Disorders of visual recognition.

Agnosias are disorders of recognition, specific to one sensory channel, that affect either the perceptual analysis of the stimulus or the recognition of its meaning. In the visual modality, objects, faces, and colors can be separately disrupted. Apperceptive object agnosia refers to failure to achieve a structured description of the shape of the object. Associative agnosia refers to inability to attribute a meaning to a correctly perceived stimulus. It must be differentiated from optic aphasia, in which the object is recognized but cannot be named in the visual modality. Associative agnosia and optic aphasia are associated with left occipitotemporal damage, and they differ more quantitatively than qualitatively. The inability to recognize familiar faces (prosopagnosia) can appear in isolation and be, in some cases, associated with a lesion confined to the occipitotemporal region of the right hemisphere. These findings are supportive of the idea that faces have a separate representation in the brain. Disorders of color cognition can affect color categorization, color-name association, and color-object association. They are linked to left hemisphere damage. The ability to recognize objects presented in the visual modality is a hierarchical process in which several cortical areas, corresponding to about 30% of the cortical mantle, participate. Their selective lesion results in a gamut of disorders whose identification provides the experienced neurologist with clues to the locus of damage and contributes to the understanding of the cognitive architecture underpinning recognition. They can result either in the inability to detect any change occurring in the visual field or in the impairment of further stages of the recognition process, from the analysis of the perceptual properties of the stimulus (form, color, motion, depth, etc.) to the achievement of its structural description and, eventually, the attribution of a meaning. In this paper, I focus on the diagnostic and clinical features characterizing the disruption of the last stage of visual information processing; that is, the failure to identify what a stimulus represents despite evidence that its three-dimensional structure has been properly reconstructed. In the literature, this impairment is traditionally referred to as associative agnosia, a psychological construct that attributes the deficit to the inability to associate a well-discriminated percept with its semantic attributes, which are stored in separate cortical areas. In the visual modality, three discrete forms of associative agnosia have been described, affecting objects, faces, and colors. These are treated separately.

Tactile morphagnosia secondary to spatial deficits.

A 73-year old man showed visual and tactile agnosia following bilateral haemorrhagic stroke. Tactile agnosia was present in both hands, as shown by his impaired recognition of objects, geometrical shapes, letters and nonsense shapes. Basic somatosensory functions and the appreciation of substance qualities (hylognosis) were preserved. The patient's inability to identify the stimulus shape (morphagnosia) was associated with a striking impairment in detecting the orientation of a line or a rod in two- and three-dimensional space. This spatial deficit was thought to underlie morphagnosia, since in the tactile modality form recognition is built upon the integration of the successive changes of orientation in space made by the hand as it explores the stimulus. Indirect support for this hypothesis was provided by the location of the lesions, which could not account for the severe impairment of both hands. Only those located in the right hemisphere encroached upon the posterior parietal cortex, which is the region assumed to be specialised in shape recognition. The left hemisphere damage spared the corresponding area and could not, therefore, be held responsible for the right hand tactile agnosia. We submit that tactile agnosia can result from the disruption of two discrete mechanisms and has different features. It may arise from a parietal lesion damaging the high level processing of somatosensory information that culminates in the structured description of the object. In this case, tactile recognition is impaired in the hand contralateral to the side of the lesion. Alternatively, it may be caused by a profound derangement of spatial skills, particularly those involved in detecting the orientation in space of lines, segments and complex patterns. This deficit results in morphagnosia, which affects both hands to the same degree.

The nature of the disorder underlying the inability to retrieve proper names.

Two patients with the syndrome of proper name anomia were investigated. Both patients were only able to produce around 50% of the names of contemporary celebrities, but performed significantly better on a task calling for naming of historical figures. The names of relatives and friends were spared in one patient, while the other retrieved names of people known since childhood much better than those of people familiar to him since the age of 25. Geographical names, names of monuments and masterpieces were preserved. The above dissociations are taken to imply that in moderately impaired patients, a temporal gradient effect concurs to modulate the severity of the naming block. A similar impairment was found in both patients when they attempted to retrieve or relearn familiar telephone numbers. This finding suggests that the core of the disorder resides in the inability to gain access to words used to identify a single entity, regardless of whether they belong to the class of proper or common names.

Let not thy left hand know what thy right hand knoweth. The case of a patient with an infarct involving the callosal pathways.

Following a cerebral vascular accident, a patient showed a classical disconnection syndrome: left-hand tactile anomia, apraxia and dysgraphia and right-hand constructional apraxia. What made the case unusual was the presence of hand asymmetry in the performance of some matching-to-sample tasks carried out in foveal vision. The left hand committed significantly more errors than the right hand when it was not possible to identify on a perceptual basis the stimulus that was to be matched, because it was removed (memory condition) or was indicated verbally (verbo-visual matching), or had the same name but not the same physical appearance as the match (capital and lower-case letter matching). No hand difference emerged when the stimulus remained in full view throughout the matching task (perceptual condition). The hand effect, however, was limited to colours and letters. Objects, geometrical shapes and unfamiliar faces were matched with equal proficiency by both hands under every condition of presentation. Left-hand errors also significantly outnumbered right-hand errors in sorting colours according to hue and colouring drawings. MRI showed an infarct in the left cingulate white matter that ran parallel to the trunk of the corpus callosum, and an infarct of the splenium. However, the latter did not prevent the transmission of colour and letter information between the two hemispheres, as shown by the performance on perceptual equivalence tasks and by the correct right-hand responses to stimuli projected to the left visual field. We propose that this pattern of deficit is contingent upon the specific role that the left hemisphere plays in categorizing a given colour patch as belonging to a definite colour region (red, blue, etc.) and in grapheme recognition. Without the assistance of the left side the right hemisphere lacks the benefit provided by meaning identification. In our patient the left brain did receive information from the right brain and was able to process it, but was prevented by the paracallosal lesion from transmitting what it knew to the right motor area. No hand effect emerged for objects and geometrical shapes, because their semantic memory is bilaterally represented.

Prosopagnosia and alexia without object agnosia.

Following a trauma causing bilateral posterior brain damage, a patient complained of dyslexia and prosopagnosia, but not object agnosia. On testing she showed intact recognition of object drawings, even when it was assessed with perceptually demanding tasks such as Ghent's overlapping figures and Street completion test. This pattern of deficit is inconsistent with Farah's (1990) prediction that the simultaneous occurrence of alexia and prosopagnosia is invariably associated with object agnosia. The patient's reading performance had the features typically found in letter-by-letter readers. On face tests, she showed a discrepancy between the impairment exhibited in familiarity recognition and famous face naming and the correct (though slow) performance in matching the names of famous persons with their photographs. This apparent contradiction was clarified by showing that the patient had maintained the ability to generate the mental images of famous faces in response to the presentation of their names. We assume that face recognition units were intact, but partially disconnected from the output of perceptual processing.

Associative agnosia and optic aphasia: qualitative or quantitative difference?

We report a patient who, following the partial removal of a tentorial meningioma, suffered a hematoma in the left occipital lobe, which was resected. He showed severe anomia for visually presented stimuli, while naming was normal when they were presented in the tactile, auditory and verbal modality. His performance on visuo-perceptual tests, not requiring meaning identification, provided evidence that he was able to access the stored representations of stimuli. When recognition was assessed with non-verbal tasks, a mixed pattern of findings emerged and the patient showed features of both associative agnosia and optic aphasia. He was severely impaired in producing pantomimes in response to pictures, but only marginally impaired in sorting figures from the same superordinate category into fine-grained subcategories. He performed within the normal range on an associative task, in which the distractors bore no semantic relation to the target, but made many errors when the distractors and the target were semantically related. We propose that the interpretation advanced by Coslett and Saffran (Brain, 1989) for optic aphasia also holds for associative agnosia and argue that both syndromes reflect the impaired access of structured representations to left hemisphere semantics, but differ in terms of the degree of compensation provided by the semantic resources of the right hemisphere. Since the anatomical basis of the two syndromes may be very similar, we submit that what makes the difference is the semantic potential of the patient's right hemisphere.

Imitation and utilisation behaviour.

OBJECTIVE: To investigate the incidence, anatomical correlates, and clinical features of imitation and utilisation behaviour, which are thought by Lhermitte and coworkers to represent a reliable and frequent index of frontal lobe disease. METHODS: 78 patients with hemispheric local lesions were tested in two separate sessions, in which their reactions to a series of gestures performed by the examiner and to the presentation of a set of objects were recorded. The patients were stratified into a frontal (n = 52) and a non-frontal group (n = 26) on the basis of their CT data. RESULTS AND CONCLUSIONS: Imitation behaviour was present in 39% of the frontal patients and was mainly associated with medial and lateral lesions, at odds with the claim of Lhermitte et al that it is a constant accompaniment of lower, mediobasal lesions. In the non-frontal group it was found in three patients, all with damage to the deep nuclei region. Utilisation behaviour was a much rarer phenomenon, present in only two patients, both of whom had frontal damage. Neither imitation behaviour nor utilisation behaviour were found in patients with retrorolandic cortical lesions.

Sparing of verbs and preserved, but ineffectual reading in a patient with impaired word production.

We report a patient who, following a left fronto-temporal lesion, showed a complete sparing of the semantic store and a deficit of word production characterized by two types of dissociations. First, oral and written output was severely disrupted in naming and partially impaired in repetition, writing to dictation and oral spelling, with the exception of verbs, which were normally produced in every modality and condition of stimulation. Second, reading was normal for all type of words as well as non-words. This pattern of deficits suggests two functional lesions, one affecting the connections between the semantic store and the phonological lexicon and the other damaging the sublexical route that converts sound to sound and sound to print. It also implies that words are independently organized in the phonological lexicon, based on their grammatical class and have discrete connections with the semantic store. However, CT scan evidence does not support the hypothesis that this functional dissociation finds its anatomical correlate in the specialization of the frontal premotor cortex for verbs and the antero-medial temporal cortex for nouns. In spite of his normal reading performance, both in terms of comprehension and of accuracy and speed in word production, the patient complained that he met with great difficulty in reading newspapers and books, to the point that he had to renounce to this previously favourite activity. It was found that it took him time and effort to grasp the meaning of complex sentences and passages and it was speculated that, contrary to single words and elementary sentences, comprehension of this type of material cannot be achieved by the mere access of orthographic stimuli to semantics, but requires the retrieval of word-forms. It would appear that a patient, whose lexical route is blocked, can only read passages, by first converting print to sound via the sublexical route and then re-entering the semantic store with oral input.

Persistent retrograde, amnesia following a minor trauma.

A 19-year-old man showed dense retrograde amnesia (RA) for autobiographical and public events covering his entire life, following a motor car accident that caused no apparent brain damage. His learning abilities remained excellent and permitted him to recover knowledge of the past, based on information gathered from other people and the media, although he never reacquired the sense of personal experience. At the onset he also showed occasional gaps in his semantic memory, e.g., he failed to recognize a few objects and was unable to provide factual information about sports he had long been playing. Eventually, his amnesia was predominantly restricted to autobiographical events or facts. After 29 months the deficit has remained unchanged. CT, MRI and PET were negative. Psychogenic amnesia was considered, but eventually ruled out for want of any evidence pointing to emotional problems, stressful situations or secondary gains. We posit that for reasons unknown a mild trauma can cause functional inhibition of the access to the engrams that are already stored, leaving intact the ability to encode and retrieve new memories.

An experimental investigation on the nature of extinction.

Under conditions of bilateral simultaneous presentation a right brain-damaged patient consistently extinguished visual stimuli presented for 200 msec to the left visual field. He was submitted to a series of experiments aimed at assessing the variables that influence this phenomenon. Extinction persisted unmodified when stimulus presentation was lengthened to 500 msec, when the size of the left stimulus was four-fold that of the right stimulus and for whatever position the stimuli occupied on the two halves of the display. Single stimuli were always perceived, but their localization was transposed rightward. The only condition in which extinction was partially reduced was when the instructions called for total disregard of the right stimulus. The patient also extinguished the leftmost of two stimuli presented in the left space, while he always perceived both stimuli when they were presented in the right space. However, if attention was covertly moved to a point in the right space and two stimuli were displayed to either side of it, left stimuli were neglected 70% of the time. Though a left stimulus was never perceived, it slowed the RT to the identification of a right stimulus. These data are interpreted in the frame of the directional bias theory, which attributes extinction to a shift of attention towards the side ipsilateral to the lesion, caused by the imbalance between the opponent turning processors, controlled by the right and left hemisphere. However, the striking difference in the rate of extinction when both stimuli were presented to the left or to the right field implies a stepwise decrement in the deployment of attention by the damaged processor, when it enters in the opposite space.

Primary amnesia of insidious onset with subsequent stabilisation.

A patient had a slowly developing amnesic syndrome that remained substantially unchanged during the two and a half years of observation. Intellectual skills were excellent and there was no language, perception, praxis, or calculation deficit. The memory impairment involved verbal and visual learning, sparing spatial learning and, to a large extent, retrograde memory. Magnetic resonance imaging was normal, but PET showed a hypometabolism of the left temporal mesial region and thalamus. This case extends the spectrum of monosymptomatic cognitive disorders, previously reported in the area of language, praxis, and visual recognition, to amnesia.

Prosopagnosia can be associated with damage confined to the right hemisphere--an MRI and PET study and a review of the literature.

The early position that prosopagnosia is predominantly associated with right hemisphere (RH) injury was challenged by the finding that in practically all cases that come to autopsy pathological data point to bilateral damage. Yet the rejection of the RH hypothesis may have been too hasty. We report three prosopagnosic patients in whom MRI and CT documented a lesion confined to the right occipito-temporal areas and PET confirmed that hypometabolism involved the RH only. A review of the literature brought out 27 cases with neuroimaging evidence that prosopagnosia was associated with RH damage plus four cases with surgical evidence. It remains, however, that the inability to recognize familiar faces is a rare disorder, not manifested by the majority of patients with right temporo-occipital injury. We submit that right-handers differ in the degree of their RH specialization in processing faces and that in only a minority of them is it so marked that it cannot be compensated for by the healthy left hemisphere.

Amyotrophic lateral sclerosis with dementia.

Four patients with amyotrophic lateral sclerosis (ALS) and dementia are reported. Mental symptoms antedated motor signs and were investigated with a neuropsychology battery, which brought out different patterns of cognitive impairment. A case presented with frontal dementia, another with a predominant aphasic, apraxic, amnesic syndrome, while the remainders showed cognitive decline in association with blunt affect. Motor signs were characterized by a precocious involvement of the upper motor system.

Are semantic systems separately represented in the brain? The case of living category impairment.

Following herpes encephalitis, a patient showed impaired knowledge of animals, fruits and vegetables, flowers and food (so called living things categories), whatever the modality in which stimuli were presented and responses were given. A series of experiments showed that the deficit specifically affected the ability to retrieve the perceptual features of the living stimuli defining their shape, while knowledge of their functional-encyclopedic properties was preserved. The patient had no problems with man-made objects, except when the recall of their colour, or the identification of their sound was requested. It is argued that the retrieval of the perceptual features was potentially disrupted for every type of category, but that the block was compensated for man-made objects, because the close correspondence between shape and function that characterises them provided an alternative route to access their structured form representations. On this account, the selective deficit for living categories seems contingent on the interaction between an overall cognitive impairment--the deficit in retrieving perceptual features--and some intrinsic properties of the stimulus--the factors that have modelled its form--and cannot be taken as evidence that semantic systems are allotted to separate cerebral areas.

Dense retrograde amnesia, intact learning capability and abnormal forgetting rate: a consolidation deficit?

Following a thoracic trauma, which caused brain hypoxia, a 24-year-old man presented with a dense retrograde amnesia for events, persons and environments that spanned his whole life before injury. Knowledge acquired at school or through the media were equally lost, with the exception of arithmetical skills and some geographical knowledge. No deficit was apparent in language, object recognition, motor skills and intellectual tests. Anterograde memory was marked by very efficient learning capacity, an almost perfect retention of information at delay interval of 4 hours and pathologically rapid forgetting at longer delays. Informal observations agreed with test performance in showing that he could relearn facts of the past and easily acquire new information, but tended to lose these memories if the information was not frequently rehearsed. PET showed a hypometabolism of the posterior temporal lobes. Though some points remain obscure, we propose that a consolidation deficit hypothesis provides the most sensible interpretation of this peculiar pattern of memory deficit.

The fuzzy boundaries of apperceptive agnosia.

Following a trauma that mainly involved the right hemisphere, a 21-year-old girl showed a profound impairment in visual object recognition, without language and intellectual deficit. Her elementary sensory functions were preserved and she performed in the normal range on visual matching tasks, on taks requiring to detect small differences between similar complex shapes and in copying drawings, without any evidence of a line by line approach. Her deficit emerged with tests that, though not implying identification of meaning, demanded to disentangle a form from a confused background and to achieve a highly structured description of the stimulus. In addition to this high-level perceptual processing disorder, there was a deficit in recovering from the visual store the shape of an object, also when the performance did not involve perceptual discrimination, e.g., in drawing from memory or telling the physical difference between two named stimuli. Knowledge of the semantic and contextual attributes of objects was intact. The case is taken as evidence that the borders of apperceptive agnosia may be ampler than usually thought and its distinction from associative agnosia less rigid, with some patients laying in-between the two syndromes.

Primary dyscalculia after a medial frontal lesion of the left hemisphere.

A patient had an infarct in the territory of the left anterior cerebral artery, which destroyed the medial cortex of the frontal lobe, and presented with a picture of primary dyscalculia. Lexical and syntactic processing of verbal and arabic numbers and comprehension of operation symbols were intact, but retrieval of basic, over-learned facts was mildly impaired and execution of calculation procedures was more severely impaired. As the same type of procedure could be passed or sometimes failed it suggests a deficit of activation of the appropriate procedures. The location of the lesion was unusual and suggested participation of medial frontal areas in calculation processes.

Proper name anomia.

Following a left thalamic infarct, a patient showed a marked impairment in retrieving person proper names in response to faces and to verbal description, despite being able to provide precise information about the persons he could not name and to point to their photograph when the name was provided by the examiner. The patient was also impaired in generating proper names, but could easily retrieve common names as well as geographical names and names of monuments. It is hypothesized that the arbitrary nature of the link between proper names and their referents makes access of phonological forms from the semantic store particularly labile. In agreement with this interpretation is the patient's inability to recall telephone numbers and to learn semantically arbitrary paired associates.

The incidence of the grasp reflex following hemispheric lesion and its relation to frontal damage.

The grasp reflex and related prehensile phenomena were investigated in two studies. In Study 1, 491 patients consecutively admitted to the neurological wards of our department were tested with a standardized procedure. Grasping was found in 8% of brain-damaged patients and occurred predominantly in patients with single (14 cases) or multiple (10 cases) hemispheric lesions, accompanied by stable neurological symptoms. The locus of the lesion was either in the frontal lobe or in the deep nuclei and subcortical white matter. Grasping never occurred when the disease was confined to the retrorolandic regions. Study 2 was designed to identify the frontal structures responsible for grasping. Forty-four patients who either showed grasping or had CT scan documentation of frontal damage were investigated. Grasping was found in 70% of patients who had involvement of the medial areas and was always associated with damage to the cingulate gyrus; the supplementary motor cortex was less consistently encroached upon. It also occurred following damage to the lateral motor and premotor areas, but only in 26% of cases. A few patients with grasping had deep lesions. The clinical analysis of the symptom showed that it usually affected both hands, even when the lesion was unilateral, and that in most patients it did not manifest itself as a forced phenomenon, but could be modified by will, although it showed up again as soon as the patient's attention was diverted. Groping was more rarely found and the so-called grasp reaction was unusual. No hemispheric asymmetry in the occurrence of this set of phenomena was apparent.

The aphasic isolate. A clinical-CT scan study of a particularly severe subgroup of global aphasics.

This paper outlines the clinical and CT scan features of a subtype of global aphasia, characterized by an extreme loss of communicative abilities, verbal as well as nonverbal. Three to four weeks after a left hemisphere stroke, 17 patients were completely unable to communicate with people addressing them. Though there were differences in their willingness to interact with the environment, they were characterized by complete loss of speech output and by inaccessibility to any kind of message, whether given verbally or through gestures. Patients who survived were reassessed 6 and 12 mos later and half of them were still found in a state of complete communicative isolation. The remainder had somewhat improved, but remained globally aphasic. The attempt to find a CT scan basis for this picture was disappointing. Only 35% of patients had a lesional pattern in agreement with the traditional view that ascribes global aphasia to the involvement of Broca's and Wernicke's areas. The location of lesion in the other cases spanned from anterior cortical damage, to posterior cortical damage, to deep nuclei damage and none of the lesions that have been proposed to account for subcortical global aphasia was consistently observed.