N. Tzourio

The cortical representation of speech

In this study, we compare regional cerebral blood flow (rCBF) while French monolingual subjects listen to continuous speech in an unknown language, to lists of French words, or to meaningful and distorted stories in French. Our results show that, in addition to regions devoted to single-word comprehension, processing of meaningful stories activates the left middle temporal gyrus, the left and right temporal poles, and a superior prefrontal area in the left frontal lobe. Among these regions, only the temporal poles remain activated whenever sentences with acceptable syntax and prosody are presented.

Mental navigation along memorized routes activates the hippocampus, precuneus, and insula

POSITRON emission tomography was used to investigate the functional anatomy of mental simulation of routes (MSR) in five normal volunteers. Normalized regional cerebral blood flow was measured while subjects mentally navigated between landmarks of a route which had been previously learned by actual navigation. This task was contrasted with both static visual imagery of landmarks (VIL) and silent Rest. MSR appears to be subserved by two distinct networks: a non-specific memory network including the posterior and middle parts of the hippocampal regions, the dorsolateral pre-frontal cortex and the posterior cingulum, and a specific mental navigation network, comprising the left precuneus, insula and medial part of the hippocampal regions.

Cerebral activations during number multiplication and comparison : a PET study

Positron emission tomography was used to examine the cerebral networks underlying number comparison and multiplication in eight normal volunteers. Cerebral blood flow was measured within anatomical regions of interest defined in each subject using magnetic resonance imaging. Three conditions were used: rest with eyes closed, mental multiplication of pairs of arabic digits and larger-smaller comparison of the same pairs. Both multiplication and comparison activated the left and right lateral occipital cortices, the left precentral gyrus, and the supplementary motor area. Beyond these common activations, multiplication activated also the left and right inferior parietal gyri, the left fusiform and lingual gyri, and the right cuneus. Relative to comparison, multiplication also yielded superior activity in the left lenticular nucleus and in Brodmanns area 8, and induced a hemispheric asymmetry in the activation of the precentral and inferior frontal gyri. Conversely, relative to multiplication, comparison yielded superior activity in the right superior temporal gyrus, the left and right middle temporal gyri, the right superior frontal gyrus, and the right inferior frontal gyrus. These results underline the role of bilateral inferior parietal regions in number processing and suggest that multiplication and comparison may rest on partially distinct networks.

Functional Anatomy of Spatial Mental Imagery Generated from Verbal Instructions

Positron emission tomography (PET) was used to monitor regional cerebral blood flow variations while subjects were constructing mental images of objects made of three-dimensional cube assemblies from auditorily presented instructions. This spatial mental imagery task was contrasted with both passive listening (LIST) of phonetically matched nonspatial word lists and a silent rest (REST) condition. All three tasks were performed in total darkness. Mental construction (CONS) specifically activated a bilateral occipitoparietal–frontal network, including the superior occipital cortex, the inferior parietal cortex, and the premotor cortex. The right inferior temporal cortex also was activated specifically during this condition, and no activation of the primary visual areas was observed. Bilateral superior and middle temporal cortex activations were common to CONS and LIST tasks when both were compared with the REST condition. These results provide evidence that the so-called dorsal route known to process visuospatial features can be recruited by auditory verbal stimuli. They also confirm previous reports indicating that some mental imagery tasks may not involve any significant participation of early visual areas.

Reopening the mental imagery debate: lessons from functional anatomy.

Over the past few years, the neural bases of mental imagery have been both a topic of intense debate and a domain of extensive investigations using either PET or fMRI that have provided new insights into the cortical anatomy of this cognitive function. Several studies have in fact demonstrated that there exist types of mental imagery that do not rely on primary/early visual areas, whereas a consensus now exists on the validity of the dorsal/ventral-route model in the imagery domain. More importantly, these studies have provided evidence that, in addition to higher order visual areas, mental imagery shares common brain areas with other major cognitive functions, such as language, memory, and movement, depending on the nature of the imagery task. This body of recent results indicates that there is no unique mental imagery cortical network; rather, it reflects the high degree of interaction between mental imagery and other cognitive functions.

Functional anatomy of human auditory attention studied with PET.

Positron emission tomography was used to investigate the functional anatomy of selective auditory attention in 17 right-handed male volunteers who submitted to different tasks: silent rest (REST) listening to frequent low- or rare high-pitched tones (LIS) delivered randomly to the right or the left ear, selective auditory attention where subjects had to attend to deviants in one ear, right (ATTR) or left (ATTL). Six subjects had the series REST, LIS, ATTR twice, eight subjects the series REST, LIS, ATTL, and the last three subjects the sereis REST, ATTR, ATTL. Event-related potentials were simultaneously recorded with PET and showed significant task and electrode site effects on the N100 amplitude. When compared to REST, LIS elicited bilateral temporal activations of the Heschls gyri and the planum temporale, with a significant rightward asymmetry, and of the posterior part of the superior temporal gyrus. Significant right precentral and anterior cingulate gyri normalized regional cerebral blood flow increases were observed in the frontal lobe. Both the ATTR and the ATTL conditions, compared to LIS, activated the supplementary motor area, bilateral precentral, and left postcentral cortices without any temporal cortex activation. In addition, the ATTL condition resulted in a right prefrontal cortex activation. Pooling the 14 subjects revealed an asymmetry in the superior temporal gyrus favoring the cortex contralateral to the attended ear. Two major networks seem thus to be involved during selective auditory attention: (1) a local temporal network, on which selective attention produces a modulation of the functional lateralization, and (2) a frontal network that could mediate the temporal cortex modulation by attention.

Cortical anatomy of mental imagery of concrete nouns based on their dictionary definition.

THE functional anatomy of the interactions between spoken language and visual mental imagery was investigated with PET in eight normal volunteers during a series of three conditions: listening to concrete word definitions and generating their mental images (CONC), listening to abstract word definitions (ABST) and silent REST. The CONC task specifically elicited activations of the bilateral inferior temporal gyri, of the left premotor and left prefrontal regions, while activations in the bilateral superior temporal gyri were smaller than during the ABST task, during which an additional activation of the anterior part of the right middle temporal gyrus was observed. No activation of the occipital areas was observed during the CONC task when compared either to the REST or to the ABST task. The present study demonstrates that a network including part of the bilateral ventral stream and the frontal working memory areas is recruited when mental imagery of concrete words is performed on the basis of continuous spoken language.

Functional anatomy of dominance for speech comprehension in left handers vs right handers.

In order to study the functional anatomy of hemispheric dominance for language comprehension we compared the patterns of activations and deactivations with PET and H(2)15O during a story-listening task in two groups of normal volunteers selected on the basis of their handedness. The reference task was a silent rest. The results showed asymmetrical temporal activations favoring the left hemisphere in right handers (RH) together with Brocas area and medial frontal activations. A rightward lateralization of deactivations located in the parietal and inferior temporal gyrus was also observed. In left handers (LH) the temporal activations were more symmetrical as were the parietal and inferior frontal deactivations. Brocas area and medial frontal gyrus activations were present in LH. The direct comparison of RH and LH activations revealed larger activations in the left superior temporal, in particular in the left planum temporale and temporal pole of RH, while LH activated an additional right middle temporal region. Individual analysis of LH differences images superimposed on individual MRI planes demonstrated an important variability of functional dominance, with two LH leftward lateralized, two symmetrical, and one showing a rightward lateralization of temporal activations. There was no relationship between functional dominance and handedness scores. These results are in accordance with data from aphasiology that suggest a greater participation of the right hemisphere in language processing in LH. In addition, the presence of bilateral deactivations of the dorsal route could support the assumption that LH ambilaterality concerns, in addition to language, other cognitive functions such as visuospatial processing.

Left planum temporale surface correlates with functional dominance during story listening

THE relationship between anatomical asymmetry of the planum temporale (PT) and functional lateralization for language comprehension was studied in 14 normal volunteers, including five left-handers (LH). PT surfaces and asymmetry were measured in each subject using structural MRI, while functional lateralization was assessed on individual regional cerebral blood flow (rCBF) difference images of a PET-H215O activation protocol in which a story listening condition was contrasted with a control state. Significant positive correlations were found between the left PT surface and the amount of NrCBF increase during the story listening in the left superior temporal gyrus as well as with the left-right activation index in the superior temporal and the temporal pole. Functional imaging data were correlated neither with the right PT surface nor with the right–left PT surface asymmetry index. However the latter index was correlated with handedness scores. The present results indicate that the size of the left PT is the relevant anatomical landmark for language dominance, and demonstrate that anatomical asymmetries are part of the functional variability for language.

A positron emission tomography study of visual and mental spatial exploration

We measured normalized regional cerebral blood flow (NrCBF) using positron emission tomography (PET) and oxygen-15-labeled water in eight young right-handed healthy volunteers selected as high-imagers. during 2 runs of 3 different conditions: 1, rest in total darkness 2; visual exploration of a map 3; mental exploration of the same map in total darkness. NrCBF images were aligned with individual magnetic resonance images (MRI), and NrCBF variations between pairs of measurements (N = 15) were computed in regions of interest having anatomical boundaries that were defined using a three-dimensional (3-D) reconstruction of each subject MRI. During visual exploration, we found bilateral activations of primary visual areas, superior and inferior occipital gyri, fusiform and lingual gyri, cuneus and precuneus, bilateral superior parietal, and angular gyri. The right lateral premotor area was also activated during this task while superior temporal gyri and Brocas area were deactivated. By contrast, mental exploration activated the right superior occipital cortex, the supplementary motor area, and the cerebellar vermis. No activation was observed in the primary visual area. These results argue for a specific participation of the superior occipital cortex in the generation and maintenance of visual mental images.