Gnosia (Gr. gnosis, knowledge) refers to the higher synthesis of sensory impulses, with resulting perception, appreciation, and recognition of stimuli. Agnosia refers to the loss or impairment of the ability to know or recognize the meaning or import of a sensory stimulus, even though it has been perceived. Agnosia occurs in the absence of any impairment of cognition, attention, or alertness. The patients are not aphasic and do not have word finding or a generalized naming impairment. Hughlings-Jackson saw agnosia as a non-language form of aphasia. Agnosias are usually specific for a given sensory modality and can occur with any type of sensory stimulus. Agnosias that involve the primary sensory modalities may represent disconnection syndromes that disrupt communication between a specific cortical sensory area and the language areas, causing a restricted anomia. Tactile agnosia refers to the inability to recognize stimuli by feel, visual agnosia the inability to recognize visually, and auditory (acoustic) agnosia the inability to know or recognize by audition. Body-image agnosia (autotopagnosia) is loss or impairment of the ability to name and recognize body parts. Finger agnosia is a type of autotopagnosia involving the fingers. Auditory agnosia is the loss of recognition of sounds; phonagnosia is the loss of recognition of familiar voices. Time agnosia refers to loss of time sense without disorientation in other spheres. Visuospatial agnosia is loss or impairment in the ability to judge direction, distance, and motion and to understand three dimensional spatial relationships. Because of the impaired spatial judgment and visual disorientation, the patient cannot find his way in familiar surroundings. Multimodal agnosias may occur with dysfunction of the association areas in the parietal and temporal lobes that assimilate sensory information from more than one domain.

Astereognosis (stereoanesthesia) is loss of the ability to recognize and identify an object by touch despite intact primary sensory modalities. There is no loss of perceptual ability. The patient can feel the object, sensing its dimensions, texture, and other relevant information. However, he is unable to synthesize this information and correlate it with past experience and stored information about similar objects in order to recognize and identify it. Stereognosis is tested by asking the patient to identify, with eyes closed, common objects placed into their hand (coin, key, button, safety pin, paper clip). The most convincing deficit is when the patient is able to identify with the other hand an object they were unable to identify with the tested hand. When primary sensory modalities in the hand are impaired, as by radiculopathy or neuropathy, failure to identify an object by touch is not astereognosis. Astereognosis usually indicates a lesion involving the contralateral parietal lobe. Rarely, a lesion of either parietal lobe can produce astereognosis bilaterally. It has also
been reported to occur with lesions involving the anterior corpus callosum and the thalamic radiations. If there is hand weakness, the examiner may hold and move the object between the patient’s fingers. It is striking to see a patient with a paralyzed hand from a pure motor capsular stroke demonstrate exquisitely intact stereognosis when tested in this fashion. In tactile agnosia, the patient is unable to identify the object with either hand, but can identify it visually. Graphesthesia is a similar function. It is tested by writing numbers on the patient’s palm or fingertips. The inability to recognize the numbers is referred to as agraphesthesia; in the presence of intact primary sensory modalities, it usually indicates a lesion involving the contralateral parietal lobe.

Finger agnosia refers to the loss or impairment of the ability to recognize, name, or select individual fingers of the patient’s own hands or the hands of the examiner. The patient loses the ability to name individual fingers, point to fingers named by the examiner, or move named fingers on request, in the absence of any other naming deficit. Testing for finger agnosia may be conveniently combined with assessment of right-left orientation. The simplest test of right-left orientation is to ask the patient to raise a specific hand. More challenging is to have the patient touch a body part on one side, e.g., the right ear, with a specific digit of the other side, e.g., the left thumb. Even more strenuous is when the examiner faces the patient, crosses his forearms with hands and fingers extended, and requests the patient to touch one of the examiner’s fingers on a specific side, e.g, the left index. A very challenging test is to ask the patient to touch a specific finger as the examiner faces away from the patient with forearms crossed behind his back, using a confusing syntax, e.g., “with your left hand touch my right index finger.” Finger agnosia and right-left confusion, along with agraphia and acalculia, make up Gerstmann syndrome. Finger agnosia alone is not highly localizing, but when all components of the syndrome are present the lesion is likely to lie in the region of the dominant angular gyrus.

In the visual agnosias, there is loss or impairment of the ability to recognize things visually, despite intact vision (psychic blindness or mindblindness). Areas 18 and 19 are particularly important for visual gnostic functions. Visual agnosia is not a sensory defect but a problem in recognition. There is impairment in the higher visual association processes necessary for recognition and naming, not explicable by any deficit in visual perception. Patients can see but cannot make sense of the visual world. Teuber said visual agnosia was a “percept stripped of its meaning.” Oliver Sacks provided an entertaining and informative description of the clinical picture of visual agnosia in The Man Who Mistook His Wife For a Hat (Touchstone Books, 1985).

Some occipital lobe lesions, particularly of the primary visual cortex, cause color blindness (central achromatopsia). Lesions of the association areas may cause color agnosia. In color agnosia, the patient cannot name or identify colors, although he is not color blind and can discern the numbers on color plates. Patients may not be able to remember the color of common things, e.g., the sky. In prosopagnosia (face or facial agnosia, face blindness), there is an inability to recognize familiar faces. The patient may not be able to identify people by looking at their faces, even close family members, but may immediately identify the person by the sound of their voice. The patient may recognize a face as a face but cannot associate it with a particular individual. They learn to identify people using other cues. In extreme examples, the patient is unable to recognize himself in a mirror or photograph. Patients with prosopagnosia, and other visual agnosias, usually have bilateral lesions of the occipitotemporal area involving the lingual, fusiform, and parahippocampal gyri. Prosopagnosia can occur with unilateral right posterior hemispheric lesions.