Respiration frequency
Respiration data were acquired using a MR-compatible respiration belt sampling at 500 Hz (Philips model 452213117812). Respiration data were first smoothed using a 1 s zero phase low-pass filter window and then mean-corrected. Breath frequency was then estimated using a fast Fourier transform (FFT) of the respiration period. As a first level covariate, frequency was estimated using a 10 s sliding window across the time series, generating a frequency value for each volume acquired in the timeseries. Trial-specific frequencies were also estimated across each task period to serve as covariates at the second (group) level of analysis.