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BLINDSIGHT: THE ROLE OF THE VISUAL CORTEX




One of the ways that neuroscientists have come to understand the brain is by studying unusual conditions. Case studies which focus on unique individuals offer a great deal in understanding the brain and its functions. In the article above, there is a description of the features of blindsight, a condition in which individuals have no apparent conscious awareness of an object (so they could not describe it), but would still be able to reach out and grab the object or even avoid bumping into it. Consciously unaware, but subconsciously aware.


Larry Weiskrantz created experiments for a patient known as D.B. which tested the limitations of the latter's vision. Surgeons had removed abnormal cells from D.B.'s brain, specifically in the right visual cortex. It meant that D.B. was blind to anything shown to his left visual field; what we see in the left visual field goes (via neural connections) to the right visual cortex. In experiments, when a researcher asked D.B. to name something in his left visual field, D.B. could not; when asked to reach out and grab the researcher's hand in the left visual field, D.B. could do it (Ramachandran and Blakeslee, 1998, p.76-7).


To understand blindsight, neuroscientists have concentrated on the function of the primary visual cortex. The primary visual cortex is located in the Occipital Lobe, at the back of the brain. Its role is to process visual information, the kind that we are conscious about. If for instance you are reading a menu and deciding what to eat for lunch, your eyes will move up and down the page. As you stop at each dish your eyes look at the words, you imagine the food describing and try to decide if that is what you want. Perhaps there is an image of the dish or you might look around the restaurant to see what other customers have chosen.





At the point in which your eyes focus on the dish, in words or in reality, the light signals activate rods and cones in the retina. These signals are then passed, via the optic nerve, to a midbrain structure called the lateral geniculate nucleus (LGN) which then project to the primary visual cortex. It is when the visual cortex activates that you are in a position to consciously recognise, discuss and ask questions about what you are looking at (generally be aware of it). This ventral stream takes visual information to the primary visual cortex. Another route, the dorsal stream, takes the visual information, via the optic nerve, to the superior colliculus, and then to the parietal lobes. The parietal lobes helps to coordinate balance and orientation. You need the parietal lobes to steady your hand in order to catch a ball.


The ventral stream is newer in evolutionary terms, whereas the dorsal stream is older. Whilst the ventral stream leads to conscious awareness of the world around, the dorsal stream does not. In blindsight cases like D.B., since the ventral route is damaged on the right side of his brain (primary visual cortex removed), he cannot consciously process what he is seeing. However, the unconscious dorsal route is still functioning, the one which aids balance and coordination, which is why D.B. can reach out and hold what is being shown. D.B. can navigate the 'where' even though he cannot name the 'what'.



The primary visual cortex located in the occipital lobe.
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In undamaged, healthy brains, where the primary visual cortex is functioning it shows that this part of the brain is there to show conscious awareness of objects. The primary visual cortex projects neural connections to the temporal lobe where the fusiform face area is located, which is the area needed to identify faces. One of the key functions of the primary visual cortex then is to show awareness of an object, beyond simply knowing where it is.




References


Blakeslee, S. and Ramachandran V. S. (1998) Phantoms in the Brain, (Harper Perennial: London, 2005)


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