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A Revision of Vision

emily's picture

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      Most people presume that what one sees, with the exception of pathological cases and drug induced visions, is closely related to what is really out there, that our vision depends directly on what our eyes tell us. But this assumption does not take into account the long chain of events within the nervous system in which information from our eyes is converted to what we perceive. Vision is a direct function of our brains, which are only aided by our eyes; without our brains to process information sent from the eyes, we would have no way of understanding what our eyes are trying to show us, and therefore we could not see.


      Indeed, vision starts in the eyes: the lens focuses light from the outside world, which hits the photoreceptors in our retinas, which in turn send a message to the visual cortex within our brain regarding the type of light received. This message is a "hint" to the brain about what the outside world looks like, a sort of "reflection" of what is really there. The neurons within the brain respond differently, depending on the information they receive from the retinal optic nerves. Within the visual cortex, there are different modules that process visual aspects like form, movement, and color separate yet parallel and which "somehow pool their results to give a single, generally consistent perception" (1). Scientists have even found that artificially stimulating (i.e. with an electrode) regions of the visual cortex yields sensations such as flashes of light and visual memories like seeing a face or hands (1).


      Not only are different sections of the visual cortex responsible for different aspects of vision, but also single cells within the visual cortex have specific functions and will only fire when they stimulated in a specific way. Physiologists David Hubel and Torstin Wiesel did an experiment recording the activity from single cells within a cat's visual cortex when presenting the eye with simple shapes. Hubel and Wiesel discovered that some cells within the visual cortex of the first stage of processing were only activated when a bar was presented in a certain orientation and with specific aspects like movement and direction. In other conditions, those cells will not react (1). These findings importantly show that "specific mechanisms in the brain select certain features of objects" (1). When our retina detects light from, or, information regarding a vertical edge, like a book on a shelf, only specific neurons within our visual cortex will respond to this information. The firing of those specific neurons is what tells us we see a vertical line, not the actual light information received by the retina. In this way, our actual perceptions depend directly on what cells fire within our visual cortex.


      Day-to-day vision is linked to the outside world because the firing of cells within our visual cortex relies on the stimulation of photoreceptors. But, the brain can stimulate itself activity without being directly signaled by sensory receptors like our eyes. Cases like this are called hallucinations. Hallucinations can be provoked by drugs like LSD or by other chemical reactions within the brain. Charles Bonnet syndrome (CBS) provides an example of visual hallucinations. CBS is a condition in which people with serious sight loss start to see things they believe are not real, or, start to hallucinate. People with CBS have cited seeing a range of forms from simple patterns to people. Because these people are not receiving sensory stimulation, the brain is more apt to stimulate itself and these firings are experienced as "normal" vision. For the most part, CBS patients experience these hallucinations when they participate in activities with little stimulation, such as lying in bed or sitting somewhere quiet (2).


        The cause of CBS illuminates an interesting point: in the absence of sensory input, our brains are able to stimulate themselves and provide a visual experience without any link to the outside world. When we are receiving sensory input from our eyes, our brains do not have the chance to stimulate themselves because they are already responding to stimulation. Outputs from the brain independent from sensory input are also the case in schizophrenia, in which the affected individual makes little contact with the outside world (3). This isolation provides the brain with the opportunity to stimulate itself since it is receiving less sensory input, and thus schizophrenic patients often experience hallucinations.


      Drug-induced hallucinations work in a similar manner and are generally caused by changes in the normal volume of neurotransmitters or the blockage of signals from different parts of the brain. In the case of LSD, an LSD molecule greatly resembles a neurotransmitter molecule called serotonin. LSD blocks serotonin receptors on the neurons and thus reduces the amount of serotonin receptors available for serotonin binding (4). Because of serotonin's decreased ability to bind to its receptors, less "messages" can be passed on between neurons. And, since neuronal messages cannot be transmitted as frequently, information received from the outside world that normally requires serotonin in order to be passed along cannot be transmitted. This isolates the brain from the outside world and thus provides the brain with the opportunity to generate its own visual outputs, or, hallucinations. Other hallucinatory drugs isolate the participant from the outside world even if they do not affect serotonin binding.


      Hallucinations call us to question the definition of reality. If our brains are responsible for what we see in day-to-day life but also during hallucinations, then what is real? Can we trust that our brain is conveying an accurate portrayal of what exists from the outside world? Hallucinations often provide illusions, or, departures from the world we accept as real; but, there are also things we actively see and process, like in many optical illusions, that we do not believe are hallucinations, rather only our brains being "tricked". The line between seeing and hallucinating is vague because what we receive as sensory input does not always match our expectations of reality. From this we can conclude that what we see has as much to do with what is going on within the nervous system as what is going on outside.




1) Gregory, Richard L. Eye and Brain: The Psychology of Seeing. Princeton, NJ: Princeton University Press, 1997.


2) "Charles Bonnet Syndrome." RNIB: Supporting Blind and Partially Sighted People. 10 December 2009. Cited 14 February 2010.


3) Fischman, Lawrence G. "Dreams, Hallucinogenic Drug States, and Schizophrenia: A Psychological and Biological Comparison". Schizophrenia Bulliten 9.1 (1983): 73-94


4) Duckworth, Ken. "Schizophrenia." National Alliance on Mental Illness. February 2007. Cited 14 February 2010.



Paul Grobstein's picture


"From this we can conclude that what we see has as much to do with what is going on within the nervous system as what is going on outside."
Maybe even more to do?  From which we can conclude that ... there is less difference between "hallucinations" and "illusions" and "reality" than we might think?  And therefore?  For some related thoughts, see Psilocybin, hallucinations, and the spiritual enlightenment.