Mapping motor representations with positron emission tomography

Brain activity was mapped in normal subjects during passive observation of the movements of an 'alien' hand and while imagining grasping objects with their own hand. None of the tasks required actual movement. Shifting from one mental task to the other greatly changed the pattern of brain activation. During observation of hand movements, activation was mainly found in visual cortical areas, but also in subcortical areas involved in motor behaviour, such as the basal ganglia and the cerebellum. During motor imagery, cortical and subcortical areas related to motor preparation and programming were strongly activated. These data support the notion that motor learning during observation of movements and mental practice involves rehearsal of neural pathways related to cognitive stages of motor control.

Localization of grasp representations in humans by PET: 1. Observation versus execution

Positron emission tomography (PET) was used to localize brain regions that are active during the observation of grasping movements. Normal, right-handed subjects were tested under three conditions. In the first, they observed grasping movements of common objects performed by the experimenter. In the second, they reached and grasped the same objects. These two conditions were compared with a third condition consisting of object observation. On the basis of monkey data, it was hypothesized that during grasping observation, activations should be present in the region of the superior temporal sulcus (STS) and in inferior area 6. The findings in humans demonstrated that grasp observation significantly activates the cortex of the middle temporal gyrus including that of the adjacent superior temporal sulcus (Brodmann's area 21) and the caudal part of the left inferior frontal gyrus (Brodmann's area 45). The possible functional homologies between these areas and the monkey STS region and frontal area F5 are discussed.

Brain activity during observation of actions. Influence of action content and subject's strategy

PET was used to map brain regions that are associated with the observation of meaningful and meaningless hand actions. Subjects were scanned under four conditions which consisted of visually presented actions. In each of the four experimental conditions, they were instructed to watch the actions with one of two aims: to be able to recognize or to imitate them later. We found that differences in the meaning of the action, irrespective of the strategy used during observation, lead to different patterns of brain activity and clear left/right asymmetries. Meaningful actions strongly engaged the left hemisphere in frontal and temporal regions while meaningless actions involved mainly the right occipitoparietal pathway. Observing with the intent to recognize activated memory-encoding structures. In contrast, observation with the intent to imitate was associated with activation in the regions involved in the planning and in the generation of actions. Thus, the pattern of brain activation during observation of actions is dependent both on the nature of the required executive processing and the type of the extrinsic properties of the action presented.

Discrimination between Alzheimer dementia and controls by automated analysis of multicenter FDG PET

A new diagnostic indicator of FDG PET scan abnormality, based on age-adjusted t statistics and an automated voxel-based procedure, is presented and validated in a large data set comprising 110 normal controls and 395 patients with probable Alzheimer's disease (AD) that were studied in eight participating centers. The effect of differences in spatial resolution of PET scanners was minimized effectively by filtering and masking. In controls FDG uptake declined significantly with age in anterior cingulate and frontolateral perisylvian cortex. In patients with probable AD decline of FDG uptake in posterior cingulate, temporoparietal, and prefrontal association cortex was related to dementia severity. These effects were clearly distinct from age effects in controls, suggesting that the disease process of AD is not related to normal aging. Women with probable AD had significantly more frontal metabolic impairment than men. The new indicator of metabolic abnormality in AD-related regions provided 93% sensitivity and specificity for distinction of mild to moderate probable AD from normals, and 84% sensitivity at 93% specificity for detection of very mild probable AD (defined by Mini Mental Score 24 or better). All regions related to AD severity were already affected in very mild AD, suggesting that all vulnerable areas are affected to a similar degree already at disease onset. Ventromedial frontal cortex was also abnormal. In conclusion, automated analysis of multicenter FDG PET is feasible, provides insights into AD pathophysiology, and can be used potentially as a sensitive biomarker for early AD diagnosis.

Dyslexia: cultural diversity and biological unity

The recognition of dyslexia as a neurodevelopmental disorder has been hampered by the belief that it is not a specific diagnostic entity because it has variable and culture-specific manifestations. In line with this belief, we found that Italian dyslexics, using a shallow orthography which facilitates reading, performed better on reading tasks than did English and French dyslexics. However, all dyslexics were equally impaired relative to their controls on reading and phonological tasks. Positron emission tomography scans during explicit and implicit reading showed the same reduced activity in a region of the left hemisphere in dyslexics from all three countries, with the maximum peak in the middle temporal gyrus and additional peaks in the inferior and superior temporal gyri and middle occipital gyrus. We conclude that there is a universal neurocognitive basis for dyslexia and that differences in reading performance among dyslexics of different countries are due to different orthographies.

A cultural effect on brain function

We present behavioral and anatomical evidence for a multi-component reading system in which different components are differentially weighted depending on culture-specific demands of orthography. Italian orthography is consistent, enabling reliable conversion of graphemes to phonemes to yield correct pronunciation of the word. English orthography is inconsistent, complicating mapping of letters to word sounds. In behavioral studies, Italian students showed faster word and non-word reading than English students. In two PET studies, Italians showed greater activation in left superior temporal regions associated with phoneme processing. In contrast, English readers showed greater activations, particularly for non-words, in left posterior inferior temporal gyrus and anterior inferior frontal gyrus, areas associated with word retrieval during both reading and naming tasks.

The bilingual brain. Proficiency and age of acquisition of the second language

Functional imaging methods show differences in the pattern of cerebral activation associated with the subject's native language (L1) compared with a second language (L2). In a recent PET investigation on bilingualism we showed that auditory processing of stories in L1 (Italian) engages the temporal lobes and temporoparietal cortex more extensively than L2 (English). However, in that study the Italian subjects learned L2 late and attained a fair, but not an excellent command of this language (low proficiency, late acquisition bilinguals). Thus, the different patterns of activation could be ascribed either to age of acquisition or to proficiency level. In the current study we use a similar paradigm to evaluate the effect of early and late acquisition of L2 in highly proficient bilinguals. We studied a group of Italian-English bilinguals who acquired L2 after the age of 10 years (high proficiency, late acquisition bilinguals) and a group of Spanish-Catalan bilinguals who acquired L2 before the age of 4 years (high proficiency, early acquisition bilinguals). The differing cortical responses we had observed when low proficiency volunteers listened to stories in L1 and L2 were not found in either of the high proficiency groups in this study. Several brain areas, similar to those observed for L1 in low proficiency bilinguals, were activated by L2. These findings suggest that, at least for pairs of L1 and L2 languages that are fairly close, attained proficiency is more important than age of acquisition as a determinant of the cortical representation of L2.

The neural correlates of verb and noun processing. A PET study

The hypothesis that categorical information, distinguishing among word classes, such as nouns, verbs, etc., is an organizational principle of lexical knowledge in the brain, is supported by the observation of aphasic subjects who are selectively impaired in the processing of nouns and verbs. The study of lesion location in these patients has suggested that the left temporal lobe plays a crucial role in processing nouns, while the left frontal lobe is necessary for verbs. To delineate the brain areas involved in the processing of different word classes, we used PET to measure regional cerebral activity during tasks requiring reading of concrete and abstract nouns and verbs for lexical decision. These tasks activated an extensive network of brain areas, mostly in the left frontal and temporal cortex, which represents the neural correlate of single word processing. Some left hemispheric areas, including the dorsolateral frontal and lateral temporal cortex, were activated only by verbs, while there were no brain areas more active in response to nouns. Furthermore, the comparison of abstract and concrete words indicated that abstract word processing was associated with selective activations (right temporal pole and amygdala, bilateral inferior frontal cortex), while no brain areas were more active in response to concrete words. There were no significant interaction effects between word class and concreteness. Taken together, these findings are compatible with the view that lexical-semantic processing of words is mediated by an extensive, predominantly left hemispheric network of brain structures. Additional brain activations appear to be related to specific semantic content, or, in the case of verbs, may be associated with the automatic access of syntactic information.

Value of [11C]choline-positron emission tomography for re-staging prostate cancer: a comparison with [18F]fluorodeoxyglucose-positron emission tomography

PURPOSE

We compared [11C]choline-positron emission tomography (PET) with [18F]fluorodeoxyglucose-PET for re-staging prostate cancer in a group of 100 patients.

MATERIALS AND METHODS

A total of 100 consecutive patients referred for whole body [18F]fluorodeoxyglucose-PET for clinical prostate re-staging after radical treatment for prostate cancer were retrospectively included in the study. Mean prostate specific antigen (PSA) was 6.57 ng./ml. In all cases [11C]choline-PET was also performed. PET studies were done with a multiring device 5 minutes after intravenous injection of approximately 370 MBq. [11C]choline and 60 minutes after injection of approximately 370 MBq. [18F]fluorodeoxyglucose. PET findings were compared with those obtained with different conventional imaging and with PSA assessed at the time of PET and 1 year later.

RESULTS

Areas of abnormal focal increases were noted in 47% of patients on [11C]choline-PET and in 27% on [18F]fluorodeoxyglucose-PET. Of the 100 patients 49 had positive conventional imaging findings. All except 14 [11C]choline-PET findings were concordant with conventional imaging, including 6 negative and 8 positive conventional imaging results. All except 1 [11C]choline-PET negative cases also had negative conventional imaging after 1 year. PSA at 1 year remained stable or decreased in 80% and 62% of [11C]choline-PET negative and positive cases, respectively.

CONCLUSIONS

[11C]choline-PET seems to be useful for re-staging prostatectomy cases with increasing serum PSA levels. It is superior to [18F]fluorodeoxyglucose-PET and complementary to conventional imaging but with the advantage of staging disease at a single step.

Brain processing of native and foreign languages

We used positron emission tomography to study brain activity in adults while they were listening to stories in their native language, in a second language acquired after the age of seven, and in a third unknown language. Several areas, similar to those previously observed in monolinguals, were activated by the native but not by the second language. Both the second and the unknown language yielded distinct left-hemispheric activations in areas specialized for phonological processing, which were not engaged by a backward speech control task. These results indicate that some brain areas are shaped by early exposure to the maternal language, and are not necessarily activated by the processing of a second language to which they have been exposed for a limited time later in life.

Word and picture matching: a PET study of semantic category effects

We report two positron emission tomography (PET) studies of cerebral activation during picture and word matching tasks, in which we compared directly the processing of stimuli belonging to different semantic categories (animate and inanimate) in the visual (pictures) and verbal (words) modality. In the first experiment, brain activation was measured in eleven healthy adults during a same/different matching task for textures, meaningless shapes and pictures of animals and artefacts (tools). Activations for meaningless shapes when compared to visual texture discrimination were localized in the left occipital and inferior temporal cortex. Animal picture identification, either in the comparison with meaningless shapes and in the direct comparison with non-living pictures, involved primarily activation of occipital regions, namely the lingual gyrus bilaterally and the left fusiform gyrus. For artefact picture identification, in the same comparison with meaningless shape-baseline and in the direct comparison with living pictures, all activations were left hemispheric, through the dorsolateral frontal (Ba 44/6 and 45) and temporal (Ba 21, 20) cortex. In the second experiment, brain activation was measured in eight healthy adults during a same/different matching task for visually presented words referring to animals and manipulable objects (tools); the baseline was a pseudoword discrimination task. When compared with the tool condition, the animal condition activated posterior left hemispheric areas, namely the fusiform (Ba 37) and the inferior occipital gyrus (Ba 18). The right superior parietal lobule (Ba 7) and the left thalamus were also activated. The reverse comparison (tools vs animals) showed left hemispheric activations in the middle temporal gyrus (Ba 21) and precuneus (Ba 7), as well as bilateral activation in the occipital regions. These results are compatible with different brain networks subserving the identification of living and non-living entities; in particular, they indicate a crucial role of the left fusiform gyrus in the processing of animate entities and of the left middle temporal gyrus for tools, both from words and pictures. The activation of other areas, such as the dorsolateral frontal cortex, appears to be specific for the semantic access of tools only from pictures.

Different neural systems for the recognition of animals and man-made tools

Using positron emission tomography, we mapped brain activity in normal volunteers during the recognition of visual stimuli representing living (animals) and non-living (artefacts) entities. The subjects had to decide whether pairs of visual stimuli were different representations of the same object, or different objects. Animal recognition was associated with activations in the inferior temporo-occipital areas, bilaterally, whereas artefact recognition engaged a predominantly left hemispheric network, involving the left dorsolateral frontal cortex. These findings, which concur with clinical observations in neurological patients, provide in vivo evidence for a fractionation of the neural substrates of semantic knowledge in man.

Prosopagnosia can be associated with damage confined to the right hemisphere--an MRI and PET study and a review of the literature

The early position that prosopagnosia is predominantly associated with right hemisphere (RH) injury was challenged by the finding that in practically all cases that come to autopsy pathological data point to bilateral damage. Yet the rejection of the RH hypothesis may have been too hasty. We report three prosopagnosic patients in whom MRI and CT documented a lesion confined to the right occipito-temporal areas and PET confirmed that hypometabolism involved the RH only. A review of the literature brought out 27 cases with neuroimaging evidence that prosopagnosia was associated with RH damage plus four cases with surgical evidence. It remains, however, that the inability to recognize familiar faces is a rare disorder, not manifested by the majority of patients with right temporo-occipital injury. We submit that right-handers differ in the degree of their RH specialization in processing faces and that in only a minority of them is it so marked that it cannot be compensated for by the healthy left hemisphere.

Different brain correlates for watching real and virtual hand actions

We investigated whether observation of actions reproduced in three-dimensional virtual reality would engage perceptual and visuomotor brain processes different from those induced by the observation of real hand actions. Participants were asked to passively observe grasping actions of geometrical objects made by a real hand or by hand reconstructions of different quality in 3D virtual reality as well as on a 2D TV screen. We found that only real actions in natural environment activated a visuospatial network including the right posterior parietal cortex. Observation of virtual-reality hand actions engaged prevalent visual perceptual processes within lateral and mesial occipital regions. Thus, only perception of actions in reality maps onto existing action representations, whereas virtual-reality conditions do not access the full motor knowledge available to the central nervous system.

Syntax and the brain: disentangling grammar by selective anomalies

Many paradigms employed so far with functional imaging in language studies do not allow a clear differentiation of the semantic, morphological, and syntactic components, as traditionally defined within linguistic theory. In fact, many studies simply consider the brain's response to lists of unrelated words, rather than to syntactic structures, or do not neutralize the confounding effect of the semantic component. In the present PET experiment, we isolated the functional correlates of morphological and syntactic processing. The neutralization of the access to the lexical-semantic component was achieved by requiring the detection of anomalies in written sentences consisting of pseudowords. In both syntactic and morphosyntactic processing, the involvement of a selective deep component of Broca's area and of a right inferior frontal region was detected. In addition, within this system, the left caudate nucleus and insula were activated only during syntactic processing, indicating their role in syntactic computation. These findings provide original in vivo evidence that these brain structures, whose individual contribution has been highlighted by clinical studies, constitute a neural network selectively engaged in morphological and syntactic computation.

Functional heterogeneity of left inferior frontal cortex as revealed by fMRI

Using functional magnetic resonance imaging (fMRI), we mapped brain activity in six normal volunteers during two silent verbal fluency tasks, one with a phonemic (letter) cue and one with a semantic (category) cue. In comparison with resting state, both tasks activated the anterior triangular portion of the left inferior frontal gyrus (IFG or F3, for third frontal gyrus) and the left thalamus. There were also areas activated in one task but not in the other: the posterior opercular portion of the left IFG for phonemic fluency, and the left retrosplenial region for semantic fluency. Our findings concur with normal psychophysical data and neuropsychological observations to suggest the recruitment of two overlapping but dissociable systems for the two tasks, and demonstrate functional heterogeneity within the left IFG (Broca's area), where the opercular portion is responsible for obtaining access to words through a phonemic/articulatory route.

Rapid assessment of regional cerebral metabolic abnormalities in single subjects with quantitative and nonquantitative [18F]FDG PET: A clinical validation of statistical parametric mapping

The [18F]fluorodeoxyglucose ([18F]FDG) method for measuring brain metabolism has not the wide clinical application that one might expect, partly because of its high cost and the complexity of the quantification procedure, but also because of reporting techniques based on region of interest (ROI) analysis, which are time-consuming and not fully objective. In this paper we report a clinical validation of statistical parametric mapping (SPM) using rCMRglc (quantitative) and radioactivity distribution (nonquantitative) [18F]FDG PET data. We show that a 10-min noninteractive voxel-based SPM analysis on a standard workstation enables objective assessment, including localization in stereotactic space, of regional glucose consumption abnormalities, whose reliability can be assessed on statistical and clinical grounds. Clinical validity was established using a small series of patients with degenerative or developmental disorders, including probable Alzheimer's disease, progressive aphasia, multiple sclerosis, developmental specific language impairment, and epilepsy. Analysis of quantitative and nonquantitative data showed the same pattern of results, suggesting that, for clinical purposes, quantitation and invasive arterial cannulation can be avoided. This should facilitate a wider application of the technique and the extension of SPM clinical analysis to H215O PET or high resolution SPECT perfusion studies.

[18F]FDG PET study in obsessive-compulsive disorder. A clinical/metabolic correlation study after treatment

BACKGROUND

We used [18F]FDG and PET in patients with obsessive-compulsive disorder (OCD) to evaluate cerebral metabolic involvement before and after treatment with serotonin-specific reuptake inhibitors.

METHOD

In 11 untreated, drug-free adults, regional cerebral metabolic rate for glucose (rCMRglu) was compared with that of 15 age-matched normal controls.

RESULTS

rCMRglu values were significantly increased in the cingulate cortex, thalamus and pallidum/putamen complex. After treatment a significant improvement in obsessive-compulsive symptoms on the Y-BOC scale (t = 3.59, P < 0.01) was associated with a significant bilateral decrease of metabolism in the whole cingulate cortex (P < 0.001). Clinical and metabolic data were significantly intercorrelated (Kendall's tau = 0.65; P < 0.01).

CONCLUSIONS

These findings indicate that OCD is associated with functional hyperactivity of a selected neuronal network and that treatment to reduce symptoms may have a selective neuromodulatory effect on cingulate cortex.

Brain abnormalities underlying altered activation in dyslexia: a voxel based morphometry study

Voxel-based morphometry was used to assess the consistency among functional imaging and brain morphometry data in developmental dyslexia. Subjects, from three different cultural contexts (UK, France and Italy), were the same as those described in a previous PET activation paper, which revealed a common pattern of reduced activation during reading tasks in the left temporal and occipital lobes. We provide evidence that altered activation observed within the reading system is associated with altered density of grey and white matter of specific brain regions, such as the left middle and inferior temporal gyri and the left arcuate fasciculus. This supports the view that dyslexia is associated with both local grey matter dysfunction and with altered connectivity among phonological/reading areas. The differences were replicable across samples confirming that the neurological disorder underlying dyslexia is the same across the cultures investigated in the study.

Aphasia and neglect after subcortical stroke. A clinical/cerebral perfusion correlation study

Sixteen patients with unilateral subcortical haemorrhagic or ischaemic stroke, confirmed by CT, were evaluated for the presence of aphasia and neglect. Compared with patients without neuropsychological deficits, left brain-damaged aphasic and right brain-damaged neglect patients showed a significantly greater reduction of cortical perfusion on N,N,N1-trimethyl-N1-(2)-hydroxy-3-methyl-5-(I-123) iodobenzyl-1,3-propanediamine 2 HCl I-123 (HIPDM) and single photon emission computerized tomography (SPECT). These results suggest that major cortical derangement is the crucial factor for the appearance of aphasia or neglect after a subcortical stroke. These remote effects, which are related to the size of the subcortical lesion, are interpreted in terms of interruption of neural connections (diaschisis).

Executive dysfunction and avoidant personality trait in myotonic dystrophy type 1 (DM-1) and in proximal myotonic myopathy (PROMM/DM-2)

A previous study in proximal myotonic myopathy (PROMM/DM-2) and myotonic dystrophy type 1 (DM-1) using brain positron emission tomography demonstrated a reduced cerebral blood flow in the frontal and temporal regions associated with cognitive impairment. The objective was to investigate further cognitive and behavioural aspects in a new series of patients with DM-1 and PROMM/DM-2. Nineteen patients with genetically determined PROMM/DM-2 and 21 patients with moderately severe DM-1 underwent neuropsychological testing and neuropsychiatric interviews. DM-1 and PROMM/DM-2 patients had significantly lower scores on tests of frontal lobe function compared to controls. Neuropsychiatric interviews demonstrated an avoidant trait personality disorder in both patient groups. Brain single photon emission computed tomography showed frontal and parieto-occipital hypoperfusion. The results suggest that there is a specific cognitive and behavioural profile in PROMM/DM-2 and in DM-1, and that this profile is associated with hypoperfusion in frontal and parieto-occipital regions of the brain.

Education and occupation as proxies for reserve in aMCI converters and AD: FDG-PET evidence

BACKGROUND

Previous reports have shown that higher education is associated with more severe brain pathology in patients with Alzheimer disease (AD), suggesting that these individuals have a functional reserve provided by education, which masks the clinical expression of a higher degree of neurodegeneration. It is unknown if a similar reserve mechanism exists in patients with amnestic mild cognitive impairment (aMCI). The aim of this study was to assess the impact of education and occupation on brain glucose metabolism (rCMRglc) measured with FDG-PET in aMCI and in a very large sample of subjects with probable AD (pAD).

METHODS

A total of 242 patients with pAD, 72 with aMCI, and 144 healthy controls participated in the study. At follow-up, 21 subjects with aMCI progressed to AD. A regression analysis was conducted (SPM2), with education and occupation as independent variables, and rCMRglc as dependent variable, adjusting for demographic data, global cognitive status, and neuropsychological scores.

RESULTS

The analysis showed a significant association between higher education/occupation and lower rCMRglc in posterior temporoparietal cortex and precuneus in pAD and aMCI converters, and no correlation in aMCI nonconverters and healthy controls. This means that, when submitted to FDG-PET for diagnostic evaluation, pAD and aMCI converters with higher education/occupation had, for comparable cognitive impairment, a more severe rCMRglc reduction than the ones with lower education/occupation.

CONCLUSIONS

This study suggests that education and occupation may be proxies for brain functional reserve, reducing the severity and delaying the clinical expression of Alzheimer disease (AD) pathology. The results in aMCI converters suggest that functional reserve is already at play in the predementia phase of AD.

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.

Acupuncture produces central activations in pain regions

Acupuncture is largely used for pain control in several pathological conditions. Its effects on the central nervous system are not well defined. We investigated the effect of the application of acupuncture to 13 normal subjects (males, 21-32 years). H2(15)O bolus PET scans were read before the application of the needles (Rest, R) and after 25 min of needle insertion. Data were acquired by scanning in 3-D mode. The acupuncture application, true acupuncture (TA), was alternated to a placebo needle application (PA) in two different sequences (seven and six subjects, respectively), either R,PA,R, TA or R,TA,R,PA, a period of 15 min being left after every first TA or PA to allow for the recovery of basal conditions. Here we show that classic acupuncture activates the left Anterior Cingulus, the Insulae bilaterally, the Cerebellum bilaterally, the left Superior Frontal Gyrus, and the right Medial and Inferior Frontal Gyri. Most of the activated areas are shared with areas activated in acute and chronic pain states as described in the literature. Thus acupuncture appears to act by activating areas also involved in pain. This indicates that acupuncture could relief pain by unbalancing the equilibrium of distributed pain-related central networks.

Interhemispheric transmission of visuomotor information in humans: fMRI evidence

Normal human subjects underwent functional magnetic resonance imaging (fMRI) while performing a simple visual manual reaction-time (RT) task with lateralized brief stimuli, the so-called Poffenberger's paradigm. This paradigm was employed to measure interhemispheric transmission (IT) time by subtracting mean RT for the uncrossed hemifield-hand conditions, that is, those conditions not requiring an IT, from the crossed hemifield-hand conditions, that is, those conditions requiring an IT to relay visual information from the hemisphere of entry to the hemisphere subserving the response. The obtained difference is widely believed to reflect callosal conduction time, but so far there is no direct physiological evidence in humans. The aim of our experiment was twofold: first, to test the hypothesis that IT of visuomotor information requires the corpus callosum and to identify the cortical areas specifically activated during IT. Second, we sought to discover whether IT occurs mainly at premotor or perceptual stages of information processing. We found significant activations in a number of frontal, parietal, and temporal cortical areas and in the genu of the corpus callosum. These activations were present only in the crossed conditions and therefore were specifically related to IT. No selective activation was present in the uncrossed conditions. The location of the activated callosal and cortical areas suggests that IT occurs mainly, but not exclusively, at premotor level. These results provide clear cut evidence in favor of the hypothesis that the crossed-uncrossed difference in the Poffenberger paradigm depends on IT rather than on a differential hemispheric activation.