Proefschrift_Holstein

General introduction

Box 2.4 | Functional magnetic resonance imaging Functional magnetic resonance imaging (fMRI) is a non-invasive imaging technique used to measure neural activity. Neural activity is accompanied by an increase in oxygen and glucose consumption. Functional MRI allows the mapping of brain function by making use of differences in magnetic properties between oxygenated and de-oxygenated blood. This difference is captured in the blood oxygen level dependent signal, or BOLD signal, which is used as a proxy for neural activity (Logothetis et al., 2001). FMRI offers good spatial resolution (~3 mm in this thesis), but blood flow changes are slow and delayed compared to when the neural activity took place. Therefore, changes in BOLD response start approximately 2 seconds after the neural activity took place and peak after 6 – 12 seconds. Nevertheless, with the help of modelling techniques, which use the shape and delay of the BOLD response, we can implement rapid event-related fMRI. I am interested in brain activity associated with specific functions. Therefore, in the work described in this thesis, fMRI scanning took place while subjects performed a computer task ( box 2.3 ). To assess neural responses associated with a specific function, I compared different conditions on a task. For example, to assess which parts of the brain are activated when a reward is anticipated, I assessed the BOLD response when people expect to earn a high reward, and compared this to their BOLD response when they expect to earn only a small reward. All other things being equal, the difference in BOLD response between these conditions is then due to changes in the reward prospect. However, and crucial to keep in mind when using fMRI, is that this co-occurrence of changes in BOLD and task-related processing does not imply causality. If we want to assess whether a region is crucial for a certain function, we will need to perturb activity in this region (e.g. using TMS; box 2.5; chapter 7 ) or apply lesions to a region ( box 2.5 ; chapter 6 ).

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