A modern map of the human cerebral cortex
The human brain's cerebral cortex is crucial for sensory and motor processing, as well as for mental functions such as interpreting language and logical reasoning, the complexity of which distinguishes us from other animals. In a paper online in Nature, Glasser et al. describe an updated map of the human cerebral cortex. This longawaited advance provides a reference atlas that will allow those researching brain structure, function and connectivity to work within a common, systems neuroscience framework. Regional differentiation within the cerebral cortex has long prompted attempts to identify the cortex's distinct compartments, from classical neuroanatomical studies at the beginning of the twentieth century to modern noninvasive, in vivo methods based on magnetic resonance imaging (MRI) Such endeavours are complicated by the fact that every location in the brain can be described by an almost infinite set of features, including density of receptor proteins for various neurotransmitter molecules, long range connections to other parts of the brain, and specialization for neural computations that support specific functions. Almost all previous studies have attempted to delineate cortical compartments using a single feature (Fig. 1) By contrast, Glasser and colleagues capitalize on the unprecedented quality and breadth of MRI data gathered by the Human Connectome Project, the aim of which is to elucidate the neural pathways that underlie brain function and behaviour using cutting edge brain imaging methods. MRI provides unparalleled access to the living brain. A single MRI machine can take many different measurements (known as modalities) from establishing the relative density of neuron insulating myelin sheaths to determining the thickness of the cortex, both of which can vary sharply between cortical areas. Furthermore, functional MRI (fMRI) can measure the changes in blood flow that accompany mental tasks, as well as whole brain activity in resting states, providing information about regional neural activity that accompanies different brain states. The authors' integration of information from several MRI modalities not only moves this work closer than previous attempts to the classical definition of a cortical area, but also has several key advantages over other investigations. First, some modalities reveal borders not clearly reflected in others. For instance, the border between areas 3a and 3b of the somato sensory cortex (which processes information about touch and pain) is easily delineated by myelin mapping, but not by resting state fMRI. As another example, Glasser et al. developed a resting state fMRI technique that maps topographic neural connectivity within the visual cortex. The sharp transition between levels of topographic connectivity across area boundaries allows much clearer delineation of discrete areas involved in early stages of visual processing than do myelin maps or conventional resting state fMRI approaches. Second, convergence across different MRI modalities reduces the likelihood of misdefining borders as a result of feature specific noise or bias. This is important, given the indirect nature of most modalities for example, fMRI measures the blood flow changes that accompany neuronal activity as a proxy for neuronal activity itself. Consequently, complex computational pre processing is often necessary to differentiate signal from noise. Agreement across modalities increases confidence that borders reflect biological reality rather than measurement biases. Finally, an integrative approach better equips researchers to describe the properties of each area, as exemplified by Glasser and colleagues' comprehensive supplementary material. The authors find, for instance, that a cortical area characterized in the 1950s by its low myelin content seems to be involved in language processing as measured by task based fMRI a finding consistent with a recent meta analysis of more than 10,000 imaging experiments across 83 behavioural tasks. Therefore, Glasser and colleagues' map represents the convergence of decades of classical neuroanatomical studies with modern non invasive studies. In contrast to the burgeoning field of resting state fMRI mapping, which has largely