Figure Caption: Sensory areas of the cortex. Represented here are somatosensory perception (touch, hot/cold, position), vision and hearing. The dark blue sections represents brain areas that mediate the association of these perceptions. The colored areas in the figure on the right depict the injured areas of the occiptal and temporal lobes associated with prosopagnosia (inability to recognize faces).

Agnosia: This refers to the inability to recognize people or objects even when basic sensory modalities, such as vision, are intact. For example, patients with visual agnosia may not recognize an object upon visual presentation even though they can describe its shape and other visual characteristics. It is apparent that sensory perception of the object is disconnected from memories associated with the object. The patient can perceive the object but has no meaningful associations to it.

Patients with aphasia may have difficulty naming objects and accurately describing them. However, this difficulty is not considered agnosia. Presumably their memories and associations to the object are intact. Their difficulty is confined to the language aspects of expressing knowledge about the object. Their ability to name the object does not improve when information about the object is presented in another modality. In contrast, patients with agnosia will often improve when information is presented in other modalities.

Visual Agnosia

The following are types of visual agnosia:

Term, Content:

• Prosopagnosia, Faces
• Agnostic Alexia, Reading Material
• Color Agnosia, Colors
• Object Agnosia, 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 injuryto the left occipital lobe. It is common for patients to have visual agnosia but intact recognition in the tactile and auditory modalities.

Patients with color anomia perform normally on tasks that require discrimination of colors but cannot name colors or point to colors named by the examiner. There is a distinction between color perception versus color recognition. A pure deficit in color perception is called Central Achromatosia (See the Perceptual Disorders section).

Patients with prosopagnosia can identify faces as such but they do not recognize whose face is present before them. They may not even be able to recognize their own face in the mirror. It often exists in combination with central achromatopsia, topographical processing deficits and general object agnosia. Competing theories of prosopagnosia stress that it is an object agnosia for specific faces, 2) a type of perceptual disorder involving a system specific for face perception and 3) a type of amnesia syndrome in which the perception of a face is dissociated from previous memories of the face.

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.

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Auditory Agnosia

This refers to the inability to recognize specific sounds in the context of intact hearing. There is a distinction in this syndrome between pure word deafness, that is considered an agnosia for auditory/verbal information, and auditory agnosia, that involves an agnosia for environmental, nonverbal sounds. Cortical deafness is also a term applied to patients who essentially do not respond to any auditory information even when hearing is intact. Finally, receptive amusia is a term that refers to agnosia for music.

Somatosensory Agnosia (Astereognosis)

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.