Pathways of attention: synaptic relationships of frontal eye field to V4, lateral intraparietal cortex, and area 46 in macaque monkey

Abstract

The frontal eye field (FEF) of the primate neocortex occupies a pivotal position in the matrix of inter-areal projections. In addition to its role in directing saccadic eye movements, it is the source of an attentional signal that modulates the activity of neurons in extrastriate and parietal cortex. Here, we tested the prediction that FEF preferentially excites inhibitory neurons in target areas during attentional modulation. Using the anterograde tracer biotinylated dextran amine, we found that the projections from FEF terminate in all cortical layers of area 46, lateral intraparietal area (LIP), and visual area V4. Axons in layer 1 spread extensively, those in layer 2/3 were most numerous, individual axons in layer 4 formed sprays of collaterals, and those of the deep layers were the finest caliber and irregular. All labeled synapses were the typical asymmetric morphology of excitatory synapses of pyramidal neurons. Dendritic spines were the most frequent synaptic target in all areas (95% in area 46, 89% in V4, 84% in LIP, 78% intrinsic local FEF). The remaining targets were one soma and dendritic shafts, most of which showed characteristics of inhibitory neurons with smooth dendrites (5% of all targets in area 46, 2% in V4, 9% in LIP, and 13% in FEF).

Figure 1.

Location of injection sites and labeled boutons. A–D, Schematic drawing of a macaque brain (A) showing cortical areas V4 (left, dark gray), LIP (middle, gray), and area 46 (right, light gray). Also shown are schematic sections made in the coronal plane through the following: tip of prearcuate gyrus (dotted line), shown in B (as, arcuate sulcus); dorsal prelunate area (dashed line, left), shown in C; and prefrontal cortex (dashed line, right), shown in D. The injection sites in FEF are indicated in B (arrows). Labeled boutons were taken (curved arrows, C, D) from areas LIP, V4, and 46, as well as the FEF injection site area. Small axes, L, lateral; D, dorsal.

Figure 2.

Light microscopic reconstruction of BDA-labeled terminals and their boutons in area V4. The reconstruction is composed from two adjacent 80-μm-thick sections. Laminae and their boundaries are indicated to the left. Scale bar, 100 μm.

Figure 3.

Electron micrograph of BDA-labeled bouton located in layer 1 of area V4. A labeled bouton forms an asymmetric synapse (solid arrowhead) with a spine (sp1) that can be traced back to the parent dendrite (d). A second spine (sp2) can be seen to connect with the same dendritic shaft (d). Scale bar, 0.5 μm.

Figure 4.

Electron micrograph of BDA-labeled axon and boutons located in layer 2/3 of area V4. The boutons form asymmetric synapses (solid arrowheads) with spines (sp1 and sp2). One of the synapse-bearing spines (sp1) can be traced back to the large dendrite (d). The second spine (sp2) was not connected to the dendrite, and a third spine (sp3) just begins to emerge from the large dendrite (d). Scale bar, 0.5 μm.

Figure 5.

Electron micrographs of BDA-labeled boutons located in layers 4 and 5/6 of area V4. A, A labeled bouton in layer 4 forms an asymmetric synapse (solid arrowhead) with a spine (sp). The spine also forms a symmetric synapse (small arrow). B, A labeled bouton in layer 5/6 forms two asymmetric synapses with spines (sp). Scale bars, 0.5 μm.

Figure 6.

Electron micrographs of BDA-labeled axons and synaptic boutons in layers 1 and 2/3 of area 46. A, A labeled bouton forms an asymmetric synapse (solid arrowhead) with a small spine (sp) in layer 1. B, A labeled bouton forms a perforated asymmetric synapse (solid arrowheads) with a medium-sized spine (sp) in layer 1. C, A labeled bouton terminal forms an asymmetric synapse (solid arrowhead) with a small spine (sp) in layer 2/3. The labeled bouton can be traced back to the main axon. D, A labeled bouton en passant forms an asymmetric synapse (solid arrowhead) with a small spine (sp) in layer 2/3. The bouton forms part of the path of the labeled axon (ax) and emerges from an interruption in the myelin sheath (m). Scale bars, 0.5 μm.

Figure 7.

Electron micrographs of BDA-labeled boutons in layer 4 and 5/6 of area 46. A, A labeled bouton forms an asymmetric synapse (solid arrowhead) with a small spine (sp) in layer 4. The spine (sp) can be traced back to the parent dendrite (d). B, A labeled bouton forms an asymmetric synapse (solid arrowhead) with a neuronal soma (sm) in layer 5/6 that also forms a second asymmetric synapse (small arrow) with an unidentified bouton. The soma forms numerous asymmetric synapses and contains many perikaryal organelles and rough endoplasmic reticulum. These features are characteristic of neurons with smooth dendrites that are GABAergic. Scale bars, 0.5 μm.

Figure 8.

Electron micrographs of a BDA-labeled bouton in layer 2/3 of area 46. The labeled bouton forms an asymmetric synapse (solid arrowhead) with a dendritic shaft (d). The dendrite was seen to contain numerous mitochondria and forms asymmetric synapses with unidentified boutons in subsequent sections. These features are characteristic of neurons with smooth dendrites that are GABAergic. Scale bar, 0.5 μm.

Figure 9.

Electron micrographs of BDA-labeled boutons in layers 1, 2/3, 4, and 5/6 of area LIP. A, A labeled bouton of layer 1 forms an asymmetric synapse (solid arrowhead) with a large spine (sp) that can be followed back to the parent dendrite (d). B, A labeled bouton in layer 2/3 forms an asymmetric synapse (solid arrowhead) with a large-diameter dendritic shaft (d). The dendrite formed asymmetric synapses with unidentified boutons and contained numerous mitochondria. These features are characteristic of neurons with smooth dendrites and that are GABAergic. C, A slender, labeled bouton in layer 4 forms a perforated synapse (solid arrowhead) with a spine (sp). D, A vesicle-packed labeled bouton in layer 5/6 forms a synapse (solid arrowhead) with a spine (sp). Scale bars, 0.5 μm.

Figure 10.

A, LM reconstruction of axon shown in B. B, Three-dimensional reconstruction of BDA-labeled axon (red) in layer 2/3 of area V4 from serial ultrathin sections. The main trajectory of the axon is right to left and is roughly orthogonal to the radially aligned large-caliber dendrite (gray) (approximately 2 μm in diameter). The dendrite is spine bearing (also gray), and all reconstructed spines formed a synapse (green). At the point of intersection of the dendrite and the axon, the latter produces a flurry of mostly bouton terminals. Eleven of these labeled boutons form synapses with spines coming from the reconstructed dendrite. Ten more of the labeled boutons formed synapses with spines (data not shown) that could not be traced back to the large dendrite. One labeled bouton formed a synapse with a small dendrite and one formed no synapse. The large dendrite shows many features characteristic of the apical dendrite of a large pyramidal neuron. Scale bar (A): 12 μm.

Figure 11.

Histograms of the distribution of postsynaptic areas (μm²) formed with spines and labeled FEF boutons in layers 1, 2/3, 4, and 5/6 of areas V4, 46, and LIP.

Figure 12.

Histograms of the synaptic targets of labeled FEF boutons in layers 1, 2/3, 4, and 5/6 of areas V4 (147 spines, 19 dendrites), 46 (190 spines, 10 dendrites, 1 soma), and LIP (169 spines, 31 dendrites).

Figure 13.

Histogram of the number of synapses formed per labeled FEF bouton in layers 1, 2/3, 4, and 5/6 of areas V4, 46, and LIP.