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. More on Prosopagnosia Click
Here
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.
Counseling
and Psychotherapy [ See
Therapy Section ]:
