Oxford University Press 1998
Neurocase (1998) Vol. 4, pp. 71–87
A Functional Model of Visuo–verbal Disconnection and the Neuroanatomical Constraints of Optic Aphasia Claudio Luzzatti1,3, Raffaella I. Rumiati2 and Graziella Ghirardi3 1
Department of Psychology, School of Medicine, University of Milano, 20134 Milano, 2Programme in Neuroscience, International School of Advanced Studies, 34014 Triest and 3Rehabilitation Unit, Rho-Passirana Hospital, Azienda USSL 33, 20017 Passirana di Rho, Italy
Abstract In this paper, we discuss the case of a patient, AB, who presented a pattern of performance corresponding to that usually known as optic aphasia. In particular, her visual object naming was severely impaired, while tactile naming and naming to definition were significantly better. In addition to the classical visual anomia, the patient also showed a deficit in tasks requiring categorization and access to associative knowledge. We interpret the results of our patient in line with the explanation proposed by Coslett and Saffran (Brain, 1989; 112: 1091–110), i.e. a disconnection between right hemisphere and left hemisphere semantic knowledge. Damage to the left occipital region requires the initial processing of visual information to be carried out in the right hemisphere only, and a lesion of the splenium of the corpus callosum interrupts the flow of information from the right to the left hemisphere. However, the pattern of symptoms observed in our patient can only be fully explained by combining this framework with a model which distinguishes visual from verbal semantics (Shallice, 1988. From Neuropsychology To Mental Structure, Cambridge University Press). While the right hemisphere has a complete visual semantic organization, it has only a basic and concrete associative semantic representation. AB’s difficulties in categorizing and in accessing associative knowledge as the result of a visuo–verbal disconnection were also interpreted in this light. Furthermore, we suggest that the variable patterns of optic aphasia and the different behaviour of associative visual agnosic patients may be explained by interindividual differences in the levels of verbal and visual semantics in the right hemisphere.
Introduction In 1889, Carl Samuel Freund published a paper in which he described several cases on the borderline between anomic aphasia and visual agnosia (Seelenblindheit), some from his own observations, some from the contemporary literature. The paper is very important historically because it is the first theoretical account of disconnection disorders in the brain. In particular, he described the case of a patient (Carl Schluckwerder) who suffered from right hemianopia and a progressive language deficit characterized by anomia as well as severe reading and writing disorders. One peculiar aspect of the case was that although the patient could identify objects on sight, he was not able to name them. In fact, he was unable to name approximately half of the objects that were shown to him, but was often able to demonstrate the use of the same objects he was unable to name, and could name them easily and quickly through
tactile manipulation. Post-mortem examination demonstrated the presence of a left (parieto)–occipital sarcoma which extended through the splenium of the corpus callosum to the right parietal lobe. Freund called this pattern of behaviour optic aphasia (OA), and suggested that the deficit observed in the first stage of the disease was due both to a left (parieto)– occipital lesion determining right side hemianopia, and to a splenial lesion causing a disconnection of the intact right occipital lobe from the left hemisphere (LH) speech areas. According to Freund’s account, this lesional pattern resulted in a disconnection of a spared visual knowledge of objects and the corresponding (intact) lexical representation. It is important to remember that, in spite of detailed fractioning of the lexical components, neuropsychological models at that time were very imprecise with regard to the
Correspondence to: C. Luzzatti, Department of Psychology, School of Medicine, University of Milano, Via Tommaso Pini 1, 20134 Milano, Italy. Tel.: +39 2 21210-206; e-mail:
[email protected]
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meaning aspect. The ‘concept of an object’, i.e. the semantic knowledge of an object, was held to be the interaction of the corresponding multiple sensory representations (visual, tactile, acoustic, etc.), in the absence, however, of any non-sensory semantic knowledge. In other words, using modern terminology, there was no mention of verbal semantics for objects [a more accurate review of the state of the art at the end of the 19th century may be found in Sigmund Freud’s (1891) discussion of Freund’s account]. One year later, Lissauer (1890) discussed the existence of a modality specific visual disorder that he called associative visual agnosia. In contrast to OA, where the deficit has been explained as a visuo–verbal disconnection, patients suffering from associative visual agnosia show a primary impairment in the recognition of objects on sight. The explanation of OA as a visuo–verbal disconnection has not always been accepted. For instance, Freud (1891), Wolff (1904), Goldstein (1906) and Kleist (1934) argued that it was not possible to distinguish OA from associative visual agnosia or from anomia, whereas Norman Geschwind (1965) explained visual agnosia itself in terms of visuo–verbal disconnection, contending that most cases of visual agnosia should be reinterpreted as a ‘confabulatory visual anomia interfering with otherwise intact gnostic capacities’ (Geschwind and Fusillo, 1966). [Note: in his re-analysis of visual associative agnosia, Norman Geshwind described the major features and aspects of the disorder we now call optic aphasia. Curiously enough, however, he never used this term and he did not quote Freund’s paper. Thus, when writing his seminal paper on disconnection syndromes in animals and humans, he may not have been aware of Freund’s description and interpretation of OA.] More recently, other authors have described patients with behavioural characteristics similar to those first described by Freund, but in an information processing frame. In this paper, we shall give a brief summary of these latter accounts.
OA as a disconnection between visual and verbal semantic systems The first account of OA was put forward by Beauvois and colleagues (Lhermitte and Beauvois, 1973; Beauvois, 1982). They suggested that OA is due to damage to the interaction between a modality-specific visual and a verbal semantic system (see Shallice, 1988). This account was supported by two arguments: a physical separation of the right occipital lobe from speech areas in the LH, and a functional separation of the visual from the verbal semantic system. Verbal semantic knowledge was considered to be intact because patients were able to name objects under tactile presentation and in response to definition. A further piece of evidence in favour of an intact visual semantic system was their patients’ ability to mime the use of the objects they were not able to name. [Note that Beauvois and
colleagues emphasized the miming capacities of OA patients more than Freund actually did. In fact, Freund did not consider a spared miming ability to be a critical symptom of OA; he simply recorded the presence of this phenomenon in his patient. This was before Liepmann described apraxia (1900) and identified the left parietal lobe as being crucial for the planning of complex motor skills. Due to the parietal component of the lesion, Freund’s patient was occasionally able to demonstrate the use of an object, but it is very likely that he also suffered from ideomotor and ideational apraxia.] The defective naming of visually presented objects was ascribed to faulty mapping of the visual semantic representation of an object onto the corresponding verbal semantic representation. The multiple semantic systems theory does not contemplate a direct link between the semantic visual system and the phonological output lexicon, so difficulty in accessing the verbal semantic system from the visual semantic system will also result in a failure to access the corresponding phonological representation. The model also implies a bi-directional disconnection between visual and verbal semantics, with a consequently anomalous performance on a word-to-picture matching task. Beauvois (Beauvois, 1982; Beauvois and Saillant, 1985) also described a case of OA for colours only, which they interpreted within the theoretical frame of OA for objects.
OA as access visual agnosia A different account was put forward by Riddoch and Humphreys (1987) who interpreted the pattern of OA as ‘access visual agnosia’. They described the case of a patient (JB) who had difficulty in naming visually presented objects and in carrying out tasks requiring access to associative/ functional knowledge relative to those objects (e.g. pictureto-picture matching task). Riddoch and Humphreys interpreted the patient’s deficits as a visual agnosic impairment in accessing complete semantic representations of objects. However, JB was able to demonstrate the use of objects, and, since they had claimed that the patient could access only partial semantic knowledge about objects, the authors had to explain the preserved miming as being based on direct links between the shape representation of the object and the corresponding motor representation. In other words, JB’s gestures in response to visually presented objects were based on a preserved structural description system. JB’s good performance on the object decision task (i.e. the decision whether a drawing depicts a real object or not) was given as evidence for this preservation of the structural description system. However, JB was not able to perform a word-to-picture matching task, i.e. to recover the structural description of objects from their names. Consequently, as in the case reported by Beauvois (Beauvois, 1982; Beauvois and Saillant, 1985), the damage causing OA in this patient was bi-directional.
Visuo–verbal disconnection in optic aphasia Access visual agnosia as an explanation of OA has also been proposed by Hillis and Caramazza (1995) and, under the label of ‘visual modality specific naming impairment’, by Leek et al. (1994).
OA as a disruption of a non-semantic route for naming In analogy with a non-semantic route for reading, Ratcliff and Newcombe (1982) proposed the existence of a direct route for picture naming, bypassing the semantic system and leading from the pictogen (i.e. the structural description of an object) to the phonological output lexicon. Thus, OA could be caused by damage to a non-semantic route for naming (Kremin, 1986; Brennen et al., 1996), just as deep dyslexia is due to damage to a non-semantic route for reading. However, while some authors have provided reliable evidence for the presence of a lexical non-semantic route in word naming (see Schwartz et al., 1980), analogous evidence for picture naming has not yet been clearly documented (see Kremin, 1986). This damage alone, however, cannot account for the full symptomatology of OA. The major limitation of this account is that it does not explain why OA patients do not use this pathway when naming pictures if there is a spared connection from the pictogen to the semantic system and from the semantic system to the phonological output lexicon. Thus, in order to account for these problems and to explain why their patient made semantic errors, Ratcliff and Newcombe (1982) also had to assume a fuzziness in mapping semantic onto phonological representations. Damage to the direct non-semantic route for picture naming would explain the modality specificity of the naming impairment which is typical of OA. In addition, the noise in the transmission of information throughout the semantic system would predict semantic errors in naming, regardless of the modality. Therefore, a disturbance within the semantic system would affect all the output systems equally. Finally, Ratcliff and Newcombe (1982) argued that the fact that OA patients preserve the ability to mime the use of objects does not necessarily prove the integrity of the semantic system. The superiority of miming over naming could be explained more satisfactorily in terms of a possible ambiguity which arises in the former task. In fact, semantically related objects, for instance shoes, socks and boots, can require similar actions.
OA as an additive access deficit of the semantic system and of the phonological output lexicon Farah (1990) suggested that OA is subsequent to a lesion of two separate functional loci. A first deficit is in the accessing of semantics from the structural description system, while the second is in mapping semantic onto lexical phonological representations. According to Farah, naming
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is more affected in the visual than in other modalities because the effect of the damage to the two loci is superadditive. A similar account has recently been proposed by Manning and Campbell (1992; Campbell and Manning, 1996) and by Raymer et al. (1997).
OA as a disconnection between right and left hemisphere semantics Following Freund’s original theory, Coslett and Saffran (1989b, 1992) reintroduced the concept of a visuo–verbal disconnection in the light of a cognitive approach. In the case of a left occipital lesion, all visual information is processed in the right hemisphere (RH), i.e. early processing, the accomplishment of episodic object representation, the matching of that representation to the corresponding structural description and access to semantics. They suggested, however, that once processed in the RH, the information cannot be transferred to the speech areas of the LH because of the callosal disconnection. The performance of their patient (EM) on categorization and on associative matching tasks therefore relied on the residual lexical–semantic capacities of the isolated RH. The naming deficit is attributed to an inability to access the LH phonological output lexicon. Coslett and Saffran’s hypothesis also accounts for EM’s reading performance which was similar to the profile of deep dyslexic patients (Saffran et al., 1980; Coltheart, 1983) and pure alexic patients (Shallice and Saffran, 1986; Coslett and Saffran, 1989a). Studies on deep dyslexia and pure alexia suggest that the RH has only partial lexical orthographic capacities and mainly for concrete words (Schweiger et al., 1989; Zaidel, 1991).
The study The major merit of Coslett and Saffran’s account of OA is that they recognized the importance of anchoring a model of normal cognitive functions to its neuroanatomical foundation. The limit of their approach, however, is that they did not specify the concept of right and left hemisphere semantics. The best cognitive model giving an account of the concept of right and left hemisphere semantics would appear to be the multiple semantic system theory (Shallice, 1988). In this frame, it can be suggested that visual semantics, as well as the other sensory semantic systems, are represented bilaterally and symmetrically in the brain (within or close to the respective associative areas), whereas the organization of verbal semantics is asymmetrical. Although the LH is incontestably the main seat of lexical semantics and of the phonological and orthographic input and output lexicons, the RH too contains phonological and orthographic input lexicons as well as some degree of lexical semantics, even if only for concrete words (e.g. Schweiger et al., 1989; Zaidel, 1991).
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In order to support the claim that OA can only be explained by a theory that takes into account both a detailed cognitive model and its anatomical constraints, we shall discuss the case of a patient, AB, who showed the typical OA pattern. In fact, her visual object naming was grossly impaired, while her tactile naming and naming to definition were normal. Unlike the patients studied by Riddoch and Humphreys (1987) and Hillis and Caramazza (1995), AB’s difficulties in naming and in tasks requiring access to semantic knowledge, were not due to faulty access to the semantic system from vision, but, as we suggest, to a disconnection of the RH visual semantics from the LH verbal semantics.
Case history AB was a 74-year-old right-handed Italian housewife with 5 years of education. Despite her low educational level, she was a passionate reader of novels. She was admitted to her local hospital in August 1993 with paresthesias and mild hyposthenia affecting her right limbs. She also presented complete dense right homonimous hemianopia and anomic aphasia. A cerebral CT scan (31/8/93) showed an ischaemic lesion of the left occipital inferior and mesial cortex and of the underlying white matter, in the vascular territory of the left posterior cerebral artery (see Fig. 1). During the following months, AB’s disorders evolved to a resolution of the language deficits, whereas reading remained severely impaired. After she left the ward, AB first spent a few weeks with her daughter and then went home, where she was able to manage her day-to-day routine with no difficulty. She remained totally independent and a brilliant cook. In spite of her severe visual anomia and her complete alexia, neither the rehabilitation team nor the patient herself or her relatives reported any object misidentification. The only symptom that emerged was the occasional impairment in using tools, which, however, she was able to choose with no uncertainty. The patient died of a primary liver cancer 16 months after onset of the cerebrovascular disease. Post-mortem pathology was not performed.
Neuropsychological testing (October 1993) AB’s language deficits were assessed using the Italian version of the Aachen Aphasia Test (AAT) (Luzzatti et al., 1996). Her spontaneous speech was only mildly impaired, with a few anomic latencies. She showed a minimal deficit in repetition of single words and sentences (96th percentile of the aphasic distribution), although her processing of written language was severely impaired. However, the comparison of the tasks that compose this subtest showed that reading aloud was almost null (6th percentile), whilst writing under dictation was only moderately affected (56th percentile). The third part of the written language subtest, writing by composition, in which the patient had to write
words dictated by the examiner choosing the correct letters out of a set of 16, produced a curious result: AB was able to choose, but not to name, the majority of the letters required to write down the target words (24/31 correct choices out of 31 letters). Her performance on the Token Test was also impaired (18 errors out of 50; 43th percentile). Confrontation naming and description of 10 pictures depicting simple scenes was severely disrupted (20th percentile). AB named 3/20 artefact objects and 2/10 colours. A qualitative analysis of her responses revealed five perseverations, one semantic error (bed 0.05). With regard to errors, a qualitative analysis of the responses revealed that for the visual condition omissions represented 56% and perseverative behaviour 22%, while in the tactile condition, circumlocutions accounted for 42% and perseverations for 28.5%. A summary of the results is given in Table 1.
Task 2: naming to definitions AB was asked to name objects (e.g. umbrella), abstract concepts (e.g. happiness), and actions (e.g. to sleep) defined by the examiner (see Gavazzi et al., 1986). Her performance was at ceiling for concrete objects and actions (30/30), and
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Fig. 1. (a, b and c) CT-scan of AB (31/8/93) showing a left hemisphere ischaemic lesion of the occipital inferior and mesial temporal cortex and of the underlying white matter, in the vascular territory of the posterior cerebral artery; (d) lateral map. The map has been drawn up using the device described by Luzzatti et al. (1979). ( ) Occipital lateral+mesial cortex and underlying white matter; ( ) only mesial cortex and underlying white matter. The lesion does not affect the corpus callosum directly; however, the extension of the lesion along the antero–posterior dimension on the mesial surface and the underlying white matter, clearly results in a functional disconnection of the callosal pathways.
her performance on abstract nouns (25/30) was still within the normal range.
Task 3: naming of line drawings We asked AB to name 103 drawings from Snodgrass and Vanderwart’s norms (1980; see Luzzatti and Davidoff, 1994, for a description of the battery). The stimuli, presented in random order, were 44 natural items (six fruits, six vegetables, six domestic mammals, six wild mammals, six non-mammal animals, six birds, eight body parts)
and 59 artificial objects (22 tools or other artefacts, eight items of furniture, five household electrical appliances, eight items of clothing, eight types of vehicle and eight musical instruments). To reduce the rate of null responses obtained in Task 1, we increased the time limit for each item to 30 s. When responses to natural items and to artificial items were compared, AB’s naming of artefacts did not differ from her naming of natural objects [÷2(1) = 0.84; n.s.]. Table 2 shows a qualitative analysis of the naming responses. We distinguished between a lack of reaction,
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Table 1. Types of error made by AB when naming real objects in the visual and in the tactile modality (Task 1) Correct responses
Visual modality 7 (23%)
Tactile modality 23 (77%)
Type of errors Omissions Circumlocutions Semantic or verbal paraphasias Visual errors Visual–semantic errors Perseverations Total number of errors
13 1 1 1 2 5 23 (77%)
1 3 1 — — 2 7 (23%)
word finding deficits with circumlocutions, co-ordinate semantic responses, visual errors and perseverations. Circumlocutions were divided into good, medium and poor. A good circumlocution is an answer in which sufficient information is given to allow the unequivocal identification of the item. A medium circumlocution is an answer in which the description does not capture the core of the concept inherent to the given item, while a poor one is an answer in which the description does not capture at all the core of the concept. Overall, the large majority of errors made by AB were circumlocutions (n = 43; 42%) and perseverations (n = 23; 22%). Circumlocutions were tested for adequacy by asking five naive control subjects to identify objects from AB’s descriptions. Almost all circumlocutions started with the name of the sovraordinate category which, in some cases, was left alone without any further specification. There was no difference between natural and artificial items [÷2(1) = 0.42; n.s.], but the distribution of errors was not identical: while the rate of perseverations [÷2(1) = 1.22; n.s.] was similar, the incidence of circumlocutions was higher for natural than for artificial objects [÷2(1) = 8.2; P < 0.01].
Discussion of section A AB showed a naming deficit specific to visual modality (23%, Task 1), but performed better on naming from manipulation (77%, Task 1) and in response to definition (100%, Task 2). When analysing her performance for error type, we found the majority of errors were circumlocutions (42%) and perseverations (22%), but only one visual and few semantic errors. Circumlocutions were either appropriate for a clear identification of the item or incomplete, but often specific enough to signal the recognition of the object. Overall, this pattern of disruption is not compatible with Riddoch and Humphreys’s (1987) interpretation of OA as an access visual agnosia, which would require a prevalence of visual and visual–semantic errors. However, the different pattern of performance observed for their patient (JB) could also be explained by the inclusion of circumlocutions and superordinate responses into the semantic and visual–semantic category of errors.
The prevalence of perseverations shown by AB is in line with the results obtained by Plaut and Shallice (1993) in a simulation study of OA. The authors devised a network for a picture naming task using 44 indoor man-made objects, and then damaged it, obtaining the typical error pattern of OA. The network produced a prevalence of ‘vertical errors’ over ‘horizontal errors’, i.e. of naming responses biased by the response given to previous objects (Lhermitte and Beauvois, 1973). Selective impairment of visually presented objects suggests that AB’s naming disorder was not secondary to an output lexicon impairment or to a loss of knowledge within a unitary semantic system. If AB’s naming deficits had been located at either of these processing levels, her naming performance should have been affected equally for all input modalities. Since this was not the case, the naming disorder could be ascribed to a problem arising at any sub-stage of the recognition of a seen object, prior to access to semantics, i.e. to damage of the patient’s ability to construct an episodic visual representation of an object, access to the stored structural description system and/or to visual semantics, access to a unitary semantic system.
Section B: early visual processing AB’s ability to produce an on-line representation of visual stimuli was assessed by the Poppelreuter test and the reproduction of geometric line drawings.
Task 4: Poppelreuter–Ghent’s test This test is used to assess figure–ground discrimination as a stage of early visual processing. The patient had to identify three to five overlapping line drawings by pointing to each of the target drawings among 10 individual alternatives displayed underneath (Della Sala et al., 1995). The test material consists of nine patterns of meaningful overlapped line drawings (36 objects) and nine patterns of meaningless forms (35 abstract line drawings). AB’s performance was within the normal range and there was no difference between real objects and meaningless drawings (29.87 and 23.38 respectively; 12–25th percentile of the normal distribution).
Task 5: copying of line drawings This task is used to assess whether a patient is able to construct and hold an episodic representation of a geometrical shape (De Renzi and Faglioni, 1967). AB’s copies of 2- and 3-D geometric line drawings (17/20) were within the normal range (r16). She was also able to copy quite accurately line drawings of natural and artificial items taken from the Snodgrass and Vandewart set (1980). AB’s performance on Tasks 4 and 5 shows that her naming deficits of visually presented objects cannot be attributed to a disorder in low-level processing.
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Table 2. Types of error produced by AB when naming line drawings (Task 3)
Correct responses (inclusive of anomic latencies) Types of error Null responses Good circumlocutionsa Medium circumlocutionsb Poor circumlocutionsc Circumlocutions+inappropriate naming from circumlocutiond Coordinate semantic responsese Supraordinate responses without circumlocutionf Unrelated verbal paraphasias Visual errorsg Perseverations Visual errors and/or perseverations and/or semantic paraphasiash Total number of errors
Natural (n = 44)
Artificial (n = 59)
Total (n = 103)
7 (16)
14+1 (25)
22 (21)
1 (1) 10 (23) 12 (27) 4 (9) — 3 (7) — — — 7 (16) — 37 (84)
1 (1) 6 (10) 6 (10) 4 (7) 1 (2) 4 (7) 1 (2) 1 (2) 1 (2) 16 (25) 3 (5) 44 (75)
2 (2) 16 (16) 18 (17) 8 (8) 1 (1) 7 (7) 1 (1) 1 (1) 1 (1) 23 (22) 3 (3) 81 (79)
Figures in parentheses are percentages. Examples: aTie: men wear it around their necks, it may be made of silk or of wool. b Broom: people use it in the kitchen. c Anchor: it is used to pick up things. d Bus: you get on it . . . a ladder! e Glove: a sock. f Drum: a musical instrument. g Record player: a balance. h Scissors: a clothes peg (perseveration from previous item, but both are X shaped).
Section C: access to the structural description system
Section D: from an object name to the underlying visual representations
After ascertaining that AB’s low-level visual processing was not impaired, we then assessed her ability to access the structural description system. For identification to occur, the episodic description obtained from the image of an object needs to match its corresponding stored structural description (see Marr, 1980). The existence of a system for structural descriptions has been supported by the evidence from neuropsychological studies (see Riddoch and Humphreys, 1987) as well as by evidence from priming experiments with normal subjects (see Schacter et al., 1990; Schacter and Cooper, 1993). The integrity of the structural description system was tested using an object decision task (Task 6).
A word-to-picture matching task and drawing from memory were used to tap the relationship between words and pictures.
Task 6: object decision task (line drawings) In this task, AB was asked to decide whether the object depicted by a line drawing was real or not. Non-real objects were obtained by substituting a part of an animal with a part belonging to another animal. In performing this task (i.e. is the stimulus real?), a subject relies on the structural description system where knowledge of the structure of objects is stored. If AB’s naming deficits were due to impaired access to the structural description system, she would have performed poorly on this task. Thirty-four line drawings, 17 of real animals and 17 of chimerae, were presented to the patient one after the other in random order. AB’s performance on this task was flawless (34/34, 100%).
Task 7: spoken word-to-picture matching In order to match a name to a target picture, a subject has to retrieve the corresponding structural representation of the object and match it to one of the pictures from the set. AB was presented with a matrix of six-to-eight line drawings from Snodgrass and Vanderwart’s set (1980). She was asked to point to the picture corresponding to a name spoken aloud by the examiner. In addition to the target picture, five to seven distractors were taken from the same category. A total of 109 items from different categories (44 natural and 65 artificial) was presented in random order. AB performed this task without any hesitation or error (109/109, 100%).
Task 8: drawing from memory In order to draw an object from memory, a subject has to activate the stored visual knowledge of the shape of the object. The examiner spoke aloud the name of 14 artificial and 12 natural objects and AB was asked to draw them with her right hand. Ten independent control subjects judged the accuracy of AB’s drawings, which was quantified as the rate of correct identifications of the objects made by the judges. Her performance was compared with that of
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Fig. 2. Examples of drawing of named objects: (a) natural items: artichoke, strawberry, grape, banana; (b) artificial items: flag, bicycle, broom, umbrella.
five normal subjects of comparable age, whose drawings were evaluated by five different independent judges. The rate of identification of AB’s drawings (85/260, 33%) did not differ from that of the five control subjects (249/650, 38%) [÷2(1) = 2.28, n.s.]. However, on separation of the results for natural and artificial items, her natural drawings (42/120, 35%) did not differ from those of the controls (89/300, 30%) [÷2(1) = 0.90; n.s], but her performance on artificial items (43/140, 31%) was lower (160/350, 46%) [÷2(1) 8.65; P < 0.01]. Figure 2 shows some examples of AB’s drawings from memory.
Discussion of sections B–D AB’s performance in low-level visual object processing (Tasks 4–5) and in accessing stored structural knowledge (Task 6) is normal. In addition, the results obtained from Tasks 7 and 8 showed that AB was also able to access visual knowledge of objects from their names. Furthermore, normal performance on a word-to-picture matching task is not compatible with an explanation of OA as based on a bi-directional disconnection between visual and verbal semantics (Lhermitte and Beauvois, 1973). [Note: the problem of mono-/bi-directionality in the disconnection was considered by Freund. In his review and discussion on the
nature of OA, he described five possible forms, in two of which he clearly predicted a unidirectional deficit, i.e. a picture naming deficit with spared word-to-picture association.]
Section E: access to semantics The aim of this section is to verify whether AB’s deficit in naming pictures (or visually presented objects) is due either to damage in the accessing of semantic knowledge from structural descriptions or to a loss of semantic knowledge itself. The multiple semantic system theory requires a further fractioning of semantics into visual and verbal components, and so the naming deficit could arise either at visual semantic or at verbal semantic level, or it could be caused by damage to the mapping of visual onto verbal semantics.
Task 9: picture-to-picture matching In order to test AB’s access to semantics from pictures we used a test devised by Visch-Brink and Denes (1993), adapted from the Pyramids and Palm Trees Test developed by Howard and Patterson (1992). AB was asked to match a picture to one of four alternatives. Correct matching
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Table 3. Summary of AB’s performance on a picture-to-picture and word-to-word association task (Tasks 9 and 10) 3rd evaluation
(n = 30)
1st evaluation (pictures only)
2nd evaluation (pictures only
(pictures)
(words)
Correct responses
17 (53%)
22 (73%)
20 (67%)
27 (90%)
Type of errors Semantic errors Visual errors Visual and/or semantic Unrelated responses No response
8 — 1 2 2
4 — 1 2 1
6 — 2 — 2
2 — — — 1
involved the choice of a picture semantically related but not visually similar to the target. Three types of distractors were used: semantically distant, visually similar, and unrelated pictures. Thirty items were presented one after the other, each on an A4 page. AB’s performance was compared with that of 23 controls (mean age = 75 years) without neurological impairments (Visch-Brink and Denes, 1993: normal range r24). She showed severe impairment in performing this task, making only 17 correct matchings out of 30 (57%). In the light of her poor performance on the first session, the test was repeated a week later, and this time AB scored 22/30 (73%). The consistency across sessions was very low [÷2(1) < 1.0; n.s.]. The majority of the errors made involved the choice of distantly related distractors (see Table 3). When compared with results obtained from a sample of aphasic patients, AB’s score was below that of 15 Broca’s (mean = 24.3; SD = 5.4), of 21 Wernicke’s (23.0 7.0), of 15 anomic (23.5 4.4), and came close to that of 13 global aphasic patients (22.9 5.4) (E.G. Visch-Brink and G. Denes, in preparation).
Task 10: word-to-word matching Given AB’s severe reading deficit, it was not possible to use the standard written verbal version of the task and so an oral version was used. AB’s score on the oral verbal task (see Table 3) was better than her performance on a third visual presentation [÷2(1) = 5; P < 0.05] and no consistency was found between the verbal and the pictorial version of the test [÷2(1) < 1.0; n.s.].
Task 11: selecting pictures belonging to the same category AB was shown four pictures of objects and was asked to single out two belonging to the same category. Distractors were taken from other categories (e.g. violin, bicycle, watering can, accordion). AB performed this task without hesitation and with near perfect scores: 19/20 (95%).
Task 12: sorting pictures into categories In this task, AB was asked to sort pictures into categories. The task was divided into five blocks. Each block comprised 20–24 pictures to be sorted into three categories, making a total of 106 pictures to be sorted. Pictures and categories were the same as in Task 3. On a first run, the patient was not given the name of the categories. AB performed the task very slowly, complaining that it was too difficult and that she could not understand what to do; she kept correcting herself over and over again, and only eventually did she give a final answer. She classified only 78/106 pictures (74%) correctly; making a total of 28 errors, of which she rectified 13 after generical prompting. [Errors were as follows: vegetables: one item was classified as a tool; tools: three items as musical instruments; musical instruments: two as tools, three items were classified as pieces of furniture; means of transportation: three as tools, one as piece of furniture; pieces of furniture: one as tool, one as means of transportation; 13 items were not classified.]
Task 12a: sorting pictures into categories (after the names of the categories were supplied) AB’s poor performance on Task 12 (sorting pictures into categories) was unexpected, considering the ease of the task and her successful performance on Task 11. Her difficulties could be explained by the fact that she was not supplied with the actual name of the categories into which to sort the pictures and that she was not able to deduce them from the pictures. We repeated the task a week later, starting with the same procedure, and AB showed the same slow and uncertain behaviour. This time, however, in view of her repeated lack of success, the names of the target categories were supplied. AB’s ability to perform the task improved dramatically and she was able to sort 99 pictures out of 106 (93%) correctly and without any hesitation. [Errors were as follows: tools: two as vegetables; means of transportation: four as musical instruments; pieces of furniture: one as musical instrument.] Of the seven errors,
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one was repaired after generical prompting. Performance on Task 12a was significantly better than on task 12 [÷2(1) = 13.6; P < 0.001].
Discussion of section E In this section, we attempted to assess AB’s access to semantic knowledge from vision and the integrity of the semantic system itself. In compliance with the multiple semantic theory, we also assessed the integrity of the visual and verbal components and of the mapping of visual onto verbal semantics. AB’s performance on an associative matching task with pictures (Task 9) was severely impaired and there was no consistency between the responses over time (Exams 1 and 2). Her performance on a spoken version of the same task (Task 10) was significantly better and there was no consistency between the visual and verbal conditions; finally, sorting items into categories benefited from lexical cueing. In fact, AB was able to sort pictures when she was given the names of the categories to which the pictures belonged (Task 12a), whereas she hesitated and made many mistakes when the names of the categories were withheld (Task 12). It is usually claimed that a demonstration of preserved access to semantic knowledge in tests which do not require verbal responses is crucial to any account of OA as a modality specific visual naming deficit. Typical examples are a normal performance on picture-to-picture association tasks and on picture categorization tasks. AB’s behaviour, however, did not correspond to this pattern. Results could be interpreted as a deficit in accessing complete semantic knowledge from vision. However, the explanation suggested by Riddoch and Humphreys (1987) and Hillis and Caramazza (1995) does not account for the fact that this defect only emerged in categorical/ associative tasks, while there were no visual and visual– semantic errors in visual confrontation naming. Furthermore, the explanation of OA as an access deficit from vision to semantics does not account for the differences in AB’s performance on the sorting task with and without lexical cueing. We suggest that this difference could be explained by assuming that AB’s performance deteriorates when a task requires the retrieval of associative and categorical knowledge which is actually lexical in nature. Rephrasing this concept in terms of a multiple semantic systems theory, AB was unable to access verbal semantics from visual semantics. We believe that in a categorization task a subject will usually (though not necessarily) adopt a lexical strategy. Therefore, on being presented with a table, a chair or a cupboard interspersed with items from other categories, the subject will sort the items after having identified (in point of fact, after having named implicitly) the corresponding categories (‘a table is a piece of furniture . . . the chair and the cupboard are also pieces of furniture, etc.’). This could
also explain AB’s poor performance on the picture-topicture association task, where her scores were lower than those of a sample of severe (global and Wernicke’s) aphasic patients. In spite of the purely visual characteristics of the stimuli used in a picture-to-picture matching task, we suggest that the task may be made easier when a lexical– semantic association is used rather than a visual association of the two objects in the same image. For instance, when a normal subject sees a pyramid, he may associate this image with the concept of Egypt and the concept of Egypt with a group of palm trees (‘Pyramids are in Egypt . . . also palm trees are typical of Egypt’). Therefore, AB’s low performance on this task can be explained as the result of an impaired access to lexical and/or lexical–semantic knowledge. The explanation of OA as a disconnection between visual and verbal semantics (Beauvois, 1982) integrates Coslett and Saffran’s (1989b) theory by specifying the concept of an interruption between right and left hemisphere semantics as a disconnection of the RH visual semantics from the LH verbal semantics and phonological output lexicon. The left occipital lesion forced AB to process pictorial stimuli in the right visual area, after which the callosal disconnection prevented the visual information processes reaching the LH. Therefore, she was not able to sort pictures into categories, since this task requires the activation of abstract categorical verbal representations which are not available in the RH. However, lexical cueing, which provided her LH with the relevant information (the names of the categories), produced an almost normal performance (see Coslett and Saffran, 1989b; Davidoff, 1991, p. 150, for a similar account). Likewise, her performance on a picture-to-picture associative task was impaired since the LH verbal associative knowledge could not be accessed.
Section F: reading
Task 13: word naming AB’s ability to read words was assessed by the reading battery devised at the Rehabilitation Unit where she attended her rehabilitation programme. The stimuli consist of 86 words (29 regular concrete nouns; 18 concrete nouns with irregular stress; 24 abstract nouns; 20 function words) and 28 non-words. AB was not able to read aloud either words or non-words, nor was she able to match written words to pictures.
Task 14: letter naming AB was asked to read 48 upper- or lowercase letters presented in random order. Her performance on this task was severely affected. AB could read only 9/24 (38%) uppercase and 8/24 (34%) lowercase letters. An analysis of errors revealed a predominance of perseverations.
Visuo–verbal disconnection in optic aphasia
Task 15: pointing to letters An evaluation carried out in April 1994 showed that AB’s ability to name letters was unchanged and that she was still able to name only 10/24 (42%) uppercase and 9/24 (38%) lowercase letters. During the same session, we also tested her with a letter name-to-letter symbol matching task. The examiner spoke the name of a letter aloud and AB had to point to the corresponding letter among 24 alternatives displayed in random order. She was able to point to 17/24 (71%) uppercase and 16/24 (67%) lowercase letters, and this performance was significantly better than her letter naming [÷2(1) = 10.7; P < 0.002].
Discussion of section F AB’s ability to read letters, words and non-words was severely impaired, while she showed only mild impairment on a letter pointing task, thus reproducing the unidirectional pattern of impairment observed with pictures of objects.
Section G: colour naming and object colour knowledge As mentioned in the Introduction, Beauvois and Saillant (1985) demonstrated disconnection between visual and verbal semantics in a patient with OA for colours. The presence of verbal disorders that involve specifically the processing of colours is predicted by cognitive psychological models, on the basis of the prevailing visuo–perceptive knowledge underlying their lexical representation. In the light of the particular status of colours, we also tested AB’s abilities in colour processing and object–colour knowledge.
Task 16: colour naming AB was asked to name 10 patches of prototypical colours. The task was repeated three times, and AB consistently scored 2/10. Errors were mostly perseverations, semantic paraphasias (names of other colours), word finding difficulties and anomic latencies.
Task 17: colour name-to-patch of colour matching AB was asked to point to a colour named by the examiner from 10 alternatives. The task was presented three times and AB performed almost flawlessly each time, obtaining a total score of 25/30 (83.5%). She pointed to the wrong patch of colour on two occasions only; the remaining failures were a latency and two repairs.
Task 18: object colour knowledge Object colour knowledge was tested by asking the patient to retrieve from memory the typical colour of an object
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(stimulus: ‘what is the typical colour of a . . .?’). Objects were natural (fruit, vegetables, animals) or artificial (conventional colours of man-made objects, e.g. a fire engine = red) (see Luzzatti and Davidoff, 1994, for a more extensive description of the task). On a first run, AB was asked to give a verbal response (the name of the colour), then she was asked to choose the closest colour from a set of 18 crayons. Overall, AB’s performance was quite accurate: on the verbal condition she gave 18/22 (82%) correct responses for the natural and 12/13 (92%) for the artificial objects; on the visual condition 17/22 (77%) for the natural and 11/13 (100%) for the artificial objects. There was no difference between visual and verbal responses. The apparent difference between natural and artificial items (35/44 versus 25/26) did not reach the level of significance [÷2(1) = 2.42].
Discussion of section G When processing colour information AB showed a pattern of impairment which was quite similar to that observed on objects and line-drawings of objects, i.e. a mono-directional visuo–verbal disconnection and spared visual colour knowledge of objects: naming of patches of colour was severely impaired, whereas pointing to colours named by the examiner as well as the retrieval of (visual) object–colour knowledge was preserved.
Section H: apraxia The tendency of some patients to compensate for their naming difficulties by trying to demonstrate the use of the objects they fail to name has been taken as a critical feature of OA. It has been claimed (see Beauvois, 1982) that the preserved ability of a patient to demonstrate how to use objects supports the existence of an intact semantic system. However, not all patients diagnosed as optic aphasics are able to mime the use of an object (e.g. Assal and Regli, 1980; Coslett and Saffran, 1989b).
Task 19: ideomotor apraxia AB was given a limb-apraxia test devised by De Renzi et al. (1980, 1982). The test consists of 24 gestures, performed by the examiner, which a patient is asked to reproduce with the left arm. Each gesture may be shown by the examiner up to three times; three points are scored for each item if the subject produces the correct gesture after the first presentation, two points after the second, one point after the third presentation, while a score of zero is given if the subject is not able to reproduce the gesture correctly. Therefore the total score ranges from 0 to 72 (61 is the cut-off score for a mild deficit, 52 for a moderate deficit). AB’s performance on the task was severely impaired (37/72). Her errors were mainly perseverations of actions—or parts of actions—that had been presented earlier in the session.
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Task 20: ideational apraxia In this task, AB was asked to demonstrate the use of objects and tools. The examiner placed each object in front of the patient and asked her to grasp it and to show how to use it. AB scored 11/14. Compared with the normative data (De Renzi et al., 1968) her performance was poor, falling within the range of a mild ideational apraxic behaviour.
Task 21: oral apraxia AB was given the oral apraxia test devised by De Renzi et al. (1966). The patient had to reproduce 10 oral movements performed by the examiner. Each item was scored 0–2 according to the accuracy of reproduction, the maximum score therefore being 20. AB’s performance on the task was almost perfect (19/20; normal range: r16).
Discussion of section H AB manifested a severe deficit in imitating gestures (Task 19) and a mild impairment in demonstrating the use of objects (Task 20), whereas no bucco–facial apraxia (Task 21) was observed. There are two possible explanations for the ideomotor limb apraxic symptoms and for the mild deficit in using objects (ideatory/utilization apraxia). The first explanation is that the LH lesion may also have damaged the centres that are involved in planning and programming complex movements. The crucial areas for limb apraxia are in the left parietal (and frontal) lobe (Liepmann, 1920; Basso et al., 1985; Faglioni and Basso, 1985). However, AB’s lesion was confined to the left occipital pole, the inferior and mesial part of the left temporal lobe and the underlying white matter and did not extend to the parietal lobe. The second explanation is that the lesion which disconnected the RH visual centres from the language areas in the left temporal lobe also disconnected the same visual centres from the LH ideatory and ideomotor areas in the left parietal lobe. This would explain the severe deficit in imitating gestures on visual presentation (for a similar account, see Raymer et al., 1997). However, the cause of the mild ideational apraxic disorder is less clear. We are inclined to believe that AB’s mild deficit in using objects could be ascribed to an interference of the visuo–praxic disconnection with the spared tactile processing of objects (see also Assal and Regli, 1980, for a similar account). Unfortunately we did not specifically test the use of objects from tactile stimuli only, but in the tactile naming condition (Task 1b) AB’s manipulation of the objects she had to name was normal and she demonstrated their use flawlessly. A final consideration can be made from AB’s completely normal imitation of oral movements. We suggest that the dissociation between a severe ideomotor and the absence of oral apraxia is the consequence of the different localization
in the LH of the centres involved in the control of the oral movements, i.e. the left insular cortex, the infero–posterior frontal cortex and the underlying subcortical nuclei (Tognola and Vignolo, 1979), with respect to the limb movements. Owing to the left occipital lesion, the visual images of the oral movement to be imitated project to the right visual cortex only. Moreover, we suggest that the path projecting to the left insular area crosses the corpus callosum at a more anterior point and is therefore not affected by splenial anatomical functional damage. A separate anatomical path for the processing of oral and limb movement is also to be considered as theoretically sound on the basis of the different types of knowledge underlying oral and limb movements, since the latter are oriented in a three-dimensional space and are more crucially related to the use of objects. We therefore suggest an explanation similar to that proposed by Manning and Campbell (1992; Campbell and Manning, 1996) in their discussion on dissociation by type of knowledge in an OA patient with severe visual anomia for objects but not for actions.
General discussion We have described the case of a patient, AB, who had a lesion of the left occipital inferior and mesial cortex and of the underlying white matter. She presented the symptoms described by Freund as optic aphasia (OA), that is, a modality specific deficit in naming line drawings and real objects from sight (Table 4, Tasks 1a and 3), but better tactile naming (Task 1b) and spared naming-to-definition (Task 2). We demonstrated that her modality specific naming deficit was not due to an early recognition problem, since AB was able to perceive visually presented objects adequately (Tasks 4–5). Accurate performance on the object decision task also suggests that AB was able to access the structural description of objects (Task 6). AB performed promptly and without errors on a word-topicture matching task (Task 7) and drew natural and artificial objects from memory fairly accurately on verbal stimulus (Task 8). A normal performance on these tasks indicates that AB was able to access structural knowledge of objects from verbal stimuli. Her performance on Task 7 is not consistent with the performance of other OA patients (e.g. Beauvois, 1982; Riddoch and Humphreys, 1987) who were impaired on a word-to-picture matching task. The presence of this deficit led Beauvois to assume bi-directional damage in her patient: from the shape of an object to its name and from the name of the object to its shape. AB’s preserved object–colour knowledge indicates normal access to the structural description system and/or to the visual semantic knowledge from the phonological input lexicon. Unidirectional damage is confirmed by normal performance on the colour name-to-colour patch matching (in contrast with the severe disorder in retrieving colour
Visuo–verbal disconnection in optic aphasia Table 4. Summary of AB’s results Tasks Section A: naming 1. Naming real objects visual tactile 2. Naming to definition 3. Naming line drawings Section B: early visual processing 4. Poppelreuter–Ghent 5. Copying of geometrical drawings Section C: access to structural description 6. Object decision Section D: from words to the underlying visual knowledge 7. Spoken word-to-picture matching 8. Drawing from memory Section E: access to semantics 9. Picture-to-picture matching 10. Word-to-word matching 11. Selecting pictures belonging to the same category 12. Sorting pictures into categories (a) without name of categories (b) with name of categories Section F: reading and writing 13. Word naming 14. Letter naming 15. Pointing to letters spelling from a visual multiple choice (AAT) [compare with handwriting under dictation (AAT)] Section G: colour naming and object colour knowledge 16. Colour naming 17. Colour name to patch of colour matching 18. Object colour knowledge visual response (pointing to a crayon) verbal response (name of the colour) Section H: apraxia 20. Reproduction of gestures (ideomotor apraxia) 21. Use of objects 22. Oral apraxia
Normal
Mild deficit
Severe deficit
+ +
+ + + +
+ +
+
+ + +
+
names from patches of colour) and on the letter name-toletter symbol matching (in contrast with the severe disorder in letter naming). However, as opposed to some other OA patients described in the literature (see De Renzi and Saetti, 1977, for a review), AB could only occasionally demonstrate retained recognition of an object by an appropriate gesture or by her ability to sort objects into categories or to associate semantically related pictures. AB’s deficit on the picture-to-picture matching task (Task 9) and on the sorting of pictures into categories task (Tasks 11–12a) could be accounted for by an access visual agnosia (Riddoch and Humphreys, 1987; Hillis and Caramazza, 1995). However, this would not account for the dramatic improvement after lexical cueing (i.e. after the names of the categories to which the objects belonged were
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supplied). We agree that AB showed a deficit in accessing semantic knowledge from visual stimuli, but we prefer to explain these results in the framework of a multiple semantic systems theory. AB was able to access her visual semantic knowledge without difficulty, but not her verbal categorical and associative knowledge that also seems to be required in a ‘purely’ visual task as a picture-to-picture matching task. Coslett and Saffran’s (1989b) account of OA as a disconnection between right and left hemisphere semantics provides a more satisfactory explanation of AB’s pattern of performance. They assumed that as a consequence of the left occipital lesion, visual information in OA is analysed in the RH only, from the early processing stages to semantics and to object identification. This information cannot access the LH semantic and lexical representations due to the posterior disconnection of the callosal pathways. Coslett and Saffran’s theory, however, does not completely account for the pattern of symptoms observed in OA. In fact, the theory assumes its full heuristic capacity only after the concept of left and right hemisphere semantics has been integrated into a more detailed cognitive model of visuo– verbal interaction. A cognitive model based on a multiple semantic systems representation appears to supply the best theoretical framework for the explanation of our patient’s data and, more in general, of OA. Figure 3 shows a model that describes the cognitive units which are involved in each hemisphere for the processing of different kinds of information (pictures, written and spoken words, etc.). We also suggest which of these units AB might not have been able to access. It is well known that cognitive functions in the human brain are distributed asymmetrically, and this is particularly true for language. It follows that some operations may be performed equally well by either hemisphere, while other functions are lateralized. With regard to word processing, the orthographic and phonological output lexicons are usually represented in the LH only. This appears to be the case for all sub-word-level processing routines also. Another important difference between the two hemispheres concerns the organization of the phonological and orthographic input lexicons and verbal semantics. In fact, while the LH has a full lexical and verbal–semantic representation, the RH has a more basic and concrete representation of imageable and morphologically simple content words only. A partial lexical ability of the RH has already been proposed to explain residual RH lexical and semantic processing after complete callosal disconnection (Zaidel, 1982, 1991), and after left hemispherectomies (Patterson et al., 1989). The same explanation has also been suggested to account for the pattern of performance in deep dyslexia (Coltheart, 1980, 1983; Schweiger et al., 1989) and the spared lexical decision and implicit semantic processing in pure alexia (Coslett and Saffran, 1989a, 1994) after focal lesions of the LH. This model adequately accounts for the pattern of symptoms usually associated with OA. After a lesion of the left
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Fig. 3. Flowchart representing the functional pathways for object identification and naming in the left and right hemispheres. The diagram also shows the functional lesions explaining AB’s symptomatology.
occipital lobe, the image of an object is analysed and identified by the RH which can subserve visual analysis, from the early processing stages to the level where the episodic representation of the object is matched to stored structural representations, and activate underlying visual semantic knowledge. In order to retrieve the name of the object or to access categorical associative knowledge, visual semantic information has to cross through the corpus callosum to the LH and reach the corresponding complete verbal semantic and lexical representations. In OA, such activation of the LH lexical–semantic and lexical representations is impaired by a functional disconnection of the callosal pathways. This damage results in faulty access to the name of the visually presented object, and impaired retrieval of the more abstract component of lexical semantic knowledge, which are usually stored in the LH only. The model also explains AB’s spared ability to match words to pictures and to draw objects from memory. In the first instance the RH has sufficient lexical and lexical– semantic capacity to process the name of concrete objects
and to retrieve the corresponding RH visual semantic knowledge (and to point to the correct picture). The same path could also explain the spared ability—or only minor impairment—in drawing objects from memory. However, we suggest that drawing from memory may have been performed along another path in the LH that still had enough visual semantic knowledge to allow the patient to carry out this task. An account of OA that considers the linguistic capacity of the RH and the variability of this capacity between subjects would also explain the variable pattern of symptoms that may follow a left occipital lesion with functional damage of the splenial callosal pathways. In fact, an almost identical lesion may be the cause either of pure alexia (Dejerine, 1892) or of OA (Freund, 1889) or associative visual agnosia (Hahn, 1895). While Schnider et al. (1994) tried to explain this ambiguity with a differential impairment of the splenium, De Renzi (1996) first suggested that a premorbid variability of RH lexical–semantic competence could explain the continuum of symptoms ranging from OA to associative visual agnosia: ‘It is this premorbid
Visuo–verbal disconnection in optic aphasia individual feature that determines the degree to which the RH can compensate for the left side’s lack of contribution to visual semantic processing and whether the clinical profile of the patient is more skewed towards optic aphasia or visual agnosia. The issue of individual differences tends to be neglected by the neuropsychological literature, mainly because they cannot be ascertained in the healthy subject and are only inferred post-hoc, on the basis of different patterns of impairment associated with the same pathological findings.’ In this frame we may postulate a continuum of symptoms ranging from an isolated right hemianopia (for a full linguistic equivalence of the two hemispheres) to associative visual agnosia (in the case of an extreme poverty of lexical and visual semantic processing by the RH) passing through pure alexia, pure alexia with OA for colours, and OA also for objects, respectively (De Renzi et al., 1987; De Renzi and Saetti, 1997). Finally, the model represented in Fig. 3 can also explain AB’s deficit in imitating gestures or miming the use of visually presented objects, as the ideatory and the ideomotor representations are usually lateralized in the LH. The visual representation of a gesture or of an object has to cross to the LH in order to activate the appropriate ideatory and ideomotor representation. In general, demonstration of the use of objects has been taken as evidence of an intact semantic system (e.g. Beauvois, 1982; Ratcliff and Newcombe, 1982). However, if OA is the result of a lesion which disconnects the RH visual areas from the LH language areas, this same lesion will also disconnect the RH from the ideational representations of gestures in the left parietal lobe (see Assal and Regli, 1981; Raymer et al., 1997 for a similar conclusion). Thus, a preserved capacity to mime the use of a visually presented object should not be considered a crucial element of OA; on the contrary, the presence of limb apraxia seems to be rather the rule than the exception. Unfortunately the presence or absence of these symptoms has not been tested systematically in the various studies of OA currently available (see De Renzi and Saetti, 1997, for a review). In conclusion, the nature of OA can only be elucidated by an explanation which takes into account both a detailed information processing model of object recognition and naming, and its functional–anatomical constraints.
Acknowledgements We are indebted to Jane Riddoch and Tim Shallice for comments and suggestions on a preliminary version of the paper and to the three anonymous referees of Neurocase for their insightful criticisms. We also thank Frances Anderson for her patient and accurate review of the English version of the manuscript. Portions of this paper were presented at TenNet 7th Meeting, Montreal, August 14–16, 1996 (Brain and Cognition 1996; 32: 199–202) and at the ASI-Symposium (NATO) on the Role of the Human
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Corpus Callosum, Lucca (Italy), September 1–10, 1996. This work was supported by a grant from the Italian National Research Council (CNR) and from the Ministero dell’Universita’ e della Ricerca Scientifica (MURST) to C.L.
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Received on 27 January, 1997; resubmitted on 13 November, 1997; accepted on 18 November, 1997
Visuo–verbal disconnection in optic aphasia
A functional model of visuo–verbal disconnection and the neuroanatomical constraints of optic aphasia C. Luzzatti, R. I. Rumiati and G. Ghirardi Abstract
In this paper we discuss the case of a patient, AB, who presented a pattern of performance corresponding to that usually known as optic aphasia. In particular, her visual object naming was severely impaired, while tactile naming and naming to definition were significantly better. In addition to the classical visual anomia, the patient also showed a deficit in tasks requiring categorization and access to associative knowledge. We interpret the results of our patient in line with the explanation proposed by Coslett and Saffran (Brain, 1989; 112: 1091–110), i.e. a disconnection between right hemisphere and left hemisphere semantic knowledge. Damage to the left occipital region requires the initial processing of visual information to be carried out in the right hemisphere only, and a lesion of the splenium of the corpus callosum interrupts the flow of information from the right to the left hemisphere. However, the pattern of symptoms observed in our patient can only be fully explained combining this framework with a model which distinguishes visual from verbal semantics (Shallice, 1988. From Neuropsychology To Mental Structure, Cambridge University Press). While the right hemisphere has a complete visual semantic organization, it has an only basic and concrete associative semantic representation. AB’s difficulties in categorizing and in accessing associative knowledge as the result of a visuo–verbal disconnection were also interpreted in this light. Furthermore, we suggest that the variable patterns of optic aphasia and the different behaviour of associative visual agnosic patients may be explained by interindividual differences in the levels of verbal and visual semantics in the right hemisphere.
Journal
Neurocase 1998; 4: 71–87
Neurocase Reference Number: O105
Primary diagnosis of interest Optic aphasia
Author’s designation of case AB
Key theoretical issue
Optic aphasia can arise because of a disconnection between right and left hemisphere semantic networks.
Key words: aphasia; vision; disconnection; semantics; lexicon; agnosia
Scan, EEG and related measures CT
Standardized assessment
Italian version of the Aachen Aphasia Test (AAT): Luzzatti et al., 1996
Other assessment
Dyslexia, naming, extensive assessment of lexical–semantic abilities
Lesion location
Left occipital inferior and mesial cortex and underlying white matter
Lesion type CVA
Language English
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