sections include: visual agnosias, auditory and tactile agnosias, diagnosis and treatment
Agnosia is a relatively rare disorder involving recognition. An agnosic person can see and hear normally, and seems to think and speak adequately, but cannot recognize someone or something he or she once knew. Careful testing cannot attribute that inability to any problems with sensing the world, general thinking, language, or attention. As one scientist has described agnosia, a normal part of the person’s life “has somehow been stripped of its meaning.” Agnosia is most commonly seen in patients with dementia or stroke.
For the vast majority of people with agnosia, the problem affects only one information pathway in the brain. For example, an individual may be shown a picture of a harmonica and be unable to name it or demonstrate how to use it. But that same individual can recognize a harmonica immediately on the basis of touch, sound, or hearing a description of its appearance or function. Another person may be unable to recognize a fork by means of touch, but can name the object when he or she sees it.
Based on these differences in how we take in information, we identify three major types of agnosia:
- visual (seeing)
- auditory (hearing)
- tactile (touching)
Perhaps because the human brain devotes more processing resources to vision than to any other sense, visual agnosia is the most common type.
We classify visual agnosias according to the level of processing at which the problems seem to arise. H. Lissauer came up with this system more than a century ago when he distinguished between apperceptive and associative agnosias. He theorized that recognizing something is a two-stage process: first our minds assemble all the incoming information into an image, and then we link that image to what we know from the past to understand its meaning.
People with apperceptive agnosia, Lissauer suggested, are impaired at the first stage. They have such obvious difficulties with visual perception that some are assumed to be blind—but they still manage to avoid bumping into things. Individuals with apperceptive agnosia typically have a lesion in the occipital lobe or the posterior temporal lobes on both sides of the brain. This condition often affects people who are recovering from cortical blindness, which has caused them to lose their sight not because of damage to their eyes or optic nerves but because of damage to the brain regions that process visual information.
Associative agnosia, in contrast, is a disorder characterized by relatively well preserved perception but an inability to access the meaning of what one perceives. Distinguishing between apperceptive and associative agnosias is classically based on a person’s ability to copy a figure or draw an object. Apperceptive agnosics typically cannot perform that task at all. In contrast, associative agnosics may be capable of producing an elaborate and accurate copy of a picture—but they cannot identify the object they have drawn. Associative agnosics do not produce their drawings normally: they typically draw (or write) in a laborious and piecemeal fashion, as if they were seeing only one small component of the object at a time.
We can also classify visual agnosics on the basis of the specific things they do not recognize. In prosopagnosia, for example, the problem is relatively specific: people cannot identify faces. These agnosics may recognize cars, words, cups, and other objects quite normally yet cannot recognize their family members or even themselves. People with this disorder have even been seen to speak to their own reflections in mirrors. Prosopagnosics often identify others by means of their clothes or voices, showing that they have not lost their knowledge of the person in question. What is missing is the connection between the face and the person they know. This disorder is associated with a lesion involving the posterior, inferior temporal lobe (fusiform gyrus) in the right hemisphere.
Other visual agnosics may be particularly impaired in the recognition of words (agnosic alexia), objects, or colors. Some visual agnosics may be impaired in recognizing all of these things. Explanations for why people can identify one type of thing and not another are controversial. One proposal is that our brains process different stimuli in different ways. We process faces holistically, as one object rather than a collection of features. Words, in contrast, are made up of a finite set of distinct components (letters), each of which must be identified correctly; thus, our brains must break down a word rather than processing it as a unit. Because the processing of these sights is so different, a lesion in a part of the brain that affects one ability may have no effect on another.
Another form of visual agnosia is called simultanagnosia. People with this disorder can recognize objects when seeing them alone but cannot process those same objects when they appear together or in the context of a scene. Studies have shown that some simultanagnosics have “implicit” knowledge that several things are present but are unable to identify them on a conscious level. This may imply that there is not simply one but multiple pathways for recognition.
Auditory and Tactile Agnosias
People with auditory agnosia still have the ability to detect and make simple judgments about sounds, but they cannot identify the sound sources. On hearing an airplane, for instance, people with this disorder may describe the noise as loud and low-pitched but be unable to name its source. In some instances the agnosia may be restricted to speech, a condition termed “pure word deafness.” In this condition, people can identify sounds such as a car horn but can’t understand spoken language. However, they can read and write. Auditory agnosics typically have bilateral lesions in the superior temporal lobes. Pure word deafness may be associated with bilateral lesions or with a lesion of the left temporal lobe involving the auditory cortex, which prevents information from the right hemisphere from reaching the language cortex in the left hemisphere.
Tactile agnosia is characterized by an inability to identify an object by touch despite being able to manipulate and feel the object. Often individuals with this disorder are able to identify objects by vision, so they can get by in most circumstances. That may be one reason tactile agnosia is so rarely identified.
Diagnosis and Treatment
Although agnosics are rare, they are nonetheless probably underdiagnosed. Many people with agnosia are initially thought to be “confused.” They or their loved ones may assume the problem is a form of dementia: Alzheimer’s disease or a lesser-known condition. Or the problem may appear to be aphasia—a disorder of language rather than recognition.
To diagnose agnosia, physicians must first exclude other potential causes of an individual’s problems recognizing people or things. This requires careful testing of the person’s mental status and general cognitive abilities so as to exclude dementia or aphasia. Additionally, the doctor must carefully evaluate the person’s ability to perceive visual or other stimuli. In the case of visual agnosias, this process should include assessment of the person’s visual fields, visual acuity, color perception, reading, facial recognition, drawing, and recognition of line drawings and real objects. A person with poor vision who cannot construct an adequate mental picture of an object does not have an agnosia. Similarly, someone who has forgotten the function and other properties of everyday objects is most likely to be suffering from a progressive degenerative disorder of the brain, such as Alzheimer’s disease.
Agnosia is a symptom of brain disorder rather than a disease in itself. The damage to the brain that produces the problem may be vascular, meaning a stroke; toxic; degenerative; or otherwise. Treating agnosias should start with treating the underlying disorder. Often the damage is reversible, but in some patients it is not. Many patients with agnosia benefit from physical and occupational therapy to manage the practical difficulties of daily life.
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