User:Tysongersh/Visual Plasticity

Visual Plasticity refers to the brains capacity to alter the structure and function of the visual system. The ideology behind Visual Plasticity is that the visual systems of the brain are malleable and able to “re-wire” themselves.

Visual Plasticity is derived from neuroplasticity, also known as experience dependant plasticity,. Neuroplasticity is the idea that the brain makes adjustments in the structure and function of neural pathways in response to experiences. The idea of a “plastic” and “malleable” brain extends to sensory and perceptual experience. The plasticity of these neurons also shapes the sensory and perceptual systems, which includes the visual system.  Visual Plasticity in early development When an organism is born, a significant part of the nervous system has already been “hard-wired” into the brain. Throughout the lifespan, the ability to rearrange these synaptic connections is present through what is considered plasticity. This plasticity allows an organism to learn and form new neuronal pathways via experience and interaction. However, this plasticity, or flexibility, is not always accessible and may present a time frame for which rearrangement can occur. This time frame most often occurs in early development, known as the critical period.  Post-early development experience and the visual system Following the early developmental Critical periods of development the visual system becomes much less susecptable to change. After the critical period, it is often difficult for the brain to re-wire itself with exposed to new environmental stimulus. in a study conducted by Blakemoor and Cooper, kittens were raised in an environment that only contained horizontal, but not vertical lines. when they were eventually exposed to vertical lines, post the critical period, they did not react to them. their brains not perceive anything vertical and they were unable to learn to see them. With this study in mind, the importance of a full range of visual stimulation during the critical period is essential to develop fully functional visual systems.

While some aspects of the visual system seem to have a limited period of plasticity, others remain plastic throughout the lifetime of the individual.

 Methodology

FMRI Scans FMRI (Functional Magnetic Resonance Imaging) is a vital tool in the research and conception of Visual Plasticity. FMRI is a specialized MRI scan that assigns the hemodynamic response caused by neuronal activity to a visual output, thus allowing researchers to assess the stimulation on the brain. FMRI scanning is necessary for research in Visual Plasticity as it allows real-time assessment of environmental factors and identification of cross modal pathways established through visual plasticity.

Cortical Mapping A cortical map is comprised of a collection of the cortical layers of the brain that undergo a specific processing function (usually a firing response from one of the sensory systems). Cortical mapping provides a somatotopic representation of sensory inputs, therefore “mapping” the brain. With regards to visual sensory system, cortical mapping allows for the identification of neuronal pathways that are activated given various stimuli and the plasticity (rearrangement) that ensues. Cortical mapping enabled the detection of cross modal plasticity; currently an important area of interest within the spectrum of neuroplasticity. Environmental stimulus to the brain observed by cortical mapping is vital in the research of visual plasticity because it generates “maps” that enable the identification of common pathways within the human brain as well as the instant formation of neuronal pathways by specific subjects.

History in 1978 M Constantine-Paton and MI Law conducted experiments attaching a third eye’s to embryonic frogs. typically, the right eye innervates the left optic tectum and the left eye innervates the right optic tectum. This structure is the frog version of the superior colliculous in humans. The presence of a third eye results in competition between the axons of the third eye and the axons of another eye for connection with the optic tectum. as a result, the competing axons connect with alternating bands on the tectum. This same process can be seen in the development of the human visual system. early in development, teh cells of the LGN and Primary Visual Cortex (V1) receive input from both eyes. during this time, input from the two eyes is not segregated into the layers of the LGN, however, as development continues each layer is specified to a single eye, and highly specialied neural pathways are formed. David Hubel and Tortsen Wiesel conducted a similar study where they sutured the eyes of kittens shut, thus allowing them to observe the effects from cells in the visual cortex.

In the experiment conducted by Hubel and Wiesel (1965,1977) the two scientists demonstrated how manipulating the sensory environment could influence the segregation of inputs into ocular dominance columns in cats and rhesus monkey. by suturing one eye of the newborn kittens and one eye of the monkey closed, Hubel and Wiesel could record the synapses from cells present in the visual cortex. Their work received the Nobel Prize 1981 for discovering ocular dominance columns. By depriving kittens from using one eye, Hubel and Wiesel showed that columns in the primary visual cortex receiving inputs from the non-sutured eye took over the areas that would normally receive input from the deprived eye. These kittens did not develop the areas that would normally receive input from both eyes; a feature needed for binocular vision. Hubel and Wiesel saw that the portion of the kitten’s brain associated with the shut eye was not idle, as expected. Instead, it processed visual information from the open eye. It was“… as though the brain didn’t want to waste any ‘cortical real estate’ and had found a way to rewire itself.” Hubel and Wiesel's experiments showed that the ocular dominance develops irreversibly early in childhood development. If portions of the brain are not receiving signals from an eye, then the brains “available space” will be utilized by other neural pathways. This occurred in the study conducted by Hubel and Wiesel. The experiment concluded that visual plasticity was only present during the critical period of the kittens; therefore, the somatotopic arrangement of the brain changed to allow for new pathway formation (i.e. plasticity during the critical period). The results of Hubel and Wiesel allowed for further understanding in the areas of childhood cataracts and strabismus. These findings also contributed to present day cortical mapping.

Modern Research

perceptual learning is the plasticity based concept that sensation and perception and plastic are plastic processes. perceptual learning occurs when we learn to perceive with more acuteness or in a new way and in the process develop new brain maps and structures

Visual plasticity and the blind. an interesting experiment conducted by Pascual Leonne involves the plasticity of the visual system. Leone had heard about a school for the blind in Spain that required its instructors to be immersed into total darkness, so that they may experience blindness first hand prior to teaching. They reported that the instructors tactile senses and ability to judge space became remarkable sensitive within just a week. they claimed the instructors could differentiate between the makers of motorcycles just by the sounds they made and distinguish the shapes of objects in their paths by their echos. once the blindfolds were removed the newly acquired abilities seemed to disappear as quickly as it came.

These reports inspired Pascual to conduct an experiment were he could make sighted people absolutely blind and track their neurological response with brain scans. in as little as two day, the subjects began to process tactile and auditory sensations with their visual cortex, similar to blind people Reading braille. the subjects even reported experiencing visual hallucinations when they moved, touched things, or were touched. when the blind folds were removed, it only took 12-24 hours for the visual cortex to stop processing any tactile or auditory stimulus. this experiment exemplifies perceptual learning.

Implications and Clinical applications The implications of research in visual plasticity are being incorporated into a variety of specialties present in modern medicine. Understanding the mechanisms behind visual plasticity have proven highly successful on a clinical level. . Optometric visual therapy uses the ideology behind neuroplasticity to develop functional programs which assist those with a spectrum of blindness. Utilizing the idea of neuroplasticity, scientists have begun designing experiments that will test visual development, visual perception and overall visual functionality.

Using brain scanning and cortical mapping, researchers have identified alternative ways the brain can wire itself to produce normal or functional vision in the presence of cognitive deficits. this is illustrated first hand with a recent discovery of a person born with half their brain. In most humans, both hemispheres of the brain are required to perceive left and right visual fields; however, the patient was able to see all fields through a single eye. The visual plasticity was apparent when scientists conducted an FMRI scan that revealed the left hemisphere’s visual cortex was processing for both right and left sides. The brain wired itself to process the missing right hemisphere’s visual field in the left hemisphere, where “portions” of the brain were designated to interpreting these signals. The implications of research done in the field of visual plasticity have proven immeasurably useful in the diagnoses of patients and the development of optometric visual therapy.