Try to identify the following object based on a description of its parts: four wheels, four doors, a front and rear windshield, headlights, taillights, seatbelts, and a steering wheel. Clearly, what is being described is a car. Now imagine being able to look at a car and identify all of those separate parts, and be able to understand the car’s purpose, but not be able to identify the object as a whole. This curious disorder is referred to as agnosia.
First coined by Austrian neurologist Sigmund Freud (1856—1939) in 1891, agnosias are disorders of recognition in patients who have fully intact primary sensations, meaning patients are not blind, deaf, or have any other sensory deficits. Criteria for diagnosis of agnosia include failure to recognize an object, normal perception of the object excluding an elementary sensory disorder, ability to name the object once it is recognized, and absence of generalized dementia.
Agnosias usually only affect one sensory modality, for example, one may not be able to identify a cookie by sight, but will be able to identify it by taste and smell. They are defined in terms of the specific sensory modality affected and generally fall into the categories of visual, auditory, or tactile.
Visual agnosias are the best studied form of this disorder and are associated frequently with forms of brain injury such as a lesion, stroke, and/or brain trauma. Visual object agnosia falls into two categories: apperceptive visual object agnosia and associative visual object agnosia.
Apperceptive agnosia is exemplified in the situation described in the introduction. Patients can pick out features of an object correctly, but are unable to appreciate the whole object, they literally only see the trees, not the forest. The right parietal cortex has been identified as important to visual processing of objects; however, this syndrome has also been identified in patients with bilateral occipital lesions, where vision is primarily located.
Associative agnosia has to do with recognition of appropriately perceived objects. Patients with associative agnosia may be able to copy or match drawings of an object they cannot name, or identify it using other senses, such as seen in the cookie example. This deficit is usually associated with bilateral posterior hemisphere lesions involving the fusiform or occipitotemporal gyri. Additionally patients with this form of agnosia often have other related recognition deficits such as color agnosia (cannot name or identify a color by sight, but this is not to be confused with being color-blind), prosopagnosia (see the following paragraph), and alexia (inability to see words or to read because of a brain defect).
Prosopagnosia is an interesting form of this disorder in which patients are unable to recognize faces, even of family and friends, and instead must focus on specific details associated with individuals. In severe cases of prosopagnosia, a patient may not even recognize his or her own face. Clearly in this patient population small changes to things such as hair color and aging can be problematic.
Similar to visual agnosia, auditory disorders range from primary auditory syndromes of cortical deafness to partial deficits or recognition of specific types of sounds. These disorders are also associated with bilateral cerebral lesions involving the temporal lobes, as this region contains the primary auditory cortex. Auditory agnosias can be divided into pure word deafness, pure auditory nonverbal agnosia, phonagnosia, and pure amusia.
Pure word deafness involves the inability to comprehend spoken words, but with the ability to hear and recognize nonverbal sounds. The area of the brain most often associated with this disorder is Wernicke’s area in the left hemisphere. Patients with this disorder may also experience paraphasic speech, meaning they use unintended or inappropriate words in an attempt to communicate. For example, they may say “purple” when they were meaning to say “friend.”
Auditory nonverbal agnosia refers to patients who have preserved hearing and language comprehension but have lost the ability to identify nonverbal sounds. For example, a patient may not be able to identify animal sounds, or sounds associated with specific objects such as an alarm.
Phonagnosia is similar to prosopagnosia; however, instead of not being able to recognize faces, these patients have difficulty recognizing familiar people by their voices. Failure to recognize a familiar voice may involve a right parietal lobe locus corresponding to the specific area for recognition of faces. Related to this defect is auditory affective agnosia, or failure to recognize emotional intonation of speech.
Amusia is the loss of musical abilities after focal brain lesions. Recognition of melodies and musical tones is a right temporal lobe function, whereas analysis of pitch, rhythm, and tempo involves the left temporal lobe. Famously, the composer Maurice Ravel (1875—1937) suffered a progressive aphasia, which took his ability to read or write music, but not his capacity to listen and appreciate it.
Patients with lesions of the parietal cortex may have preserved the ability to feel pinpricks, temperature, vibration, and proprioception, but fail to identify objects in the contralateral hand or recognize numbers or letters written by the opposite side of the body. These deficits are called astereognosis (if both hands are affected, it results in the inability to identify an object by active touch and without any other sensory input) and agraphesthesia (the inability to know what number or letter is drawn on their skin by an examiner) and tend to represent deficits of cortical sensory loss rather than full tactile agnosias.
See also: Color Blindness; Sacks, Oliver Wolf
Farah, Martha. (2004). Visual agnosia. Cambridge, MA: MIT Press.
Kirshner, Howard S. (2002). Agnosias. In Behavioral neurology: Practical science of mind and brain. Boston, MA: Butterworth Heinemann.
National Institute of Neurological Disorders and Stroke (NINDS). (2007). NINDS agnosia information page. Retrieved from http://www.ninds.nih.gov/disorders/agnosia/agnosia.htm