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Agnosia alludes to the impairments of a person's ability to recognize familiar objects, sounds, shapes and smells often encountered in life. In most cases, agnosia usually affects one particular sensory domain, so that a person, who even though could describe its visual features as accurately as a normal human being could, is unable to recognize an object by sight, may be able to do so by touch or smell. Thus, it is safe to assume that the afflicted individual’s knowledge of the objects they see every day (name, function, etc) is not linked somehow to one of the existing sensory domains.

There are currently three identified types of agnosia: visual agnosia, where the afflicted person is bereft of the ability to recognize objects by sight, auditory agnosia, where the individual is unable to recognize sounds, and somatosensory agnosia, where the individual could not identify objects by touch.

Visual agnosia could be divided into the following subtypes:

1. Prosopagnosia – The inability to recognize faces even though the person is familiar with the afflicted individual

2. Agnostic Alexia – the inability to fully understand the printed word even though it could be seen by the sufferer

3. Achromatopsia – the inability to identify colours

4. Inability to draw objects

5. Inability to reproduce drawings of objects

Three subtypes exist under auditory agnosia as well, namely:

1. Linguistic agnosia – the inability to comprehend the spoken word

2. Classical agnosia – the inability to make out sounds from the individual’s surroundings, such as cars roaring by or the birds’ warble in the trees

3. Amusia – the inability to comprehend music

Somatosensory agnosia, also known as astereognosis, involves individuals facing difficulties in discerning objects of various shapes, textures, weights and sizes via touch (tactile stimulation), though they are able to do so with much ease via sight.

Causes of agnosia include stroke, dementia and certain neurological disorders which would result in damage to specific areas of the brain; in the case of visual agnosia, it is usually the visual-processing region of the brain being damaged. There are patients too who received a dramatic recovery from blindness and are then diagnosed with agnosia.

Currently there is no known cure for agnosia, though patients could adapt to their condition or may even improve by perceiving information through unimpaired modalities, or by undergoing occupational or speech therapies.

Neurologist cum author Oliver Sacks had presented a fascinating case of a man with visual agnosia in the book ‘The Man Who Mistook His Wife for a Hat and Other Clinical Tales’, the title of which was derived from the aforementioned individual, Dr. P, who had literally mistaken his wife as a hat by grabbing her face and trying to lift it off.

Reference: http://en.wikipedia.org/wiki/The_Man_Who_Mistook_His_Wife_for_a_Hat