Auditory agnosia is a form of agnosia that manifests itself primarily in the inability to recognize or differentiate between sounds. It is not a defect of the ear or "hearing", but rather a neurological inability of the brain to process sound meaning. While auditory agnosia impairs the understanding of sounds, other abilities such as reading, writing, and speaking are not hindered. It is caused by bilateral damage to the anterior superior temporal gyrus, which is part of the auditory pathway responsible for sound recognition, the auditory "what" pathway.
Persons with auditory agnosia can physically hear the sounds and describe them using unrelated terms, but are unable to recognize them. They might describe the sound of some environmental sounds, such as a motor starting, as resembling a lion roaring, but would not be able to associate the sound with "car" or "engine", nor would they say that it was a lion creating the noise. All auditory agnosia patients read lips in order to enhance the speech comprehension.
It is yet unclear whether auditory agnosia (also called general auditory agnosia) is a combination of milder disorders, such auditory verbal agnosia (pure word deafness), non-verbal auditory agnosia, amusia and word-meaning deafness, or a mild case of the more severe disorder, cerebral deafness. Typically, a person with auditory agnosia would be incapable of comprehending spoken language as well as environmental sounds. Some may say that the milder disorders are how auditory agnosia occurs. There are few cases where a person may not be able to understand spoken language. This is called verbal auditory agnosia or pure word deafness. Nonverbal auditory agnosia is diagnosed when a person's understanding of environmental sounds is inhibited. Combined, these two disorders portray auditory agnosia. The blurriness between the combination of these disorders may lead to discrepancies in reporting. , 203 patients with auditory perceptual deficits due to CNS damage were reported in the medical literature, of which 183 diagnosed with general auditory agnosia or word deafness, 34 with cerebral deafness, 51 with non-verbal auditory agnosia-amusia and 8 word meaning deafness (for a list of patients see).
History A relationship between hearing and the brain was first documented by Ambroise Paré, a 16th-century battlefield doctor, who associated parietal lobe damage with acquired deafness (reported in Henschen, 1918). Systematic research into the manner in which the brain processes sounds, however, only began toward the end of the 19th century. In 1874, Wernicke was the first to ascribe to a brain region a role in auditory perception. Wernicke proposed that the impaired perception of language in his patients was due to losing the ability to register sound frequencies that are specific to spoken words (he also suggested that other aphasic symptoms, such as speaking, reading and writing errors occur because these speech specific frequencies are required for feedback). Wernicke localized the perception of spoken words to the posterior half of the left STG (superior temporal gyrus). Wernicke also distinguished between patients with auditory agnosia (which he labels as receptive aphasia) with patients who cannot detect sound at any frequency (which he labels as cortical deafness).
In 1877, Kussmaul was the first to report auditory agnosia in a patient with intact hearing, speaking, and reading-writing abilities. This case-study led Kussmaul to propose of distinction between the word perception deficit and Wernicke's sensory aphasia. He coined the former disorder as "word deafness". Kussmaul also localized this disorder to the left STG. Wernicke interpreted Kussmaul's case as an incomplete variant of his sensory aphasia.
In 1885, Lichtheim also reported of an auditory agnosia patient. This patient, in addition to word deafness, was impaired at recognizing environmental sounds and melodies. Based on this case study, as well as other aphasic patients, Lichtheim proposed that the language reception center receives afferents from upstream auditory and visual word recognition centers, and that damage to these regions results in word deafness or word blindness (i.e., alexia), respectively. Because the lesion of Lichtheim's auditory agnosia patient was sub-cortical deep to the posterior STG (superior temporal gyrus), Lichtheim renamed auditory agnosia as "sub-cortical speech deafness".
The language model proposed by Wernicke and Lichtheim wasn't accepted at first. For example, in 1897 Bastian argued that, because aphasic patients can repeat single words, their deficit is in the extraction of meaning from words. He attributed both aphasia and auditory agnosia to damage in Lichtheim's auditory word center. He hypothesized that aphasia is the outcome of partial damage to the left auditory word center, whereas auditory agnosia is the result of complete damage to the same area. Bastian localized the auditory word center to the posterior MTG (middle temporal gyrus).
Other opponents to the Wernicke-Lichtheim model were Sigmund Freud and Carl Freund. Freud (1891) suspected that the auditory deficits in aphasic patients was due to a secondary lesion to cochlea. This assertion was confirmed by Freund (1895), who reported two auditory agnosia patients with cochlear damage (although in a later autopsy, Freund reported also the presence of a tumor in the left STG in one of these patients). This argument, however, was refuted by Bonvicini (1905), who measured the hearing of an auditory agnosia patient with tuning forks, and confirmed intact pure tone perception. Similarly, Barrett's aphasic patient, who was incapable of comprehending speech, had intact hearing thresholds when examined with tuning forks and with a Galton whistle. The most adverse opponent to the model of Wernicke and Lichtheim was Marie (1906), who argued that all aphasic symptoms manifest because of a single lesion to the language reception center, and that other symptoms such as auditory disturbances or paraphasia are expressed because the lesion encompasses also sub-cortical motor or sensory regions.
In the following years, increasing number of clinical reports validated the view that the right and left auditory cortices project to a language reception center located in the posterior half of the left STG, and thus established the Wernicke-Lichtheim model. This view was also consolidated by Geschwind (1965) who reported that, in humans, the left planum temporale is larger in the left hemisphere than on the right. Geschwind interpreted this asymmetry as anatomical verification for the role of left posterior STG in the perception of language.
The Wernicke-Lichtheim-Geschwind model persisted throughout the 20th century. However, with the advent of MRI and its usage for lesion mapping, it was shown that this model is based on incorrect correlation between symptoms and lesions. Although this model is considered outdated, it is still widely mentioned in Psychology and medical textbooks, and consequently in medical reports of auditory agnosia patients. As will be mentioned below, based on cumulative evidence the process of sound recognition was recently shifted to the left and right anterior auditory cortices, instead of the left posterior auditory cortex.
Related disorders After auditory agnosia was first discovered, subsequent patients were diagnosed with different types of hearing impairments. In some reports, the deficit was restricted to spoken words, environmental sounds or music. In one case study, each of the three sound types (music, environmental sounds, speech) was also shown to recover independently (Mendez and Geehan, 1988-case 2). It is yet unclear whether general auditory agnosia is a combination of milder auditory disorders, or whether the source of this disorder is at an earlier auditory processing stage.
Cerebral deafness Cerebral deafness (also known as cortical deafness or central deafness) is a disorder characterized by complete deafness that is the result of damage to the central nervous system. The primary distinction between auditory agnosia and cerebral deafness is the ability to detect pure tones, as measured with pure tone audiometry. Using this test, auditory agnosia patients were often reported capable of detecting pure tones almost as good as healthy individuals, whereas cerebral deafness patients found this task almost impossible or they required very loud presentations of sounds (above 100 dB). In all reported cases, cerebral deafness was associated with bilateral temporal lobe lesions. A study that compared the lesions of two cerebral deafness patients to an auditory agnosia patient concluded that cerebral deafness is the result of complete de-afferentation of the auditory cortices, whereas in auditory agnosia some thalamo-cortical fibers are spared. In most cases the disorder is transient and the symptoms mitigate into auditory agnosia (although chronic cases were reported). Similarly, a monkey study that ablated both auditory cortices of monkeys reported of deafness that lasted 1 week in all cases, and that was gradually mitigated into auditory agnosia in a period of 3–7 weeks.
Pure word deafness Since the early days of aphasia research, the relationship between auditory agnosia and speech perception has been debated. Lichtheim (1885) proposed that auditory agnosia is the result of damage to a brain area dedicated to the perception of spoken words, and consequently renamed this disorder from 'word deafness' to 'pure word deafness'. The description of word deafness as being exclusively for words was adopted by the scientific community despite the patient reported by Lichtheim's who also had more general auditory deficits. Some researchers who surveyed the literature, however, argued against labeling this disorder as pure word deafness on the account that all patients reported impaired at perceiving spoken words were also noted with other auditory deficits or aphasic symptoms. In one review of the literature, Ulrich (1978) presented evidence for separation of word deafness from more general auditory agnosia, and suggested naming this disorder "linguistic auditory agnosia" (this name was later rephrased into "verbal auditory agnosia"). To contrast this disorder with auditory agnosia in which speech repetition is intact (word meaning deafness), the name "word sound deafness" and "phonemic deafness" (Kleist, 1962) were also proposed. Although some researchers argued against the purity of word deafness, some anecdotal cases with exclusive impaired perception of speech were documented. On several occasions, patients were reported to gradually transition from pure word deafness to general auditory agnosia/cerebral deafness or recovery from general auditory agnosia/cerebral deafness to pure word deafness.
In a review of the auditory agnosia literature, Phillips and Farmer showed that patients with word deafness are impaired in their ability to discriminate gaps between click sounds as long as 15-50 milliseconds, which is consistent with the duration of phonemes. They also showed that patients with general auditory agnosia are impaired in their ability to discriminate gaps between click sounds as long as 100–300 milliseconds. The authors further showed that word deafness patients liken their auditory experience to hearing foreign language, whereas general auditory agnosia described speech as incomprehensible noise. Based on these findings, and because both word deafness and general auditory agnosia patients were reported to have very similar neuroanatomical damage (bilateral damage to the auditory cortices), the authors concluded that word deafness and general auditory agnosia is the same disorder, but with a different degree of severity.