A more sophisticated approach is based on parametric task manipulations and single-trial EEG-fMRI covariation [150, 151]. A range of parametrically graded experimental conditions are employed to identify cortical regions for which the BOLD response shows the same modulation across conditions as a specific single-trial ERP component. More specifically, external stimuli are designed to induce the variation of single-trial EEG responses. This single-trial variability is specific to each time point within a trial. For a specific time point, the signal varying over trials can serve as the predictor in the fMRI regression analysis, yielding a map of fMRI correlates. Repeating the analysis at multiple or potentially all latencies, one can obtain a series of fMRI maps, each of which is associated with a specific instant within the time scale of a single trial. This approach, although interesting, is theoretically demanding. The single-trial variability can result from spontaneous activity and noise that may or may not be differentiable from that driven by the external stimuli. Moreover, it requires a careful design of stimulus properties to allow for an effective control of single-trial amplitudes.
