Tuesday, February 15, 2011

Vision Related Neural Changes as We Age

In the previous post, we reviewed the anatomical changes in the eyes, associated with human aging. However, the aging effects on the vision are not limited to the anatomy. All the signals, received from the receptors, are directed to the brain for the data elaboration and processing. Therefore, we have to review also the neural changes, associated with vision, to cover thoroughly this topic.

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The term "neural" refers to components of the nervous system, which includes the brain, spinal cord, and nerves. The nervous system is made up of specialized cells called neurons. Neurons are unique because they are the only cells in the body that communicate through chemical and electrical signals. Perception relies heavily on the nervous system because neurons pass the information to the brain, which then processes the information.

The retina is an extension of the brain at the back of the eye and is made up of several layers of neurons. The back layer of the retina is made up of photoreceptors, special receptor cells that turn light energy into a neural signal. The loss of retinal cells with age causes the retina to thin, especially in the periphery. The cells that survive often exhibit irregular orientations. This change in orientation means that light entering the eye no longer hits the photoreceptors at the same angle as in younger, healthier eyes. This contributes to increased glare in elderly eyes.

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Other parts of the brain experience cell loss with age as well. Many different neural pathways make up the human brain's visual system. Each of these pathways sends sensory information to specific part of the brain responsible for a specific aspect of vision. All of these pathways are likely to lose neurons due to age. Because neurons in the brain do not regenerate, this cell death results in diminished abilities to perceive different aspects of visual stimuli. Another result of neural cell death is a general slowing of response time, meaning that the visual systems of older persons respond to light much more slowly than younger persons.

Age-related declines in the pursuit eye movement system have also been noted.  The young people can accurately track moving targets at velocities up to 30 degrees/sec, whereas the fixational accuracy of pursuit eye movements began to breakdown for older people when target velocity exceeded 10 degrees/sec.

Along with changes in the retina, changes in the brain affect the way the aging eye responds to light stimuli. Studies using electroencephalograms have provided evidence that reaction times to stimuli increase with age. The results showed that fewer neurons respond to a given stimulus in older eyes than in younger eyes and that older neurons do not respond as quickly as younger neurons. This change has many effects on the elderly person's ability to perform daily tasks.

The brain also undergoes chemical changes that can affect the older visual system's overall functioning. Neurons throughout the body communicate through the use of chemicals called neurotransmitters. When the body produces too many or too few neurotransmitters, neurons do not communicate properly. As we age, the brain begins to produce neurotransmitters in abnormal proportions. This affects the visual system by changing the abilities of neurons in the visual pathways to communicate.

Dark adaptation represents a neuro-chemical process helping to the visual system increases its gain (i.e., absolute sensitivity) in order to resolve the very small differences in luminance helping to adapt to the nighttime environment.  Dark adaptation is slow, and it slowly diminishes with aging.

The loss of short-term memory is another neural change that affects vision in older persons. This change can impact vision in the elderly when they must divide their attention among multiple stimuli. Short-term memory loss can also impair vision when the elderly must organize incoming stimuli. These changes and impairments make several implications for the senior citizens' ability to drive as well as other tasks.

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While many neural and anatomical changes are independent of each other, some changes are a combination of both types of change. As a result of both neural and anatomical changes, the older eye has limited motility as compared to younger, healthier eyes. Motility refers to the ability of the eye to move properly. Older individuals tend to have less control over their voluntary eye movements than younger people. Also, the older eye has a more limited range of movement. Aging eyes have decreased ability to perform smooth pursuit eye movements, which are used when the eye tries to visually track a moving object. During saccadic eye movements, which are used to scan stationary objects, the eye jumps from one part of the scene to the next. The older eye performs the visual jumps used in saccadic eye movements more slowly than the younger eye. These changes in the ability to control eye movements can affect a number of tasks, such as reading.

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