Shown are two modules (regions of interest, ROI₁ and ROI₂), represented by ellipses containing pools of excitatory neurons (red triangles) and inhibitory neurons (green circles), synaptically connected within and between pools (black arrows).

<p>The extrinsic (associational) axons between ROI₁ and ROI₂ originate and end on single neurons and are either excitatory to excitatory (red arrows) or excitatory to inhibitory (green arrows). The subcortical inputs are also excitatory; computationally, this input is represented by a Poisson train of firings, the strength and frequency of which can be varied. (B), extrinsic (intermodular, associational) connections in an 8 modular network (NI). The bold line within each module indicates separation of excitatory and inhibitory neurons within the module with all extrinsic fibers between the modules forming excitatory synapses, primarily on excitatory neurons within the modules but not exclusively so. (C), extrinsic (intermodular, associational) connections in a 7 modular network (NII). The connectivity due to the associational axons follows the criteria described in A above. NII can be taken to represent the Default Mode Network (DMN). In this case the modules may be identified as follows: 1, right lateral temporal cortex; 2, right inferior parietal lobule; 3, left inferior parietal lobule; 4, posterior cingulate cortex; 5, dorsal medial prefrontal cortex; 6, ventral medial prefrontal cortex; 7, left lateral temporal cortex. The number and diversity of the associational fibers were chosen so as to reflect the reported weight of such connections between the modules of the DMN (see [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0144796#pone.0144796.ref049" target="_blank">49</a>], their Fig 8 and associated Table; also Fig 4A and 4C in [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0144796#pone.0144796.ref050" target="_blank">50</a>]. (After Fig 1 in (15) with permission.)