A test of time-resolved functional MRI with subsecond event durations

Functional neuroimaging (fMRI) signals are famously sluggish, unfolding over several seconds after even brief sensory (and neural) events. Relative timing differences between different evoking conditions, however, can distinguish between changes in the duration or intensity of neural activity (Henson et al., 2002). Specifically, increases in stimulus duration cause a delayed response peak with a greater amplitude, whereas increases in stimulus intensity affect the peak magnitude alone. These relationships have been empirically demonstrated using stimulus durations of several seconds (Friston, 2005), but many processes relevant to cognitive psychology occur on sub-second timescales. Here we tested whether the predicted relationship between stimulus properties and the hemodynamic response held for sub-second stimuli and sub-second changes in stimulus duration. In Experiment 1 (n=15), we presented visual and auditory stimuli at three durations (100, 300, and 900 ms) and three intensities in a slow event-related design with a rapid TR (625 ms). In Experiment 2 (n=14), the stimulus durations were extended (1000, 1200, and 1800 ms) to increase the evoked signal. By fitting the observed fMRI signals to parameterised hemodynamic response functions, we found that changes in stimulus duration caused differences in peak latency and magnitude, whereas changes in stimulus intensity affected only peak magnitude. Significant effects were found for duration differences as small as 200 ms, with clearer results when a sufficiently large base signal was evoked and when a bootstrap resampling technique was used. Our results support the validity of existing models of the hemodynamic response. They also support the use of time-resolved fMRI for mental chronometry in cognitive neuroscience, thereby allowing researchers to draw additional inferences about brief mental events. ### Competing Interest Statement The authors have declared no competing interest.