Statistical analyses of activation images across participants revealed greater activation of multiple structures, including the left amygdala, by faces compared to shapes in the 100-200ms and 150-250ms post-stimulus windows (). For Experiment 1, blocked and randomized presentation of negative faces elicited an evoked response of the left amygdala 100-200ms post-stimulus onset (). For Experiment 2, left amygdala activation was also observed in both runs (within 1 mm according to the Talairach Daemon, Lancaster et al., 2000
), with its peak response occurring in the 150-250ms window for the blocked run and the 100-200ms window for the randomized run (). Other structures activated within these time windows by negative faces relative to shapes in Experiment 1 were as follows: right insula, right superior temporal gyrus (Brodmann Area (BA) 39), left fusiform gyrus (BA 19), left cerebellum, and right posterior cingulate (BA 29). In Experiment 2, regions displaying greater activity to faces over shapes included the following: right fusiform gyrus (BA 37), left fusiform gyrus (BA 37), right inferior parietal lobule (BA 40), left cingulate gyrus (BA 31), right insula (BA 13), right cerebellum, right middle temporal gyrus (BA 39), right middle occipital gyrus (BA 18), right middle occipital gyrus (BA 19) and left parahippocampal gyrus (BA 36). Regions commonly activated in both experiments by negative faces, besides the left amygdala, were the left fusiform gyrus, right insula, and right superior/middle temporal gyrus (). No other time windows revealed differentially greater activity for faces over shapes consistently across blocked and randomized runs.
Peak differential activations occurring 100-200ms relative to stimulus onset comparing face-evoked activity to shape-evoked activity.
Figure 2 Axial and coronal views of left amygdala activity for both blocked and randomized runs in Experiment 1 and Experiment 2. Color map represents magnitude of normalized log10 power ratios. Anatomical underlays are averaged anatomical images across participants (more ...)
Figure 3 Axial and sagittal views showing significantly greater evoked activity to negative faces compared to shapes 100-200ms post-stimulus onset that was common to both the blocked and randomized runs and both experiments. Color map represents mean normalized (more ...)
Virtual sensor time series were created for the left amygdala and left fusiform gyrus sources with coordinates based on the group statistical local maxima in the 100-200ms time window for the blocked and randomized runs for both experiments (). For the left amygdala source, local maxima were as follows: -21, -2, -14 mm (Experiment 1, blocked run), -28, -2, -14 mm (Experiment 1, randomized run), and -21, -1, -11 mm (Experiment 2, both runs). For the left fusiform source, local maxima were as follows: -28, -63, -9 mm (Experiment 1, both runs), -31, -45, -15 mm (Experiment 2, blocked run), and -46, -50, -15 mm (Experiment 2, randomized run). Each time course revealed a greater evoked response to faces compared to shapes with little variability in peak latency across blocked and randomized trial runs. Grand-averaged evoked responses in the left amygdala peaked between 136-146 ms following face onset in Experiment 1, and between 183-188ms in Experiment 2. Grand-averaged evoked responses of the left fusiform gyrus showed peaks at 125-130 ms and 177-183 ms following face onset in Experiment 1. For Experiment 2, there was a single peak response for faces at 170 ms for both runs in the left fusiform gyrus.
Figure 4 Grand-averaged waveforms of left amygdala and left fusiform gyrus activity for the blocked (top) and randomized runs (bottom) and for Experiment 1 (left) and Experiment 2 (right). Virtual sensor time series for each structure were projected from the location (more ...)
Based on the peak latencies of the evoked left amygdala responses, we defined a 10-ms window and conducted volumetric analyses using the same covariance matrices as in the main analyses. presents single-subject images for 5 participants for which local maxima in the left amygdala were evident. The majority of participants showed activations that extended into, but peaked outside, the left amygdala.
Single-subject images, in axial and coronal views, showing local maxima in the left amygdala. Normalized power ratio data are overlayed on individual anatomical MRIs in original subject space, shown in radiological orientation.
Effect sizes were computed at peak latencies for the left amygdala response to compare the difference in evoked activity across experiments. Cohen’s d’s were as follows: 1.08 (Experiment 1, blocked), 1.41 (Experiment 1, randomized), 2.43 (Experiment 2, blocked) and 2.08 (Experiment 2, randomized). For both runs, negative faces elicited larger left amygdala responses in Experiment 2 relative to Experiment 1.