1.3 Odor detection
1.3.3 fMRI results
1.3.3.1 Sequence parameters: F map
To verify that the specific sequence used for the acquisition of olfactory signal was efficient, we constructed an F-map of the all conditions (12 odors and 1 control), and approximated the maximal brain coverage by using the maximal threshold of p = 1 (unselective). The unthresholded signal as
104 extensively covers ventral frontal and temporal portions of the brain in which olfactory areas are included (e.g. rectus gyrus, Figure 33).
Figure 33. F map for all the 13 conditions, p = 1. Sagittal ( X = -6,left) and horizontal (z = -18, right) views.
1.3.3.2 Odor perception
We determined which cerebral regions were sensitive to olfactory stimulation by comparing all events in which an odor was sent with respect to a control odorless event (Odor vs No Odor, O>NO, Figure 34a). This contrast led to suprathreshold activations in olfactory areas such as the medial OFC – extending to the subgenual ACC, the lateral OFC, the left amygdala – extending to the hippocampus, the bilateral putamen – extending to the insula on the left side and the medial frontal gyrus (Figure 34a), in line with previous studies (see section 2.4, theoretical part). Furthermore, we contrasted all a priori defined positive and negative odors vs. control events (Figure 34 b and c).
When contrasted against control events, Pleasant odors (P) elicited a pattern comparable to that of the O>NO contrast, with slightly more pronounced activation in the medial OFC and reduced clusters in the putamen. Negative (N) odors vs control elicited marked activity in the bilateral putamen, insula, and post central gyrus, lateral OFC but not in the medial OFC. In both contrasts, the amygdalar activation was preserved, in line with previous results (see In). Finally, we contrasted pleasant vs.
unpleasant odors. In the P>U contrast (Figure 34d) only a small cluster of 12 voxels in the medial OFC survived, while in the opposite contrast (U>P, Figure 34e), gave rise to activations in the bilateral anterior insula, the inferior frontal gyrus (IFG; extending to the lateral OFC and the anterior insula), the medial superior frontal gyrus, the supplementary motor area (SMA), and the bilateral thalamus.
None of the contrasts showed a preferential activation of the amygdala. Taken together, these results confirm the reliable elicitation of pleasant and unpleasant olfactory sensation, with differentiated neural activation patterns in the medial OFC and the lateral OFC / insula respectively.
Interestingly, the amygdala appeared to respond to olfactory stimulation in general (both pleasant
105 and unpleasant odors), but not to valence, in line with previous findings advocating for a broader role as mentioned in the introduction (see section 2.4.4., theoretical part)
Figure 34. Prestudy: fMRI data.
Brain sections displaying differential activity for the odor>non-odor (O>NO, a), pleasant (P)>NO (b), unpleasant (U)>NO (c), P>U (d), and U>P (e) (contrasts for 20 trial repetitions for each condition. Activations are overlaid on a template anatomical brain scan (height level threshold corresponding to P<0.005, whole brain [WB], uncorrected).
1.3.3.3 Influence of stimulus repetition on the olfactory signal
As a manipulation check, we assessed the influence of stimulus repetition on olfactory related signal, by comparing all events in which an odor was sent with respect to a control odorless event (Odor vs No Odor, O>NO), considering 12, 16 or all 20 trials per condition. Although olfactory related
106 activations were observable from 12 repetitions onwards (Figure 35a), the size of the clusters remained relatively small when compared to more repetitions (e.g. 32 vs. 435 voxels in the medial OFC for 12 and 20 repetitions, respectively, Figure 35 a and c). These results confirm that a high number of repetitions of olfactory stimulations dramatically improves the quality of the recorded signal, despite the subsequent lengthening of the experiment (>80 min instead of 48 in the case of 20 and 12 repetitions respectively). The potential drawback of olfactory adaptation caused by sustained odor exposure seems to have been circumvented by the use of a great variety of odors.
Figure 35. Effect of stimulus repetition on olfactory signal.
Brain sections displaying differential activity for the odor>non-odor (O>NO) contrast for 12 (a), 16(b) and 20(c) trial repetitions for each condition. Activations are overlaid on a template anatomical brain scan (height level threshold corresponding to P<0.005, whole brain [WB], uncorrected).
1.3.3.4 Influence of physiological parameters
A second model was designed in which the raw time courses of respiration and heart rate were included as non-interest, continuous nuisance regressors along with the movement parameters. The results did not show any significant improvement of the signal, therefore this correction was not implemented as of when the smells were used as a sole condition as this was the case in study 1.
1.3.3.5 FIR & Time courses of activation
The time courses extracted from the olfactory ROIs identified in the O>NO contrast revealed differential temporal patterns of activity (Figure 36) in accordance with the path of olfactory information throughout the brain. For instance, amygdala (cyan curve) and insula (dark blue)
107 activities peaked around 3.2s after the onset of odor stimulation, while medial (red curve) and lateral OFC (green curve) activities were more delayed. This temporal evolution was echoed in the results obtained with the FIR model: activations of the amygdala and the insula (cyan and dark blue circles) started to appear at the second time bin (1.6-3.2s) and gradually faded away while activations in the lateral and medial OFC (green and red circles) consistently emerged at the 4th and 5th time bins.
Figure 36. Dynamic time course and FIR.
Time course of the BOLD response extracted from the main areas identified in the main analysis Odor>no Odor (O>NO) contrast. FIR 5 x 1.6 s time bins displaying the temporary evolution of odor perception (O>NO contrast) were added below with corresponding color-coded circles. Activations are overlaid on a template anatomical brain scan (height level threshold corresponding to P<0.005, whole brain [WB], uncorrected.
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