Discrete 2 s periods of odorant (or sham) valve events (Figure 1) were modeled in a within subject (fixed-effects) general linear model using as basis functions SPM’s canonical hemodynamic response function (HRF). Initial testing showed that piriform and orbitofrontal responses to odorants (compared to odorless sniffing) were maximized when the HRF onset was delayed by one second after the sniff instruction, with time and dispersion derivatives of the HRF accounting for slight variations in response onset and duration. Movement parameters from realignment were included as regressors to account for residual movement-induced effects. A high-pass filter with a cut-off of 1/128 Hz was applied to each voxel’s time series to remove low frequency noise; auto-regression was not used due to the long inter-stimulus interval (Della-Maggiore et al., 2002). This within subject model yielded contrast images of activation within an odorant condition (AO, NApO, and ApCO) for each subject, with each odorant set contrasted against sniffing of an odorless control event (i.e, control valve opening without odorant delivery). This permitted quantifying the extent to which the BOLD response from an odorant class was different from stimulation (auditory commands, sniffing, attentional processing, motor response) without a chemosensory stimulus.
