Stefano F. Cappa

Identification of the central vestibular projections in man: a positron emission tomography activation study

The cerebral representation of space depends on the integration of many different sensory inputs. The vestibular system provides one such input and its dysfunction can cause profound spatial disorientation. Using positron emission tomography (PET), we measured regional cerebral perfusion with various vestibular stimulations to map central vestibular projections and to investigate the cerebral basis of spatial disorientation. We showed that the temporoparietal cortex, the insula, the putamen, and the anterior cingulate cortex are the cerebral projections of the vestibular system in man and that the spatial disorientation caused by unilateral vestibular stimulation is associated with their asymmetric activation.

A PET Follow-up Study of Recovery after Stroke in Acute Aphasics ☆

The neural correlates of recovery from aphasia are largely unknown. Several different sources of evidence, from clinical studies to neurophysiological investigations, have suggested a contribution of the contralateral, undamaged hemisphere in recovery from aphasia. Eight patients with unilateral left hemispheric stroke were submitted to a standard language examination and to a [18F]FDG PET study in the recent phase after stroke (within 2 weeks) and 6 months later. All patients had a substantial recovery of specific aspects of language functions at the follow-up. Analysis of regional glucose metabolism showed hypometabolism in structurally unaffected regions both in the left and in the right hemisphere (diaschisis), in the acute stage. Glucose metabolism increased significantly on both sides in all patients at the second PET study. Regional analysis showed significant positive correlations between changes in metabolic values in several cortical and subcortical regions in the right hemisphere and changes in language performance at follow-up. The present findings show that an extensive, bihemispheric depression of metabolism is found in the acute stage after stroke in aphasic patients. Language recovery in the first months after aphasia onset is associated with regression of functional depression (diaschisis) in structurally unaffected regions, in particular in the right hemisphere.

The Effects of Semantic Category and Knowledge Type on Lexical-Semantic Access: A PET Study ☆

Neuropsychological studies of patients with category-specific recognition disorders, as well as PET investigations of semantic category effects in visual recognition tasks, have led some authors to the hypothesis that visual-perceptual knowledge plays a crucial role in the recognition of natural items, such as animals, while functional-associative information is more important for the recognition of man-made tools. To study the cerebral correlates of the retrieval of different types of semantic knowledge about living and nonliving entities, we performed a PET experiment in which normal subjects were required to access visual- and functional-associative information related to visually presented words corresponding to animals and tools. The experimental conditions were the following: (1) Rest. (2) Baseline: letter detection in pseudo-words. (3) Animal, visual knowledge: decide whether the animal has a long or short tail with respect to the body. (4) Animal, associative knowledge: decide whether the animal is typically found in Italy. (5) Tool, visual knowledge: decide whether the object is longer than wider or vice versa. (6) Tool, functional knowledge: decide whether the object is typically used as a kitchen tool. Lexical-semantic access (all lexical conditions pooled) activated the prefrontal cortex on the left and the parietal-occipital junction and posterior cingulate cortex bilaterally. An analysis of the individual experimental conditions in comparison with the nonword baseline showed that accessing visual versus associative knowledge was associated with different activation patterns: predominantly frontal in the case of visual features, temporoparietal for associative knowledge. While the activation patterns involved similar areas for living and nonliving entities, in the case of the latter they were restricted to the left hemisphere. The analysis of main effects confirmed these findings: there were several significant differences in the visual-associative comparison, while category-related differences were less prominent. These findings indicate that the retrieval of different types of knowledge is associated with distinct patterns of brain activation; on the other hand, category-related differences were less evident than in picture matching and naming tasks.

A fMRI study of word retrieval in aphasia.

The neural mechanisms underlying recovery of cognitive functions are incompletely understood. Aim of this study was to assess, using functional magnetic resonance (fMRI), the pattern of brain activity during covert word retrieval to letter and semantic cues in five aphasic patients after stroke, in order to assess the modifications of brain function which may be related to recovery. Four out of five patients had undergone language recovery, according to standard testing, after at least 6 months of rehabilitation. The cerebral activation of each patient was evaluated and compared with the activation pattern of normal controls studied with the same fMRI paradigm. In the patients, the pattern of brain activation was influenced by the site and extent of the lesion, by the degree of recovery of language, as reflected by task performance outside the scanner, and by task requirements. In the case of word retrieval to letter cues, a good performance was directly related to the activation in Brocas area, or in the right-sided homologue. On the other hand, in the case of semantic fluency, the relationship between performance level and activation was less clear-cut, because of extensive recruitment of frontal areas in patients with defective performance. These findings suggest that the performance in letter fluency is dependent on the integrity of the left inferior frontal cortex, with the participation of the homologous right hemispheric region when the left inferior frontal cortex is entirely of partially damaged. Semantic fluency, which engages the distributed network of semantic memory, is also associated with more extensive patterns of cerebral activation, which however appear to reflect retrieval effort rather than retrieval success.

Temporary remission of left hemianesthesia after vestibular stimulation. A sensory neglect phenomenon

In three right-brain damaged patients with contralesional neglect vestibular stimulation induced a temporary remission of left hemianesthesia, in addition to the well-known transient recovery of extrapersonal and personal neglect. These findings indicate that in neglect patients attentional factors may play an important role in producing apparently primary sensory deficits, which may be interpreted in terms of defective access to conscious processing.

Neural control of fast-regular saccades and antisaccades: an investigation using positron emission tomography

Abstractu2002Regional cerebral blood flow changes related to the performance of two oculomotor tasks and a central fixation task were compared in ten healthy human subjects. The tasks were: (a) performance of fast-regular saccades; (b) performance of voluntary antisaccades away from a peripheral cue; (c) passive maintenance of central visual fixation in the presence of irrelevant peripheral stimulation. The saccadic task was associated with a relative increase in activity in a number of occipitotemporal areas. Compared with both the fixation and the saccadic task, the performance of antisaccades activated a set of areas including: the superior and inferior parietal lobules, the precentral and prefrontal cortex, the cingulate cortex, and the supplementary motor area.The results of the present study suggest that: (a) compared with self-determined saccadic responses the performance of fast regular, reflexive saccades produces a limited activation of the frontal eye fields; (b) in the antisaccadic task the inferior parietal lobes subserve operations of sensory-motor integration dealing with attentional disengagement from the initial peripheral cue (appearing at an invalid spatial location) and with the recomputation of the antisaccadic vector on the basis of the wrong (e.g., spatially opposite) information provided by the same cue.

Distinct brain loci in deductive versus probabilistic reasoning

Deductive versus probabilistic inferences are distinguished by normative theories, but it is unknown whether these two forms of reasoning engage similar cerebral loci. To clarify the matter, positron emission tomography was applied during deductive versus probabilistic reasoning tasks, using identical stimuli. Compared to a language comprehension task involving the same stimuli, both probabilistic and deductive reasoning increased regional cerebral blood flow (rCBF) bilaterally in the mesial frontal region and in the cerebellum. In the direct comparison, probabilistic reasoning increased rCBF in left dorsolateral frontal regions, whereas deductive reasoning enhanced rCBF in associative occipital and parietal regions, with a right hemispheric prevalence. The results suggest that reasoning about syllogisms engages distinct brain mechanisms, depending on the intention to evaluate them deductively versus probabilistically.

Metabolic Impairment in Human Amnesia: A PET Study of Memory Networks

Human amnesia is a clinical syndrome exhibiting the failure to recall past events and to learn new information. Its “pure” form, characterized by a selective impairment of long-term memory without any disorder of general intelligence or other cognitive functions, has been associated with lesions localized within Papezs circuit and some connected areas. Thus, amnesia could be due to a functional disconnection between components of this or other neural structures involved in long-term learning and retention. To test this hypothesis, we measured regional cerebral metabolism with 2-[18F]fluoro-2-deoxy-d-glucose ([18F]FDG) and positron emission tomography (PET) in 11 patients with “pure” amnesia. A significant bilateral reduction in metabolism in a number of interconnected cerebral regions (hippocampal formation, thalamus, cingulate gyrus, and frontal basal cortex) was found in the amnesic patients in comparison with normal controls. The metabolic impairment did not correspond to alterations in structural anatomy as assessed by magnetic resonance imaging (MRI). These results are the first in vivo evidence for the role of a functional network as a basis of human memory.

Visual and spatial perception in the early phase of Alzheimer's disease

A battery of visuospatial perception tests was administered to 27 mild probable Alzheimers disease (AD) patients; 21 were reassessed after 8 months. At the first evaluation, AD patients were impaired only in an object-naming task. After 8 months, the performance in the subtests of object perception was unchanged, while there was a significant decline in the total score of the items tapping space perception. A significant worsening was also observed in the Reys figure copy score and was correlated with the decrease in the spatial perception score. This study confirms that an impairment in visual perceptual tests requiring access to semantic and lexical knowledge is present in the earliest phase of AD, whereas visuospatial and constructional impairments became evident only later. This pattern of progression may represent the clinical correlate of increasing pathological involvement of posterior associative cortex.

The Mini-Mental State Examination in Alzheimer's disease and multi-infarct dementia.

The aim of this study was to compare the performances on each item of the Mini-Mental State Examination (MMSE) of patients with Alzheimers disease (AD) and multi-infarct dementia (MID). In order to identify the items that could better distinguish the two groups of patients, 70 AD and 31 MID patients matched for disease severity, age, and education were evaluated. The scores of the 101 patients on each of the MMSE items were entered into a principal component factor analysis using varimax rotation, and two main components were derived. Component 1 was probably representative of recently acquired information, whereas component 2 represented educational level. A score summing the items that loaded on component 1 and the recall item was calculated to generate a measure of episodic memory. Performing analysis of variance and covarying for age and education revealed that this score was statistically different in the two groups, with AD patients having lower values. The data suggest that the MMSE may demonstrate a pattern of impairment of memory that differs between AD and MID. Possible explanations of this finding should take into account the different neuroanatomical impairments and the different degrees of motivation, due to depression or attentional deficits, toward external stimuli.