Interhemispheric communication in a case of total visuo-verbal disconnection

The influence of misnaming on object recognition: a case of multimodal agnosia

We present a case of multimodal agnosia in the visual and tactile modality due to an infarction in the territory of the left posterior cerebral artery. The patient's ability to recognize objects fluctuated depending on his verbal activity. When he misnamed presented objects, he tended to use them and to draw them in keeping with the wrong name. We submit that the mechanism causing associative agnosia is more dynamic than it was hitherto considered. It originates from the rivalry between top-down central regulation and bottom-up peripheral flow.

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.

Modality specific naming and gesture disturbances: a case with optic aphasia, bilateral tactile aphasia, optic apraxia and tactile apraxia

This study reports a patient who manifested optic aphasia, tactile aphasia, optic apraxia, and tactile apraxia following an operation for epidural left parietal haematoma. He could neither name nor pantomime the use of objects presented visually or tactually, but correctly performed semantic association tasks, thus demonstrating preserved recognition. He could name and pantomime the use of auditorily presented objects. Experimental results disproved that pantomime disorders were secondary to naming disorders, and suggested that modality specific aphasia and modality specific apraxia are independent clinical syndromes. CT scans showed injury to the posterior callosal radiations, the white matter of the angular gyrus, and the medial portion of the occipital lobe in the left hemisphere. We suggest that modality specific aphasia and modality specific apraxia can be explained by assuming a common semantic memory store.

Visual agnosia and optic aphasia: are they anatomically distinct?

A patient with left infero-medial occipital-temporal infarct suffered a visual agnosia that, by a minor change of the task, could be manipulated to optic aphasia. Tools in actual use and pantomimes of tool use were better named than stationary tools, a dissociation that suggests differences in the ability of stimuli to evoke associations over multiple modalities. Based on this case and analysis of previous reports we suggest that optic aphasia differs from visual agnosia primarily in the degree of callosal disconnection and that the preserved demonstration of tools use and semantic classification of optic aphasia reflect right hemisphere contribution to visual processing.

Visual associative agnosia and optic aphasia. A single case study and a review of the syndromes

The case is presented of a patient who showed visual naming disturbances caused by a left occipital infarction. His performance on tests of visual naming, of recognition not requiring a verbal response, and of verbal-visual matching demonstrated a wide range of qualitatively different errors, including complete inability to recognize the object, access to partial semantic knowledge, and mere name finding difficulty. On the basis of the present case and of a review of the recent literature, the clinical distinction between visual associative agnosia and optic aphasia and the relation of these disorders with the anatomical site of lesion are discussed.

Interhemispheric disconnection effects in multiple sclerosis

Patients with multiple sclerosis reported less left ear numbers but more right ear numbers than controls in a dichotic listening test. The multiple sclerosis patients were also relatively impaired on three learning tasks; one of these, a test for paired-associate learning of names and faces, correlated with left ear findings; the results are interpreted as supporting a hypothesised disconnection mechanism.