Anatomical Connections of the Functionally Defined "Face Patches" in the Macaque Monkey

Abstract

The neural circuits underlying face recognition provide a model for understanding visual object representation, social cognition, and hierarchical information processing. A fundamental piece of information lacking to date is the detailed anatomical connections of the face patches. Here, we injected retrograde tracers into four different face patches (PL, ML, AL, AM) to characterize their anatomical connectivity. We found that the patches are strongly and specifically connected to each other, and individual patches receive inputs from extrastriate cortex, the medial temporal lobe, and three subcortical structures (the pulvinar, claustrum, and amygdala). Inputs from prefrontal cortex were surprisingly weak. Patches were densely interconnected to one another in both feedforward and feedback directions, inconsistent with a serial hierarchy. These results provide the first direct anatomical evidence that the face patches constitute a highly specialized system and suggest that subcortical regions may play a vital role in routing face-related information to subsequent processing stages.

Figure 1

Distribution of retrogradely-labeled cells in the temporal lobe after CTB-555 injection into left PL (case Monkey D). (A) Lateral view of the left hemisphere indicating the trajectory of the electrode (and injection cannula) into PL, located above the posterior middle temporal sulcus (pmts). (B, C) Trajectory of the actual electrode in this face patch in coronal and sagittal-like MR images. (D) Baseline-subtracted, normalized responses of 14 cells along one penetration (spaced 150 µm apart) into PL to 96 images, grouped into 6 categories (faces, bodies, fruits, gadgets, monkey body parts, and scrambled patterns). (E) Rostro-caudal level of coronal MR slices illustrated in this figure. (F) Coronal MRI slice showing the location of PL in the left hemisphere. (G) Corresponding histology slice in the frozen brain block taken while slicing it (light yellow outline shows contour of MRI slice in F; note that the two hemispheres were registered independently). The red spot indicates the CTB-555 injection site, and the dark yellow outline surrounding the injection site is the outline of face patch PL shown in F. (H) High-power fluorescent photomicrograph of the injection site, which involved all cortical layers and was well localized within PL. (I, L, O, R, R’) Coronal MR images spanning IT to prestriate cortex, with face-selective activation overlaid. Dotted yellow lines indicate area borders, following the Saleem and Logothetis atlas (Saleem and Logothetis, 2012). (J, M, P, S, S’) Plottings of retrogradely-labeled neurons in corresponding histology sections (thick white lines). Each dot represents one labeled cell; dashed line indicates layer IV. The dark yellow outlines illustrate the contours of corresponding MR images in I, L, O, R, and R’; the bright yellow outlines indicate the different fMRI face patches. (K, N, Q, T, T’) Low-power photomicrographs of CTB-labeled cells from regions of IT cortex indicated by rectangular boxes in J, M, P, S, S’. Insets: high-power photomicrographs of labeled neurons. (U) Flatmap of left occipito-temporal areas with face-selective activation overlaid. (V) Flatmap with density of labeled cells overlaid. (W) Flatmap with density of labeled cells superimposed on face patches. Note that some clusters of labeled neurons were localized within AM and ML face patches (shown in W; see also I/J and O/P) but no other patches. Also note the strong intrinsic connections beyond the halo of the injection site, and posterior visual areas (R, S). Abbreviations in U–W: ios, inferior occipital sulcus; ips, intraparietal sulcus; ls, lunate sulcus; ots, occipitotemporal sulcus; sf, sylvian fissure; sts, superior temporal sulcus. For the abbreviation of all the face patches and other area names see the “list of abbreviations” in Supplementary Material.

Figure 2

Distribution of retrogradely-labeled cells in prefrontal and temporal cortex after FB injection into left ML (case Monkey D), conventions as in Figure 1. (A) Lateral view of the left hemisphere indicating the trajectory of the electrode (and injection cannula) into ML, located at the lip of the mid superior temporal sulcus (sts). (B, C) The trajectory of actual electrode into this face patch, in coronal and sagittal-like MR images. (D) Normalized responses of cells along one penetration (spaced 150 µm apart) into ML. (E) Rostrocaudal level of coronal MR slices illustrated in this figure. (F) Coronal MRI slice showing the location of the left ML at the ventral lip of the sts. (G) Corresponding histology slice in the frozen brain block. The blue spot indicates the FB injection site. (H) High-power fluorescent photomicrograph of the injection site, which was confined within ML. (I, L, O, R, R’) Coronal MR images with face-selective activation overlaid. (J, M, P, S, S’) Plottings of retrogradely-labeled neurons in prefrontal, inferotemporal, and prestriate cortical areas after this injection. (K, N, Q, T, T’) Low-power photomicrographs of FB-labeled cells from the selected regions of the orbitofrontal (rectangular box in J), inferotemporal (rectangular boxes in M, P, S’), and ventral prestriate (rectangular box in S) areas. Insets: high-power photomicrographs of labeled neurons. (U) Flatmap of left occipito-temporal areas with face-selective activation overlaid. (V) Flatmap with density of labeled cells overlaid. (W) Flatmap with density of labeled cells superimposed on face patches. Note that clusters of labeled neurons were localized within four of the six face patches: AM, AF, ML and PL (shown in W; see also L/M and R’/S’).

Figure 3

Distribution of retrogradely-labeled cells in the temporal lobe after FB injection into right AL (case Monkey N), conventions as in Figure 1. (A) Lateral view of the right hemisphere indicating the trajectory of the electrode (and injection cannula) into AL, located at the lip of the anterior sts. (B, C) The trajectory of actual electrode into this face patch in coronal and sagittal-like MR images. (D) Normalized responses of cells along one penetration (spaced 200 µm apart) into AL. (E) Rostrocaudal level of coronal MR slices illustrated in this figure. (F) Coronal MRI slice showing the location of both left and right AL at the ventral lip of the sts, and AF in the fundus/dorsal bank of the sts. (G) Corresponding histology slice in the frozen brain block. The blue spot indicates the FB injection site in right AL. (H) High-power fluorescent photomicrograph of the injection site, which was confined within AL. (I, L, O, R) Coronal MR images with face-selective activation overlaid. (J, M, P, S) Plottings of retrogradely-labeled neurons. (K, N, Q, T) Low-power photomicrographs of FB-labeled cells from the temporal pole (rectangular box in J) and area TE (rectangular boxes in M, P, S). Insets: high-power photomicrographs of labeled neurons. (U) Flatmap of right occipito-temporal areas with face-selective activation overlaid. (V) Flatmap with density of labeled cells overlaid. (W) Flatmap with density of labeled cells superimposed on face patches. Note that most clusters of labeled neurons were localized within the face patches AM, AF, AL, ML and MF (shown in W; see also L/M, O/P, and R/S).

Figure 4

The distribution of retrogradely labeled cells in the temporal lobe after CTB-488 injection into left AM face patch (case Monkey N). (A) Lateral view of the left hemisphere indicating the trajectory of the electrode (and injection cannula) into AM, located close to the amts. (B, C) The trajectory of actual electrode into this face patch, in coronal and sagittal-like MR images. (D) Normalized responses of cells along one penetration (spaced 100 µm apart) into AM. (E) Rostrocaudal level of coronal MR slices illustrated in this figure. (F) Coronal MRI slice showing the location of both left and right AM. (G) Corresponding histology slice in the frozen brain block. The green spot indicates the CTB-488 injection site. (H) High-power fluorescent photomicrograph of the injection site, which was confined within AM. (I, L, O, R) Coronal MR images with face-selective activation overlaid. (J, M, P, S) Plottings of retrogradely-labeled neurons in prefrontal and inferior temporal cortex. (K, N, Q, T) Low-power photomicrographs of CTB-labeled cells from the selected regions of the orbitofrontal cortex (rectangular box in J), and area TE and sts (rectangular boxes in M, P, S). Insets: high-power photomicrographs of labeled neurons. (U) Flatmap of left occipito-temporal areas with face-selective activation overlaid. (V) Flatmap with density of labeled cells overlaid. (W) Flatmap with density of labeled cells superimposed on face patches. Note that most clusters of labeled neurons were localized within face patches AM, AF, AL, and MF (shown in W; see also L/M and R/S). Other details are the same as in figure 1.

Figure 5

Subcortical connections of different face patches. (A) Coronal MR image with claustrum outlined. (B) Distribution of retrogradely-labeled cells in the claustrum after CTB-555 injection into PL and Fast Blue into ML in Monkey D. The light yellow line indicates the contour of the corresponding MR image in A. Note the non-overlapping distribution of CTB (red) and Fast Blue (blue) labeled neurons in the ventral part of the claustrum (see also C). (C) Low and high-power photomicrographs of CTB and Fast Blue labeled neurons (rectangle in B). (D, G, J) Coronal MR images with rostrocaudal subdivisions of the pulvinar indicated. (E, H, K) Distribution of labeled neurons in the rostrocaudal extent of the lateral pulvinar (pl) after retrograde tracer FB injection into AL (D–F and J–L), and bidirectional tracer LY into ML (G–I). Note the complementary and discontinuous patches of Fast Blue-labeled neurons at the rostrocaudal levels AP+3 and +1 (blue dots in E and K), and LY-labeled neurons at AP +2 (green dots in H). (F, I, L) Low and high-power photomicrographs of FB- and LY-labeled neurons (rectangles in E, H, K). Note the LY positive anterogradely-labeled axon terminals with neurons in the lateral pulvinar in (I). (M, P) Coronal MR images with subdivisions of the amygdala indicated. (N, Q) Distribution of labeled neurons in the subnuclei of the amygdala after Cascade Blue (CB) injection into AL (M-O), and LY into AM (P-R). Note that the labeled neurons are mainly observed in the magnocellular division of the accessary basal nucleus (ABmc) after AL injection. In contrast, the labeled neurons are located in the intermediate portion of the basal (Bi) and parvicellular division of the accessory basal nucleus (ABpc) after AM injection. (O, R) High-power photomicrographs of CB- and LY-labeled neurons (rectangles in N, Q, respectively). Abbreviations: bsc, brachium of the superior colliculus; pm, medial pulvinar; pi, inferior pulvinar; pl, lateral pulvinar; lv, lateral ventricle; cl, claustrum; COa, anterior cortical nucleus of the amygdala; AAA, anterior amygdaloid area; Bmc, basal nucleus of the amygdala, magnocellular subdivision; Bpc, basal nucleus of the amygdala, parvicellular subdivision; Ld, Lateral nucleus of the amygdala, dorsal subdivision; Lv, Lateral nucleus of the amygdala, ventral subdivision.

Figure 6

Summary of the connections of the face patches. (A–E) Flatmaps of left and right temporal cortex in three monkeys (cases D, B, and N) show the face-selective activation (orange-red patches), location of the injection sites in different face patches (indicated by “X”), and the origin of projections to injected face patches (different contour lines). The contours of the projections to each face patch were obtained from Figures 1–4V, S1–S9V. Red = injection in PL, Green = injection in ML, Blue = injection in AM, Cyan = injection in AL, and Black = control injection, outside of the face patches (within the sts). (F) Summary of the wiring diagram of face patches and their inputs and outputs (both red and black arrows). The direction of the arrows indicates the direction of the axons.