10.1. INTRODUCTION AND HISTORICAL OVERVIEW

Sensory agnosias are relatively uncommon clinical syndromes characterized by a failure of recognition that cannot be attributed to the loss of primary sensory function, inattentiveness, general mental impairment, or lack of familiarity with the stimulus (Fredericks 1969; Bauer 2003). In other words, sensory agnosias are disorders that are bracketed by failures of early sensory processing on the input side and inability to attend to or comprehend the output of high-level sensory processing on the output side. For example, a blind subject who fails to recognize a rose or a deaf subject who does not recognize the sound of a hammer driving a nail are not agnosic; similarly, a demented person who no longer knows what a hammer is would not be considered agnosic if he failed to recognize the object.

Although the term “agnosia” was coined by Freud (1891) in his discussion of aphasia and related disorders, descriptions of the disorder predate Freud. Finkelnburg (1870) described the syndrome of “asymbolia” in which a percept failed to contact the knowledge relevant to that percept and Hughlings Jackson (1876) described the phenomenon of “imperception” in the context of a patient with a large tumor of the posterior portion of the right hemisphere who was unable to navigate in familiar places and did not recognize people or places. He postulated that the posterior portion of the right hemisphere was crucial for visual memory and recognition. Munk (1881) provided an early and influential description of dogs with parieto-occipital lesions that were able to navigate about their surroundings without bumping into objects or getting lost yet didn’t recognize objects of obvious relevance such as food; he termed this disorder “Seelenblindheit” or mindblindness. Interestingly, similar findings were subsequently reported in macaques after lesions of the occipital lobe and temporal projections (Horel and Keating 1972).

An important early theoretical contribution was made by Lissauer (1890), who distinguished between two forms of the disorder: “apperceptive” and “associative” agnosias. As this terminology continues to be widely employed in the clinical literature, it will be briefly reviewed. For Lissauer “apperception” referred to the “stage of conscious awareness of a sensory impression” (translated into English by Jackson [1932]). Apperceptive agnosias, in Lissauer’s scheme, represented a disorder in which the early visual processing of a stimulus is disrupted, with a resulting failure to generate a fully specified perceptual representation. Associative agnosias, in contrast, were characterized by preserved ability to compute a representation of a visual stimulus but an inability to recognize the object, as indicated by the ability to name the object or produce verbal or non-verbal information that would unambiguously identify the stimulus. Presaging a number of contemporary accounts of the processes involved in recognition (e.g., Damasio 1989; Simmons and Barsalou 2003; Farah 2000; Humphreys, Riddoch, and Fortt 2006), for Lissauer recognition entailed the simultaneous activation of multiple attributes (e.g., sound, touch, smell, etc.) that were linked to the visual form. That is, for Lissauer as for a number of contemporary theorists, “recognition” of a telephone entailed the contemporaneous activation of the visual image, sound, heft, texture, manner of use, and function of a telephone (Allport 1985).

Although the boundaries between agnosia and primary visual loss on the input side and disorders of semantics at a higher level were hazy in Lissauer’s account (and remain so today), he offered a clear distinction between apperceptive and associative visual agnosias: the ability to copy a visual stimulus. Reflecting their inability to generate an adequate sensory representation of the stimulus, apperceptive agnosics are unable to copy a stimulus whereas associative agnosics are able to copy a figure but remain unable to recognize what they have copied (Rubens and Benson 1971). As has been emphasized by Farah (2004) in her authoritative account of the visual agnosias, the reproductions of associative agnosics may be extremely detailed but appear to be slavish copies of the stimulus, uninformed by stored knowledge of the stimulus. As indicated in Figure 10.1, when asked to copy a drawing that has been deliberately distorted, associative agnosics may include the distortion in their drawing without appearing to be aware of the error.

FIGURE 10.1

A copy of a drawing by a visual agnosic of a carrot with a line drawn through it. The patient failed to recognize the carrot; reflecting the lack of top-down processing from stored knowledge of the object, his copy incorporated the out of place line. (more...)

Unlike other classical neurologic syndromes such as aphasia or neglect, the status of the concept of agnosia has varied substantially over the century since its description. In the early portion of the twentieth century, the discussion of the disorder was largely framed by the prevailing Gestalt theory of psychology; Poppelreuter (1923) and Goldstein (Goldstein 1943; Goldstein and Gelb 1918), for example, interpreted their subjects’ behaviors with respect to such concepts such as “closure” and “invariance.”

The very existence of the phenomenon of visual agnosia was questioned by a number of investigators. This was expressed by Pavlov (1927), for example, who, in response to Munk’s phrase “the dog sees but does not understand,” countered that “the dog understands but does not see well enough.” In later years Bay (1953) and Bender and Feldman (1972) argued that apparent visual agnosias were attributable to perceptual impairment (e.g., “tunnel vision”), general cognitive impairment, or some combination thereof.

In recent years, there has been a resurgence of interest in the topic for several reasons. One is the development of more sophisticated and nuanced models of “recognition” according to which perception is taken to be a multi-stage interactive process; on such accounts, the distinction between perceptual disorders and agnosia is seen not as dichotomous but rather as a process in which sensory inputs give rise to progressively more elaborated representations in which different types of information (e.g., shape, color, location) may be emphasized (Heinke and Humphreys 2003; Ellis and Young 1988). Second, in light of the increasingly complex and interactive models of recognition, the heuristic value of data from patients with agnosia has proven to be substantial. As illustrated by the influential contributions of Farah (2004), Riddoch and Humphreys (1987), and others, data from agnosic subjects may serve to indicate the fault lines in the process of recognition and offer important constraints for accounts based on animal work, modeling, and studies of normal subjects.

10.2. TYPES OF AGNOSIA

As sensory agnosia is defined as a modality-specific disorder of recognition, the syndrome may be encountered in any sensory channel that permits entities to be identified. Reflecting the central role of vision in humans as well as the fact that vision has been studied more extensively than other sensory modalities, most work has focused on visual agnosia. Tactile, auditory, and even olfactory and gustatory agnosias have also been described. In this chapter we focus on different types of visual agnosia and present a theoretical framework for understanding them before discussing other types of agnosia.

10.2.1. Visual Agnosia

Most studies of visual agnosia have emphasized the recognition of man-made objects. More recently, however, investigations of agnosic subjects have demonstrated remarkable specificity with respect to the types of stimuli with which subjects may be impaired. Agnosias for objects, faces (“prosopagnosia”), words (“pure word blindness”), colors, and the environment (including landmarks) have all been described. Finally, the disorder of simultanagnosia—an inability to “see” more than one object at a time—is often regarded as a type of visual agnosia. Consistent with these behavioral dissociations, functional imaging in normal subjects demonstrates that different parts of the brain may, at least to some degree, be optimized for the processing of different classes of visual stimuli. We briefly discuss these types of visual agnosia in turn. Before considering the specific disorders of higher-level visual processing, however, we present an information-processing account of visual processing. A detailed account of visual processing is beyond the scope of this chapter but can be found in Chapter 8; the model described briefly below is intended only to provide a framework within which the complex and rich clinical literature can be understood.

10.2.1.1. Visual Object Recognition: A Theoretical Overview

The most important insight regarding visual processing during the last century has been the recognition that different types of visual information are segregated at the retina and that different types of information are processed in parallel. Although there are numerous and strong interactions between the processing streams, the segregation persists until late in visual processing, as indicated by the fundamental distinction between the ventral “what” stream and the dorsal “where” (Ungerleider and Mishkin 1982) or “how to” (Milner and Goodale 1995) streams. A cartoon depicting the basic architecture of the visual processing system is presented in Figure 10.2.

FIGURE 10.2

A cartoon of the processes involved in visual object recognition.

Physiologic, anatomic and, more recently, imaging studies have demonstrated conclusively that different visual attributes such as color, angle, motion, depth, orientation, and length are processed in parallel in different brain regions. Low-level visual routines such as “boundary marking” (e.g., Ullman 1984; Borenstein and Ullman 2008) serve to group different stimuli leading to visual forms. Although these processes were intensively investigated by Gestalt psychologists, the neural bases of the routines remain poorly understood. These processes, including grouping of visual features into candidate “objects” and marking the boundary of candidate objects, appear to be crucial for object recognition. These processes do not require attention or effort and are assumed to be “automatic.”

Although there is convincing evidence that visual attributes are processed in parallel, the visual environment consists of stimuli or regions of space in which the attributes are integrated; that is, in order to generate an interpretable, coherent picture of the environment, information that is processed in parallel in different brain regions must be integrated so that, for example, the color red and oval shape are linked to generate the percept of an apple. Abundant experimental evidence demonstrates that this “binding” of visual feature information is mediated by a limited-capacity operation typically referred to as “visual attention” (the neural mechanisms of which are described in detail in Chapter 8). The binding function of visual attention is illustrated by the phenomenon of “illusory conjunctions.” Treisman (Treisman and Gelade 1980; Treisman and Souther 1985) and others (Cave 1999; Prinzmetal et al. 2002) have demonstrated that when visual attention is “overloaded,” visual attributes can miscombine. For example, when presented with an array containing red X’s and green T’s for 200 ms, normal subjects may report seeing a red T despite the fact that no such stimulus was present. These and a host of similar findings suggest that a limited-capacity, relatively fast but not infallible, “glue” links visual feature information computed in parallel. This glue is visual attention. As suggested by the commonly used “spotlight” metaphor (Eriksen and Hoffmann 1973; Posner 1980), visual attention appears to be spatially based under most circumstances. Experimental evidence suggests that attention can also be allocated to other visual attributes, including color (Cave 1999), motion, and even objects (Duncan 1984; Vecera and Farah 1994).

Limited-capacity operations that serve to select visual information for additional processing (including integration) have been demonstrated at many levels of neural processing, ranging from relatively high-level vision (Corbetta 1998), intermediate vision (Moran and Desimone 1985), and even the primary visual cortex (Vidyasagar 1998). Despite the fact that these operations differ somewhat from the procedure by which visual features are integrated, this process is also typically described as visual attention. As will be discussed below, disruption of visual attention at different levels of visual processing may give rise to distinctly different clinical syndromes.

The integration of visual feature information generates a viewer-centered representation of the orientations and depths of the surfaces of an object as well as the discontinuities between the surfaces. This representation is similar in most important respects to the “2 1/2D model” described by Marr (1982), which makes explicit the form, shape, and volume of the object as well as the hierarchical relationships between the parts. This type of representation has been termed a “structural description” (Riddoch and Humphreys 1987) and is similar to the 3-D representation of Marr (1982) in that it is assumed to be perspective independent; that is, at this level of processing, the representation computed by the brain specifies the relationships between the visual features in an “object-centered” fashion. The nature of the processes that mediate between the viewer-dependent, integrated feature representation and the perspective-independent or object-centered structural description remain unknown. As reviewed briefly by Farah (2000), a variety of accounts have been proposed, from connectionist architectures to template-matching procedures. Details of these proposals are beyond the scope of this chapter.

As will be discussed below, a large number of imaging studies suggest that the fusiform gyri and lateral occipital region are crucial anatomical substrates for the integrated feature representation and structural description systems (Grill-Spector, Knouf, and Kanwisher 2004; Haxby, Hoffman, and Gobbini 2000 for reviews).

Familiar objects are quickly and effortlessly recognized when viewed across a wide range of angles and perspectives. A bowl, for example, may be viewed from the side when sitting in a dishwasher, from below when stacked on a high shelf, or from above when clearing a table.

Although the low-level visual information regarding surfaces, color, form, etc. is remarkably different in these circumstances, under normal circumstances these stimuli are immediately recognized as the same. In the context of Figure 10.2, the mechanism that supports object constancy is termed the view normalization system.

The visual processing discussed to this point is in the service of object recognition—that is, knowledge of the form, function, name, and other attributes of entities in the environment. Following Lissauer (1890) as well as recent accounts of semantic representations (Allport 1985; Damasio 1989; Rogers et al. 2004; Saffran and Schwartz 1994; Warrington and Shallice 1984; Simmons and Barsalou 2003), I suggest that recognition entails the simultaneous activation of the many aspects of knowledge-specific knowledge; thus, as argued by Lissauer (1890), for example, recognition of a violin consists of the activation of stored knowledge regarding the sound, heft, feel, and manner of manipulation of the instrument.

With this overview of the series of processes underlying visual recognition in mind, we now turn to a consideration of the different types of visual agnosia. Rather than use the traditional “apperceptive–associative” distinction described above, we discuss the clinical phenomena with reference to the putative locus of the processing deficit exhibited by the patient.

10.2.1.2. Disorders of Low-Level Vision

A variety of clinical disorders have been described in which the primary deficit is a disruption of processing of different types of visual feature information. These are reviewed briefly in this section.

10.2.1.2.1. Achromatopsia

Achromatopsia is an acquired disorder of color perception characterized by a loss of the ability to distinguish color. The disorder is probably far more common than widely appreciated because it varies in severity from a mild loss of the richness of color (e.g., “red desaturation”) to a complete loss of the sense of color. Milder forms are often not recognized by the patient whereas in more severe forms the patient may indicate that they see the world in black and white or shades of gray. The defect is often in one visual field or part thereof but may involve the entire visual field if both hemispheres are affected.

Since Verrey’s initial report of the disorder in 1888, achromatopsia has been consistently associated with lesions involving the lingual or fusiform gyri. Subsequent studies with static brain imaging (Damasio et al. 1980) and functional brain imaging have yielded generally similar results. Reflecting this anatomic substrate, achromatopsia may be observed in isolation or in association with conditions such as prosopagnosia, alexia, or superior visual field deficits that are associated with lesions to nearby cortex (e.g., Pearlman, Birch, and Meadows 1979). Interestingly, the loss of color perception is not typically associated with visual object agnosia.

10.2.1.2.2. Impaired Motion Perception (Akinetopsia)

Reflecting its crucial role in vision, motion cells that appear to be optimized for motion perception may be identified at the retina, lateral geniculate, primary visual cortex, and a number of higher-level visual cortices, including MT, a cortical region that appears to be specialized for motion processing (see below).

Relatively pure disorders of motion perception are rare. The syndrome was first reported almost a century ago (Goldstein and Gelb 1918; Potzl and Redlich 1911). In recent years, a subject with this disorder, LM, has been extensively investigated (e.g., Zihl, Cramon, and Mai 1983). LM developed a profound impairment in the ability to detect motion after bilateral infarcts involving the posterior portions of the middle temporal gyri extending into the occipital lobe as well as adjacent subcortical white matter. The deficit had profound consequences for her ability to negotiate her environment. She did not perceive movement as a continuous process but stated that objects seemed to jump from one position to the next. When she poured water into a cup, the liquid appeared to be static, like a piece of ice. Although profoundly impaired in motion perception, LM performed well on other measures of visual processing. Her visual fields were full and she performed normally on tests of stereopsis, visual acuity, color perception, and critical flicker fusion. At least under most circumstances, LM exhibited no impairment in object recognition.

Several patients have been reported whose object recognition is influenced by motion. Botez and Serbanescu (1967) reported two patients with a “static form agnosia” characterized by a failure to recognize stationary stimuli but much improved performance when the same stimuli were moved. A similar but perhaps less striking facilitation of recognition with movement was exhibited by the “visual form agnosic” reported by Benson and Greenberg (1969) as well as a patient reported by Horner and Massey (1986). We have also observed this phenomenon in a number of patients with hypoxic brain injury or degenerative diseases preferentially involving the posterior portions of the hemispheres (that is, “posterior cortical atrophy”). In these patients, it appeared that patients were unable to reliably deploy visual attention to different stimuli in the array; movement seemed to permit the subjects to foveate the stimulus that was then recognized by normal procedures.

Data from patients with achromatopsia as well as abundant fMRI studies implicate the posterior portion of the middle temporal gyrus (“V5”) as a cortical region specialized for motion processing (Kable, Lease-Spellmeyer, and Chatterjee 2002; Watson et al. 1993).

10.2.1.3. Visual Form Agnosia

One well-described but relatively rare syndrome, “visual form agnosia,” may represent a deficit in the segregation of coherent regions of visual input. One such subject, a 24-year-old man who had suffered carbon monoxide poisoning (Mr. S), was reported by Benson and Greenberg (1969) and investigated in detail by Efron (1968). Mr. S. was relatively intact with respect to general cognitive function but was substantially impaired in recognizing visually presented objects, drawings, letters, or faces. Extensive testing of several low-level visual attributes revealed normal performance on tasks requiring the detection of differences in luminance, wavelength, area, and motion. Perhaps his most striking visual deficit was a profound impairment in the ability to discriminate shape; for example, if presented with a square and a tall, skinny rectangle (height:width ratio of 4:1), matched for area, color, and other visual qualities, he was unable to indicate if the stimuli were the same or different shapes.

More recently, Milner and Heywood (1989) reported a second visual form agnosic, DF; like LM, she had suffered carbon monoxide poisoning. Structural MRI demonstrated bilateral lesions involving the lateral occipital area. She exhibited profound visual recognition problems and performed poorly on tasks requiring that she discriminate between different shapes.

DF exhibited a finding of great theoretical interest that had not been described previously. Although unable to distinguish between visual forms or to name objects, she performed normally with respect to hand posture and shape when asked to pick up the objects; thus, when asked to pick up rectangles whose shape she was unable to describe, the distance between her thumb and index finger and timing of the movements of the fingers in the reach trajectory were normal. Thus, information regarding visual form that was not available for the purposes of object analysis was available to the motor system.

10.3. IMPAIRMENTS OF THE STRUCTURAL DESCRIPTION SYSTEM (“ASSOCIATIVE AGNOSIAS”)

Failures of object recognition in the context of visual processing that is adequate to support copying of a stimulus have been reported on a number of occasions (Davidoff and Wilson 1985; Levine 1978; Levine and Calvanio 1989; see Farah 2004 for a comprehensive review). Rubens and Benson (1971) reported a patient who exhibited a striking form of associative agnosia after a hypotensive episode. Elementary visual function was normal except for right hemianopia; the subject was unable to name most objects yet drew accurate and quite detailed depictions of the very objects that he was unable to name.

From Lissauer’s initial account to the present, the pattern of deficits displayed by Rubens and Benson’s patient has been attributed to disruption of the structural description system. Associative visual agnosia is differentiated from loss of knowledge of the object (semantics) by the fact that, when queried verbally or by means of auditory or tactile input, the patients are able to provide appropriate information about objects that they are unable to recognize visually; thus, when asked to provide a verbal description of a hammer, for example, an associative agnosic would be expected to describe its function, heft, and sound as well as demonstrate its manner of manipulation. Similarly, the patient would be expected to be able to name the hammer after handling it.

Despite the fact that they are able to describe or copy visual stimuli quite adequately, it seems unlikely that they are contacting normal structural descriptions for several reasons. First, as emphasized by Farah (2000), copies generated by visual agnosics are typically produced in a “slavish” and painstaking fashion, presumably reflecting a reliance on the exact physical attributes of the stimulus with little or no “top-down” input from stored knowledge of object appearance. This point is illustrated by the copy depicted in Figure 10.2; this subject copied the stimulus—including the extraneous, sinusoidal line—without distinguishing between the irrelevant and object-specific information. Second, it is noteworthy that the quality of the copies generated by associative agnosics is typically strongly influenced by the richness of the visual image; subjects generally perform best with real objects, less well with pictures, and worst with line drawings. This hierarchy would not be expected if all three types of stimuli contacted the same structural description.

Finally, it should be noted that when asked to match or sort complex non-object figures, all associative agnosics of which we are aware exhibit impairments (Humphreys and Riddoch 1987; Ratcliff and Newcombe 1982; see Farah 2004 for discussion). This observation suggests that these subjects have some degree of impairment in the processes by which structural descriptions are accessed.

It should be emphasized that although the model depicted in Figure 10.1 provides a useful framework for considering the general principles involved in visual processing and their breakdown, it does not readily accommodate the full range of agnosic performance. One well-studied “integrative agnosic,” HJA (Riddoch and Humphreys 1987), illustrates this fact. Although the details of the subject’s performance are beyond the scope of this chapter, it is noteworthy that HJA appears to be an associative agnosic (Farah 2004), whereas in other regards his performance is more typical of apperceptive agnosia (Riddoch and Humphreys 2003).

10.4. CATEGORY-SPECIFIC RECOGNITION DEFICITS

As noted previously, for some patients the ability to recognize visually presented stimuli is significantly influenced by the semantic category of the item. Thus, a number of patients have been reported who are able to name man-made objects but are severely impaired in naming naturally occurring items such as animals, fruits, and vegetables. A number of competing hypotheses have been proposed to explain these category-specific deficits. The modality-specific hypothesis (Warrington and Shallice 1984), later named the Sensory-Functional Theory (SFT; Caramazza and Shelton 1998), postulates that the animate versus inanimate dissociation follows from a selective impairment of the sensory or functional attributes that subserve the processing of either of these two categories. By this account, the identification of animate objects relies more on sensory attributes and would be disproportionately impaired by damage to the processing of sensory features associated with these objects. In contrast, inanimate objects may be known primarily by virtue of their function and the manner in which they are used or manipulated. As a consequence, a deficit in the recognition of inanimate objects would be disrupted by loss of information regarding the function of an object or sensory-motor knowledge regarding the manner in which the object is manipulated. A competing hypothesis is that the semantic knowledge is organized categorically in the brain (Caramazza and Shelton 1998). Clearly, teleologic explanations for this organization can be made. Evolutionary pressures would favor an animal that could easily recognize and distinguish other animals that are potential predator or prey, or plants that are potential sources of food. Further, developmental data support the idea that infants as young as 3 months of age can differentiate living from non-living things.

We recently studied a 50-year-old patient who exhibited a category-specific visual agnosia. After suffering a stroke of the right temporal lobe in the context of hypotension, he developed amnesia, prosopagnosia, and an inability to recognize living things. He was significantly impaired naming animate as compared with inanimate items. Additional testing revealed that his ability to name objects was strongly predicted by the nature of his experience with the object. Those objects— whether animate or inanimate—that he knew by virtue of using or manipulating (e.g., hammer) were named relatively accurately whereas those items that he knew primarily by sight (e.g., elephant) were named poorly. We argued that he exhibited a mild visual agnosia but that “motor knowledge” could compensate at least in part for the deficits in the object recognition system.

10.5. FUNCTIONAL IMAGING CORRELATES OF OBJECT RECOGNITION

In the past few years, functional imaging in healthy subjects has helped to elucidate the anatomy of object recognition. Malach et al. (1995) described the lateral occipital cortex (LOC), which is located at the lateral and ventral aspects of the occipito-temporal cortex. This area is activated preferentially by objects compared with scrambled objects or textures, regardless of the nature of the object (e.g., faces, cars, common objects, and even unfamiliar abstract objects) (Malach, Levy, and Hasson 2002). Further processing of the visual stimuli occurs in specific brain areas according to stimulus category. For example, faces are processed in the fusiform face area (Kanwisher, McDermott, and Chun 1997) and in the occipital face area (Gauthier, Tarr, and Moylan 2000); the former is more selective for faces than the latter. Places and/or spatial layouts are processed in the parahippocampal place area (Epstein and Kanwisher 1998), whereas objects like animals and tools are processed in specific loci in the fusiform and the superior and middle temporal gyri (Chao, Haxby, and Martin 1999). Orthographic stimuli are processed in the left inferior occipito-temporal cortex on or near the left fusiform gyrus (Polk et al. 2002; Allison et al. 1994), although this localization is highly debated (Price and Devlin 2003). Thus, the specific perceptual categories of visual stimuli dictate very different ways of their processing in the brain and might give an anatomic basis for the different agnostic syndromes.

10.6. AUDITORY AGNOSIAS

Like the visual agnosias discussed above, auditory agnosias are characterized by an inability to recognize a stimulus—in this case, a sound—that cannot be explained by inadequate elementary sensory processing, generalized loss of knowledge (e.g., dementia), or inadequate attention to the task. Perhaps reflecting the fact that in the context of normal vision, audition is not usually needed for object recognition, auditory agnosias have been reported far less commonly than visual agnosias. Indeed, in most naturalistic settings, one is rarely required to identify an object on the basis of sound alone. Even in those circumstances in which this is necessary—for example, identifying the sound of a telephone—the range of potential stimuli is narrow, limiting the complexity of the processing required. Perhaps because of these factors, complaints of poor recognition of objects from sound are uncommon. In our experience, when questioned about their auditory recognition deficit, most subjects will concede that sounds simply “aren’t right” and, when pressed, attribute the disorder to hearing loss.

There is, however, reason to believe that auditory agnosias are under-reported. On a number of occasions, we have observed patients with auditory recognition deficits evident on formal testing who were utterly unaware of the deficit. Additionally, most patients with documented auditory sound agnosia do not complain of an inability to recognize sounds (Saygin et al. 2003; Vignolo 2003). For example, although aphasic subjects rarely complain of difficulty understanding non-speech sounds, acquired language disorders are frequently associated with a generalized disorder of auditory recognition (Vignolo 1982). Saygin et al. (2003), for example, recently reported an elegant study in which 30 left-hemisphere-damaged aphasic subjects were asked to match environmental sounds (sound of a cow mooing) or linguistic phrases (“cow mooing”) to pictures. They found that the aphasic subjects were impaired with both word and sound stimuli; furthermore, there was a high performance correlation between severity of aphasia and accuracy on the environmental sound-matching task. Lesion overlay analysis demonstrated that damage to posterior regions in the left middle and superior temporal gyri and inferior parietal lobe was a predictor of impaired performance on both tasks.

Although accounts of auditory recognition are, in general, less well-developed than those in the visual domain, several investigators have argued that two forms of auditory sound agnosia may be identified (Kleist 1928). Echoing Lissauer’s distinction between apperceptive and associative visual agnosias, Vignolo (1982) distinguished between a perceptual or discriminative agnosia linked to right hemisphere lesions and an associative agnosia observed with left hemisphere lesions. Although subjects with both conditions exhibit the same fundamental deficit, an inability to recognize objects from sound, support for the claim that they arise at different levels of processing comes from analyses of the errors (Vignolo 1982; Schnider, Benson, and Scharre 1994). In the right hemisphere “apperceptive” form of auditory agnosia, confusions are typically based on the features of the sound; for example, a car horn may be confused with a musical instrument. In left hemisphere “associative” auditory agnosia, errors appear to be semantically based; for example, subjects may confuse a police whistle with a siren.

Vignolo (2003) recently reported data that support this lateralization of function. Tasks assessing music and identification of environmental sounds were administered to 40 subjects with unilateral stroke; right hemisphere lesions tended to disrupt the apperception of environmental sounds whereas left hemisphere lesions disrupted the semantic identification of sounds. Finally, recent evidence from functional imaging studies in normal subjects is also consistent with this claim. Lewis et al. (2004) reported data from an fMRI study in which participants listened to a wide range of environmental sounds while undergoing BOLD imaging. The contrast between recognizable and unrecognizable stimuli revealed activity in a distributed network of brain regions previously associated with semantic processing, most of which were in the left hemisphere.

Auditory information is not only relevant to the recognition of words and entities in the environment but also to spatial processing; that is, just as vision may be used in parallel for the identification and localization of objects, sound may also be employed in the service of spatial processing. Although beyond the purview of this chapter, it should be noted that both functional imaging and lesion studies have suggested that distinct processing pathways may underlie these capacities (Griffiths et al. 1998). Clarke et al. (2000), for example, reported detailed investigations of four subjects with left hemisphere lesions who exhibited substantial dissociations in the ability to recognize and localize auditory stimuli, suggesting that the capacities to identify and localize auditory stimuli are, at least in part, distinct and dissociable.

In the following sections we briefly review the major syndromes of auditory agnosia starting with cortical deafness and culminating in disorders of auditory word recognition and recognition of auditory affect.

10.7. CORTICAL DEAFNESS AND GENERALIZED AUDITORY AGNOSIA

Cortical deafness is a rare disorder characterized by profound loss of awareness of sound; in its most profound form, patients appear to be deaf. Patients with this disorder have typically suffered extensive bihemispheric lesions that destroy superior temporal cortex crucial for the analysis of sound (Leicester 1980). More recently, several patients with extensive subcortical bilateral lesions that undercut and, presumably, disconnect the auditory cortex from input from the medial geniculate have been reported (e.g., Kazui et al. 1990; Kaga et al. 2005).

As in the visual domain, the distinction between a primary disorder of auditory processing and “higher-level” disorders of auditory recognition remains controversial. Michel et al. (1980) proposed a number of criteria to differentiate these conditions, including auditory evoked potentials as well as mapping of the lesions to koniocortex as opposed to pro- and parakoniocortex. Neither of these approaches has been demonstrated to provide an adequate account of the distinction.

Consistent with the animal literature demonstrating that bilateral ablations of auditory cortex do not consistently produce complete deafness (e.g., Neff 1961; see also Celesia 1976), most subjects who present with substantial bilateral temporal strokes also do not exhibit cortical deafness. Similarly, the rare subjects who present with cortical deafness typically regain awareness of sound but continue to exhibit profound auditory recognition deficits for all kinds of stimuli. With formal testing, the errors of subjects with generalized auditory agnosia typically represent confusions based on acoustic properties rather than meaning; thus, in the context of the two-stage model described above, the disorder may be described as an “apperceptive” disorder.

10.8. OTHER AGNOSIAS

There has been an explosion of interest in the human neurobiology of taste and smell in recent years (Small 2006; Gottfried 2006). As the brain mechanisms underlying these disorders have been investigated, patients who meet traditional criteria for olfactory or gustatory agnosia have been identified. Small et al. (2005) reported a thorough investigation of a subject with longstanding bilateral temporal lobe dysfunction who developed a gustatory agnosia after surgical resection of the left anterior medial temporal lobe for control of seizures. Prior to surgery the patient performed normally on tasks assessing her ability to detect and estimate the intensity of taste; additionally, although recognition thresholds were elevated, she successfully named basic tastes (e.g., sweet, sour). After resection of the left anterior medial temporal lobe she retained the ability to detect, discriminate, and react hedonically to tastants but lost the ability to recognize the tastant. Based on these and other data (Small et al. 1997), the investigators argued that recognition of taste involves interactions between the primary gustatory centers located in the insula/opercular region and the regions of the anterior medial temporal lobe that are critical for taste recognition.

Olfactory agnosias have also been described. For example, Mendez and Ghajarnia (2001) reported a patient who was unable to recognize faces and odors in the context of right temporal lobe dysfunction. Jones-Gotman, Rouleau, and Snyder (1997) reported data from a smell recognition test from 70 subjects who had undergone temporal lobectomy for seizure control. They found that damage to the anterior temporal lobe of either hemisphere was associated with impairment of odor identification with impairment of odor identification. However, because basic smell detection was not formally assessed, one cannot state with certainty that these subjects met the criteria for olfactory agnosia. Eichenbaum et al. (1983) demonstrated that patient HM, who had undergone bilateral anterior temporal lobe resections, exhibited preserved early olfactory processing but substantial impairment in higher-order olfactory processing. Gottfried and Zald (2005) have recently reviewed the contributions of orbitofrontal cortex to olfactory processing and the implications of lesions in this region for odor recognition in humans and non-human primates.

10.9. AGNOSIA AND CONSCIOUS AWARENESS

As previously noted, agnosia is a disorder of stimulus recognition. Traditionally, evidence for “recognition” has come from explicit report: subjects name the object or provide a verbal description that is sufficiently detailed so that the object can be unambiguously identified. As in other domains in neuropsychology (e.g., Schacter 1987; McGlinchey-Berroth 1996), there is now abundant evidence that some agnosic subjects may derive substantial information from stimuli that they fail to “recognize.” This phenomenon, which has been termed “covert recognition” or “processing without awareness,” has been studied most extensively in prosopagnosics (see Bruyer 1991 for review). Bruyer et al. (1983) demonstrated that a prosopagnosic subject exhibited greater difficulty learning to associate familiar but unrecognized faces with fictitious as opposed to real names. Similar data from a face-name interference test were reported by DeHaan, Young, and Newcombe (1987) as well as Young et al. (1986). Additional evidence from psychophysical tasks was provided by Bauer (1984), who demonstrated that a prosopagnosic subject exhibited the largest galvanic skin (“lie detector”) response to unrecognized faces when the face was paired with the correct name (see also Bauer and Verfaellie 1986; Tranel and Damasio 1985).

Similar findings have been reported in other agnosic syndromes. For example, Coslett and Saffran (1989; Coslett et al. 1993) reported that patients with written word agnosia (“pure word deafness”) responded significantly better than chance on a variety of semantic tasks to words that they claimed not to have “seen.” Several investigators have demonstrated that simultanagnosic subjects derive information from unreported stimuli (Robertson et al. 1997). Coslett and Lie (2008), for example, demonstrated that a simultanagnosic patient’s report of two items in an array was significantly influenced by the semantic relationship between items in an array: like a previously reported subject (Coslett and Saffran 1993), this subject reported items from the same semantic class (e.g., hammer, pliers) more reliably than items from different categories (e.g., skirt, apple).

The interpretation of these and other data demonstrating dissociations between implicit and explicit knowledge remains controversial. It is clear from these and multiple other sources of data that patients with brain injury derive substantially more information than they realize. In light of this observation, one may question whether this implicit knowledge influences the behavior of these subjects. For example, is a visual agnosic who can’t discriminate between a shotgun and fire-poker on formal testing likely to pick up a shotgun to stoke a fire? Although one might expect that implicit knowledge would constrain the behavior of agnosic subjects, we are unaware of any data that speak directly to this point.

Although beyond the scope of this chapter, there is a substantial (but controversial) body of literature suggesting that unconscious thought is an important and efficient tool for decision making, problem solving, and attitude formation (see Dijksterhuis et al. 2006). In a series of experiments, Dijksterhuis and colleagues have demonstrated that “unconscious” thought may, under some circumstances, be more rational and efficient than “conscious” thought. In one experiment (Dijksterhuis 2004), subjects were asked to rate the desirability of different apartments after being told of 12 dimensions (e.g., cost, size) on which the apartments differed. One group of subjects was asked to respond immediately upon hearing about the apartments, another group was given three minutes to explicitly consider the apartments, and a third group performed a distracting (2-back) task for three minutes before responding. Subjects who were distracted for three minutes and therefore not able to consciously consider the apartments performed significantly better in discriminating between the most and least desirable apartments than those subjects who responded immediately or carefully considered the options. Dijksterhuis and colleagues argue for a fundamental discrepancy between “conscious” and “unconscious” thought and claim that the latter may be more efficient for certain types of mental operations. The implications of “unconscious” thought for the processes underlying sensory agnosia are, at present, not clear, but they represent a fertile domain for future research.

10.10. CONCLUSION

Sensory agnosias are relatively uncommon but often striking disorders of recognition that are of considerable interest for several reasons. Not only are they important from a clinical perspective but also, as emphasized by a number of investigators (e.g., Farah 2004; Bauer 2003), by dissecting perceptual systems at the fault lines, the disorders may provide important insights regarding the basic mechanisms of information processing and their anatomic bases. Using visual agnosia as the prototype, we have attempted to demonstrate that the confusion that arises from a consideration of the phenomenology of the disorders can be mitigated by considering the disorders in the context of a theoretically motivated account of basic perceptual mechanisms. Such an approach is, in principle, applicable to agnosias in other modalities. Finally, there are several areas for future research that appear particularly promising. The study of high-level recognition deficits including the study of “category-specific” agnosias offers great promise in the exploration of semantics, including the role of “grounded cognition” (see Barsalou 2008).

Additionally, in light of recent demonstrations of the role of unconscious operations in perception and thought, the study of agnosic subjects may contribute to the understanding of the manner in which motivations, goals, and drives are influenced by factors about which subjects remain unaware. For example, the extent to which Pavlovian conditioning is present in agnosic patients is not clear. The demonstration that a prosopagnosic patient could form a conditioned response between a face that they could not identify and an unconditioned stimulus would suggest that reward-based conditioning does not require recognition but could be based on lower-level sensory processing. To this end, Chapter 16 explicitly considers the novel idea that many of the behavioral deficits observed in patients with frontal lobe dysfunction reflect an agnosia for object “value.”

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