Cerebral Cortex Advance Access originally published online on May 11, 2005
Cerebral Cortex 2006 16(2):268-279; doi:10.1093/cercor/bhi106
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The Human Parietal Operculum. II. Stereotaxic Maps and Correlation with Functional Imaging Results
1 Institute of Medicine, Research Center Jülich, 52425 Jülich, Germany, 2 C. & O. Vogt-Institute for Brain Research, University of Düsseldorf, 40001 Düsseldorf, Germany and 3 Department of Psychiatry and Psychotherapy, RWTH Aachen University, Germany
Address correspondence to Professor Karl Zilles, Institut für Medizin, Forschungszentrum Jülich GmbH, D-52425 Jülich, Germany. Email: K.Zilles{at}fz-juelich.de.
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Abstract |
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In this study we describe the localization of the cytoarchitectonic subdivisions of the human parietal operculum in stereotaxic space and relate these anatomically defined cortical areas to the location of the functionally defined secondary somatosensory cortex (SII cortex) using a meta-analysis of functional imaging results. The human parietal operculum consists of four distinct cytoarchitectonic areas (OP 1–4) as shown in the preceding publication. The 10 cytoarchitectonically examined brains were 3-D-reconstructed and spatially normalized to the T1-weighted single-subject template of the Montreal Neurological Institute (MNI). A probabilistic map was calculated for each area in this standard stereotaxic space. A cytoarchitectonic summary map of the four cortical areas on the human parietal operculum which combines these probabilistic maps was subsequently computed for the comparison with a meta-analysis of functional locations of SII. The meta-analysis used the results from 57 fMRI and PET studies and allowed the comparison of the functionally defined SII region to the cytoarchitectonic map of the parietal operculum. The functional localization of SII showed a good match to the cytoarchitectonically defined region. Therefore the cytoarchitectonic maps of OP 1–4 of the human parietal operculum can be interpreted as an anatomical correlate of the (functionally defined) human SII region. Our results also suggest that the SII foci reported in functional imaging studies may actually reflect activations in either of its architectonic subregions.
Key Words: human cerebral cortex • meta-analysis • neuroimaging • probabilistic maps • SII • somatosensory cortex
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Introduction |
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The human secondary somatosensory cortex (SII) has been identified on the parietal operculum by functional magnetic resonance imaging (fMRI), positron emission tomography (PET) and magnetoencephalography (MEG). In these experiments, various paradigms have elicited activations of SII, including electrical nerve stimulation (Boakye et al., 2000
; Del Gratta et al., 2002), vibration (Burton et al., 1993; Coghill et al., 1994; Francis et al., 2000) and a light touch on the skin (Burton et al., 1997; Disbrow et al., 2000). It has been described that SII is also engaged in the perception of pain (Coghill et al., 1994, 1999; Svensson et al., 1997) and visceral sensations (Aziz et al., 2000; Lotze et al., 2001). The reported activation foci are widely distributed on the parietal operculum. For example, Peyron et al. (1999) reported the SII activation following painful heat stimulation of the hand at x = 30, y = 10, z = 16. The coordinates reported by Xu et al. (1997) for the same task (x = 52, y = –34, z = 20) on the other hand were located >20 mm apart. Both sets of coordinates were registered into Talairach space (Talairach and Tournoux, 1988). Although some variability may be explained by differences in the image preprocessing and the details of the applied contrasts, the magnitude of the spatial discrepancy raises doubt about whether these foci indeed reflect activations of the same cortical area.
Electrophysiological and tracing studies in monkeys have provided evidence that SII can be subdivided into at least two distinct areas, containing separate, mirror-image body representations (Burton et al., 1995; Krubitzer et al., 1995). These two areas share a common border at the representations of the face (superficial), hands (intermediate) and feet (deep). The existence of multiple areas within the SII region was also reported in other non-human primates (Krubitzer and Kaas 1990; Krubitzer et al., 1993) and is now considered a general feature of the somatosensory cortex of anthropoid primates (Kaas and Collins, 2003). In the most commonly used nomenclature for the cortical areas on the parietal operculum of non-human primates the area immediately adjacent to SI is labeled as ‘parietal ventral area’ (PV). It is caudally followed by area SII. Importantly, this ‘area SII’ is not identical with the historically earlier defined and much larger ‘SII region’, i.e. the term ‘SII’ is used for the whole region as well as for an individual area within this region. Finally, area VS (ventral somatosensory) is located deeper in the Sylvian fissure, medially to area SII and area PV.
Disbrow et al. (2000) showed that the human SII region also consists of multiple somatotopically organized areas by demonstrating multiple activation foci within the SII region for various body parts in an fMRI study. These results may explain the diversity of stimuli shown to activate SII as well as its variable location in functional imaging experiments since activity from any of these body representations would be labeled as ‘SII activation’.
How does such functional segregation relate to the cytoarchitectonic organization of the parietal operculum? In order to answer this question, we examined the cytoarchitecture of the human SII region in a sample of ten human post-mortem brains using an observer-independent approach for the definition of areal borders and showed that the human SII region consists of four different cytoarchitectonic areas (see Eickhoff et al., 2005b). Based on this parcellation, the aim of the present study was to analyze the stereotaxic location and intersubject variability of these four areas and to calculate a probabilistic cytoarchitectonic summary map of the human parietal operculum. The summary map was then compared with the results of a meta-analysis of functional imaging studies in the same spatial reference system in order to test if the cytoarchitectonically defined areas correspond to the functional localization of the SII (hand) region as defined in human functional imaging studies. Given that there is rarely a distinction between the different subdivisions of SII in human functional imaging studies, this meta-analysis was designed to explore the relationship between OP 1–4 and the functional activations of human SII as a whole, i.e. to test whether the defined areas cover the functionally defined SII region. Since in monkeys the location of the hand representations is found at the border between the most important subdivisions (area SII and area PV; Kaas and Collins, 2003), we hypothesize that the main focus of the meta-analysis will be located at the border between OP 1 and OP 4, which are the most likely analogues of SII (OP 1) and PV (OP 4) based on cortical topography (Eickhoff et al., 2005b).
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Material and Methods |
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Cytoarchitectonic Mapping and 3-D Reconstruction
Ten post-mortem human brains were histologically processed and the parietal operculum was cytoarchitectonically mapped using an observer-independent method for detecting significant changes in the lamination pattern (for details see Eickhoff et al., 2005b). The brain volumes were subsequently 3-D-reconstructed by using (i) images of each 60th silver-stained histological section (distance between sections: 1.2 mm) digitized with a CCD camera (this histological data set is marked by a high contrast, but inevitable distortions caused by histologic techniques); (ii) a data set of block-face images corresponding to the digitized sections which have been taken from the unstained surface of the paraffin block just before sectioning together with a reference system to establish the 3-D integrity of the histologic volume; and (iii) a structural magnetic resonance data set which was obtained before paraffin-embedding of the brain (3-D FLASH sequence covering the entire brain; flip angle = 40°; repetition time TR = 40 ms; echo time TE = 5 ms for each image; 128 sagittal sections; spatial resolution 1 x 1 x 1.17 mm; 8 bit gray value resolution corresponding to 256 gray values). A comparison of these three data sets then allowed to correct for the deformations and shrinkage inevitably caused by the histological techniques, e.g. embedding, sectioning and mounting on glass slides in the reconstructed volumes by using both linear and nonlinear transformations (Henn et al., 1997; Mohlberg et al., 2003; Amunts et al., 2004). The cytoarchitectonically defined borders were subsequently transferred to the respective reconstructed sections of each brain (Amunts et al., 1999, 2000, 2004; Geyer et al., 2000; Grefkes et al., 2001). To visualize the location and extent of the parietal operculum, the temporal lobe and the perisylvian gray matter were interactively removed in the reconstructed volume of each brain (Fig. 1). This step enabled a good view on the location of the opercular areas in the depth of the Sylvian fissure. After smoothing with an isotropic Gaussian kernel [full-width half-maximum (FWHM) = 1.2 mm], the surface was rendered using AVS Express (AVS Inc., Waltham, MA).
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Spatial Normalization and Probabilistic Maps
The 3-D reconstructed volumes of the 10 brains were spatially normalized (warped) to the T1-weighted single-subject template of the Montreal Neurological Institute (MNI) (Evans et al., 1992) using a nonlinear elastic registration algorithm (Henn et al., 1997; Mohlberg et al., 2003). The origin of the MNI reference space, however, does not coincide with the location of the anterior commissure (AC) of this specific template brain but is located 4 mm more caudally (y-axis) and 5 mm more dorsally (z-axis). In order to keep the AC as the anatomical reference of the coordinate system, the origin of the cytoarchitectonic maps was corrected for this displacement. The maps are thus registered in the ‘anatomical MNI space’, which differs from the original MNI space only by the linear shift of the origin (coordinates 0, 0, 0), a transformation, which can easily be applied when combining functional and anatomical data (Eickhoff et al., 2005a). The corresponding areas of the different subjects were superimposed and a probabilistic map was generated for each area. It describes, for each voxel of the reference brain, how many individual brains overlapped with the displayed cytoarchitectonically defined areas in that particular voxel.
Quantitative Analysis of Volumes and Stereotaxic Location
The volumes of areas OP 1–4 were analyzed with respect to inter-hemispheric and inter-areal differences by an analysis of variance (ANOVA). The statistical design of this two-way ANOVA was as follows: the two factors were ‘hemisphere’ (left, right) and ‘area’ (OP 1–4); the blocking factor was ‘brain’. The level of significance was P < 0.05. If the effect of a factor was significant, we used a subsequent pairwise multiple comparison procedure (Tukey test) to isolate the conditions in which the levels of this factor differed significantly (P < 0.05, corrected for multiple comparisons). Interhemispheric differences in the location of OP 1–4 with respect to the anterior–posterior, the medial–lateral and the inferior–superior axes were tested in using the same procedure.
Maximum Probability Map (MPM)
A maximum probability map of the parietal operculum was computed for the comparison with functional imaging data by analyzing the probabilities for each area (i.e. the numbers of overlapping representations) in each voxel (Eickhoff et al., 2005a). Hereby each voxel was assigned to the most likely area. If different areas showed equally high probabilities for occurring in the same voxel, this voxel was assigned to that area which showed the higher average probability in the directly adjacent voxels. In rare cases where still no definitive allocation to a single area was possible, the procedure was repeated on probability maps, which were smoothed using an isotropic Gaussean kernel (FWHM 6 mm). This approach alone, however, would overestimate the spatial extent of OP 1–4 at the borders towards the surrounding, unmapped cortex (cf. Fig. 3), since the probability for a voxel of belonging to e.g. the insular cortex is unknown. In order to determine an unbiased probability threshold, we examined the probabilities at the border zones where the probabilistic maps of OP 1–4 overlapped. The median probability used to assign such voxels was 4.22, i.e. the majority of voxels assigned at these border zones had a probability of 4/10 or more. Assuming a similar probability distribution at the borders towards unmapped regions the threshold for including a voxel into the MPM was set to 4/10 (40% probability). To ensure the continuity of the resulting summary map voxels were also included if the cumulative probability of this voxel across all areas was at least 60% (i.e. OP 1–4 were found at this position in more than half of the examined brains) or more than two-thirds of the surrounding voxels were assigned. Volume and location of the MPM representation of each area were then compared with the means values for that area after normalization to evaluate the quality of representation. For this evaluation the volume of the MPM representations were expressed in percent relative to the mean volume of the respective areas after spatial normalization. A value of <100% then indicates that the MPM representation is smaller than the mean areal volume, values of >100% indicate an MPM representation, which is larger than the mean volume.
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Meta-analysis of Functional Imaging Studies Reporting SII Activations
We performed a meta-analysis of reported SII activations based on 22 PET and 35 fMRI studies published between 1993 and 2004 (Table 1) and compared the results with the MPM of OP 1–4. The analysis was restricted to activations of the hand region to avoid influences of somatotopy. All foci labeled as ‘SII’, ‘BA 43’ or ‘parietal operculum’ were included, since these terms are often used as equivalents in the literature. Since the relationship between the reported activation foci and the different subdivisions within SII defined in non-human primates were rarely discussed in the used literature, these foci were thought to represent activations within the ‘SII region’ not of any particular subdivision. In total, 181 stereotaxic coordinates (95 on the left, 86 on the right hemisphere) were analyzed. Experimental conditions included pain (56 foci), light touch, including tactile discrimination and object manipulation (49), electrical stimulation of skin or nerves (35) and motor tasks (41). MNI coordinates were translated to match the anatomical MNI space. Coordinates reported in ECHBD space (Roland and Zilles, 1994) were adjusted by an affine transformation of the ECHBD reference brain to the MNI single-subject template (in anatomical MNI space).
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The reported foci were not treated as single points but as localization probability distributions centered at the given coordinates (Turkeltaub et al., 2002
), which were modeled by three-dimensional Gaussean functions. Since virtually all functional imaging data is preprocessed by spatial filtering using Gaussian kernels, this use of three-dimensional Gaussean functions should allow the best approximation of the underlying volumes from the published maxima data. The FWHM of these functions (12 mm) is well within the range of the smoothing filters used in the original studies, which were included in the meta-analysis. Subsequently an ‘activation likelihood estimate’ (ALE), given by the union of the probabilities associated with the different foci, was calculated for each voxel. The obtained ALE values were compared with those obtained from random sets of foci to differentiate ALE values representing nonrandom clustering of foci (i.e. a true convergence of reported coordinates) from those representing random clustering (i.e. noise): 20 000 sets of random coordinates (each set comprising 95 coordinates on the left and 86 on the right hemisphere) within the range of the reported coordinates were generated and processed identically to the literature data. The histograms of the obtained ALE values were averaged to obtain a single noise histogram representing the null hypothesis distribution. The p-value associated with a specific ALE score (the probability for rejecting the null hypothesis when it is actually true, i.e. for incorrectly declaring a voxel active) is now given by the probability of obtaining equal or higher values in the noise histogram, which represents the null hypothesis. The anatomical location of functionally defined SII was visualized by projecting the significant (p < 0.00001) voxel on the MPM and quantified by calculating the overlap of the significant clusters with OP 1–4. Given the functional evidence for a separated representation of touch and pain within the human SII region (Coghill et al., 1994; Ferretti et al., 2003), we examined the relative association of the significant voxel to painful and non-painful stimuli: For each voxel the probabilities for the pain-related foci and the probabilities for the non pain-related foci were compared using Student's t-test (P < 0.05, uncorrected). |
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Volumes and Stereotaxic Localization of Areas OP 1–4
There was a high intersubject variability in size and location of all areas (Fig. 2, Tables 2 and 3). The volumes of OP 1 for example ranged from 3026 mm3 (left hemisphere in case 8) to 12 862 mm3 (right hemisphere in case 4). No volume differences were found between the two hemispheres for any area (no significant interaction between the factors area and side: F = 1.242, df = 3, P > 0.05). There was a statistically significant difference in the mean volumes of OP 1–4 after allowing for effects of interindividual variations (effect of factor area: F = 24.08, df = 3, P < 0.001). In the subsequent pairwise comparison the volume differences between OP 1 and OP 4 and between OP 2 and OP 3 respectively did not reach statistical significance, but OP 1 and OP 4 were both significantly larger than OP 2 and OP 3 (P < 0.05, corrected for multiple comparisons). The coordinates for the centers of gravity of OP 1–4 in anatomical MNI space are given in Table 3. No significant interhemispheric differences in the medial–lateral (F = 2.315), anterior–posterior (F = 0.736) or superior–inferior (F = 1.708) location were found of any area (P > 0.05).
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Probabilistic Maps of OP 1–4
Probabilistic maps were computed for OP 1–4 (Fig. 3). These maps showed that high probabilities, corresponding to locations where a specific area was found in e.g. 9 out of 10 examined brains, are only observed for a small number of voxels. In contrast, the volumes for low probabilities, e.g. made up by voxel containing an area in 2 out of 10 brains, were considerably larger. As a consequence, the probabilistic maps of OP 1–4 overlapped considerably at lower probabilities, i.e. it was impossible to assign all voxel in the parietal operculum unequivocally to a specific cytoarchitectonic area at lower thresholds.
The MPM of OP 1–4, however, represents a contiguous, non-overlapping parcellation of the human parietal operculum (Fig. 4). Importantly the MPM does not show the parcellation of an exemplary or ‘typical’ hemisphere as in the classical architectonic brain maps (e.g. Brodmann, 1909), but reflects the most likely area at each positions in a sample of ten post-mortem brains. In total, 24 810 voxels (1 mm3 each) were assigned to the MPM. Of these voxels, 23 263 (93.8%) were assigned based on the relative probabilities, 1525 (6.1%) were assigned by comparing the neighboring voxels and the remaining 22 (0.1%) were assigned by using the smoothed probability maps. The volumes of OP 1–4 in the MPM represented between 81% (i.e. 19% smaller) and 102% (i.e. 2% larger) of the mean volumes of the individual areas after spatial normalization (cf. Table 2). There is also a good match in stereotaxic location between OP 1–4 and their representations in the MPM. All centers of gravity of the MPM representations are located within one standard deviation of the mean location of the respective area (Table 3).
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The Localization of SII Activations
The localization of the activation foci for the SII hand representation (Fig. 5) corresponded well with the described MPM: 92% of the functional foci were located less than 5 mm apart from the nearest voxel included in the MPM representations of OP 1–4. The mean stereotaxic coordinates of the SII hand representation on the right hemispheric were x = –49, y = –19, z = +18. At this location, OP 1 and OP 4 were both found with a probability of 40%. The probability for OP 3 was 20%. The mean functional activation was thus located at the border between OP 1 and OP 4. The mean stereotaxic location of SII on the left hemisphere was x = +51, y = –21, z = +19. Here the probability for OP 1 was 60%, whereas the probabilities for OP 4 and OP 3 were 20% each. In the MPM the corresponding voxel was located within area OP 1, close to the borders to OP 3 and OP 4.
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The ALE threshold corresponding to a significance level of P < 0.00001 was 0.004 for the left and 0.0038 for the right hemisphere. One statistically significant cluster was identified on each hemisphere reflecting the functionally defined location of SII. Note, that there is no differentiation between the different subdivisions of the SII region as described for the macaque in this meta-analysis, since this subdivisions are rarely indicated or discussed in the functional imaging literature. Thus these clusters should represent the most likely localization of the whole ‘SII region’. Both clusters were almost completely located within the cytoarchitectonically examined region as defined by the MPM containing OP 1–4 (Fig. 5): 77% of the significant voxel on the left hemisphere and 80% of those on the right hemisphere were located in the MPM representation of one of the four areas. Those of the significant voxel which matched the cytoarchitectonic areas were mainly found in areas OP 1 (50%) and OP 4 (42%). The highest ALE score (representing the most likely functional localization of SII) on the left hemisphere was found at x = –52, y = –23, z = +18 (ALE: 0.0109). The highest ALE score on the right (0.0098) was located at x = 54, y = –22, z = +20. The location of both of these maxima corresponded to voxels just posterior to the border between OP 1 and OP 4.
A subset of voxels was identified within each of these two clusters which showed a significantly higher probability for activation in pain-related paradigms than in non-pain related ones. In both hemispheres these voxel were located in the caudal portion of the functional cluster within OP 1. Bilaterally a second group of voxels showing a significantly higher probability for non-pain related foci was identified more rostrally. The voxels belonging to this group were located at either side of the border between area OP 1 and OP 4 (Fig. 5).
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Discussion |
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Stereotaxic Maps of the Parietal Operculum
Size, spatial location and intersubject variability of four cytoarchitectonic areas on the human parietal operculum have been analyzed in this study. Stereotaxic probabilistic maps have been calculated for each area and combined into a maximum probability map (MPM) of the parietal operculum by assigning each voxel of the reference brain to the most likely anatomical area. The MPM enables the definition of non-overlapping volumes of interest for each area based on a sample of 10 post-mortem human brains. The volumes assigned to OP 1–4 in the MPM of the parietal operculum represent 
90% of the mean volumes of the individual areas (Table 2) and thus adequately reflect the size of the different areas. The lack of an overlap between different areas is a main advantage compared with a simple thresholding of the probabilistic maps at e.g. 40%. Although the volumes of the 40% representations are also close to the mean volumes (representing between 88 and 105% of the mean, i.e. being from 12% smaller to 5% larger), these thresholded maps overlap by 12% of their volume. Thus, a significant portion of the voxel is ambiguously defined by this method. Based on this good correlation in both size and location between the individual areas and their representations in the MPM, we conclude, that the MPM represents an appropriate tool for the comparison of microstructurally defined areas with functional imaging data.
Methodological Considerations with Respect to the Meta-analysis
Inherently, the spatial resolution of meta-analyses is limited due to differences between the original studies with respect to spatial normalization, filtering and statistical models (Turkeltaub et al., 2002). Nevertheless the algorithmical approach used in this study has major advantages over other meta-analytic methods, as it allows the quantification of the level of concordance at different locations and the use of significance thresholds. Since the obtained ALE scores were tested against a noise distribution obtained from random sets of foci, it can be assumed that significant voxel represent a non-random clustering of functional activation foci. This method provides statistically testable conclusions on the functional localization of the SII region, which can subsequently be compared with the architectonic localization of OP 1–4 to verify that their location does indeed correspond to the functionally defined SII region. However, questions requiring a higher spatial resolution, e.g. exploring the total number of somatotopic representations in the parietal operculum and neighboring cortices, their location relative to each other and their relation to cytoarchitectonic areas, are beyond the capacity of a meta-analysis. This limitation includes an examination, whether painful stimuli are uniquely and exclusively processed in OP 1, as indicated by our meta-analysis and by previous lesion studies of other groups (e.g. Greenspan and Winfield, 1992; Greenspan et al., 1999), whereas non-painful stimuli are represented in multiple areas.
Correlation of OP 1–4 with Functional Imaging Results
The location of functional activations of ‘SII’, which presumably represent activations from all subareas of the SII region, matched the location of OP 1–4 in both hemispheres very well. Less than 20% of the significant voxels were found outside of the volumes assigned to OP 1–4 in the MPM. Most of these voxels were located inferiorly to the operculum in the CSF of the Sylvian fissure, whereas the match between the activation foci and the MPM was almost perfect in the medial–lateral and anterior–posterior directions. This notion may relate to differences in the templates used for spatial normalization. The observation that OP 1–4 almost completely cover the functionally defined SII region renders the hypothesis very unlikely that further cortical areas are also part of the anatomical substrate of the SII region.
The conclusion that all four areas actually contribute to the functions assigned to SII, however, should not be drawn from this meta-analysis of somatosensory data. This is particular true for areas OP 3, which may correspond to the only vaguely characterized primate area VS, and OP 2, which cannot be related to a specific area in the SII region. In non-human primates complex response characteristics have been described for these parts of the parietal operculum, including responses to visual, auditory and vestibular stimuli (Robinson and Burton, 1980; Akbarian et al., 1988; Krubitzer et al., 1995; Guldin and Grusser, 1998). Thus areas OP 2 and OP 3 (VS) may not be activated optimally in response to pure somatosensory stimuli, as applied in the included studies. Rather more specifically designed studies employing stimuli which have been shown to activate these regions in non-human primates might be necessary to demonstrate specific activations within these areas. This also raises the question whether a term like ‘SII region’ or ‘secondary somatosensory area’ is indeed appropriate, given that it becomes more and more apparent that a whole number of anatomically and physiologically different areas have been lumped under the label ‘SII’.
The high probability of SII activations in the border region between OP 1 (i.e. the most likely human equivalent of area SII) and OP 4 (i.e. the most likely human equivalent of area PV) may be explained by the low spatial resolution of meta-analyses, which is not able to resolve two closely neighboring activation sites in areas OP 1 (area SII) and OP 4 (area PV). This close neighborhood can be assumed since the hand representations are located at the border between areas SII and PV in all examined primates (Krubitzer and Kaas, 1990; Krubitzer et al., 1995; Manger et al., 1997). Accordingly, in a recent fMRI study two hand representations (presumably within areas SII and PV) were identified on the human parietal operculum in such close neighborhood that they could not be separated statistically at group level (Disbrow et al., 2000). It can thus be hypothesized that the hand representations in the human equivalents of SII and PV should be merged on the coarse spatial scale of a meta-analysis as well. The functional representation of the hand in the human parietal operculum at the border between OP 1 and OP 4 is therefore in good accordance with the interpretation of OP1 and OP4 as human analogues of primate areas SII and PV respectively.
The anatomical localization of painful and non-painful stimuli suggests one possible functional aspect of the architectonic heterogeneity of the SII region: voxels which were associated with pain-related activity were located only within OP 1 (area SII). In contrast, voxels significantly associated with non-painful activity were found more anteriorly at the border between OP 1 (area SII) and OP 4 (area PV). This observation is in good accordance with a recent fMRI work where the existence of two separate foci within SII was demonstrated. Here only the posterior activation site showed a modulation due to pain intensity (Ferretti et al., 2003). The response characteristics of all four areas still have to be studied in more detail by examining the cortical responses to a wider range of painful and non-painful stimuli in functional imaging experiments and comparing the evoked activations to cytoarchitectonically defined OP 1–4.
Perspectives
It can be expected that at least some of these different cytoarchitectonic areas contain separate whole body representations, reflect specialized receptive field somatotopy or serve different functional aspects within the somatosensory system. This functional relevance of OP 1–4 with respect to pain perception, tactile discrimination, sensorimotor integration and tactile attention, however, has yet to be investigated. One possibility for this task is matching the probabilistic cytoarchitectonic maps with functional imaging results. This combination may establish the correlation between different stimulation paradigms or tasks and individual cortical areas, and will help to further advance our understanding of the correlated functional and architectonic organization of the human SII region. To encourage the use of the probabilistic cytoarchitectonic maps of OP 1–4 in the context of functional imaging experiments, these maps, together with a software tool (Eickhoff et al., 2005a) for their integration into the widely used functional image analysis software SPM (The Wellcome Department of Imaging Neurosciences, www.fil.ion.ucl.ac.uk/SPM), are freely available to all researchers at http://www.fz-juelich.de/ime/spm_anatomy_toolbox.
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Acknowledgments |
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This Human Brain Project/Neuroinformatics research was funded jointly by the National Institute of Mental Health, of Neurological Disorders and Stroke, of Drug Abuse, the National Cancer Center, the Deutsche Forschungsgemeinschaft (KFO-112) and the Volkswagenstiftung.
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