Gil Assal

Auditory agnosia and auditory spatial deficits following left hemispheric lesions: evidence for distinct processing pathways

Auditory recognition and auditory spatial functions were studied in four patients with circumscribed left hemispheric lesions. Patient FD was severely deficient in recognition of environmental sounds but normal in auditory localisation and auditory motion perception. The lesion included the left superior, middle and inferior temporal gyri and lateral auditory areas (as identified in previous anatomical studies), but spared Heschls gyrus, the acoustic radiation and the thalamus. Patient SD had the same profile as FD, with deficient recognition of environmental sounds but normal auditory localisation and motion perception. The lesion comprised the postero-inferior part of the frontal convexity and the anterior third of the temporal lobe; data from non-human primates indicate that the latter are interconnected with lateral auditory areas. Patient MA was deficient in recognition of environmental sounds, auditory localisation and auditory motion perception, confirming that auditory spatial functions can be disturbed by left unilateral damage; the lesion involved the supratemporal region as well as the temporal, postero-inferior frontal and antero-inferior parietal convexities. Patient CZ was severely deficient in auditory motion perception and partially deficient in auditory localisation, but normal in recognition of environmental sounds; the lesion involved large parts of the parieto-frontal convexity and the supratemporal region. We propose that auditory information is processed in the human auditory cortex along two distinct pathways, one lateral devoted to auditory recognition and one medial and posterior devoted to auditory spatial functions.

Face recognition and postero-inferior hemispheric lesions

Two patients with severe and lasting prosopagnosia were studied for visuo-cognitive functions and anatomo-clinical correlations. Both patients were deficient in recognizing familiar faces and items of previously well known categories (plants and fish for patient 1, mountains for patient 2). Patient 2, but not patient 1, was also deficient for matching of unknown faces. Patient 1, but not patient 2, was achromatopsic. Both patients had bilateral symmetrical lesions. Patient 1 had a lesion of the inferior occipito-temporal cortex, including inferior parts of early stage visual areas. Patient 2 had a lesion of the inferior temporal and fusiform gyri anterior to the early stage visual areas. When compared in Talairach space, the lesions of both patients had minimal overlap. Thus, severe and lasting prosopagnosia was associated with two almost exclusive lesion sites in the postero-inferior part of the hemispheres. Comparison between activation studies of face processing (by others) and our lesion study uncovered several paradoxes. Lesions of regions involved in a given task in normal subjects do not produce a deficit in this task, as shown here for gender discrimination and partially for face matching. Conversely, lesions of a region not specifically involved in a given task in normal subjects can produce a deficit in the task, as shown here for face identification.

NON-VERBAL AUDITORY RECOGNITION IN NORMAL SUBJECTS AND BRAIN-DAMAGED PATIENTS : EVIDENCE FOR PARALLEL PROCESSING

Three different aptitudes involved in sound object recognition were tested in 60 normal subjects and 20 brain-damaged patients: (i) capacity to segregate sound objects on different cues (intensity steps, coherent temporal modulations or signal onset synchrony); (ii) asemantic recognition of sounds of real objects by judging whether two different sound samples belonged to the same object; and (iii) semantic identification of sounds of real objects as judged by means of a multiple choice response test. In 12 patients, different aptitudes involved in auditory recognition were disrupted separately and in a way which speaks in favour of parallel rather than hierarchical processing. There was no strong association between deficits in non-verbal auditory recognition and aphasia or the side of lesion.

Dissociations in aphasia: A case report

Abstract This paper is a case study of an aphasic patient who displays two interesting characteristics, namely, (1) a dissociation between his oral and written disorders (i.e., oral Wernicke vs. written Broca); (2) an agraphia-alexia syndrome in which the picturability of the entities referred to by words plays an important role. Some of the characteristics shown by this patient have been well described concerning reading but never concerning writing. This second aspect is particularly emphasized.

Klüver-Bucy syndrome after left anterior temporal resection

A 70-year-old right-handed woman developed a complete Klüver-Bucy syndrome including psychic blindness, aberrant sexual behavior, hypermetamorphosis, aphasia and visual agnosia following left anterior temporal lobectomy for an anaplasic oligodendroglioma. MRI showed no argument for a contralateral ischemic infarct, tumoral growth or white matter damage. Thus the possibility that a unilateral anterior temporal lesion can cause the whole picture of Klüver-Bucy syndrome must therefore be considered.

Interhemispheric transfer of visual motion information after a posterior callosal lesion: a neuropsychological and fMRI study.

Abstract. Interhemispheric transfer of visual information was investigated behaviourally and with functional magnetic resonance imaging (fMRI) 6 months after a lesion of the posterior two-thirds of the corpus callosum. On tachistoscopical left hemifield presentation, the patient was severely impaired in reading letters, words and geographical names and moderately impaired in naming pictures and colours. In contrast, interhemispheric transfer of visual motion information, tested by verbal report of the direction of short sequences of coherent dot motion presented within the left hemifield, was preserved. The pattern of cerebral activation elicited by apparent motion stimuli was studied with fMRI and compared to that of normal subjects. In normal subjects, apparent motion stimuli, as compared to darkness, activated strongly striate and extrastriate cortex. When presented to one hemifield only, the contralateral calcarine region was activated while regions on the occipital convexity, including putative area V5, were activated bilaterally. A similar activation pattern was found in the patient with a posterior callosal lesion; unilateral left or right hemifield stimulation was accompanied by activation in the contralateral and ipsilateral occipital convexity. Ipsilateral hemifield representation in the extrastriate visual cortex is believed to depend on callosal input. Our observation suggests that this is not the case for visual motion representation and that other, probably parallel, pathways may mediate visual motion transfer after posterior callosotomy.

Left hemisphere strategies in visual recognition, topographical orientation and time planning

Right hemisphere lesions are known to impair, in many cases, visual recognition and visuospatial orientation. We report here on the compensatory strategies used by a patient whose posterior part of the right hemisphere was either destroyed or visually deafferented. She presented a slight appreceptive agnosia, memory disorders, and severe topographical disorientation. Her strategy relied on detail-by-detail analysis of buildings for recognition and on identification of landmarks and memorization of their sequences for finding routes. Time planning was impaired; the patient was unable to use a bi-dimensional timetable, but relied on a (linear) list of assignments. The linear, speech-related strategy she used was sustained uniquely by left hemisphere networks, since no processing of visual information occurred in the right hemisphere.

Dissociation of ear preference in monaural word and voice recognition

The ambiguity in the literature concerning cerebral dominance for voice recognition, has been investigated with a monaural go-no go task, using male and female voices. A left ear advantage for the male, and a right ear advantage for the female voice was found, suggesting that both hemispheres can perform voice recognition. A monaural word recognition test showed the expected left hemisphere dominance.