Data analysis

At the first level analysis estimates of functional activation during conditions (Experimental, Control and Rest) were obtained using a blocked analysis. A high-pass filter (cut-off 520 s) was applied. We then used an approach based on our region of interest (ROI), bilateral amygdala. The bilateral amygdala mask was derived from the Automated Anatomic Labelling atlas 2 (Rolls et al., 2015) and had a volume of 3,744 mm³.

We extracted the beta values for each of the contrasts (Experimental > Rest and Control > Rest) within the bilateral amygdala, using the marsbar toolbox [version 0.44 (Brett et al., 2002)]. As previously reported, we observed a significant interaction in the MIST between environment and time in amygdala pooled activity of the Experimental and Control condition, which descriptively decreased after the walk in nature and remained stable after the walk in the urban environment (Sudimac et al., 2022). In the present study we examined the change in amygdala activity in the MIST in male and female subsample separately. In both subsamples a two-way ANOVA was conducted with environment as a between-subject factor (urban vs. natural) and time as a within-subject factor (before vs. after the walk) in amygdala pooled activity of Experimental and Control condition. Subsequently, two-tailed post-hoc t-tests were conducted in order to examine if the environment-by-time interaction was driven by a change in amygdala activity after the walk in the urban or in the natural environment.

At the first level analysis of the FFT, estimates of functional activation during each condition (unmasked Fear, unmasked Neutral, masked Fear, masked Neutral, Response) were modelled using an event-related paradigm. A high-pass filter (cut-off 128 s) was applied and the ROI-based approach was used, focusing on the bilateral amygdala.

We reasoned that the intervention, namely a one-hour walk, would globally affect the stress levels and therewith stress-related brain activity, not only when contrasting the Fear > Neutral condition. Therefore, we examined amygdala activity in the Fear and in the Neutral condition separately, by extracting the blood-oxygen-level dependent (BOLD) signal within the bilateral amygdala using the marsbar toolbox [version 0.44 (Brett et al., 2002)]. We averaged data from unmasked and masked stimuli, because the results were similar.

Within both the male and female subsamples we conducted a two-way ANOVA with environment as a between-subject factor (urban vs. natural) and time as a within-subject factor (before vs. after the walk) in amygdala activity pooled from the Fear and Neutral conditions. Two-tailed post-hoc t-tests were performed within the urban and the natural environment to examine if the environment-by-time interaction was driven by a change in amygdala activity after the urban walk or the walk in nature.