Independent projection streams from macaque striate cortex to the second visual area and middle temporal area

Abstract

The interareal wiring of the neocortex is usually depicted as a network of single point-to-point connections, often side-stepping the possibility that some neurons may project to multiple cortical areas. The prevalence of such neurons is unknown; if they are abundant, cortical circuits are more likely to be connectionally diffuse. We used a dual-tracer approach to determine whether single neurons in the macaque primary visual cortex (V1) project to two extrastriate areas, the second visual area (V2) and the middle temporal area (MT). We found two large intermingled groups of single-labeled neurons in layer 4B of V1 projecting independently to either V2 or MT. A third, sparser group of double-labeled neurons projected to both areas; we termed these manifold neurons. We also found that MT-projecting cells were distributed indiscriminately with respect to cytochrome oxidase compartment in layer 4B, revealing a subpopulation that provides a potential source of patch input from V1 to MT. The results demonstrate that primary sensory cortices can use multiple projection strategies to distribute signals to higher areas, and suggest that feedforward projections may route signals with more specificity than feedback pathways.

Figure 2.

Injection locations in V2 and MT. a, Lateral view of a macaque right hemisphere, illustrating pipette entry points. The blue sagittal plane is diagrammed in b. The dotted line is the V1–V2 border. b, Tracer deposit sites for WGA–HRP in V2 and CTB in MT, as they might appear in a sagittal section. c, Single flatmount section from the right hemisphere of Monkey 2. The blue dashed line encircles MT, as estimated from the entire series of sections. Anatomical axes: posterior–anterior, left–right; dorsal–ventral, top–bottom. d, Same section as in c, with the injection sites from an adjacent CTB/WGA–HRP-reacted section aligned and superimposed digitally. Sulci outlines are drawn in white (IOS, inferior occipital sulcus). The black box outlines the region in which a deeper section was photographed for Figure 3. e, Left hemisphere of Monkey 1, with sulcal outlines and injection sites. The black box is a region in V1 in which overlapping labeled cells appeared in a deeper layer 4B section (shown at higher magnification in Figure 4). Scale bar, 1 cm.

Figure 1.

Location of V1 projection zones in V2 and MT. a, Single flatmount section cut through the superficial layers of the right occipital cortex of a macaque, showing the CO pattern in V1, V2, and MT. b, Same section as in a with areas identified and sulci outlined in white. Sulcal boundaries are tracked during the unfolding process. The blue dotted line indicates area MT as judged by the CO pattern. c, Adjacent section to a, processed for autoradiography and photographed in dark-field. The large [³H]proline injections in V1 lead to anterogradely labeled fields of axon terminals centered in V2 and MT. Scale bar, 1 cm.

Figure 5.

Distinct cell types project to V2 and MT. a, b, WGA–HRP-labeled cells were most often small stellate (a, asterisk) or pyramidal neurons and less frequently medium-sized pyramids (b). CTB-labeled neurons included the large Meynert cells characteristic of layer 4B (c) and pyramids ranging in size (d). e, f, Double-labeled cells were always medium-sized cells. g, h, Single-labeled neurons could often be found situated within the same cortical column. i, Distribution of cell body areas for each type of labeling across all animals, showing that double-labeled cells (red) formed a subpopulation among the smaller V2-projecting cells (gray) and MT-projecting cells (white). Only one of every three fields were measured for the single-labeled cells. Scale bar: a–h, 20 μm.

Figure 3.

MT-projecting cells in V1. a, Brightfield image of a CO-stained section containing CTB-labeled cells. Many layers are visible in this area because the plane of section was oblique to the cortical surface. b, Inverted dark-field image of the same section in a, showing that CTB labeling was found only in layers 4B and 6. Laminar boundaries (blue) are taken from the CO pattern. c, High magnification view of the boxed field in b, showing light, medium, and heavily labeled neurons. The two top labeled cells were also CO-rich, as evident in the proximal dendrite staining. d, Pyramidal cell from the next section. Its apical dendrite was cut off where it left the section. Stellate cells (c, top) had no apical process. Scale bars: a, b, 500 μm; c, d, 50 μm.

Figure 4.

Three different output populations in layer 4B. a, A single section cut tangentially through layer 4B of Monkey 1, processed for WGA–HRP and CTB. Individually lettered neurons are shown at higher magnification in Figure 5. b, Desaturated image of the CO pattern in layer 3, showing the distribution of labeled cells in a in patch (gray outline) and interpatch compartments. Blue dots indicate WGA–HRP-labeled cells projecting to V2, green squares indicate CTB cells projecting to MT, and red squares indicate double-labeled cells. Black arrows point to blood vessels used for alignment. Scale bar, 200 μm.

Figure 6.

MT-projecting cells are independent of the CO patch system. a, Inverted dark-field image of CTB-labeled cells in layer 4B. The top left corner of the section passed out of layer 4B. Black arrows point to blood vessels used for alignment. b, CO section from layer 3 aligned with the section in a. c, Gray-level rendered image of CO density, divided into three zones of equal area forχ² analysis: patch (inside black contours), border (between black and white contours), and interpatch. Cells (red) plotted over the density image from a, showing no tendency to cluster in any zone. Spatial correlation analysis showed that the mean CO density in the neighborhood of MT-projecting cells was not clearly related to patches in this case (d), nor in three other hemispheres (e–g) at different eccentricities (Table 2). Spatial correlation values were contrast enhanced because they were of very low amplitude, ranging within ± 3 from mean gray (128) as indicated by the gray-scale bar. Scale bars: a–c, 1 mm; d–g, 100 μm.