Sections include: what we see, navigating through space, visual memory
Our visual system is the primary means by which we learn about the world, and we receive more information visually than by any other means. Most people would say that the organ of vision is the eye, but in fact the eye is only the beginning of the visual system.
The processing of visual information takes place in a network of areas throughout the brain, with different functions performed in different regions. Seeing color and motion; recognizing shapes, such as printed words, objects, and faces; finding our way around our environments; and imagining an appearance from memory are among the vital and complex functions our brains perform.
The ultimate goals of vision can be divided into two very general categories:
■ to recognize what we see
■ to navigate through space
These categories of visual function are carried out by largely separate systems, each one of which involves multiple stages of visual information processing.
One consequence of this networked arrangement is that disease or damage in the brain may affect our vision in different ways, depending on what parts of the brain are harmed. Strokes, injuries, and brain tumors can all damage particular regions while leaving others intact. Damage to certain parts of the temporal lobes can impair visual recognition, while damage to certain parts of the parietal lobes can impair visual-spatial functioning.
What We See
The first part of the cerebral cortex to which the eyes and optic nerves send visual information is called, appropriately enough, the primary visual cortex. It is situated at the very back of the brain in the occipital lobes and plays a crucial role in the early stages of processing signals for form, motion, and color vision. Damage here causes cortical blindness, as opposed to forms of blindness caused by damage to the eyes or optic nerves. Partial damage causes only partial blindness, as when problems in just one occipital lobe cause a person not to be able to process information, and thus see, one side of space.
From the primary visual cortex, the brain’s representation of the visual world is split into many specialized processing pathways. Damage to these pathways can affect vision in very specific and sometimes surprising ways. For example, our
brains appear to process color and motion in distinct areas. People sometimes lose the ability to see color while retaining all other aspects of vision. This is different from inborn color blindness, which results from defects in the retina and usually lets a person perceive at least some colors. People with cerebral color blindness describe the world as looking like a black-and-white movie. A person’s ability to see motion can also be disrupted, so that he or she sees only a rapid sequence of still images when looking at objects in motion.
Impairment of visual recognition is termed visual agnosia and takes different forms depending on the brain areas affected. One area of the temporal lobes is devoted to recognizing faces, presumably because of the crucial importance of face recognition to our functioning and survival. Damage here can result in a relatively isolated but still troubling loss: individuals suffering from what is called face agnosia report that while faces continue to look like faces, they also look too similar to tell apart. One man with this condition described the experience of coaching his son’s soccer team and not knowing which boy was his son when they were in uniform.
Visual agnosia can affect how people recognize everyday objects as well, causing tremendous difficulty. People can see the sizes and shapes and colors of the things around them, yet not know the identity of these things. Because the damage is usually confined to the visual areas of the temporal lobe, such people will often put their intact touch areas to work. They will pick up an object and feel it in order to identify it.
Printed words constitute a very special category of visual object for literate humans, and it seems that our brains develop specialized areas for the recognition processes of reading. Accordingly, it is possible to sustain damage to this area alone and lose the ability to read. This form of visual agnosia is usually called pure alexia because it is a relatively isolated loss of the ability to recognize words. At first, pure alexics may feel that they need new glasses or better light for reading. Unfortunately, the problem is not external and will not be solved by large print or good light. Instead, an internal bottleneck has developed in their visual processing, allowing them to read only slowly, often letter by letter or with many errors.
Navigating Through Space
Just as the temporal lobe recognition system comprises specialized subsystems, so does the parietal lobe spatial system. One of the most fundamental spatial functions of the brain is the allocation of spatial attention to objects and locations in the visual field. Thanks to this system, we can focus our visual attention on the book in front of us, yet still notice and look up if someone walks into the room.
When a lesion to one paretial lobe disrupts this allocation, the result is “hemispatial neglect.” In this syndrome, individuals fail to pay attention to one side of space. The magnitude and persistence of this attentional impairment may astonish friends and family. After a right parietal lesion, for example, a person may fail to dress the left side of his or her body, leave food on the left side of a plate and complain about the hospital’s stingy portions, and simply ignore anyone who approaches from the left. Suggesting that the person orient to the left has little effect. Clearly, hemispatial neglect puts a person at risk for injury from walking into unnoticed objects on the affected side.
Bilateral damage to the parietal lobes can cause an even more severe loss of spatial attention, known as simultanagnosia. People with this disorder have trouble shifting their attention in any direction, left or right, and as a result see only one object at a time. In many ways, this condition is as disabling as blindness.
The visual areas of the parietal lobe are also important for guiding human action in space. This is true for small-scale spaces, where actions may include reaching for an object with the appropriate arm movement without knocking over other objects. It is also true for large-scale spaces, where actions may include moving toward destinations that may be distant and even out of sight while avoiding collisions along the way. With these functions the brain once again employs a divide-and- conquer strategy. Thus, the tasks of reaching for objects at nearby locations and of moving to distant objects are processed in different parts of the brain. Indeed, the process of navigating can itself be broken down into different components with different brain bases.
Each navigational process can therefore be disrupted separately by damage to different locations. The spatial-perception problems of simultanagnosia will impair a person’s sense of orientation within the large-scale environment. A more specific form of environmental spatial impairment can follow damage to the posterior cingulate gyrus, located in the evolutionarily older limbic cortex of the brain. In this case, the individual cannot perceive and remember the spatial relations among landmarks in the environment, or his or her orientation relative to them.
A person’s ability to navigate through the environment can also be disrupted by damage to the temporal system for recognizing objects described earlier. Such a person cannot recognize landmarks. In some cases, people become agnosic primarily for landmarks such as buildings, monuments, squares, and so on. This suggests the existence of yet another specialized processing system.
The primary function of the cortical visual system is perception, but we also rely on it to represent visual and spatial information in the absence of external stimuli—in other words, to remember or imagine things. When we imagine the appearance of an object or the layout of a scene from memory, we are using some of the same mechanisms in the occipital, temporal, and parietal lobes that are used when we recognize and localize physically present and visible stimuli.
There are many reports of both imagery and perception being impaired after brain damage, which suggests that these functions are related. For example, people with acquired cortical color blindness frequently say that their mental images are devoid of color, and they have trouble answering questions that rely on creating mental images, such as, “What color are the stars on the American flag?” Individuals with agnosias of certain types, such as for faces, also report impaired imagery for that category.
Hemispatial neglect may also extend to mental images. Researchers first demonstrated this with people living in Milan, who were very familiar with the large central square known as the Piazza del Duomo. When asked to describe the piazza viewed from the steps of the cathedral, those with hemispatial neglect affecting the left side of perceived space mentioned the buildings on the right side of the vista they were imagining and omitted the ones on the left. To show that this was not simply due to one side of the piazza’s being more memorable than the other, the scientists then asked the individuals to imagine the square from the opposite end. The people then described the buildings they had previously
omitted, and neglected those they had mentioned before.
When a person’s perception is intact but his or her imagery abilities are impaired, the underlying problem must lie somewhere other than the visual representations that are shared between perception and imagery. In some cases, the disruption appears to lie in the “image generation” process, by which we use long-term visual memory knowledge to reconstruct a visual mental image. Researchers have long noted such impairments, usually after damage to the visual areas of the left hemisphere.
Current research on vision and the brain is using functional brain imaging to confirm and extend what has been learned by studying neurological patients. In the future we are likely to see these methods combined, allowing researchers to image the brains of patients at different stages of recovery from visual-spatial impairment. Such an approach will teach us much about the development of vision beyond childhood, and may suggest new avenues of treatment to facilitate recovery.
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