Agnosia

Agnosia is a rare neurological disorder characterized by the inability to recognize familiar faces, objects, or sounds by using one or more senses. In individuals with agnosia, their senses are intact, however, their brain cannot process sensory information due to damage to the sensory cortex. Agnosias are categorized into visual, tactile, and auditory based on the sensory modality affected. 

In most cases, only one sensory modality is affected. For example, an individual may be unable to identify an object by touching it, but they may be able to describe it verbally or recognize the same object from pictures or drawings.

Agnosia is caused by damage to the brain regions responsible for processing and integrating sensory information from the body and its surroundings. Brain damage can occur after a stroke, traumatic brain injury (TBI), brain tumors, infections affecting the central nervous system (e.g., herpes simplex encephalitis), and neurodegenerative disorders (e.g., Alzheimer disease or frontotemporal dementia). Hypoxia, or low oxygen blood levels, and toxins, like carbon monoxide poisoning, can also result in brain damage and agnosia. 

The main sensory lobes of the brain are the parietal, temporal, and occipital lobes; each responsible for processing and interpreting a sensory input. The parietal lobe is dedicated to processing sensory information from the opposite side of the body, enabling individuals to interpret sensations like touch, temperature, or pain. Tactile agnosias usually result from damage to the somatosensory cortex and association areas in the parietal lobe.

On the other hand, the temporal lobe houses the primary auditory cortex, which is needed for speech comprehension. Damage to this area can result in auditory agnosia. Additionally, the ventral part of the temporal lobe helps process visual information, such as the recognition of objects and faces; whereas the medial temporal lobe, along with the hippocampus, is responsible for the formation of new memories. 

Finally, the occipital lobe is where visual information is processed in terms of form, color, and motion allowing the brain to put together a recognizable image. Visual agnosia is the most common form of agnosia and usually occurs from damage to the occipital lobe and the dorsal or ventral streams.

Signs and symptoms of agnosia depend on the areas of the brain affected. Because most areas of the brain are highly interconnected with each other, agnosia almost always accompanies other neurological deficits. 

Individuals with visual agnosia can have difficulty recognizing familiar faces, a condition called prosopagnosia. As a result, they may be unable to distinguish family members or friends from strangers based on their faces despite normal visual function. Instead, they may rely on additional information to recognize them, like their voice, hairstyle, clothing, or other physical features. Some individuals with prosopagnosia also find it challenging to identify people’s emotions through their facial expressions. 

Another form of visual agnosia is simultagnosia, which is the inability to perceive more than one object at a time. Individuals with simultagnosia are unable to perceive the overall meaning of a picture or multiple things together, however, they can identify the same objects when set apart from the rest. Often, individuals with simultagnosia have difficulty reading, as it involves viewing more than one word at a time. They may also have difficulty finding a particular object when there is a lot of clutter, or bump into furniture or other elements in a room.

Next, auditory agnosia is characterized by the inability to recognize auditory information, including sounds, music, or speech, despite having intact hearing. An example of auditory agnosia is pure word deafness, where individuals cannot understand spoken words despite being able to read, write, and speak normally. Another unique form of auditory agnosia is amusia, defined as the inability to interpret music and distinguish it from other sounds.

Diagnosis of agnosia begins by taking a thorough clinical history of the individual’s symptoms in order to identify the specific form of agnosia. This is accompanied by a complete neuropsychological evaluation that also includes an assessment of memory, attention, cognition, and language. With visual and auditory agnosias, vision and hearing are also assessed to determine if there are any sensory deficits with perception rather than recognition. Laboratory tests and imaging techniques, like a brain magnetic resonance imaging (MRI) or computed tomography (CT) scan, don’t play an important role in diagnosing agnosia, but they may be helpful in identifying the underlying cause. 

Currently, there is no specific treatment to cure agnosia. Management focuses on treating the underlying cause, when possible, such as reestablishing blood flow as quickly as possible in cases of stroke; surgery or radiation for brain tumors; and antibiotics or antivirals to treat infections of the central nervous system. 

Long-term management of agnosia focuses on finding compensatory strategies for sensory deficits. For example, individuals with visual agnosia can learn to identify objects by touch rather than seeing them; place common objects in the same place every day; and reduce the clutter in a room. On the other hand, individuals with auditory agnosia may learn to understand others through lip reading rather than listening. Occupational and speech therapists can help find each individual the coping strategies that fit best their specific needs.