Coupling of neural activity and BOLD fMRI response: new insights by combination of fMRI and VEP experiments in transition from single events to continuous stimulation.

Functional magnetic resonance imaging (fMRI) measures the correlation between the fMRI response and stimulus properties. A linear relationship between neural activity and fMRI response is commonly assumed. However, the response to repetitive stimulation cannot be explained by a simple superposition of single-event responses. This might be due to neural adaptation or the hemodynamic changes underlying the fMRI BOLD response. To assess the influence of adaptation, the BOLD responses and visual evoked potentials (VEPs) to identical stimuli were recorded. To achieve different adaptation levels, 2-s stimulus epochs alternated with different interstimulus intervals (ISI = 0.0, 0.4, 0.8, 2.0, and 12 s) were presented. Neural adaptation during the checkerboard reversal paradigm used for fMRI measurements is demonstrated. Even if the measured VEP amplitude is used as the weighting function for a linear model, the measured BOLD fMRI signal time-course is not adequately predicted.