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Magnetic resonance imaging (MRI) is a popular technique to measure the structure and function of the brain. Recent advancements in acquisition hardware and analysis techniques have enabled the study of information processing between different layers of the cerebral cortex, i.e. laminar MRI.

One of the fundamental assumptions of MRI data-analysis and interpretation is that there is a linear relationship between neuronal activity and the measures MRI signal. This is the linearity assumption. The local MRI signal in one cortical layer is dependent on the local neuronal activation, but also on the signals from deeper-lying layers. The linearity assumption might thus be violated for laminar MRI. This could result in wrongfully over- or underestimated results, potentially leading to incorrect interpretations.

In his thesis, Jelle van Dijk shows that the linearity assumption holds for laminar MRI. Van Dijk then uses laminar MRI to study the information processing of numerosity across cortical layers. The current analysis shows that laminar information processing in numerosity-selective cortex appears to be different from laminar information processing in early visual cortex. Subsequently, he confirms these results using electrodes that are placed directly on the brain.

Van Dijk: "In summary, I show that the linearity assumption holds for laminar fMRI. This enables wider application of laminar fMRI to study sensory and cognitive processes. Using our own methods, I conclude that laminar fMRI is a useful tool to study cognitive processes -in this case numerosity perception- and that this leads to new insights about information processing in the human brain."