Functional magnetic resonance imaging (fMRI) helps us study human brain function by measuring changes in blood oxygenation. In our research, it provides both a map for stimulation and a way to measure network responses.
What does the BOLD signal measure?
The blood-oxygen-level-dependent (BOLD) signal reflects changes in blood oxygenation associated with neural activity. It is an indirect measurement, shaped by blood flow, oxygen use and the local vasculature. A brighter activation map is therefore not a direct count of neurons firing.
The response unfolds over seconds. This matters when combining fMRI with TMS: we can choose the time of a stimulation pulse very precisely, but the imaging response remains slower. We compare responses across repeated experimental conditions to test the effect of that timing.
Task fMRI and resting-state fMRI
Task fMRI compares defined conditions, such as naming pictures, listening to language or retaining information. The design and comparison determine what an activation map can tell us. A region responding during a task is a candidate for further investigation; its activation alone does not establish that it is necessary for the task.
Resting-state fMRI examines spontaneous signal fluctuations without an explicit task. Correlations between regions describe functional connectivity. These maps can help identify candidate network targets for stimulation, but they do not by themselves reveal a direction of influence.
How we use fMRI with brain stimulation
In Vienna, we use individual imaging to formulate network hypotheses and concurrent TMS-fMRI to test responses to stimulation. Chronometric experiments add a comparison between task phases, linking changes in behaviour with the measured network response.
What makes an fMRI result interpretable?
Head movement, breathing, cardiac fluctuations and scanner artefacts can influence the signal. Good experimental controls, data-quality checks and an analysis matched to the question are essential. Positive and negative BOLD changes cannot simply be equated with neural excitation and inhibition.
Background: Logothetis et al., Neurophysiological investigation of the basis of the fMRI signal (2001); Woolgar et al., concurrent TMS-fMRI consensus guidelines (2026).