Figure 4. Functional connectivity analysis of network activation during retrieval of passive avoidance.

Thirty regions of interest (ROIs) showing significant increases in regional cerebral blood flow in conditioned compared to control rats were entered into connectivity analysis. Abbreviations are as indicated in Figure 3 and Table 1. (A) Interregional correlation matrix for the control group. Pearson's correlation coefficients were color-coded. The matrix is symmetric across the diagonal line from upper left to lower right. Significant correlations (P < 0.05) were marked with white dots. Note the strong positive connections among cortical ROIs including PrL, Cg1, Cg2, RS, and striatal ROIs (NAcc, adCPu). Amygdala (AMYG) was negatively connected with this corticostriatal cluster. (B) Functional network for the control group is represented with a graph, in which vertices represent ROIs and edges represent significant correlations (connections). Solid lines denote positive correlations, whereas dashed lines negative correlations. The size of each vertex is proportional to its betweenness centrality, a measurement of how central a vertex is in a network. ROIs with the highest betweenness centrality were considered hubs of the network. Key vertices are color-coded to facilitate between-group comparison (unrelated to the color scale of A and C). (C) Interregional correlation matrix for the conditioned group. Note the reduced number of significant connections. AMYG was positively correlated with the cerebellum, and negatively correlated with PrL and Cg in the corticostriatal cluster. (D) Graph representation of the functional network in the conditioned group. NAcc, aINS, and AMYG were identified as hubs of the network. ROIs in the right or left hemisphere are denoted with a suffix “R” or “L”, respectively.