Overview of agnosias

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Agnosia refers to the inability to recognize people or objects even when basic sensory modalities, such as vision, are intact. As an example, patients with visual agnosia may not recognize an object upon visual presentation even though they can describe its shape and other visual characteristics or the person cannot recognize their wife's face and in some cases even their own. It is therefore apparent that sensory perception of the object / face is disconnected from memories associated with the object / face. The patient can perceive the object but has no meaningful associations to it. In agnosia, while perception itself, feeling an objects shape, normal, recognition of objects is not.

There are basically three different forms of agnosia; visual agnosia where the person has difficultly recognizing objects, faces and words. The second is Auditory Agnosia which involves the inability to recognize sounds and thirdly, Somatosensory Agnosia where the person has difficulty perceiving objects through tactile stimulation.

The characteristic and sub-types of the three forms of Agnosia are as follows:


Visual Agnosia

Visual agnosia is a neurological disorder distinguished by the inability to recognize familiar objects. There are a number of sub-types of visual agnosia. The most common are:

- Prosopagnosia - Inability to identify faces even though the person is known to the sufferer. (See below)

- Agnostic Alexia - Reading Material

- Color Agnosia - Inability to discriminate between colors and therefore cannot name the colour.

- Object Agnosia - Inability to name objects.

- Simultanagnosia - Inability to recognize a whole image although individual details are recognized.

Visual agnosia is associated with lesions of the left occipital and temporal lobes. Many patients have a severe visual field defect on the right side because of the injury to the left occipital lobe. It is common for patients to have visual agnosia but intact recognition in the tactile and auditory modalities.

In prosopagnosia bilateral lesions of the gray and white matter of the occipitotemporal gyrus cause prosopagnosia. In particular, the inferior longitudinal fasciculus, a pathway that interconnects the occipital and temporal lobes, has been suggested as the lesioned area in prosopagnosia.


Auditory Agnosia

There are three sub-types of auditory agnosia:

- Auditory/verbal information Agnosia - Inability to hear words.

- Auditory Agnosia - Inability to hear environmental sounds such as a car starting or a dog barking.

- Receptive Amusia - Inability to hear music.

Cortical deafness is also a term applied to patients who essentially do not respond to any auditory information even when hearing is intact.


Somatosensory Agnosia (Astereognosis - Tactile Agnosia)

Patients with this disorder have difficulty perceiving objects through tactile stimulation although basic tactile sensation is intact. There is a distinction between the inability to recognize basic features of an object, such as size, weight, and texture, and the inability to name or recognize the object. Patients who cannot recognize an object by touch may still be able to draw the object and recognize the object pictured in the drawing. Sometimes they can describe the physical features of the object but cannot recognize it.

Pure astereognosis is thought to be caused by lesions of the somatosensory cortex (post central gyrus) in the area subsumed by the hand.


Differential Diagnosis:

Some disorders have similar symptoms. The clinician, therefore, in his diagnostic attempt, has to differentiate against the following disorders which need to be ruled out to establish a precise diagnosis.

- Not a General Memory Disorder.
- Capgras Syndrome
- Mental Retardation
- Dementia
- Head Traima
- Acoustic (Loud Noise) in the case of Auditory Agnosia.


Cause:

Damage to the brain, arising from a head injury or a stroke for example, sometimes results in highly specific impairments of the person's cognitive processes.

- Brain Damage
- Stroke
- Dementia
- Neurological Disorders


Treatment:

Treatment is generally symptomatic and supportive. The primary cause of the disorder should be determined in order to treat other problems that may contribute to or result in agnosia.

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Prosopagnosia

Prosopagnosia is a rare disorder characterized the inability to recognize faces, including one's own in the mirror, inability to recognize previously familiar faces, including, friends and family. The sufferers knows that they are looking at a face but they cannot say who the face belongs to. However, recognition of familiar people can sometimes be preserved but it depends upon such cues as; gait, clothing, voice etc. The main characteristics of the disorder are:

- Selective inability to recognize faces, although other visual objects may be correctly identified.

- The inability to identify a familiar face of a family members and friends. There may also be the inability to recognize their own face in a mirror.

- Accurate judgments about gender, age and emotion are still intact and can recall detailed information about a specific individual if prompted.

- No indications of object agnosia.

Associated Features:

- The person may be able to imagine faces of familiar people.

- May be able to differentiate between faces and other objects.

- May not be able to match unfamiliar faces.


Differential Diagnosis:

Some disorders have similar symptoms. The clinician, therefore, in his diagnostic attempt, has to differentiate against the following disorders which need to be ruled out to establish a precise diagnosis. Agnosia of which Prosopagnosis may be a symptom is a disorder which the person looses the ability ability to recognize objects, people, sounds, shapes, or smells; that is, the inability to attach appropriate meaning to objective sense-data.

- Object Agnosia.
- Not a general memory disorder.
- Capgras Syndrome
- Mental Retardation
- Dementia


Cause:

It typically results from damage to specific brain areas in the occipital or parietal lobes of the brain.


Treatment:

Treatment is generally symptomatic and supportive. The primary cause of the disorder should be determined in order to treat other problems that may contribute to or result in Prosopagnosis.