The Brain and Blindness
Vision Requires More than Just the Eyes to "See"
How Vision Works
The function of sight is much like a camera that takes photographs (Breedlove, & Watson, 2013). The cornea, which is the transparent outer layer of the eye, will bend light rays and is responsible for forming the image on the retina (Breedlove, & Watson 2013).
The retina is the receptive surface inside of the eye that contains photoreceptors, which are neural cells that respond to light, and other neurons (Breedlove, & Watson, 2013). The lens also helps to focus the image on the retina (Breedlove, & Watson, 2013).
The retina is actually located behind the lens which is held in place by a large chamber filled with vitreous humor, which is a gel type fluid (Ireland, 2012). The retina spreads out like an umbrella or cloth over the inside rear portion of the eye (Ireland, 2012). The retina is not physically attached to anything but at the very center of it, and this is where a blind spot called the optic disk is found (Ireland, 2012). This optic disk allows optical nerves to pass through it and head toward the brain (Ireland, 2012). This focus is adjusted according to the shape of the lens, which is controlled by the ciliary muscles found inside of the eye (Breedlove, & Watson, 2013).
Accommodation happens as the ciliary muscle contracts or expands according
to the distance of the object so that the image appears clearer on the retina (Breedlove,
& Watson, 2013). The pupil is an opening formed by the iris that lets light enter into the eye (Breedlove, & Watson, 2013).
The iris is the small circular structure that provides the opening that forms the pupil (Breedlove, & Watson, 2013). The size of the pupil determines the amount of light that is allowed into the eye (Breedlove, & Watson, 2013).
Eyeball movement is controlled by the extraocular muscles, which are three pairs of muscles that go from the outside of the eyeball itself all the way to the bony socket of the eye, which is also called the orbit (Breedlove, & Watson, 2013).
The very first stages in vision information processing start in the retina. The
photoreceptors in the retina detect light and can be either in rod form or cone form
(Breedlove, & Watson, 2013). Both the rod and cone have the ability to release
neurotransmitter molecules which control the bipolar cells (Breedlove, & Watson, 2013).
The bipolar cells are a type of interneuron in the retina that gets information from rods and cones and sends it on to the retinal ganglion cells (Breedlove, & Watson, 2013). The retinal ganglion cells are a type of cell in the retina that has axons which form the optic nerve (Breedlove, & Watson, 2013).
The optic nerve is also known as the cranial nerve II and is a collection of ganglion cell axons which extend from the retina to the optic chiasm (Breedlove, & Watson, 2013). The optic nerve will carry information to the brain (Breedlove, & Watson, 2013).
Horizontal cells also help with vision. These cells are specialized retinal cells that make contact with both the receptor cells and the bipolar cells (Breedlove, & Watson, 2013). When the light is dim the rods are used, and this is known as the scotopic system, and this system does not respond any different to different colors (Breedlove, & Watson, 2013).
The photopic system uses the cones and does enable color vision (Breedlove, & Watson, 2013). The central region of the retina, known as the fovea, is jam packed with the
most photoreceptors which provides high visual acuity (Breedlove, & Watson, 2013).
The ganglion cells, which are in front of the retina, produce action potentials which
Breelove and Watson (2013) state “are conducted along their axons to send visual
information to the brain" (p. 299).
The axons are what create the optic nerve that brings the visual information into the brain on each side until it eventually reaches the occipital lobe, which is found at the rear of the brain (Breedlove, & Watson, 2013). The axons close to the nose will have the optic nerve crossing to the opposite side of the brain at the midline, which is known as the optic chiasm (Breedlove, & Watson, 2013). The retina close to the side of the head will have the axon projecting toward that side of the brain (Breedlove, & Watson, 2013).
Beyond the optic chiasm the axons of the retinal ganglion cells become the optic tract (Breedlove, & Watson, 2013). Most of the axons that make up the optic tract will end on cells in the lateral geniculate nucleus (Breedlove, & Watson. 2013).
The lateral geniculate nucleus, or LGN, is the visual part of the thalamus (Breedlove, & Watson, 2013). The LGN is part of the thalamus that will receive information from the optic tract and then send it out to the visual areas in the occipital cortex (Breedlove, & Watson, 2013). Axons of postsynaptic cells in the LGN will form the optic radiations (Breedlove, & Watson, 2013). These optic radiations end in the primary visual cortex of the occipital lobe (Breedlove, & Watson, 2013).
(Photoreceptors in the Brain, 2012)
Individuals who are blind may find that other areas of the brain will jump in to help
accommodate for the loss in vision and this could be due to the brains neuroplasticity
(Proulx, 2013). Studies have shown that the occipital lobe of blind individuals is actually
active when auditory, haptic, and olfactory stimuli occur (Proulx, 2013). Other studies have
found that certain brain regions of blind individuals are enlarged (Leporé, et. al., 2010).
One of the areas that have shown the enlargement is the tonotopic region of the
auditory cortex, which was found to be almost twice the size of the one found in a brain
of a sighted person (Leporé, et. al., 2010). In individuals who became blind later in life
and those born blind studies show that the hippocampal volumes are abnormally
enlarged (Leporé, et. al., 2010). An increase in the white matter connectivity that occurs
between primary somatosensory and visual areas of the brain was also found in
individuals who are blind (Leporé, et. al., 2010). The enlargement of the frontal lobes
in groups of individuals who are blind could lend an explanation as to why some of
these individuals have enhanced skills (University of California, 2009).
Some Developmental Problems
development, specifically with language skills. The problems they have with
communication revolve around the lack of access to the environment and lack of verbal
cues from individuals around them (Gunaratne, 2002). Behavior may be a problem as well
as they do not have the ability to model performed behavior of others (Gunaratne, 2002).
This may lead a child that is born with congenital blindness to have reduced expressive
behavior (Gunaratne, 2002).
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Gunaratne, L. (2002). Visual Impairment: It’s effect on cognitive development and behaviour. Retrieved from http://www.intellectualdisability.info/physical-health/visual-impairment-its-effect-on-cognitive-development-and-behaviour
Ireland, K. (2012). Visualizing human biology (4th ed.). Hoboken, NJ: Wiley.
Johns Hopkins University. (2015). How the brain responds to stories. [Image]. Retrieved from http://www.itnonline.com/content/johns-hopkins-study-reveals-superior-adaptability-brains-vision-center
Leporé, N., Voss, P., Lepore, F., Chou, Y., Fortin, M., Gougoux, F., Lee, A., Brun, C., Lassonde, M., Madsen, S., Toga, A., Thompson, P. (2010). Brain structure changes visualized in early- and late-onset blind subjects. NeuroImage, 49(1), 134-140. doi: 10.1016/j.neuroimage.2009.07.048. Retrieved from http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2764825/
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Proulx, M. (2013, Feb.). Blindness: remapping the brain and the restoration of vision. Retrieved from http://www.apa.org/science/about/psa/2013/02/blindness.aspx
Sinha, P. (2010). Brains learn to see. [Video File]. Retrieved from https://www.youtube.com/watch?v=xeFl0RE31x0
Stanford. (2009). Understanding blindness and the brain. [Video File]. Retrieved from https://www.youtube.com/watch?v=VVgfC_FV2hI
University of California - Los Angeles. (2009, November 19). Blindness causes structural brain changes, implying brain can re-organize itself to adapt. ScienceDaily. Retrieved from www.sciencedaily.com/releases/2009/11/091118143259.htm