What is Agnosia?
Agnosia is a rare disorder characterized by an inability to recognize and identify objects or persons despite having knowledge of the characteristics of the objects or persons. People with agnosia may have difficulty recognizing the geometric features of an object or face or may be able to perceive the geometric features but not know what the object is used for or whether a face is familiar or not. Agnosia can be limited to one sensory modality such as vision or hearing. For example, a person may have difficulty in recognizing an object as a cup or identifying a sound as a cough. Agnosia can result from strokes, dementia, developmental disorders, or other neurological conditions. It typically results from damage to specific brain areas in the occipital or parietal lobes of the brain. People with agnosia may retain their cognitive abilities in other areas.
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.
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 visual 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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