Biomedical subjects

Karl Zilles

Publications and source records attributed to Karl Zilles.

At least 55 records · Page 3Linked to original sources

PubMed2005

O-(2-[18F]fluoroethyl)-L-tyrosine PET combined with MRI improves the diagnostic assessment of cerebral gliomas.

MRI is commonly used to determine the location and extent of cerebral gliomas. We investigated whether the diagnostic accuracy of MRI could be improved by the additional use of PET with the amino acid O-(2-[18F]fluoroethyl)-l-tyrosine (FET). In a prospective study, PET with FET and MRI was performed in 31 patients with suspected cerebral gliomas. PET and MRIs were co-registered and 52 neuronavigated tissue biopsies were taken from lesions with both abnormal MRI signal and increased FET uptake (match), as well as from areas with abnormal MR signal but normal FET uptake or vice versa (mismatch). Biopsy sites were labelled by intracerebral titanium pellets. The diagnostic performance for the identification of cellular tumour tissue was analysed for either MRI alone or MRI combined with FET PET using alternative free response receiver operating characteristic curves (ROCs). Histologically, 26 biopsy samples corresponded to cellular glioma tissue and 26 to peritumoral brain tissue. The diagnostic performance, as determined by the area under the ROC curve (Az), was Az = 0.80 for MRI alone and Az = 0.98 for the combined MRI and FET PET approach (P < 0.001). MRI yielded a sensitivity of 96% for the detection of tumour tissue but a specificity of only 53%, and combined use of MRI and FET PET yielded a sensitivity of 93% and a specificity of 94%. Combined use of MRI and FET PET in patients with cerebral gliomas significantly improves the identification of cellular glioma tissue and allows definite histological tumour diagnosis. Thus, our findings may have considerable impact on target selection for diagnostic biopsies as well as therapy planning.

Adolescent↗

PubMed2005

Distributions of transmitter receptors in the macaque cingulate cortex.

The primate cingulate cortex is structurally and functionally complex. Although no studies have investigated the regional densities of multiple neurotransmitter receptor systems, such information would be useful for assessing its functions and disease vulnerabilities. We quantified nine different receptors in five transmitter systems by in vitro autoradiographic mapping of the cingulate cortex of macaque monkeys with the aim to link cytoarchitectonic regions and functional specialization. Receptor mapping substantiated the subdivision of the cingulate cortex into anterior versus posterior regions. In anterior cingulate cortex (ACC) AMPA glutamatergic receptors and GABA(A) inhibitory receptors were present in significantly higher concentrations than the modulatory alpha-adrenergic and muscarinic receptors. These differences were absent in the posterior cingulate cortex (PCC). By contrast, NMDA receptor densities were significantly higher than AMPA receptor densities in PCC, but not in ACC. The midcingulate area 24' shared more features with ACC than PCC. This area was characterized by the highest ratios of NMDA receptors to alpha-adrenergic, muscarinic and 5-HT2 receptors among all cingulate regions. Compared to rostrocaudal divisions, the differences between dorsoventral subdivisions a-c were small in all regions of cingulate cortex, and only muscarinic and alpha-adrenergic receptor densities followed the degree of cytoarchitectonic differentiation. We conclude that multiple receptor mapping reveals a highly differentiated classification of cingulate cortex with a characteristic predominance of fast ionotropic excitatory and inhibitory receptors in ACC, but a strong and varied complement of NMDA and metabotropic receptors in PCC.

Animals↗

PubMed2005

High-resolution MRI reflects myeloarchitecture and cytoarchitecture of human cerebral cortex.

Maps of cytoarchitectonically defined cortical areas have proven to be a valuable tool for anatomic localization of activated brain regions revealed by functional imaging studies. However, architectonic data require observations in a sample of postmortem brains. They can only be used reliably for comparison with functional data as probabilistic maps after spatial normalization to a common reference space. The complete architectonic analysis of an individual living brain has not been achievable to date, because the relationship remains unclear between laminar gray value changes of cerebral cortex in magnetic resonance (MR) images and those of cyto- and myeloarchitectonic histologic sections. We examined intensity profiles through the cortex in five imaging modalities: in vivo T1 and postmortem T2 MRI, one cell body stain, and two myelin stains. After visualizing the dissimilarities in the shapes of these profiles using a canonical analysis, differences between the profiles from the different image modalities were compared quantitatively. Subsequently, the profiles extracted from the in vivo T1-weighted images were estimated from profiles extracted from cyto- and myeloarchitectonic sections using linear combinations. We could verify statistically the mixed nature of the cortical T1 signal obtained in vivo: The MR intensity profiles were significantly more similar to myeloarchitectonic than to cytoarchitectonic profiles, but a weighted sum of both fitted the T1 profiles best.

Brain Mapping↗

PubMed2005

Automated quality assurance routines for fMRI data applied to a multicenter study.

Standard procedures to achieve quality assessment (QA) of functional magnetic resonance imaging (fMRI) data are of great importance. A standardized and fully automated procedure for QA is presented that allows for classification of data quality and the detection of artifacts by inspecting temporal variations. The application of the procedure on phantom measurements was used to check scanner and stimulation hardware performance. In vivo imaging data were checked efficiently for artifacts within the standard fMRI post-processing procedure by realignment. Standardized and routinely carried out QA is essential for extensive data amounts as collected in fMRI, especially in multicenter studies. Furthermore, for the comparison of two different groups, it is important to ensure that data quality is approximately equal to avoid possible misinterpretations. This is shown by example, and criteria to quantify differences of data quality between two groups are defined.

Artifacts↗

PubMed2005

Age-related morphology trends of cortical sulci.

The age-related trends of the width and the depth of major cortical sulci were studied in normal adults. Ninety healthy subjects (47 males, 43 females) age 20-82 years were evaluated. Measurements of average sulcal width and depth in 14 prominent sulcal structures per hemisphere were performed with high-resolution anatomical MRI. The average sulcal width increased at a rate of about 0.7 mm/decade, while the average sulcal depth decreased at a rate of about 0.4 mm/decade. Sulcal age-related trends were found to be highly influenced by gender in the superior temporal, collateral, and cingulate sulci (P < 0.05), with males showing more pronounced age-related change in sulcal width than females. Sulcal structures located in multimodal cortical areas showed more profound age-related changes than sulcal structures in unimodal cortical areas (P < 0.05).

Adult↗

PubMed2005

Taurine-transporter gene knockout-induced changes in GABA(A), kainate and AMPA but not NMDA receptor binding in mouse brain.

The aim of this study was to determine whether the knockout of the taurine-transporter gene in the mouse affects the densities of GABA(A), kainate, AMPA and NMDA receptors in the brain. The caudate-putamen, the hippocampus and its subregions, and the cerebellum of six homozygous taurine-transporter gene knockout mice and six wild-type (WT) animals were examined by means of quantitative receptor autoradiography. Saturation studies were carried out for all four receptor types in order to find possible intergroup differences in Bmax and K(D) values. Taurine-transporter gene knockout animals showed significantly higher GABA(A) receptor densities in the molecular layer of the hippocampal dentate gyrus and in the cerebellum than did WT animals. The densities of kainate receptors were significantly higher in the caudate-putamen, the CA1 and hilus regions of the hippocampus and in the cerebellum of knockout animals. The caudate-putamen and cerebellum of these mice also contained significantly higher AMPA receptor densities. However, there were no significant differences between knockout and WT animals concerning the densities of NMDA receptors. Reduced brain taurine levels are associated with increased GABA(A), kainate and AMPA receptor densities in some of the regions we examined.

Animals↗

PubMed2005

The macaque inferior parietal lobule: cytoarchitecture and distribution pattern of serotonin 5-HT1A binding sites.

The inferior parietal lobule (IPL) of the macaque monkey integrates sensory information as a prerequisite for reaching and grasping movements. Electrophysiological data suggest that the convexity of the IPL is heterogeneous along its rostro-caudal axis. Cytoarchitecture reflects this functional diversity. However, various parcellations have been proposed so far, ranging from one homogeneous region to four areas. In order to obtain a more valid anatomical map of the IPL, we studied the distribution pattern of serotonin 5-HT1A binding sites (i.e., the domain of synaptic transmission and cortical information processing) and cytoarchitecture with an observer-independent technique (thus avoiding the subjectivity inherent in visually guided microstructural parcellations). We cut with a cryostat microtome four unfixed macaque hemispheres and processed sections for 5-HT1A binding sites with [3H]8-OH-DPAT receptor autoradiography or for cell bodies with a modified silver stain. We digitized the autoradiographs and the histological sections and extracted from the cortex of the IPL equidistant density profiles oriented vertically to the cortical layers. We then compared groups of neighboring profiles with multivariate statistics expecting to see a significant difference in profile shape at the interface between two cortical areas. These positions were compared between 5-HT1A autoradiographs (neurochemical borders) and adjacent histological sections (cytoarchitectonic borders). Neurochemical and cytoarchitectonic borders showed a good topographical correspondence and revealed three areas arranged in a rostro-caudal sequence along the convexity of the IPL. Dorsally, the areas extend approximately 1 mm into the depth of the lateral bank of the intraparietal sulcus. Their ventral border lies on the convexity of the IPL close to the shoulder of the lateral sulcus. The three areas are in close agreement with areas PF, PFG, and PG as defined by Pandya and Seltzer (J Comp Neurol 204:196-210, 1982) and Gregoriou et al. (Program No. 919.5, Abstract Viewer/Itinerary Planner, Washington, Society for Neuroscience, 2003). The neurochemical and cytoarchitectonic data show that a complex structural framework underlies the functional heterogeneity along the rostro-caudal axis of the IPL, e.g., the representation of different types of arm and hand actions in different sectors of the monkey's workspace.

Animals↗

PubMed2005

Analysis of neuroreceptor PET-data based on cytoarchitectonic maximum probability maps: a feasibility study.

Three-dimensional maximum probability maps (MPMs) of cytoarchitectonically defined cortical regions based on postmortem histological studies have recently been made available in the stereotaxic reference space of the Montreal Neurological Institute (MNI) single subject template. This permits the use of cytoarchitectonic maps for the analysis of functional in vivo datasets, including neuroreceptor positron emission tomography (PET) studies. In this feasibility study, we used 5-hydroxytryptamine 2A (5-HT2A) receptor PET to test the applicability of maximum cytoarchitectonic probability maps for quantitative analysis. As the outcome parameter, we extracted local distribution volume ratios (DVRs) from 19 cytoarchitectonically defined volumes of interest (VOIs) per hemisphere from five healthy subjects. The experimental design included a forward ('PET to atlas' normalization) and a backward ('atlas to PET' normalization) procedure to double-check the stability of transformation and overlay. Resulting DVRs were compared with receptor densities (RDs) obtained from postmortem [3H]ketanserin autoradiography of multiple areas. Correlations between the bi-directional normalization procedures (r = 0.89; 38 VOIs) as well as between in vivo and vitro data (nine VOIs; r = 0.64 and r = 0.47 for forward and backward procedure, respectively) suggest that the implementation of cytoarchitectonic maximum probability maps is a promising method for an accurate and observer-independent analysis of neuroreceptor PET data.

Adult↗

PubMed2005

Decreased prefrontal 5-HT2A receptor binding in subjects at enhanced risk for schizophrenia.

The brain serotonin-2A receptor (5-HT(2A)R) has been implicated in both the pathology of schizophrenia and the therapeutic action of atypical antipsychotics. However, little is known about the 5-HT(2A)R status before the onset of schizophrenia and before the exposure to antipsychotics. We used [18F] altanserin and positron emission tomography (PET) in a pilot study of 6 individuals suspected to be at elevated risk for schizophrenia and seven age-matched controls to test the hypothesis that regional 5-HT(2A)R binding is altered in the prodromal stages of schizophrenia. Distribution volume ratios (DVRs) as a proxy for 5-HT(2A)R availability were significantly reduced in prefrontal cortex regions of at-risk subjects, implicating early abnormalities of serotonergic neurotransmission that antecede the onset of schizophrenia.

Adult↗

PubMed2005

Bilateral, vascular and perivascular glial upregulation of heat shock protein-27 after repeated epileptic seizures.

Heat shock protein-27 (HSP-27) is an inducible stress response protein. It inhibits apoptotic cell death and is a reliable marker for oxidative stress. We studied the induction of HSP-27 in rat brains on days 1, 4 and 14 after repeated, pentylenetetrazole (PTZ)-induced seizures using immunohistochemisty. Saline treated control rats showed no induction of HSP-27. HSP-27 reactive astrocytes were rarely seen 1 or 4 days after PTZ injection. When present, single astrocytes were located in the cortex and/or the hippocampus. After 14 days PTZ treatment, a bilateral distribution of HSP-27 immunoreactive glia was present in piriform and entorhinal cortices and in the dentate gyrus of most brains. Rats with most intense HSP-27 upregulation showed HSP-27 in amygdala and thalamic nuclei. Astrocytes associated with blood vessels presented strongest HSP-27 staining, but did not show upregulation of gial fibrillary acidic protein and none responded with HSP-47 expression. Additionally, HSP-27 immunoreactivity increased in the endothelial cells of blood vessels in the affected brain regions, although no neuronal induction occurred. Contrastingly, a subconvulsive dose of the glutamine synthetase inhibitor L-methionine sulfoxime, which acts directly on astrocytes, resulted in a rapid, homogeneous astrocyte-specific HSP-27 upregulation within 24 h. Thus, repeated PTZ-induced seizure activity elicits a focal "heat shock" response in endothelial cells and astrocytes of selected cerebral regions indicating that expression of HSP-27 occurred in a seizure-dependent manner within the affected cerebral circuitries. Therefore, this PTZ-model of repeated seizure activity exhibited a cortical pattern of HSP-27 expression which is most comparable to that known from patients with epilepsy.

Animals↗

PubMed2005

Preferred stereoselective brain uptake of d-serine--a modulator of glutamatergic neurotransmission.

Although it has long been presumed that d-amino acids are uncommon in mammalians, substantial amounts of free d-serine have been detected in the mammalian brain. d-Serine has been demonstrated to be an important modulator of glutamatergic neurotransmission and acts as an agonist at the strychnine-insensitive glycine site of N-methyl-d-aspartate receptors. The blood-to-brain transfer of d-serine is thought to be extremely low, and it is assumed that d-serine is generated by isomerization of l-serine in the brain. Stimulated by the observation of a preferred transport of the d-isomer of proline at the blood-brain barrier, we investigated the differential uptake of [3H]-d-serine and [3H]-l-serine in the rat brain 1 h after intravenous injection using quantitative autoradiography. Surprisingly, brain uptake of [3H]-d-serine was significantly higher than that of [3H]-l-serine, indicating a preferred transport of the d-enantiomer of serine at the blood-brain barrier. This finding indicates that exogenous d-serine may have a direct influence on glutamatergic neurotransmission and associated diseases.

Animals↗

PubMed2005

Preferred stereoselective transport of the D-isomer of cis-4-[18F]fluoro-proline at the blood-brain barrier.

Generally, L-amino acids are preferably transported into mammalian cells compared with their D-isomers, and only L-amino acids are incorporated into proteins. Former studies, however, indicated that D-[H]proline is accumulated in the brain of mice after injection, while L-[3H]proline is not. We investigated the differential cerebral uptake of the D- and L-isomers of the PET tracer cis-4-[18F]fluoroproline (D-/L-cis-FPro) and of D-/L-[3H]proline (D-/L-Pro) in rats by dual tracer autoradiography and the uptake of D-cis-FPro in two human subjects by PET. The standardized uptake value (SUV) of D-cis-FPro in the cerebral cortex of rats 2 h p.i. was 3.05+/-1.18 (n=9) versus 0.06+/-0.01 (n=4) for L-cis-FPro (P<0.001) and 1.29+/-0.27 (n=4) for D-Pro versus 0.30+/-0.14 (n=9) for L-Pro (P<0.001). Analysis of the rat brain tissue after injection of D-cis-FPro (n=3) revealed no radioactivity in the proteins but a relevant part in the form of L-trans-FPro. The PET studies yielded a four- to five-fold higher SUV and influx rate constant in the human cortex for D-cis-FPro than for L-cis-FPro. We conclude that D-cis-FPro and D-Pro are preferably transported at the blood-brain barrier compared with their L-isomers and isomerized to the L-form within the brain. Thus, D-Pro in the plasma might be a source of intracerebral L-proline, which has been shown to act as a modulator of excitatory neurotransmission.

Animals↗

PubMed2005

Nicotine modulates reorienting of visuospatial attention and neural activity in human parietal cortex.

Prior studies in animals and humans indicate that reorienting of visuospatial attention is modulated by the cholinergic agonist nicotine. We have previously identified neural correlates of alerting and reorienting attention in humans and found that the parietal cortex is specifically involved in reorienting. This study investigates whether the alerting and reorienting systems, especially in the parietal cortex, are modulated by nicotine. We used event-related functional magnetic resonance imaging (fMRI) and studied 15 nonsmoking volunteers under placebo and nicotine (NICORETTE) polacrilex gum 1 and 2 mg). Subjects performed a cued target detection task with four different types of randomly intermixed trials (no, neutral, valid, and invalid cue trials). Alerting was captured by comparing BOLD activity and reaction times (RTs) in neutrally cued trials with no cue trials. Reorienting was isolated by comparing invalidly with validly cued trials. On the behavioral level, nicotine affected reorienting of attention by speeding RTs in invalidly cued trials; alerting was not affected by nicotine. Neurally, however, nicotine modulated both attentional systems. Pharmacologic effects on alerting-related brain activity were mainly evident as modulation of BOLD responses in the right angular gyrus and right middle frontal gyrus due to a reduction of neural activity in no cue trials. In the reorienting system, effects of nicotine were mainly evident in the left intraparietal sulcus and precuneus and due to a reduction of neural activity in invalidly cued trials. We conclude that nicotine enhances reorienting of attention in visuospatial tasks and that one behavioral correlate of speeded RTs is reduced parietal activity.

Acetylcholine↗

PubMed2005

Hierarchical processing of sound location and motion in the human brainstem and planum temporale.

Horizontal sound localization relies on the extraction of binaural acoustic cues by integration of the signals from the two ears at the level of the brainstem. The present experiment was aimed at detecting the sites of binaural integration in the human brainstem using functional magnetic resonance imaging and a binaural difference paradigm, in which the responses to binaural sounds were compared with the sum of the responses to the corresponding monaural sounds. The experiment also included a moving sound condition, which was contrasted against a spectrally and energetically matched stationary sound condition to assess which of the structures that are involved in general binaural processing are specifically specialized in motion processing. The binaural difference contrast revealed a substantial binaural response suppression in the inferior colliculus in the midbrain, the medial geniculate body in the thalamus and the primary auditory cortex. The effect appears to reflect an actual reduction of the underlying activity, probably brought about by binaural inhibition or refractoriness at the level of the superior olivary complex. Whereas all structures up to and including the primary auditory cortex were activated as strongly by the stationary as by the moving sounds, non-primary auditory fields in the planum temporale responded selectively to the moving sounds. These results suggest a hierarchical organization of auditory spatial processing in which the general analysis of binaural information begins as early as the brainstem, while the representation of dynamic binaural cues relies on non-primary auditory fields in the planum temporale.

Acoustic Stimulation↗

PubMed2005

Characterization of the rhesus monkey superior olivary complex by calcium binding proteins and synaptophysin.

This study was performed in order to characterize the main nuclei of the rhesus monkey superior olivary complex by means of antibodies against the calcium binding proteins parvalbumin, calbindin and calretinin and the synaptic vesicle protein synaptophysin. These markers revealed the neuronal morphology and organization of nuclei located within the rhesus monkey superior olivary complex. The architectural details included the distribution of axonal terminals on neurons. The medial superior olivary nucleus was present as a column of neurons. No clear segregation of calretinin-positive terminals was noticed on the medial and lateral dendritic fields of these neurons. The lateral superior olivary nucleus was characterized by a distinct nuclear shape. Calretinin-, parvalbumin- or calbindin-positive terminals contacted somata and dendrites. The medial nucleus of trapezoid body could be clearly differentiated as a distinct region in the rhesus monkey superior olivary complex. Somata of that nucleus showed calbindin- and parvalbumin-labelling whereas somatic calyces of Held were reavealed by calretinin and synaptophysin labelling. The results are discussed with respect to the processing of acoustic information in primate species and their ability to hear high and low frequencies, which is reflected by anatomical correlates.

Animals↗

PubMed2005

Regional and subtype selective changes of neurotransmitter receptor density in a rat transgenic for the Huntington's disease mutation.

Huntington's disease (HD) is an autosomal dominantly inherited progressive neurodegenerative disorder caused by a CAG/polyglutamine repeat expansion in the gene encoding the huntingtin protein. We have recently generated a rat model transgenic for HD, which displays a slowly progressive phenotype resembling the human adult-onset type of disease. In this study we systematically assessed the distribution and density of 17 transmitter receptors in the brains of 2-year-old rats using quantitative multi-tracer autoradiography and high-resolution positron emission tomography. Heterozygous animals expressed increased densities of M(2) acetylcholine (increase of 148 +/- 16% of controls; p > 0.001; n = 7), nicotine (increase of 149 +/- 16% of controls; p > 0.01; n = 6), and alpha(2) noradrenergic receptors (increase of 141 +/- 15% of controls; p > 0.001; n = 6), respectively. Densities of these receptors were decreased in homozygous animals. Decreases of receptor density in both hetero- and homozygous animals were found for M1 acetylcholine, 5-HT 2A serotonin, A 2A adenosine, D1 and D2 dopamine, and GABA(A) receptors, respectively. Other investigated receptor systems showed small changes or were not affected. The present data suggest that the moderate increase of CAG/polyglutamine repeat expansions in the present rat model of Huntington's disease is characterized by subtype-selective and region-specific changes of neuroreceptor densities. In particular, there is evidence for a contribution of predominantly presynaptically localized cholinergic and noradrenergic receptors in the response to Huntington's disease pathology.

Animals↗

PubMed2005

Cortical representations of personally familiar objects and places: functional organization of the human posterior cingulate cortex.

The recognition of both personally familiar objects and places involves nonspatial memory retrieval processes, but only personally familiar places are represented as space. Although the posterior cingulate cortex (PCC) is considered to process both types of such memories, its functional organization is poorly understood. In this event-related fMRI study, normal subjects judged familiar/unfamiliar pictures in four categories: familiar places (FP), familiar objects (FO), unfamiliar places (UP), and unfamiliar objects (UO), thus constituting a two-factorial design. A significant main effect of stimuli with greater activation in the place (FP and UP) than object (FO and UO) trials was observed bilaterally in several medial temporo-occipito-parietal regions, including the caudal PCC (cPCC) and parahippocampal gyrus. The reverse comparison revealed greater activation in the lateral inferior occipito-temporal junctions and intraparietal sulci bilaterally. A significant main effect of familiarity with greater activation in the familiar (FP and FO) than unfamiliar (UP and UO) trials was observed in the mid-dorsal PCC (mPCC), retrosplenial cortex, posterior precuneus, and the left intraparietal sulcus. Activation specific to the FP trials (as assessed by the interaction) was observed in the right posterodorsal PCC (pPCC) only. Together with data from previous functional imaging studies, the results suggest a functional segregation of human PCC with differential involvement of pPCC in spatial representations of personally familiar places and of the mPCC and retrosplenial cortex in episodic retrieval of personally familiar places and objects. Activation of the left intraparietal sulcus may reflect retrieval of memories related to object manipulation.

Adolescent↗

PubMed2005

Common and differential neural mechanisms supporting imitation of meaningful and meaningless actions.

Neuropsychological studies indicate that, after brain damage, the ability to imitate meaningful or meaningless actions can be selectively impaired. However, the neural bases supporting the imitation of these two types of action are still poorly understood. Using PET, we investigated in 10 healthy individuals the neural mechanisms of imitating novel, meaningless actions and familiar, meaningful actions. Data were analyzed using SPM99. During imitation, a significant positive correlation (p < .05, corrected) of regional cerebral blood flow with the amount of meaningful actions was observed in the left inferior temporal gyrus only. In contrast, a significant positive correlation (p < .05, corrected) with the amount of meaningless movements was observed in the right parieto-occipital junction. The direct categorical comparison of imitating meaningful (100%) relative to meaningless (100%) actions showed differential increases in neural activity (p < .001, uncorrected) in the left inferior temporal gyrus, the left parahippocampal gyrus, and the left angular gyrus. The reverse categorical comparison of imitating meaningless (100%) relative to meaningful (100%) actions revealed differential increases in neural activity (p < .001, uncorrected) in the superior parietal cortex bilaterally, in the right parieto-occipital junction, in the right occipital-temporal junction (MT, V5), and in the left superior temporal gyrus. Increased neural activity common to imitation of meaningless and meaningful actions compared to action observation was observed in a network of areas known to be involved in imitation of actions including the primary sensorimotor cortex, the supplementary motor area, and the ventral premotor cortex. These results are compatible with the two-route model of action imitation which suggests that there are at least two mechanisms involved in imitation of actions: a direct mechanism transforming a novel action into a motor output, and a semantic mechanism, on the basis of stored memories, that allows reproductions of known actions. Our results indicate that, in addition to shared neural processes, the direct and the semantic mechanisms that underlie action imitation also draw upon differential neural mechanisms. The direct mechanism underlying imitation of meaningless actions differentially involves visuospatial transformation processes as evidenced by activation of areas belonging to the dorsal stream. In contrast, imitation of meaningful actions differentially involves semantic processing as evidenced by activation of areas belonging to the ventral stream.

Adult↗