Chapter 13: Agnosia
Author's Commentary
Agnosia is a failure to recognize an object or sound with intact vision and hearing. Agnosia for taste and smell are rare. Opercular and SII lesions have caused agnosia for pain.
Parietal Lobe and Disorders of the Body Scheme
A normal person has a sense of the position of their body in space and its functional capacity. Patients also have a sense of the relation of the body in horizontal and vertical space. In the upright position, the sense of the midline of the body to a vertical axis is called the subjective visual vertical (SVV). The right parietal lobe is critical for these functions.
Excerpts From Chapter 13
Agnosia is failure to recognize an object or sound with intact vision and hearing. Agnosia for taste and smell are rare. Agnosia for pain (SII cortical lesions) has recently been described.
Examining for Agnosia
The patient is shown an array of common small objects and is asked to name them, describe their use, and to pick out specific ones named by the examiner. If the patient is unable to do this visually, he or she is allowed to palpate the object and is asked the same questions. The patient is shown several different colors and asked to name them, match them with duplicates, and then to arrange them in shades of increasing darkness or lightness. The patient is asked to walk to a specific location in a room when by doing so he or she would have to circumvent objects.
Visual Agnosia
This is a disorder of higher cortical function in which an alert, intelligent, non-aphasic patient with normal visual perception cannot recognize a visual stimulus.
The patient is unable to name or describe the function of objects shown, but immediately identifies them by touch or noise (bell) or smell (rose). A patient with a nominal aphasia cannot name the object by any modality of presentation (visual object agnosia). The usual lesion for this deficit is the second and third gyri of the dominant occipital lobe and its adjacent white matter outflow tracts. The patient may also be unable to identify or match colors, which is visual agnosia for colors.
There are several rather specific constellations of visual agnosia, which can be easily recognized and are quite striking.
Examination Technique Demonstration
Fig. 13.1 Tactile inattention. (a) The examiner simultaneously lightly touches the face and the foot. The patient will not perceive the foot with right hemispheric lesions. (b) The examiner simultaneously lightly touches the hand and the foot. The patient will not perceive the hand with right hemispheric sensory lesions.