(A–C) Linear regressions for boutons targeting spines showing significant (p < 0.01) correlations of: (A) bouton volume and spine PSD area; (B) bouton volume and spine volume; (C) spine volume and PSD area.
(D–F) Linear regressions for boutons targeting dendritic shafts of inhibitory neurons. (D) Bouton volume and dendrite PSD area were significantly correlated (p < 0.01). (E) There was no correlation between bouton volume and dendrite diameter (p = 0.53), or; (F) between dendrite diameter and PSD area (p = 0.21).

(A–B) Average maximum major diameters and; (C) volumes of the total, large, and small labeled boutons in area 9 from areas 32 and 46, measured at the light microscope (A) and EM (B, C). Error bars = SD from cluster analysis.
(D) Proportion of small (grey) and large (black) labeled boutons from areas 32 and 46; data from EM and light microscopy were pooled. Error bars = SEM.
(E–F) Normalized frequency histogram of volumes of boutons from area 32 (black bars) and area 46 (silhouette bars) terminating in: (E) layer I and; (F) layers II-IIIa of area 9.

(A) Axon terminals in area 9 were labeled after injection of distinct neural tracers in ACC area 32 (cases AY, BG, BI and BIe) shown on the medial surface of the rhesus monkey brain; and (B) in dorsolateral area 46 (cases BIr and BH) on the lateral surface (long dashes depict the upper bank of the principal sulcus). Short dashes mark areal borders. The inset in B shows the location of sites examined in area 9 with labeled axon terminals (green arrows) among excitatory (exc, green) and inhibitory (inh, red) neurons.
(C–D) Coronal sections show labeled axon terminals (brown) in area 9: (C) from ACC (area 32) and; (D) from area 46. Sections were counterstained with Nissl (blue) to show cortical layers; laminar labels are placed at the beginning of each cortical layer. Abbreviations of sulci: A, arcuate; C, central; Cg, cingulate; P, principal; Ro, rostral.

(A) Normalized distributions of all labeled synapses with spines (black) or with aspiny or sparsely spiny dendrites of inhibitory neurons (crosshatch) in area 9: axons from area 32 had more synapses on inhibitory neurons than area 46 (p < 0.01).
(B) Proportion of synapses with inhibitory neurons labeled with calbindin (CB), calretinin (CR), or parvalbumin (PV). ACC area 32 innervated significantly more CB neurons than area 46 (p < 0.01, asterisk).
(C–G) EM photomicrographs of labeled boutons (At) that form synapses (silhouette arrowheads) with distinct neurochemical classes of inhibitory neurons. (C) Large labeled bouton from area 32 (At) forms a synapse with a CB+ dendrite labeled with gold (arrows). (D1–D3) A series of three images of a small bouton from area 46 (At) that forms a synapse (arrowheads in D1 and D2) with a CB+ dendrite labeled with gold (arrows in D2 and D3). (E–F) Two large boutons (At) from area 32 form a synapse with a CR+ dendrite: one labeled with gold (E, arrow), another with TMB (F, arrows). The CR+ dendrite in E also receives synapses from two unlabeled boutons (black arrowheads). (G) A small bouton from area 46 (At) forms a synapse with a CR+ dendrite labeled with gold (arrows), which also receives a synapse from an unlabeled bouton (black arrowheads). (H) Large labeled bouton from area 32 (At) forms a synapse with a PV+ dendrite labeled with gold (arrows). (I) Small bouton from area 46 (At) forms a synapse with a PV+ dendrite labeled with TMB (arrow). Scale bars = 0.5 μm; Error bars = SEM.

(A) Low cognitive demands in working memory: Activation of the pathway from area 46 to area 9 predominantly engages CR neurons (via 46-cr), which disinhibit pyramidal neurons. This pathway is consistent with disinhibiting activity in area 9 and enhancing relevant signals (large green triangle). Innervation of CB neurons is low, but a few may be activated (via 46-cb); this pathway is consistent with suppressing moderate noise (small green triangle).
(B) High cognitive demand: The pathways from area 46 and ACC (area 32) may act in synergy. When noise increases during high cognitive demands, suppression engaged by the area 46 pathway may not be enough. ACC may be recruited to suppress excessive noise by strongly activating CB neurons through efficient large boutons (via 32-cb). ACC may also engage CR neurons to further disinhibit the relevant signal drowned out by excessive noise (via 32-cr).
(C) High cognitive demand: Large ACC boutons may overpower the effects of small area 46 boutons to reverse decisions. CB inhibitory neurons activated by large boutons from ACC (32-cb) may overpower CR disinhibitory neurons engaged by small boutons from area 46 (46-cr) to suppress a previous signal (green, old signal). CR disinhibitory neurons may be recruited by large boutons from ACC (32-cr), overpowering CB neurons engaged by area 46 (46-cb) to enhance a new signal. In addition, the two pathways may recruit PV inhibitory neurons (46-pv and 32-pv), potentially shifting the temporal dynamics to reverse the response, with the large boutons from ACC acting as drivers (32-pv).

(A) Scatter plot shows the volumes of individual boutons from area 32 (n = 91) and area 46 (n = 90) that formed synapses with distinct postsynaptic targets in layers I-IIIa of area 9.
(B) Average volume of boutons from area 32 (dark blue) and area 46 (light blue) with distinct postsynaptic targets. In the pathway from area 32, boutons were significantly larger than from area 46 when forming synapses with aspiny shafts of inhibitory neurons or with multiple postsynaptic sites (p < 0.01, asterisks), but not with synapses on single spines.
(C) Proportion of all labeled boutons with distinct postsynaptic targets. Boutons forming synapses with aspiny shafts, and spines and shafts were more prevalent from area 32 (on shafts, n = 67/345 boutons; spine + shaft, n = 23/345) than from area 46 (on shafts, n = 39/325; spine + shaft, n = 8/325); the opposite relationship was seen for boutons innervating spines (n = 235/345 from area 32; n = 261/325 from area 46). Statistically significant comparisons are marked with asterisks.
(D–K) 3D reconstructions of labeled boutons (At) in area 9 from area 32 (dark blue) and area 46 (light blue). (D–E) Small boutons that form a synapse (PSD, yellow) with a spine (sp, green) in layer I of area 9: (D) from area 32 and; (E) from area 46. (F) A large multisynaptic bouton from area 32 forms synapses with two spines (sp1 and sp2) in layers II-IIIa, branching from dendrites (den1 and den2). (G) A small multisynaptic bouton from area 46 forms synapses with two spines (sp1 and sp2) in layers II-IIIa; sp1 branches from dendrite (den). (H–I) Boutons targeting aspiny dendrites of inhibitory neurons (den, red): (H) a large bouton from area 32 and; (I) a small bouton from area 46. (J–K) Multisynaptic boutons forming synapses with an aspiny shaft (den1) and a spine (sp) branching from a dendrite (den2): (J) a large bouton from area 32 and; (K) a small bouton from area 46. Scale bars = 0.5 μm. Error bars = SEM.

(A) Diagram shows the three main types of postsynaptic targets of labeled boutons (open circles) in area 9: (1) spines of excitatory (exc, green) neurons; (2) dendritic shafts of inhibitory (inh, red); or (3) multiple postsynaptic sites, involving more than one spine (3e, excitatory), or a spine and an aspiny shaft (3m, mixed).
(B–G) EM photomicrographs show examples of labeled axon terminals (At) and their synapses (arrowheads) with morphologically identified postsynaptic targets in 2D. (B) A bouton from area 32 and; (C) a bouton from area 46, each innervating a spine head (sp); the spine in C branches from a dendrite (den) with mitochondria (m). (D) A large bouton from area 32 and; (E) a small bouton from area 46, each innervating an aspiny shaft of an inhibitory neuron (den) with a nearby shaft synapse (arrow) from an unlabeled terminal. (F, G) Large multisynaptic boutons from area 32: (F) one bouton forms synapses with two spines (sp1 and sp2) and; (G) another with a spine (sp) and an aspiny shaft (den).
(H–K) 3D reconstructions of labeled boutons and their postsynaptic densities (PSD) and targets in area 9: (H) A bouton from area 46 (At, dark blue) has a synapse (PSD, yellow) with a spine (sp, green translucent) branching from a spiny dendrite (den) of an excitatory neuron. (I) A bouton from area 32 (At, light blue) forms a synapse with an aspiny dendrite (den, red translucent) of an inhibitory neuron, which receives other shaft synapses (grey) from unlabeled terminals. (J) A bouton from area 46 (At, dark purple) forms synapses with two spines (sp1 and sp2); sp2 branches from a spiny dendrite (den). (K) A bouton from area 32 (At, light purple) forms synapses with an aspiny dendrite (den1) and two spines (sp1 and sp2) from the same spiny dendrite (den2). Scale bars = 0.5 μm.