Visual Agnosia I. Biran, MD, and H. B. Coslett, MD Address Clinical Variants Department of Neurology, University of Pennsylvania School of Medi- Visual agnosia can be specific to certain kinds of objects. cine, 3400 Spruce Street, Philadelphia, PA 19104, USA. Accordingly, there are agnosias for objects, agnosias for E-mail: [email protected] faces or “prosopagnosia,” agnosias for words or “pure Current Neurology and Neuroscience Reports 2003, 3:508–512 word blindness,” agnosias for colors, and agnosias for Current Science Inc. ISSN 1528–4042 Copyright © 2003 by Current Science Inc. the environment, including landmarks. Finally, the dis- order of simultanagnosia—an inability to “see” more than one object at a time—is often regarded as a type of The visual agnosias are an intriguing class of clinical phe- visual agnosia. nomena that have important implications for current theo- ries of high-level vision. Visual agnosia is defined as impaired Agnosia for objects object recognition that cannot be attributed to visual loss, Inability to recognize familiar objects is the most com- language impairment, or a general mental decline. At least mon of the agnostic syndromes. Patients are impaired in in some instances, agnostic patients generate an adequate naming regular objects and are often unable to describe internal representation of the stimulus but fail to recognize them or mimic their use. Object agnosias are further it. In this review, we begin by describing the classic works classified as either apperceptive (the percept is not fully related to the visual agnosias, followed by a description of constructed and, therefore, patients are unable to copy the major clinical variants and their occurrence in degener- drawings) or associative (the percept is relatively intact ative disorders. In keeping with the theme of this issue, we and patients are able to copy drawings). As the follow- then discuss recent contributions to this domain. Finally, we ing text discusses, the agnosia can be specific to a present evidence from functional imaging studies to sup- semantic category, usually living or animate objects; port the clinical distinction between the various types of agnosias for nonliving or inanimate objects have also visual agnosias. been described [8]. Prosopagnosia Introduction Prosopagnosic patients are unable to recognize the iden- The term “agnosia” was coined by Freud [1] in his discus- tity of faces (ie, to whom a face belongs), although they sion of aphasia and related disorders. Like subsequent are capable of recognizing that a face is a face and, in investigators, he described a disruption between objects some instances, the gender and age of the person. and their concepts. However, there are earlier descriptions Prosopagnosia is usually secondary to temporo-occipital of similar clinical phenomena. Munk [2] described dogs lesions, affecting the fusiform and lingual gyri. Whether with parieto-occipital lesions that were able to avoid a bilateral or a right unilateral lesion is sufficient to cause objects in their surroundings but failed to recognize the the deficit has been an issue of debate [9]. There is also objects. He termed this behavior as “Seelenblindheit” evidence that impaired face recognition could be (mind-blindness) [2]. An important early theoretical con- encountered with frontal lesions. Rapcsak et al. [10] doc- tribution was made by Lissauer [3], who offered the dis- umented both anterograde and retrograde face memory tinction between two clinical forms of impaired object impairment in subjects with frontal lesions. recognition: “apperceptive” and “associative” agnosias. On Lissauer’s account, the former reflects a failure to generate a Agnosia for words fully specified perceptual representation, whereas the latter This is also known as pure alexia, alexia without agraphia, is attributable to an inability to link an adequate percept to or pure word blindness. Although this phenomenon is stored knowledge indicating its name, function, size, and usually discussed in the context of language impairments, so forth [3] (translated into English by Jackson [4]). As it is an agnostic symptom, as subjects who suffer from this noted by Teuber [5], associative visual agnosia may be deficit show a language impairment limited to visually pre- regarded as a “normal percept stripped of its meaning.” sented stimuli (eg, reading), but not to auditorily pre- Earlier descriptions of visual agnosia were provided by sented stimuli. Accordingly, this deficit could be regarded Finkelnburg’s account [6] of “asymbolia” and Jackson’s as a failure in the visual recognition of words, and thus as concept [7] of “imperception.” an agnostic deficit [11]. Visual Agnosia • Biran and Coslett 509 Color agnosia Progressive prosopagnosia In this rare clinical syndrome, patients fail to name colors. This is a degenerative disorder presenting with progressive This deficit is not secondary to basic color perception, as impairment in the recognition of faces. This syndrome is demonstrated by tasks requiring color categorization and part of the fronto-temporal dementias (FTDs), which may hue perception. The distinction between this syndrome present as focal atrophy in any combination of the right and color anomia is not clear. Some authors suggest that and left frontal or temporal cortices. Although in the the two syndromes could be differentiated by tasks prob- semantic dementia variant of the FTDs the atrophy is ing color information (ie, matching between known prominent in the left temporal lobe, in progressive objects to colors), which is impaired with agnostic subjects prosopagnosia the atrophy is prominent in the right tem- and preserved with anomic subjects [12,13]. poral lobe [20–22]. Landmark agnosia Schizophrenia Landmark agnosia is one of the causes for topographic dis- Although not a classic neurodegenerative disorder, orientation (the loss of way-finding ability). Patients with schizophrenia is often associated with visual agnosia. In landmark agnosia are unable to use environmental fea- a recent study of 41 patients with schizophrenia, tures for orientation. This deficit is usually secondary to Gabrovska et al. [23] reported a high incidence of visual lesions of the medial aspect of the occipital lobes, either processing deficits that were similar to associative agno- bilaterally or right-sided. Pallis [14] reported a classic case sia. Schizophrenia patients also demonstrate a specific of this disorder, and the topic has recently been reviewed impairment to memorizing faces. This deficit is highly by Aguirre [15]. correlated with a reduction in the volume of the fusiform gyrus [24]. In conjunction with other reports of discrete Simultanagnosia cognitive impairments in schizophrenia [25], these con- Simultanagnosia is the inability to perceive simulta- tributions help to bridge the gap between schizophrenia neously several items in the visual scene. This piecemeal and behavioral neurology. perception of the visual environment causes severe dis- ability and patients often behave as blind subjects. Simultanagnosia can be part of Balint’s syndrome [16], Mechanisms which also includes an inability to shift gaze (psychic The visual agnosias can teach us about the way we allocate paralysis of gaze) and difficulties in reaching visualized attention to the external world and build an internal mental objects (optic ataxia). This complex syndrome is associ- representation. In the following section, we focus on recent ated with bilateral lesion of the posterior parietal and work that explores the cognitive deficits underlying visual occipital lobes [17]. agnosia. These reports not only contribute to our under- standing of the clinical disorders, but also have implications for our understanding of perception and cognition. Visual Agnosia in Degenerative Disorders The clinical phenomena discussed here were described Attention mainly through lesion studies. However, the agnosias are Simultanagnosia can teach us about the way we allocate and also prevalent in degenerative disorders, often at a stage of shift attention. According to Posner et al.’s [26] paradigm, the illness at which general cognitive abilities are at least shifting attention from a previous to a novel focus is per- relatively preserved. The following syndromes have been formed in three steps: 1) disengaging from the previous reported in neurodegenerative disorders. focus, 2) moving attention between the foci, and 3) engaging attention at the new focus. In a recent report, Pavese et al. Posterior cortical atrophy [27•] provide compelling evidence that, at least in some This disorder presents with a progressive decline in com- instances, simultanagnosia is associated with a deficit in dis- plex visual processing and relative sparing of other cogni- engaging visual attention. They studied a patient with simul- tive abilities [18]. It is associated with occipito-parietal tanagnosia whose ability to report both items in an array was atrophy and hypometabolism on single photon emission greatly facilitated by removing the item that he had initially computed tomography (SPECT) or positron emission reported. Performance was not improved by the sudden tomography (PET) scans. In most instances, the disorder is onset of a second stimulus, suggesting that once he had caused by Alzheimer’s disease. However, posterior cortical “locked onto” a stimulus, he was unable to disengage atten- atrophy differs from typical Alzheimer’s disease in that the tion and shift to a different object or location