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The human cerebral cortex is composed of a mosaic of areas thought to subserve different functions. The parcellation of the cortex into areas has a long history and has been carried out using different combinations of structural, connectional, receptotopic, and functional properties. Here we give a brief overview of the history of cortical parcellation, and explore different microstructural properties and analysis techniques that can be used to define the borders between different regions. We show that accounting for the 3D geometry of the highly folded human cortex is especially critical for accurate parcellation. We close with some thoughts on future directions and best practices for combining modalities.

There is evidence of both crossmodal and intermodal plasticity in the deaf brain. Here, we investigated whether sub-cortical plasticity, specifically of the thalamus, contributed to this reorganisation. We contrasted diffusion weighted magnetic resonance imaging data from 13 congenitally deaf and 13 hearing participants, all of whom had learnt British Sign Language after 10 years of age. Connectivity based segmentation of the thalamus revealed changes to mean and radial diffusivity in occipital and frontal regions, which may be linked to enhanced peripheral visual acuity, and differences in how visual attention is deployed in the deaf group. Using probabilistic tractography, tracts were traced between the thalamus and its cortical targets, and microstructural measurements were extracted from these tracts. Group differences were found in microstructural measurements of occipital, frontal, somatosensory, motor and parietal thalamo-cortical tracts. Our findings suggest that there is sub-cortical plasticity in the deaf brain, and that white matter alterations can be found throughout the deaf brain, rather than being restricted to, or focussed in the auditory cortex., Highlights • Diffusion weighted MRI data was acquired from congenitally deaf and hearing groups. • Mean and radial diffusivity is altered in the thalamus of congenitally deaf people. • Diffusivity in white matter thalamo-cortical radiations is altered in deafness.

We explored how developing neural artifact and animal representations in the dorsal and ventral stream play a role in children's increasingly more proficient interactions with objects. In thirty-three 6- to 10-year-old children and 11 adults, we used fMRI to track the development of (1) the cortical category preference for tools compared to animals and (2) the response to complex objects (as compared to scrambled objects) during a passive viewing task. In addition, we related a cognitive skill that improved substantially from age 6 to 10, namely the ability to recognize tools from unusual viewpoints, to the development of cortical object processing. In multiple complementary analyses we showed that those children who were better at recognizing tools from unusual viewpoints outside the scanner showed a reduced cortical response to tools and animals when viewed inside the scanner, bilaterally in intraparietal and inferotemporal cortex. In contrast, the cortical preference for tools in the dorsal and ventral visual stream did not predict object recognition performance, and was organized in an adult-like manner at six. While cortical tool preference did not change with age, the findings suggest that animal-preferring regions in the ventral visual stream may develop later, concordant with previous reports of a protracted development in similar regions for faces. We thus conclude that intraparietal and inferotemporal cortical networks that support aspects of object processing irrespective of tool or animal category, continue to develop during the school-age years and contribute to the development of object recognition skills during this period.

Cortical surface-based analysis of fMRI data has proven to be a useful method with several advantages over 3-dimensional volumetric analyses. Many of the statistical methods used in 3D analyses can be adapted for use with surface-based analyses. Operating within the framework of the FreeSurfer software package, we have implemented a surface-based version of the cluster size exclusion method used for multiple comparisons correction. Furthermore, we have a developed a new method for generating regions of interest on the cortical surface using a sliding threshold of cluster exclusion followed by cluster growth. Cluster size limits for multiple probability thresholds were estimated using random field theory and validated with Monte Carlo simulation. A prerequisite of RFT or cluster size simulation is an estimate of the smoothness of the data. In order to estimate the intrinsic smoothness of group analysis statistics, independent of true activations, we conducted a group analysis of simulated noise data sets. Because smoothing on a cortical surface mesh is typically implemented using an iterative method, rather than directly applying a Gaussian blurring kernel, it is also necessary to determine the width of the equivalent Gaussian blurring kernel as a function of smoothing steps. Iterative smoothing has previously been modeled as continuous heat diffusion, providing a theoretical basis for predicting the equivalent kernel width, but the predictions of the model were not empirically tested. We generated an empirical heat diffusion kernel width function by performing surface-based smoothing simulations and found a large disparity between the expected and actual kernel widths.

A recent study from our laboratory demonstrated that parietal cortex contains a map of visual space related to saccades and spatial attention and identified this area as the likely human homologue of the lateral intraparietal (LIP). A human homologue for the parietal reach region (PRR), thought to preferentially encode planned hand movements, has also been recently proposed. Both of these areas, originally identified in the macaque monkey, have been shown to encode space with eye-centered coordinates. Functional magnetic resonance imaging (fMRI) of humans was used to test the hypothesis that the putative human PRR contains a retinotopic map recruited by finger pointing but not saccades and to test more generally for differences in the visuospatial maps recruited by pointing and saccades. We identified multiple maps in both posterior parietal cortex and superior frontal cortex recruited for eye and hand movements, including maps not observed in previous mapping studies. Pointing and saccade maps were generally consistent within single subjects. We have developed new group analysis methods for phase-encoded data, which revealed subtle differences between pointing and saccades, including hemispheric asymmetries, but we did not find evidence of pointing-specific maps of visual space.

Though the function of prefrontal cortex has been extensively investigated, little is known about the internal organization of individual prefrontal areas. Functional magnetic resonance imaging was used to show that some frontal and prefrontal cortical areas represent visual space in orderly, reproducible, topographic maps. The map-containing areas partly overlap dorsolateral prefrontal areas engaged by working memory tasks. These maps may be useful for attending to task-relevant objects at various spatial locations, an aspect of the executive control of attention.

Several properties of the cerebral cortex, including its columnar and laminar organization, as well as the topographic organization of cortical areas, can only be properly understood in the context of the intrinsic two-dimensional structure of the cortical surface. In order to study such cortical properties in humans, it is necessary to obtain an accurate and explicit representation of the cortical surface in individual subjects. Here we describe a set of automated procedures for obtaining accurate reconstructions of the cortical surface, which have been applied to data from more than 100 subjects, requiring little or no manual intervention. Automated routines for unfolding and flattening the cortical surface are described in a companion paper. These procedures allow for the routine use of cortical surface-based analysis and visualization methods in functional brain imaging.

The surface of the human cerebral cortex is a highly folded sheet with the majority of its surface area buried within folds. As such, it is a difficult domain for computational as well as visualization purposes. We have therefore designed a set of procedures for modifying the representation of the cortical surface to (i) inflate it so that activity buried inside sulci may be visualized, (ii) cut and flatten an entire hemisphere, and (iii) transform a hemisphere into a simple parameterizable surface such as a sphere for the purpose of establishing a surface-based coordinate system.

Automatic Reconstruction of the Cortical Surface To apply surface-based techniques routinely, the cortical surface reconstruction process must be automated. Several years ago, a method was devised for recovering the complete cortical surface of each hemisphere from high resolution MRI images in the context of improved solutions to the inverse problem for EEG and MEG (2). Since then, these programs have been improved and extended to incorporate functional MRI imaging data (3) (see also http://cogsci.ucsd.edu/~sereno).