Discrete 2-second periods of odorant (or sham) presentations were modeled in a within-subjects general linear model, using SPM’s canonical hemodynamic response function (HRF). HRF onset was delayed one second after the sniff instruction based on prior empirical evidence that this optimizes the response of the piriform and orbital olfactory association areas (Kareken et al., 2010a). Time and dispersion derivatives of the HRF accounted for slight variations in response onset and duration. Movement parameters from realignment were included as regressors, while a high-pass filter with a cutoff of 1/128 Hz was applied to each voxel’s time series to remove low frequency noise. This within-subject model yielded contrast images of activation within a mood condition (Positive, Negative, Neutral), and with one odorant (e.g., AcO) compared to another odorant (e.g., AppCo) and the SO.
