Marc Thioux

Neuroanatomical Substrates of Arabic Number Processing, Numerical Comparison, and Simple Addition: A PET Study

Positron emission tomography was used to localize the cerebral networks specifically involved in three basic numerical processes: arabic numeral processing, numerical magnitude comparison, and retrieval of simple addition facts. Relative cerebral blood flow changes were measured while normal volunteers were resting with eyes closed, making physical judgment on nonnumerical characters or arabic digits, comparing, or adding the same digits. Processing arabic digits bilaterally produced a large nonspecific activation of occipito-parietal areas, as well as a specific activation of the right anterior insula. Comparison and simple addition fact retrieval revealed a fronto-parietal network involving mainly the left intraparietal sulcus, the superior parietal lobule and the precentral gyrus. Comparison also activated, but to a lesser extent, the right superior parietal lobe, whereas addition also activated the orbito-frontal areas and the anterior insula in the right hemisphere. Implications for current anatomo-functional models of numerical cognition are drawn.

The Isolation of Numerals at the Semantic Level

In this study, we examine the case of a patient (NM) who could comprehend and produce numerals despite impairment on comprehension tasks and a high degree of anemia for other categories of words. It will be claimed that NM suffered from an impairment to the semantic system affecting all categories except numerals and the series of days and months. The case of a patient presenting with the exact reverse dissociation has been described a few years ago by Cipolotti et al. (Brain 1991; 114: 619-37). We conclude that NMs pattern of performance provides evidence that numerals constitute a relevant and perhaps a distinct category at the semantic level.

Selective Preservation of Exceptional Arithmetical Knowledge in a Demented Patient

Neuropsychological methodology is generally founded on the description of functional dissociation after focal brain damage. The observation of exceptional abilities can also constitute a means of documenting the different cognitive systems. In this study, we report the performance of a demented elderly patient who, although he had impaired functions in several cognitive domains, showed exceptional calculating abilities, superior to those of young adults. This case study deals with the problem of various mathematical dissociations at different levels.

Number processing and calculation in a case of visual agnosia

We describe the performance of a brain-damaged subject who suffered from visual agnosia leading to major difficulties in generating and exploiting visual representations from long-term memory. His performance in a physical judgement task in which he was required to answer questions about the visual shapes of Arabic numerals reflected his agnosic problems. However, he showed no impairment in usual number processing and calculation tasks. This case shows that, despite some commonalities in number and object processing, actual numerical processes are not affected by visual agnosia and can be preserved even when fine visual processes are impaired.

INTRUSION OF THE VERBAL CODE DURING THE PRODUCTION OF ARABIC NUMERALS: A SINGLE CASE STUDY IN A PATIENT WITH PROBABLE ALZHEIMER'S DISEASE

In the present study, we report on the performance of a patient with probable Alzheimers disease (AD) who made a lot of verbal code intrusions during the production of Arabic numerals. For instance, she wrote 3 mille instead of 3000 [trois mille] or 15ZE instead of 15 [quinze]. These errors of shift towards the unintended code are typical in cases of AD. Here, we suggest a classification of shift errors into three categories. We try to elucidate the origin of these errors on the basis of the analysis of the patients performance in other numerical tasks, on the basis of the regularities observed in the errors, and on the basis of the evolution of shift errors with the advance of the disease. It appeared that a pattern of the sort we have observed could be better accounted for by the co-occurrence of two distinct deficits: an impairment of inhibitory processes on one hand and an impairment of the transcoding mechanism per se on the other.

Functional imaging of visual semantic processing in the human brain

Previous neuroimaging studies have identified a large network of cortical areas involved in semantic processing in the human brain, which includes left occipito-temporal and inferofrontal areas. Most studies, however, investigated exclusively the associative/functional semantic knowledge by using mainly words and/or language related tasks, and this factor may have contributed to the large left hemisphere superiority found in semantic processing and to the controversial involvement of left prefrontal structures. The present study investigates the neural basis of visual objects knowledge, accessed exclusively through pictorial information. Regional cerebral blood flow (rCBF) was assessed using positron emission tomography (PET) during 3 conditions in right-handed normal volunteers: resting with eyes closed, retrieval of semantic information related to visual properties of objects (real size), and visual categorization based on physical properties of the image. Confirming previous experiments and neuropsychological findings, most activations were found in left occipito-temporal areas during retrieval of visual semantic knowledge. The absence of any activation in the left prefrontal inferior cortex for visual semantic processing confirms recent observations which suggest that this region would not be involved in retrieval of visual semantic knowledge from living entities. Rather, such knowledge about visual properties of objects, situated closely to cortical regions mediating perception of the visual attributes, can be retrieved directly from these regions when visual images are used as entry level stimuli.