Under the Microscope: Have you ever driven home deep in thought on some topic and, on arrival at our destination, found you cannot remember any detail of your journey? asks Prof William Reville
You have driven on auto-pilot, safely negotiating traffic lights, corners, and organising yourself with respect to other traffic without being conscious of these activities. This has often happened to me and I believe it is quite a common experience. It is a general illustration of how the brain can know things without registering them consciously. The phenomenon is of great interest to scientists and philosophers who ponder the nature of consciousness and how the brain/mind works.
A particularly fascinating illustration is the phenomenon of blindsight where, following certain kinds of brain lesions, patients report an inability to see objects, but if pressed to guess at their location they can accurately point at them. This whole area is vividly described by Vilanur Ramachandran who delivered the 2003 Reith Lectures on BBC Radio 4. The lectures were published in book form - The Emerging Mind (Profile Books, 2003).
The act of seeing, which we take for granted, is an amazing and complex act of synthesis by the brain. When you look at an object, light rays from the object enter your eye and an optical image is formed on the retina, a membrane lining the back of the eye that contains photoreceptor cells. The retina is connected to the brain via the optic nerve. Electrical signals convey the image from the retina photoreceptor cells to the brain and allow us to consciously see the object. The electrical signals go to two major visual centres in the brain. Ramachandran calls one centre "the old visual centre" and the other one "the new pathway". The old centre is ancient in evolutionary terms and is located in the brain stem. The second pathway goes to the visual cortex at the back of the brain.
The new pathway does most of what we commonly associate with seeing and allows us to consciously recognise objects. The old pathway locates objects in the visual field so that you can direct the most sensitive central part of the retina towards the object and then use the new pathway to consciously identify the object and generate appropriate behaviour.
Damage to the visual cortex in the new pathway results in blindness. For example, if the left visual cortex is damaged, looking straight ahead you will see nothing to the right side of your nose. In the 1980s, when examining a patient who had this type of visual problem, Larry Weizcrantz and Alan Cowey at Oxford discovered the extraordinary neurological syndrome called blindsight.
Weizcrantz placed a spot of light in the patient's blind region and asked "what do you see?" The patient replied "nothing", not surprisingly given that he was blind to that region. Weizcrantz said "I know you can't see it, but please reach out anyway and touch it." The patient accurately located the spot of light that he couldn't consciously perceive. To the amazement of Weizcrantz and Cowey, the patient pointed to the spot of light in 99 per cent of hundreds of trials, even though he said he was only guessing on each trial.
The explanation is that although the new pathway is damaged, the old pathway remains operational. This allows objects to be located in space and sends messages to higher brain centres that guide the hand to point to the invisible object even though the patient is blind to what is happening.
The old pathway projects to the parietal lobes on the sides of the brain. These create a 3 D representation of the external world and allow us to navigate spatially, e.g. catching an object that's thrown at you. If the right parietal lobe is damaged you get a syndrome called "neglect" which is the converse of blindsight. The patient can't move his eyes towards an object that comes towards him from the left, and he can't reach out and grab it. The patient shows indifference to the left side of the world.
He is not blind to events on the left and can see them when his attention is drawn to them, but otherwise he ignores them. For example, if eating food from a plate, he eats only from the right side of the plate.
Why is it that one visual pathway in the brain, the new pathway, leads to conscious awareness, whereas nerve signals in a parallel part of the brain, the old pathway, can carry out complex calculations without being conscious? Why can't the entire brain function without consciousness?
The mental processing that constitutes visual perception is not a single capacity located in a single brain area. This may also apply to more than visual perception. The mind as a whole may be distributed throughout the brain with its various parts working in a co-ordinated but semiautonomous manner. The loss of one processing area would fragment the mind, with a whole faculty like conscious vision lost, but not its component part.
What would happen if consciousness itself were the faculty lost? That would leave us in a robotic state, but could we survive? I would say almost certainly not. Blindsight is not real sight without consciousness and possibly no perception is real without consciousness. It is very probable that without consciousness we could not perceive the environment sufficiently well for us to survive.