethan et al., 2007). This rules out all sorts of concerns about blindsight such as the suggestions that DB might be lying or falsely describing degraded vision as no vision at all. Unusually good performance can hardly be faked.
If blind-sight is possible without visual awareness or visual cortex, is it also possible without the eye’s rods and cones which turn light into neural signals? Interestingly, yes. Back in 1995 a team led by Charles Czeisler reported an unusual finding in three blind people whose eyes were damaged due to various diseases. When a bright light was shone in their face, they had less melatonin – a hormone related to the sleep cycle – in their blood. Probably a little known cell type – called intrinsically photosensitive retinal ganglion cells – turned light into neural signals and generally helps us synchronize our sleep-wake cycle with the day-night cycle.
A new article by Vandewalle and collagues shows what the potential of this newly discovered cell type is. They tested three blind people with eye damage and simply asked ‘is there a light or not?’ If a light was on for ten seconds, all three ‘guessed’ significantly differently from chance. This is remarkable as these people reported not seeing anything, electrical brain potentials following light flashes were curiously absent and their eyes were undoubtedly damaged.
When looked at together, these phenomena offer a new picture of the visual system. In the text-books you see a linear picture roughly like this:
light –> rods/cones in the eye –> visual cortex –> rest of brain
A new model is needed because a remarkable range of behaviours can still be performed when the middle elements of this account are removed. Instead of a linear picture we need a collection of parallel pathways all using light to influence the brain. The blind-sight pathway proves that circumventing the visual cortex is possible. People without rods/cones prove that not even these cells are needed to make use of light.
And now imagine that vision is one of the best-understood systems in the brain. If even vision can offer such surprises it is difficult to imagine what other brain systems hide below the surface. However, going ‘below the surface’ also comes with a considerable cost. Ask blind people what they see and they simply say ‘nothing’. Their residual abilities are hidden from them. It takes careful psychological testing to make them aware of what they can do.
So, how do blind people see? Some of them see without even knowing it.
Czeisler CA, Shanahan TL, Klerman EB, Martens H, Brotman DJ, Emens JS, Klein T, & Rizzo JF 3rd (1995). Suppression of melatonin secretion in some blind patients by exposure to bright light. The New England journal of medicine, 332 (1), 6-11 PMID: 7990870
Trevethan CT, Sahraie A, & Weiskrantz L (2007). Can blindsight be superior to ‘sighted-sight’? Cognition, 103 (3), 491-501 PMID: 16764848
Vandewalle G, Collignon O, Hull JT, Daneault V, Albouy G, Lepore F, Phillips C, Doyon J, Czeisler CA, Dumont M, Lockley SW, & Carrier J (2013). Blue Light Stimulates Cognitive Brain Activity in Visually Blind Individuals. Journal of cognitive neuroscience PMID: 23859643
Weiskrantz L, Warrington EK, Sanders MD, & Marshall J (1974). Visual capacity in the hemianopic field following a restricted occipital ablation. Brain : a journal of neurology, 97 (4), 709-28 PMID: 4434190
1) By Antonio Cruz/ABr (Agência Brasil.) [CC-BY-3.0-br (http://creativecommons.org/licenses/by/3.0/br/deed.en)%5D, via Wikimedia Commons