Auditory agnosia

Handbook of Clinical Neurology, Vol. 129 (3rd series) The Human Auditory System G.G. Celesia and G. Hickok, Editors © 2015 Elsevier B.V. All rights reserved

Chapter 32

Auditory agnosia L. ROBERT SLEVC* AND ALISON R. SHELL Department of Psychology, University of Maryland, College Park, MD, USA

INTRODUCTION Imagine chatting with a friend at a crowded outdoor cafe´, background music playing, a couple arguing at the next table, and cars driving by. The auditory information reaching your ears in this situation is a mixture from all these sources (and more), yet ultimately yields a representation of the auditory scene in which you are able to successfully follow your conversation. Although the ability to identify and process these sounds may seem nearly effortless in normal circumstances, the complexity of our auditory world becomes clear in light of neurologic conditions that can lead to strikingly distinct deficits in auditory perception. These impairments of auditory perception and/or recognition that are not attributable to hearing or cognitive deficits are termed auditory agnosia. Auditory agnosia is distinct from peripheral hearing loss and also distinct from impaired hearing acuity as a result of bilateral damage to primary auditory cortex – a condition termed cortical deafness (although perhaps more appropriately called central deafness or cerebral deafness given that it may result from damage to the projections to auditory cortex rather than cortical damage per se; Tanaka et al., 1991; Griffiths, 2002). Whereas cortically deaf patients typically behave as if they were deaf (despite normal peripheral hearing and brainstem evoked potentials), patients with auditory agnosia are aware of sounds but have difficulty with sound identification. Auditory agnosia is also distinct from language perception deficits associated with the posterior (sensory) aphasias, although both cortical deafness and sensory aphasia can evolve into auditory agnosia (e.g., Mendez and Geehan, 1988; Slevc et al., 2011).

DEFINITIONS Auditory agnosia has not received as much attention as the visual agnosias; however it is similar in that it can be a

general deficit affecting all domains of auditory perception, or can be relatively specific, largely affecting only one type of sound perception/recognition. Patients can have agnosia specific to speech perception, called verbal auditory agnosia or pure word deafness; agnosia specific to music perception, called amusia; or agnosia specific to non-speech environmental sounds, called non-verbal auditory agnosia or environmental sound agnosia. There is, unfortunately, considerable terminological variability in the literature; here we use auditory agnosia to refer broadly to all subtypes of agnosia for auditory stimuli; however note that “auditory agnosia” is sometimes used more narrowly to refer specifically to environmental sound agnosia (i.e., agnosia for environmental sounds in the presence of preserved perception of speech and (sometimes) music; e.g., Vignolo, 1982, 2003). These specific deficits of auditory perception have garnered much interest despite their rarity because they may reveal the distinct cognitive and anatomic units of auditory perception and thus inform the extent to which auditory perception relies on a modular, and possibly domain-specific, architecture. However the extent to which auditory agnosia reflects separable domains of auditory perception is still debated: claims that the agnosias reflect damage to a modular system, where separate domains of auditory stimuli are processed independently (e.g., Peretz et al., 1994), contrast with claims that the agnosias result from deficits of varying severity to a domain of a general auditory processing system (e.g., Griffiths et al., 1999). The generalist approach gains some support from the fact that dissociations between auditory processing domains in these agnosia subtypes are rarely “pure”; instead most cases show at least some degree of overlapping deficits. Although this is widely acknowledged, the prevalence of overlapping (and non-overlapping)

*Correspondence to: L. Robert Slevc, Department of Psychology, University of Maryland, College Park, MD 20742, USA. E-mail: [email protected]

574 L.R. SLEVC AND A.R. SHELL deficits is not well documented, perhaps due simply to likely requires an ad hoc approach designed for a particthe practical constraints of carrying out extensive testing ular patient to determine which (if any) specific subtype across multiple types of stimuli. In addition, it is not yet of auditory agnosia (as detailed below) is the appropriate clear exactly what constitutes an affected domain in diagnosis. these subtypes. For example, is verbal auditory agnosia Although it is difficult to make a precise estimate, specific to speech sounds or does it instead reflect a defauditory agnosia appears to be quite rare and, accordicit in some more basic (non-speech) aspect of auditory ingly, the specific subtypes are even less common. analysis that is disproportionately involved in speech Polster and Rose (1998) point out several reasons why perception? Overlapping deficits are to be expected if agnosia might be less commonly observed in the auditory they reflect difficulty with particular aspects of auditory than in the visual domain. One such reason is that there is analysis that differentially, but not exclusively, affect greater neuroanatomic redundancy in the auditory than particular domains of sound processing (Griffiths in the visual system; in contrast to vision, where each et al., 1999). As detailed below, work that has carefully hemisphere receives information from only half of the examined different aspects of auditory analysis suggests visual field, auditory information arrives to each hemithat cases of auditory agnosia do not tend to support sphere from the entire auditory scene. Because of this, domain specificity at the level of language, music, or unilateral lesions to auditory areas often leave sound environmental sounds (although there are cases with processing relatively functional overall and thus audiremarkable dissociations of these types, e.g., Metztory agnosia typically requires bilateral lesions to the latLutz and Dahl, 1984; Peretz et al., 1994). Instead, these eral temporal lobes (which usually only occurs with two cases can illuminate the types of distinct mechanisms separate cerebrovascular accidents). A second reason involved in sound processing, thereby informing our for the rarity of auditory agnosia diagnoses is that audiunderstanding about the basic cognitive and neural protory agnosia may often be obscured by other deficits; for cesses involved in auditory perception more generally example, verbal auditory agnosia may not be obviously (e.g., Zatorre et al., 2002; Scott and Wise, 2004; distinguishable from other forms of posterior aphasia Hickok and Poeppel, 2007; Giraud and Poeppel, 2012). (especially as auditory agnosia often evolves from initial Auditory agnosia typically results from cerebrovaspresentations of sensory aphasia). In any case, there is a cular accidents, although other causes are possible, relative scarcity of data on auditory agnosia. This, comincluding herpes simplex encephalitis (Buchman et al., bined with the considerable variability in clinical presen1986), Landau–Kleffner syndrome (Metz-Lutz, 2009; tation and in exactly what is tested across cases, means Stefanatos, 2011), and dementia (Otsuki et al., 1998; that there is not yet a consensus on the specific etioloGoll et al., 2010a; Gibbons et al., 2012). There are also gies, or even the specific symptoms, associated with congenital forms of agnosia, which have been documenthese subtypes of auditory agnosia. ted mostly for musical perception (congenital amusia; An influential distinction has been made between see Peretz and Hyde, 2003; Stewart, 2011; and apperceptive and associative forms of auditory agnosia Chapter 33, this volume). To arrive at a diagnosis of audi(e.g., Vignolo, 1982; Buchtel and Stewart, 1989). This tory agnosia, it is necessary to first establish that a apperceptive/associative distinction comes from work patient’s sound-processing deficit is not due to periphon visual agnosia, where Lissauer (1890; cited in Farah, eral hearing problems via audiometric testing (pure2004) suggested that object recognition could be tone, speech, or immittance audiometry), otoacoustic impaired via an apperceptive deficit, due to problems emission testing, brainstem auditory evoked potentials, with visual object processing, or via an associative defior other audiometric methods (Hall and Mueller, 1997; cit, due to problems associating intact perceptual inforsee Chapters 17 and 18 of this volume). Of course, as mation with object representations. The division of auditory agnosia typically occurs in older adults, some apperceptive and associative deficits is less straightfordegree of age-related hearing loss is to be expected ward in audition than in vision, as it is not entirely obvi(see Chapter 20, this volume); however a diagnosis of ous what constitutes an auditory object (Griffiths and auditory agnosia requires any hearing loss to be suffiWarren, 2004). Nevertheless, early conceptions of audiciently mild to not explain the patient’s deficit. tory agnosia were primarily associative (e.g., Lichtheim, It is also necessary to rule out possible “top-down” 1885; Bastian, 1897), assuming that basic perceptual prodeficits that could affect auditory perception, for examcessing (apperception) was intact and the deficit resulted ple, attentional, linguistic, or memory deficits, as from an inability to link a perceived pattern to an approassessed by standard neuropsychological tests. If both priate auditory object, or sound meaning. Other theories peripheral hearing and general cognitive and linguistic assume that auditory agnosia is best characterized as abilities are intact, then additional testing can be done an apperceptive disorder, pointing to the fact that to determine the extent and domain of agnosia. This agnosia subtypes typically overlap, which likely reflects

AUDITORY AGNOSIA underlying problems with sound analysis (Goldstein, 1974; Buchtel and Stewart, 1989; Griffiths et al., 1999; Griffiths, 2002). Still, while considerable work argues that auditory agnosia (and its subtypes) may indeed reflect relatively basic problems perceiving and/or analyzing sound patterns (e.g., Albert and Bear, 1974; Buchtel and Stewart, 1989; Phillips and Farmer, 1990; Poeppel, 2001; Vignolo, 2003; Stefanatos, 2008; Slevc et al., 2011), there are cases that seem better characterized as associative deficits (e.g., Franklin et al., 1996; Saygin et al., 2010). This apperceptive/associative distinction might thus be expected to link different forms of auditory agnosia to different anatomic etiologies (e.g., Buchtel and Stewart, 1989; Goll et al., 2010b). This chapter does not take apperceptive/associative as its primary organizational principle (but see Goll et al., 2010b, for such an approach). Instead, we divide auditory agnosia into four major categories – (1) general auditory agnosia; (2) agnosia for speech (verbal auditory agnosia or pure word deafness); (3) agnosia for environmental sounds; and (4) agnosia for music (amusia) – and discuss apperceptive and associative variants within each subtype (for other reviews, see Polster and Rose, 1998; Griffiths et al., 1999, 2012; Simons and Lambon Ralph, 1999; Griffiths, 2002; Bauer and McDonald, 2003; Lechevalier et al., 2003; Goll et al., 2010b; Stefanatos

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and DeMarco, 2012). This taxonomy of auditory agnosia subtypes is, of course, somewhat problematic as even the rare cases of domain specificity do not typically show perfect dissociations (if multiple domains are even assessed). In addition, these categories do not make up an exhaustive set of auditory perceptual deficits; for example, one auditory perceptual deficit not addressed here is selective to sound localization rather than identification (e.g., Clarke et al., 2000, 2002; Adriani et al., 2003). Although this could be considered as a type of auditory agnosia, our focus is on deficits in sound identification and/or discrimination so we do not discuss deficits in auditory spatial perception here (instead, see Chapter 31, this volume). Finally, it is important to acknowledge that cases with similar types of impairment may or may not involve disruption of the same underlying process(es), thus cases should be evaluated individually instead of attempting to make conclusions from the average performance of a group from a particular clinical category (Caramazza, 1984). That said, a division into a tractable number of subtypes may be a necessary simplification in order to make any generalizations from the highly variable and unique series of individual case studies that make up our knowledge of auditory agnosia. Table 32.1 thus indicates the common pattern of spared

Table 32.1 Subtypes of auditory agnosia Verbal

Phonagnosia

Amusia

Typical clinical features

NonGeneral Apperceptive Associative Apperceptive Associative verbal

Apperceptive Associative

Hearing ability (audiometric sensitivity) Speech comprehension Speech repetition Non-speech sound recognition Non-speech sound discrimination Pitch perception Rhythm perception Timbre perception Recognition of familiar melodies Voice discrimination Recognition of familiar voices

+

+

+

+

+

+

+

+

–

–

–

+

+

+

+

+

– –

– +

+ +

+ +

+ +

+ –

+ +/–

+ +

–

+

+

+

+

+/–

+/–

+

– – – –

+/
+/
+/
* +/


+ + + +/–*

+/
+/– +/– +

+ + + +

+/– +/– +/– +/–

– – – –

+ + + –

– –

+/
+/


+ +

– –

+ –

+ +

+ +

+ +

Typical perceptual abilities in auditory agnosia subtypes. + intact abilities; – impaired abilities; +/– mixed findings; *likely patterns, but based on insufficient evidence.

576

L.R. SLEVC AND A.R. SHELL temporal and spectral auditory analysis (Tanaka et al., 1987; Mendez and Geehan, 1988). General auditory agnosia is most often associated with bilateral temporal damage: of 27 cases cited here that report both a clear deficit in at least two domains of auditory perception and reasonably clear lesion localization, 17 involved bilateral temporal damage (including insular cortex) and 5 involved other types of bilateral damage (mostly subcortical). Although general auditory agnosia is likely the most common type of auditory agnosia, most investigations have focused on the specific subtypes (as will be evident below). In some cases, this truly reflects investigations of patients with relatively specific deficits, but in others it is rather a matter of terminology, where a patient who might be best diagnosed with general auditory agnosia is instead labeled as a case of some specific subtype due to insufficient testing to reveal the extent of cross-domain deficits. For example, in a careful analysis of a patient previously diagnosed with pure word deafness (verbal auditory agnosia), Pinard et al. (2002) found considerable deficits also in environmental sound and musical perception.

Fig. 32.1. Typical lesion laterality for auditory agnosia subtypes, divided into deficits of auditory analysis (apperceptive agnosias; bottom) and deficits of sound identification (associative agnosias; top) deficits. Most cases involve lesions to left, right, or both temporal cortices, although many cases involve subcortical and/or more extensive cortical lesions.

and impaired abilities associated with different subtypes of auditory agnosia, and Figure 32.1 indicates the typical laterality of lesions underlying different agnosia subtypes.

GENERAL AUDITORYAGNOSIA Although much of the interest in auditory agnosia comes from specific subtypes, the most common presentation is of general (or global) agnosia across all auditory domains (e.g., Oppenheimer and Newcombe, 1978; Miceli et al., 1980; Auerbach et al., 1982; Miceli, 1982; Kazui et al., 1990). General auditory agnosia can develop from what initially appears to be cortical deafness (Ziegler, 1952; Mendez and Geehan, 1988; Godefroy et al., 1995), and can sometimes resolve into more specific subtypes of auditory agnosia (e.g., Motomura et al., 1986; Engelien et al., 1995). General auditory agnosia is characterized by impaired recognition of all types of auditory stimuli, including speech, environmental sounds, and music. This broad deficit suggests an underlying problem with sound recognition that affects all domains of auditory perception, and indeed general auditory agnosia is associated with deficits in both

VERBAL AUDITORY AGNOSIA (WORD DEAFNESS) Agnosia specific to speech sounds is often called pure word deafness (and sometimes also called word sound deafness or just word deafness). This is, critically, distinct from posterior (sensory) aphasia in that other domains of linguistic processing, including reading, writing, and speaking, are relatively intact, although agnosia is often still accompanied by some degree of aphasic symptoms (for reviews, see Goldstein, 1974; Buchman et al., 1986; Poeppel, 2001; Badecker, 2005; Stefanatos et al., 2005b). Verbal auditory agnosia often evolves from Wernicke’s aphasia, in that paraphasias and reading and writing deficits resolve without corresponding improvement of speech perception and repetition (e.g., Albert and Bear, 1974; Slevc et al., 2011). The term pure word deafness is somewhat misleading as the syndrome does not typically seem to be an agnosia for words per se, but rather agnosia for speech sounds. That is, verbal auditory agnosia is most often apperceptive in nature. In addition, word deafness is rarely (if ever) pure; in a review of 37 reports of pure word deafness, Buchman et al. (1986) argued that word deafness never occurred without other auditory processing impairments (but see Coslett et al., 1984; Metz-Lutz and Dahl, 1984; Yaqub et al., 1988; Takahashi et al., 1992). We thus use the term verbal auditory agnosia here to emphasize its relationship to other auditory agnosias (Wang et al., 2000). Although there is still relatively little research given the rarity of this syndrome, verbal auditory

AUDITORY AGNOSIA agnosia is arguably the most well-studied subtype of auditory agnosia. This greater concentration of work on verbal auditory agnosia does not necessarily reflect a greater prevalence compared to other agnosia subtypes; rather, it likely reflects greater rates of diagnosis and referrals because speech perception deficits can be particularly detrimental to overall functioning. Verbal auditory agnosia is sometimes grouped with the aphasias as a language disorder, and taken as evidence for speech specificity at an early stage of auditory perception. This fits with claims that even the very early stages of speech sound perception occur in a languagespecific way (e.g., Liberman and Mattingly, 1989; Liberman and Whalen, 2000) and with evidence for very early left lateralization of speech perception (e.g., Na¨a¨ta¨nen et al., 1997; Hornickel et al., 2008). However there is also considerable evidence that the early stages of speech processing occur bilaterally (for discussion, see Hickok and Poeppel, 2004, 2007, and Chapter 9, this volume), and that lateralization differences may instead reflect hemispheric specializations for more basic auditory processes. Indeed, verbal auditory agnosia typically results from bilateral temporal lesions in the superior temporal cortex (or subcortically, affecting the relevant projecting auditory radiations; see Chapter 1, Fig. 1.9): of 63 reports with sufficiently detailed lesion data (Wang et al., 2000; J€ orgens et al., 2008; Slevc et al., 2011; and earlier cases reviewed by Poeppel, 2001), 43, or almost 70%, involve bilateral lesions. Patients with verbal auditory agnosia are aware of speech but describe it as sounding like a foreign language (Albert and Bear, 1974), as distorted and cartoonlike (Wee and Menard, 1999), as rapidly fading (Klein and Harper, 1956), or with descriptions like “voice comes but no words” (Hemphill and Stengel, 1940). Speech production in verbal auditory agnosia is relatively normal (Table 32.1), although in some cases production is overly loud and has abnormal prosody (e.g., Otsuki et al., 1998; J€ orgens et al., 2008). Speech perception is typically improved by relying on cross-modal information such as lip reading and from “top-down” contextual information (e.g., Saffran et al., 1976; Coslett et al., 1984; Buchtel and Stewart, 1989; Slevc et al., 2011; Robson et al., 2012). Perception is also sometimes improved by dramatically slowing the speech signal (Albert and Bear, 1974; Stefanatos et al., 2005a), perhaps by ameliorating some patients’ particular difficulty with rapid temporal aspects of the speech signal (see below). Although verbal auditory agnosia is often claimed to only rarely result from unilateral damage (e.g., Poeppel, 2001; Bauer and McDonald, 2003), unilateral cases are by no means vanishingly rare. In fact, the first described cases of verbal auditory agnosia were in patients with unilateral left temporal lesions (Kussmaul, 1877;

577

Lichtheim, 1885) and, as mentioned above, approximately 30% of cases with clearly identifiable lesions have only unilateral damage. These unilateral lesions are overwhelmingly left-lateralized; there is only a single report of a right-hemisphere lesion associated with verbal auditory agnosia (Roberts et al., 1987). Interestingly, this was not obviously a case where language was rightlateralized: the patient showed a strong right-hand bias and had no family history of left-handedness. (Note also that this patient’s deficit was probably not specific to speech – he reported an acquired inability to appreciate music or recognize familiar melodies as well as some difficulty recognizing environmental sounds – although it is unclear how severe these other deficits were as nonspeech perception was not formally tested.) A common way to reconcile these two anatomic etiologies is to think of verbal auditory agnosia as a disconnection syndrome (Lichtheim, 1885; Geschwind, 1965; Gazzaniga et al., 1973; Takahashi et al., 1992), as illustrated in Figure 32.2. By this account, bilateral temporal damage prevents auditory input from either hemisphere from reaching speech-processing mechanisms (i.e., Wernicke’s area; for example, see Fig. 32.2A). Unilateral damage must not only interrupt ipsilateral transmission of left-hemisphere auditory input to speech-processing areas, but also destroy contralateral projections from right-hemisphere auditory analysis (Fig. 32.2B and C). In fact, this is essentially the early conception of verbal auditory agnosia as a primarily associative deficit (Lichtheim, 1885; Bastian, 1897; see Goldstein, 1974, for a review), in which the “left auditory word center” (i.e., Wernicke’s area) is dissociated from auditory input (by this account, the “word center” itself was assumed to be spared given preserved speech production). If verbal auditory agnosia is a disconnection syndrome, then unilateral cases of verbal auditory agnosia should be accompanied by damage to cross-hemispheric whitematter pathways from primary auditory cortex in the

Fig. 32.2. Three hypothesized lesion profiles that could result in verbal auditory agnosia / word deafness, according to a disconnection approach. A, auditory area; M, medial geniculate body; W, Wernicke’s area; X, lesion. (Adapted from Takahashi et al., 1992; brain outlines courtesy of Akira O’Connor.)

578

L.R. SLEVC AND A.R. SHELL

Fig. 32.3. (A) Anatomic magnetic resonance imaging images from a patient with verbal auditory agnosia following a lefthemisphere lesion. (B) Region of interest (ROI) used to define Heschl’s convolutions in the right hemisphere, and (C) whitematter pathways passing through the ROI shown in (B) as assessed with diffusion tensor imaging (DTI). (Reproduced from Slevc et al., 2011.)

undamaged hemisphere. So far, however, the only study that has investigated white-matter connectivity patterns in a patient with unilateral verbal auditory agnosia (using diffusion tensor imaging; see Chapter 16, this volume) found preserved cross-hemispheric white-matter pathways (Slevc et al., 2011; Fig. 32.3). These preserved pathways suggest that such a disconnection is not the only cause of verbal auditory agnosia with unilateral damage (although note that the preserved existence of whitematter tracts does not necessarily imply their preserved functionality). Instead, it may be that verbal auditory agnosia (at least in cases with unilateral damage) is not an associative disorder, but rather an apperceptive deficit in some aspect of auditory processing that is both left-lateralized and disproportionately involved in speech perception.

Apperceptive verbal auditory agnosia As noted above, considerable work suggests that verbal auditory agnosia may be best characterized as an apperceptive disorder, reflecting an auditory processing deficit that disproportionately affects speech (Albert and Bear, 1974; Buchtel and Stewart, 1989; Phillips and Farmer, 1990; Griffiths et al., 1999; Poeppel, 2001; Griffiths, 2002). A particularly influential proposal is that verbal auditory agnosia might result from an underlying deficit in rapid temporal processing that is particularly important for the perception of some speech sounds, especially for some consonants that are

characterized by rapid temporal transitions on the order of tens of milliseconds (e.g., Shannon et al., 1995). A rapid temporal processing deficit could disproportionately affect speech perception as the processing of environmental sounds and of music likely relies more on pitch and spectral cues (de Cheveigne´, 2005) and on temporal cues occurring over relatively longer time periods. Evidence for a rapid temporal processing deficit in verbal auditory agnosia comes from the observation that individuals with verbal auditory agnosia often have particular difficulty with temporally dynamic aspects of speech stimuli, such as place of articulation and voicing contrasts in consonants, compared to temporally protracted aspects of speech such as vowels (Saffran et al., 1976; Auerbach et al., 1982; Miceli, 1982; Yaqub et al., 1988; Praamstra et al., 1991; Wang et al., 2000; Stefanatos et al., 2005a; Slevc et al., 2011). Verbal auditory agnosia also is associated with pronounced deficits in perception of rapid temporal changes in non-speech stimuli (Fig. 32.4), including most (5 out of 6) cases that have evaluated click fusion and gap detection thresholds (Fig. 32.4C; Albert and Bear, 1974; Tanaka et al., 1987; Otsuki et al., 1998; Yaqub et al., 1988; J€orgens et al., 2008; but see Stefanatos et al., 2005b) as well as 3 cases with impaired discrimination of non-speech sine-wave stimuli differing in rapid temporal transitions (Fig. 32.4B; Wang et al., 2000; Stefanatos et al., 2005b; Slevc et al., 2011). Temporal processing deficits are not, however, uniquely associated with verbal auditory agnosia; impairments on click fusion tasks have also been reported in other forms of auditory agnosia (in 9 of 10 tested cases: Albert et al., 1972; Auerbach et al., 1982; Motomura et al., 1986; Buchtel and Stewart, 1989; Kazui et al., 1990; Best and Howard, 1994; Godefroy et al., 1995; Mendez, 2001; Stefanatos, 2008; but see Lambert et al., 1989). In 3 of these cases, a gradual improvement in temporal processing (as assessed with click fusion thresholds) accompanied recovery from auditory agnosia (Motomura et al., 1986; Best and Howard, 1994; Godefroy et al., 1995), lending further (although correlational) support to this relationship. It is not yet clear how such an apperceptive deficit in rapid temporal processing relates to the different lesion profiles of verbal auditory agnosia. One proposal is that an apperceptive temporal processing deficit, or a prephonemic variant of verbal auditory agnosia, reflects an underlying deficit in temporal acuity due to bilateral lesions, whereas an associative, phonemic variant results from left-hemisphere damage (Auerbach et al., 1982; Poeppel, 2001). However, other theories point to a lefthemisphere specialization for rapid temporal analysis, fitting with general theories of auditory processing that propose language lateralization reflects an underlying left-hemispheric specialization for rapid temporal

AUDITORY AGNOSIA

579

other cases), raising the possibility that unilateral cases of verbal auditory agnosia might reflect a somewhat “higher-level” deficit of temporal processing despite preserved “lower-level” temporal perception. While these findings have been taken as evidence that speech perception deficits in verbal auditory agnosia often do result from an underlying deficit in rapid temporal processing (e.g., Phillips and Farmer, 1990; cf. work on developmental language deficits, e.g., Merzenich et al., 1993; Tallal, 2004), it is important to note that not all cases of verbal auditory agnosia are associated with impairments in basic temporal acuity (e.g., Stefanatos et al., 2005b), and some show deficits even with relatively slow temporal changes on the order of hundreds of milliseconds (e.g., Wang et al., 2000; J€orgens et al., 2008). Furthermore, an attempt to improve rapid temporal processing via a training paradigm in one verbal auditory agnosia patient led to improvements only on discrimination of non-speech stimuli (Slevc et al., 2011), supporting claims that the demands speech perception places on rapid temporal processing are somewhat overstated (McGettigan and Scott, 2012). Nevertheless, the bulk of evidence suggests that rapid temporal processing deficits may underlie at least some cases of auditory agnosia. A better understanding of the precise role of temporal processing deficits in auditory agnosia will depend on more consistent testing of complex sound perception in agnosia patients using wellvalidated psychoacoustic techniques (e.g., Griffiths et al., 1999). Fig. 32.4. Example stimuli for tests used to assess rapid temporal processing in (verbal) auditory agnosia. (A) Synthesized speech stimuli from a /ba/-/da/ continuum, differing in the rapid (40 ms) onset transition in the second formant (F2) and (B) non-speech analogs of F2 in (A). (Adapted from Slevc et al., 2011). (C) Schematic of gap detection and click fusion tasks.

analysis and a right-hemispheric specialization for either spectral analysis or for temporal analysis over a longer time window (e.g., Poeppel, 2001; Zatorre and Belin, 2001; Boemio et al., 2005; Hickok and Poeppel, 2007; Poeppel and Monahan, 2008; but see McGettigan and Scott, 2012). This distinction implies that cases of verbal auditory agnosia due to unilateral (left-hemisphere) damage might reflect an underlying deficit in rapid temporal processing, and, indeed, at least three case reports of unilateral verbal auditory agnosia have found particular difficulties with rapidly changing stimuli (Wang et al., 2000; Stefanatos et al., 2005b; Slevc et al., 2011). Stefanatos et al.’s (2005b) patient NH showed impaired perception of rapid temporal transitions but did not show a complementary deficit in basic click fusion threshold (click fusion data were not available in these

Associative verbal auditory agnosia Although early conceptions of verbal auditory agnosia were associative in nature (see above discussion on the disconnection approach), there are very few reports of clearly associative forms of verbal auditory agnosia. The cases that do exist are often termed word-meaning deafness, first reported by Bramwell (1897; reprinted in Ellis, 1984), or occasionally described as a variant of transcortical sensory aphasia (e.g., Heilman et al., 1981). Word-meaning deafness is characterized by intact ability to repeat speech, discriminate phonemes, and perform auditory lexical decisions, but impaired ability to comprehend spoken words (Kohn and Friedman, 1986; Franklin et al., 1994, 1996; Hall and Riddoch, 1997; Plasencia et al., 2006; Wirkowski et al., 2006). These patients thus may have the somewhat surprising ability to write down a word or phrase that they do not understand and then successfully understand what they have written (Hall and Riddoch, 1997; Francis et al., 2001). Auditory comprehension of abstract words may be worse than of concrete words (Franklin et al., 1994, 1996), although this seemingly conceptually based

580

L.R. SLEVC AND A.R. SHELL

dissociation has also been explained in terms of an apperceptive deficit that essentially causes “noisy” input to the semantic system (Tyler and Moss, 1997). “Pure” cases of word-meaning deafness are very rare, with only two prototypical patients in the literature – KW (Hall and Riddoch, 1997; Francis et al., 2001) and DrO (Franklin et al., 1996; Tyler and Moss, 1997) – and only a handful more when including less clear-cut cases (Heilman et al., 1981; Ellis, 1984; Kohn and Friedman, 1986; Franklin et al., 1994; Plasencia et al., 2006; Wirkowski et al., 2006). KW, DrO, and 5 of the 7 other cases appear to have suffered unilateral left cortical lesions, although note that similar lesions can also result in an apperceptive form of verbal auditory agnosia (e.g., Wang et al., 2000; Slevc et al., 2011). Unfortunately, word-meaning deafness is not yet well understood even by the standards of auditory agnosia, and the few reports available suggest substantial heterogeneity in the associated perceptual/cognitive deficits.

PHONAGNOSIA Agnosias for speech are not always lexical in nature; phonagnosia is an apparent auditory parallel to prosopagnosia (impaired visual recognition of faces; e.g., Gainotti and Marra, 2011), characterized by difficulty recognizing familiar voices (Van Lancker and Kreiman, 1987; Van Lancker et al., 1989; Hailstone et al., 2010). Phonagnosia can occur in an apperceptive form, characterized by difficulty discriminating between voices after either left or right temporal damage, and an associative form, characterized by an inability to recognize voices after right parietal damage (Van Lancker et al., 1989; Lang et al., 2009). Phonagnosia can also accompany frontotemporal lobar degeneration (Hailstone et al., 2010) or can arise as a developmental deficit (Garrido et al., 2009). The extent to which phonagnosia can occur in the absence of other auditory perceptual or person identification deficits remains unclear; some patients with phonagnosia do appear to be able to successfully recognize speech and environmental sounds; however the deficit is more often associated with other forms of auditory agnosia and/or with other types of person recognition deficits such as prosopagnosia (see, e.g., Table 32.2; Neuner and Schweinberger, 2000).

AGNOSIA FOR ENVIRONMENTAL SOUNDS Auditory agnosia can also occur with (relatively) intact speech perception, but impairment in perceiving and/or comprehending non-speech environmental sounds (Spreen et al., 1965; Albert et al., 1972; Motomura et al., 1986; Fujii et al., 1990; Taniwaki et al., 2000). For example, Spreen et al.’s (1965) patient identified

Table 32.2 Summary of experimental neuropsychological profiles in phonagnosia patients QR and KL

Voices

Other sounds

Faces

Domain

Case QR

Case KL

Identification Familiarity Emotion recognition Perception Musical instrument matching Environmental sound recognition Recognition Perception

# # N N #

# # N N N

N

N

N* N

## N

#

impaired performance relative to controls. impaired performance relative to both controls and other case. * When matched to voices for difficulty. N, normal performance. Reproduced from Hailstone et al. (2010). ##

the sound of scissors as “a clock ticking” and rattled keys as “a chain jingling or a bell ringing.” This selective nonspeech perceptual deficit is sometimes called auditory agnosia or non-verbal auditory agnosia (e.g., Vignolo, 1982, 2003; Saygin et al., 2010); however, here we use the more specific term environmental sound agnosia, referring specifically to difficulty in identifying familiar non-speech sounds. Many cases of environmental sound agnosia also involve deficits of musical perception (e.g., Spreen et al., 1965; Lambert et al., 1989; Eustache et al., 1990; Vignolo, 2003), although this is difficult to evaluate precisely as musical perception is often not tested at all (e.g., Clarke et al., 2000) or only tested in a very general way (e.g., successful recognition of familiar songs (Eustache et al., 1990; Saygin et al., 2010) may be possible even in the presence of abnormal musical processing). Nevertheless, at least two reports suggest that environmental sound agnosia can exist independently of both musical and speech perception deficits (Motomura et al., 1986; Saygin et al., 2010). There are relatively few investigations of environmental sound agnosia in the absence of verbal auditory agnosia, perhaps because such deficits are less detrimental to everyday functioning than verbal auditory agnosia, and so tend to go unreported. Vignolo (1982) points out the additional reason why careful tests of environmental sound perception have rarely been conducted: only recently have batteries of recorded environmental sounds been easily available for testing (e.g., Saygin et al., 2003), and many cases simply involve the observation that a patient identifies or even just reacts appropriately to some small set of easily produced sounds.

AUDITORY AGNOSIA Environmental sound agnosia has been associated with a variety of lesion locations. Typically, it occurs as a result of cortical damage, which can be unilateral (to the left or right hemisphere) or bilateral (Schnider et al., 1994). However there are also cases arising from subcortical lesions to the auditory radiations bilaterally (Godefroy et al., 1995; Taniwaki et al., 2000). These different lesion etiologies may be linked to different forms of environmental sound agnosia: an apperceptive form, where identification errors are acoustically similar to the target (e.g., mistaking cat meows for someone singing), has been associated with right-hemisphere lesions (Fujii et al., 1990), and an associative form, where targets are additionally confused with semantically related sources (e.g., choosing a dog as the source of a cat meow), has been associated with left-hemisphere lesions (Vignolo, 1982; Schnider et al., 1994; but see Clarke et al., 1996). Other work suggests more complicated dissociations in associative environmental sound processing. For example, Trumpp et al. (2013) report a patient with damage to left posterior superior and middle temporal gyrus who showed difficulty identifying everyday object sounds with high acoustic relevance (e.g., telephone), but normal identification of sounds from animals and musical instruments. This patient showed a similar dissociation in lexical decision and in verbal fluency (abnormally slow lexical decisions and generating abnormally few exemplars only for sound-related objects), suggesting that an associative environmental sound deficit may also impair aspects of conceptual representations, and fitting with the idea that modality-specific perceptual information is an important aspect of conceptual knowledge (e.g., Barsalou, 2008).

AMUSIA The testing of musical sound analysis after cerebrovascular accident (or after any other cause of acquired auditory agnosia) is more challenging than testing speech or environmental sound perception because, unlike in these other domains, experience and training with music vary widely across the population and so it is not possible to assume a common baseline level of musical ability. A majority of work that has attempted to directly test for amusia has relied on a standardized diagnostic battery: the Montreal Battery for Evaluation of Amusia (Peretz et al., 2003). Here, we only briefly discuss acquired agnosia for musical perception; for more detailed treatment of acquired amusia, see Chapter 34 and for discussion of congenital amusia, see Chapter 33. In brief, amusia can also occur in apperceptive and associative forms. Apperceptive cases of acquired amusia are characterized by perceptual deficits in musical

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perception in the face of preserved musical production (Griffiths et al., 1997; Ayotte et al., 2000). Such sensory amusia generally results from right-hemisphere lesions (Stewart et al., 2006), although cases have also been reported with bilateral damage (e.g., Peretz et al., 1994). Music is a complex auditory stimulus, and while relatively little work has contrasted different aspects of musical perception, it does appear that agnosia can be relatively specific to different musical features. For example, there are reports of amusia confined to perception of pitch (Peretz et al., 1994), timbre (Kohlmetz et al., 2003), or rhythm (Mavlov, 1980). In addition, some aspects of musical perception can be selectively spared: Mendez (2001) reports a patient with global agnosia who developed an increased appreciation for music, perhaps relying on spared rhythm perception. An associative form of amusia was reported by Peretz (1996), whose patient CN showed preserved processing of pitch, timbre, and rhythm, but an impaired ability to recognize melodies and sing from memory (despite preserved ability to recognize familiar song lyrics) following bilateral temporal lesions. Such associative cases appear to be rare; however, it is difficult to assess the prevalence of musical agnosia (of any type) as such deficits do not often greatly disrupt everyday life, and so are likely to go unreported.

AUDITORYAFFECTIVE AGNOSIA Auditory affective agnosia or sensory aprosodia refers to selective impairments in recognizing affective information in speech despite relatively normal production of emotional prosody (Heilman et al., 1975; Darby, 1993). This is sometimes classified as a paralinguistic variant of verbal auditory agnosia (e.g., Bauer and McDonald, 2003). However the deficit seems likely to reflect problems understanding how variations in pitch, timbre, and volume reflect aspects of emotional communication and/or the speaker’s emotional state, and so may be better thought of as a form of associative amusia. Auditory affective agnosia is often associated with right posterior damage, especially in the right temporal operculum (Gorelick and Ross, 1987; Ross and Monnot, 2008), and is likely associated with other kinds of pragmatic impairments associated with right-hemisphere damage (e.g., Brownell et al., 1983). Alternatively, auditory affective agnosia might reflect an apperceptive deficit in perceiving the musical aspects of speech, which would then be closely related to apperceptive amusia (Patel, 2008). These possibilities have not yet been directly contrasted, as patients with impairments in auditory affective perception have not been carefully tested on other domains of affective perception or on perception of the musical features corresponding to affective speech.

582 L.R. SLEVC AND A.R. SHELL Thus it remains unclear whether auditory affective agnoThis met with some success, although improvements sia is a separate form of associative auditory agnosia, a may have partially reflected an adopted strategy of more general consequence of affective deficits, or a subcovertly spelling and visualizing the target words. type of apperceptive amusia. Another treatment approach, based on theories of a rapid temporal processing deficit underlying verbal auditory agnosia, is to employ treatment specifically TREATMENT FOR AUDITORYAGNOSIA designed to improve temporal processing. Stefanatos There have been few attempts to treat the perceptual defet al. (2005b) found that extended formant transitions icits associated with auditory agnosia; those that exist in digitally synthesized speech stimuli did not lead to have investigated a diverse set of patients and have notable improvements in phonemic discrimination in met with only mixed success. Essentially all treatment their verbal auditory agnosia patient, but did show that studies so far focus on the remediation of speech perceptemporally extending the duration of naturally produced tion deficits, either in patients with verbal auditory agnowords and sentences led to some improvement in comsia specifically or, more commonly, in patients with a prehension (Albert and Bear, 1974). Slevc et al. (2011) variety of perceptual and/or aphasic symptoms. The employed a similar sound manipulation strategy in a most common approach along these lines has simply training task, using a treatment designed to improve been to provide training on minimal-pair discrimination rapid temporal processing in developmental language or phoneme identification tasks, sometimes supported deficits (Merzenich et al., 1993; see Tallal, 2004, for a by visual cues (Morris et al., 1996; Wee and Menard, review) over a relatively long treatment period (43 ses1999; Maneta et al., 2001; Tessier et al., 2007). Morris sions over 2.5 months). Although their verbal auditory et al.’s (1996) patient had severe difficulties with speech agnosia patient improved in the ability to discriminate perception (but intact environmental sound perception) rapid temporal transitions in non-speech stimuli (sinein the context of broader aphasic symptoms. They wave sweeps; Fig. 32.4B), this was not accompanied by administered an adaptive syllable discrimination trainany improvements in discrimination of synthesized ing, where the patient first practiced discriminating speech stimuli or in speech perception more generally. between syllables differing in three distinctive features Thus there is little evidence overall for successful treat(voice, place, and manner of articulation) then, as perment programs for speech perception in auditory agnoformance improved, syllables differing on two of these sia, although this may simply reflect the limited number features, then syllables differing in only one distinctive of treatment studies to date. feature. Although their patient did improve on minimalEven less work has investigated treatment of other pair discrimination and repetition following 6 weeks (12 types of sound processing in auditory agnosia; in fact, sessions) of this training, he did not show commensurate there do not appear to be any documented treatment improvements in comprehension. studies targeting deficits in environmental sound or Maneta et al. (2001) reported a treatment study for music perception. In terms of spontaneous recovery, another aphasic patient with a particularly severe speech some patients recover from auditory agnosia altogether perception deficit (perception of non-speech sounds was (e.g., Fujii et al. 1990), while other cases show either no not tested). This treatment program used a similar prorecovery or persistence of specific perceptual deficits gram of minimal-pair discrimination training, plus addidespite recovery of other forms of sound identification. tional training on lip reading (via diagrams of lip shapes Saygin et al. (2010) present an interesting case of a plus exaggerated lip movements during the discriminapatient with a unilateral left-hemisphere lesion to tion tasks) and on cued speech (hand signals correspondWernicke’s area (Fig. 32.5) who recovered his speech ing to voicing, manner, and place of articulation); perception abilities but was left with severe environhowever they found no improvements in speech sound mental sound agnosia. They suggest that speech percepdiscrimination after 12 sessions. Tessier et al. (2007) tion may have recruited intact auditory processing areas found more encouraging results for a patient with gentypically associated with non-verbal processing (i.e., eral auditory agnosia: phoneme recognition, discriminaspeech perception may have “taken over” environmental tion, and speech comprehension were improved after a sound perception) – a hypothesis supported by functreatment program that involved intensive phoneme distional magnetic resonance imaging evidence showing crimination and phoneme recognition tasks accompahis selective responses to speech in areas associated with nied by visual cues (written representations of the general (non-speech-specific) auditory processing in phonemes) that were gradually delayed and removed. controls (specifically, right anterior temporal cortex Francis et al. (2001) report a treatment program for a and left perilesional temporal cortex; see also Engelien patient with word-meaning deafness that involved a et al., 1995). While speculative, this suggests the possibilseries of reading and auditory comprehension exercises. ity of functional reorganization of the auditory system

AUDITORY AGNOSIA

Fig. 32.5. Magnetic resonance imaging of patient M’s lesion. Five selected axial images are shown to depict the extent of lesion. Neurologic convention. (Reproduced from Saygin et al., 2010.)

and highlights the need to consider how functional reorganization after brain damage may alter correlations between brain and behavior (Musso, 1999; Leff et al., 2002).

CONCLUSION Much of the interest in auditory agnosia comes from the apparent specificity of the agnosia subtypes, including verbal auditory agnosia (or word deafness), environmental sound agnosia, and amusia. Auditory agnosia thus plays an important role in a larger debate about the underlying nature of auditory perception. It is certainly the case, however, that clear dissociations between disordered perception of speech, of environmental sounds, and of music are quite rare. This may indicate that the auditory agnosias are not, in fact, distinct deficits at the level of language, music, and environmental sounds, but instead reflect relative degrees of impairment of underlying aspects of auditory analysis that disproportionately affect different domains of processing (e.g., Griffiths et al., 1999; Poeppel, 2001). However, it is also possible that this taxonomy does reflect distinct deficits that are simply difficult to observe in the “natural experiments” that the neuropsychologic literature provides. That is, patients rarely, if ever, have circumscribed lesions in specific regions of interest; instead, lesions often affect large areas of neural tissue, and lesion locations are a function of vascular anatomy and other physiologic constraints. In addition, lesions do not simply disrupt the lesioned region, but also affect a much broader set of functional neural networks. Thus, while lesion evidence can demonstrate that a brain region is necessary for some function, it cannot show that the region is sufficient, as that function may depend as much or more on other aspects of a larger network. These limitations are not specific to the study of auditory agnosia, but are part of a general set of caveats common to most neuropsychologic studies. A few other such

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caveats deserve mention. For one, behavioral deficits also change over time, both as a function of physiologic recovery and as patients develop compensatory cognitive processes. Because of this, it is not always trivial to separate normal functioning from abnormal (but perhaps effective) processing based on alternative adopted strategies (Francis et al., 2001; Saygin et al., 2010). A second caveat is that the interpretation of any particular pattern of performance may also be confounded due to comorbidity with other deficits. Auditory agnosia refers to a deficit in auditory perception in the face of relatively preserved hearing and cognitive/linguistic functioning; however most cases show at least some degree of impairment in other areas. In addition, most studies lack premorbid measures of the relevant abilities, making it difficult to objectively evaluate the extent of impairment. Finally, few studies comprehensively evaluate different aspects of auditory processing, resulting in cases of apparent domain specificity without appropriate evidence (for example, a doctor’s jangling of keys is not a comprehensive test of environmental sound perception; cf. Pinard et al., 2002). Fortunately, progress is being made on many of these fronts. Significant advances in the understanding of auditory agnosia are likely to accompany the use of more sophisticated behavioral assessments; for example, the use of careful psychoacoustic methods to investigate complex sound perception at levels intermediate between pure-tone audiometry and complex speech sounds (Griffiths et al., 1999). Advances will also likely accompany more sophisticated methods for brain imaging; for example, measures of white-matter tractography (see Chapter 16, this volume) provide a straightforward way to investigate disconnection accounts (Slevc et al., 2011). Additional advances will likely come with more comprehensive theories of auditory perception and cognition, informed both by the auditory agnosias and by neuroimaging work on non-disordered populations. The different types of auditory perceptual deficits grouped under the term auditory agnosia provide an important window into our understanding of auditory perception and cognition. While there may yet be more questions than answers, a growing body of work investigating auditory agnosia is increasing our understanding of our remarkable ability to process complex sounds.

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