Until about 35 years ago scientists believed there was only a single visual-processing area, called the visual cortex, situated at the back of the brain. We now know more than 30 areas in the brains of primates—including humans—are involved in handling aspects of vision such as the perception of motion, color and depth. Vision, it turns out, is a much more complex and sophisticated affair than anyone had imagined. It makes sense that responsibility for processing is divided into various areas that have different computational objectives.
We take our sight for granted because it usually seems so effortless. It is only when parts of these different visual areas are damaged, causing selective yet often profound disturbances in perception, that we begin to appreciate the range and subtlety of normal human vision. This approach parallels our study of “normal” illusions—by understanding misperceptions, whether for intact or damaged systems, we gain insight into brain processes involved in perception.
Consider the case of a man known as GY. Damage to his visual cortex resulted in complete blindness in one half of the visual field. He could not consciously see anything, not even a spot of light, shown to him in that region. Yet when asked to reach out and touch the spot, he could do so accurately; he could touch a spot he couldn’t see! It seems downright spooky, but, as you will soon learn, we can explain—at least partially—his condition, known as blindsight, in terms of the multiple specialized anatomical pathways devoted to vision that we mentioned earlier. [For more on blindsight, see “Subconscious Sight,” by Susana Martinez-Conde; Scientific American Mind, April/May 2008.]
Or consider the strange case of John, elegantly studied in 1987 by M. Jane Riddoch and Glyn W. Humphreys, both now at the University of Birmingham in England. John had served as an air force pilot. Soon after his retirement he suffered a stroke that partially damaged visual regions of both hemispheres of his brain. He could observe things around him; he was not blind in the usual sense. But when he saw his wife—or anyone else for that matter—he could not recognize her. He knew her by her voice; his brain areas for hearing were unaffected, as were his memories. Indeed, he could not visually distinguish among umbrellas, chairs or other common objects, even though he claimed to be able to see them perfectly clearly. “They are out of focus in my mind, doctor,” he would say, “not in my eye.”
The doctors confirmed this assertion by asking him to copy a drawing of, for example, St. Paul’s Cathedral that was hanging on the wall. John could produce a faithful rendering, almost a carbon copy, of the picture but had no idea what it was. He might as well have been copying a meaningless jumble of lines.
John had a condition known as visual agnosia, a phrase coined by Sigmund Freud meaning “lack of visual knowledge.” Unlike some of Freud’s more outlandish ideas such as “penis envy” or the “Oedipus complex,” this one has survived the test of time.
What must it feel like to have such a condition, seeing yet not knowing? You can get an inkling by looking at the famous old woman/young lady illustration (a). The first time you look at this illusion, you probably see the girl. But after a while, you can mentally flip the image to see an old face. The young woman’s chin becomes the hag’s nose, and the young ear becomes the old eye. Now, when you were perceiving the face as a young woman, you were also simultaneously seeing the lines and curves constituting the old hag. Yet you were not perceiving (or “knowing”) the old woman. In effect, you suffer from a temporary form of agnosia for her. Intriguingly, some people, including our colleague Stuart M. Anstis, a psychologist at the University of California, San Diego, are permanently “stuck” on the young lady and cannot see the hag. Psychologist Richard L. Gregory of the University of Bristol in England refers to this inability as visual hagnosia.
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7 Comments
Add CommentI wonder if it's possible that the "where" pathway developed separately from the "how" pathway in the visual cortex and that it functions on an unconscious level as an evolutionary adaptation. I could see the advantage in a threatening situation where having the brain begin to react to a potential threat by knowing where it's located and preparing a physiological response unconsciously would come in handy, rather than waiting for interpretation as to what the object is and how to interact with it. Perhaps the organization is useful in the way that the automatic withdrawal reaction is useful when touching a hot stove. You don't have to think about it, you just do it. Perhaps knowing where something is regardless of whether you know what it is proves useful for the same reason.
Reply | Report Abuse | Link to thisProof that all we sense is illusion can be seen in amputees, that have sensation in parts of missing limbs. How our brains can project an illusion of our surroundings OUTSIDE of us is a constant cause of fascination. When we turn our heads, our environment is detected as standing still. It will be a long time before we are able to program such sensing phenomena into robots!
Reply | Report Abuse | Link to thisi also have a blind sight disoderor visual blind sight disorder
Reply | Report Abuse | Link to thisi was watching discovery then i came to know that i have visual blind sight disorder in which a person is dong his functioning by watching the object if dont see the object i cannot do any thing.it is a risk factor for me in the scence of driving walking jams mostly tel me how can i prevent my self from blind sight dissorder this is mostly due to the defficiency of oxygen in the neurons or nerve cells nerve ceels are not active due to the defficency of oxygen. i m also a biologist.
I'd like to ask professor Ramachandran
Reply | Report Abuse | Link to thisa) if he thought consciousnee could be a spandrel of language (eg language mapping out the synthesis of long to short memories along such a route that they effectively produce a 'film' embedded within the none conscious operations of the body
b) do you think these conscious routes could be closely tracked by synesthetic connections so that particular memories associated with particular synesthetic 'landmarks' or neural connections stimulate or intensify the neural signals associated with 'multi-tasking' neural connectivity. So meaning is given to those neural events which tie together different sense perspectives of one activity.
Language being the locus of synesthetic connectivity between the senses. So a multiple of senses can find meaning in one symbol.
Or that correspondence between the neural locations(shared between sensory pathways) responsible for synesthetic correspondence i.e those neurons (in different sensory pathways) responsible for corepresenting the same object or action, collectively act as a 'switch' so to 'grab' an action or object and place it in either a conscious or sub or none conscious realm.
Reply | Report Abuse | Link to thisIn the conscious realm these signals resultant from the collective neuronal output are subsequently amplified and dissociated in such a way that a linguistic representaion is evoked. These pathways or associations being available for linguistic report.
Whilst the unconscious pathways are not so although synesthetic connectivity still allows control of the bady and its actions.
I argued that such human consciousness first arose with the use of portable mirrors or highly reflective surfaces whereby neuron systems were tuned accurrately so that these processes were first accurately coordinated through sensory feedback.
(Peter Reynolds - Didd Mirrors Cause Consciousness Tucson 2010).
Thinking on the hoof here - is it possible that deafness would have not been too big a disadvantage in the absence of language and might even have been an advantage in producing or enhancing a gesturally based language - so might have been tolerated earlier than blindness.
Reply | Report Abuse | Link to thisWould certain neural pathways have become unmasked in these individuals and when blindness came along as an 'allowable' disability, the interbreeding between the deaf and blind would cause synesthesia in the unmasked neurons associated with the enhance abilities seen in the deaf and blind.
Would this correspond to the 'new' or 'conscious' visual pathway described by Vilayanur Ramachandran and others or provide as specific type of feed into an older pathway so to distinguish it from what is now regarded as the older pathway.
These 'newer' neural pathways being associated with langauge in particular because it was the deaf in whom language was most clearly defined from a visual perspective.
Likewise this newer pathway would become more enhanced in offspring of the deaf and blind.
These pathways becoming unmasked in the general population according to the templates of cave art, whose narrative would instantiate and carve out routes that would lead to consciousness.
Being finally fully embedded with the use of mirrors and the constructive feedback from mirror use.
peter reynolds
reflectogenesis@hotmail.co.uk
Sorry the above was out of context.
Reply | Report Abuse | Link to thisI am postulating that consciousness came about from the interbreeding of the deaf and blind 50000 years ago which I am suggesting is the first time society had gained control of its evironment for the first time enough to support - firstly deaf (as this would not have been too great a disability given that language might not have existed) - and then the blind.
Bavalier et al suggesting that the remaining sense modalities in the deaf and blind are enhanced as compared to normally sensed humans)
Peter Reynolds
Reflectogenesis@hotmail.co.uk