patch AF], and cortical and subcortical systems
engaged in reward, valence, and emotional processing [caudate; amygdala; and areas 10o, 11l,
and 14r of the orbitofrontal cortex (OFC)] (figs.
S5, A and D, and S6, A and D).
Large parts of the SIN network were exclu-
sively selective for social interactions and did not
respond to any other stimulus condition in the
context of the present design. Areas of this ex-
clusively social interaction network (ESIN) in-
cluded a cluster in mPFC, ACC, and dmPFC; a
cluster in vlPFC; area 7a in the inferior parietal
lobule; and OFC areas 10o and 14r (Fig. 3, A and
C, and figs. S5, A and E, and S6, A and E). The
ESIN was even deactivated in all but the social
interaction conditions (Fig. 3D). We did not find
such exclusivity of functional specialization for
any other stimulus category anywhere in the brain.
However, the joint characteristic of the ESIN—
social cognition focus and general deactivation
during visual stimulation—bear resemblance to
the human theory of mind (ToM) and the human
default mode network (DMN) (17, 18). Curiously,
ToM and DMN intersect in the human brain at
regions of quite plausible homology to ESIN areas
of the macaque brain. Thus, the macaque ESIN
shares functional and anatomical characteristics
of human ToM and ESIN (fig. S4).
We found three networks engaged in interaction analyses, each with distinct functional characteristics and internal organization. Are there
overarching principles of organization for all areas
processing interactions? We conducted principal
component analysis (PCA) across the different
movie categories on 43 regions of interest (Fig.
4, A and C). The areas in this space could be
grouped into object, body, and face patches and
classical MNS, SIN (without the ESIN), and ESIN.
This analysis, as well as an analysis of correlation
distances to the classical MNS and ESIN (Fig. 4B),
showed a greater similarity of face patches to the
ESIN than any to other ROIs. Because of their
functional homogeneity, we then performed PCA
in each of the six aforementioned groups of brain
areas across the same stimulus conditions (Fig.
4D). SIN and ESIN separated the social interac-
tion condition from all others along PC1. They
did not differentiate the other two agency condi-
tions (“acting” and “nonacting”) along this dimen-
sion. Both properties were shared by the face
patches (Fig. 4D). The body patches, instead,
separated all three agency conditions along PC1
and the three object conditions jointly along PC2
and PC1. This functional similarity suggests that
face patches are putative entry points to the SIN
and ESIN. Body and object patches turned out to
be functionally related to the MNS (Fig. 4, A, B,
and D). It has been proposed that the MNS might
provide inputs to the ToM network in humans
(19). Functionally, however, the MNS differed sub-
stantially from the SIN and ESIN and was more
similar to object and body patches, whereas the
SIN and ESIN were closer to face areas (Fig. 4,
A, B, and D). Therefore, interaction analysis by
two streams—already segregating inside the STS
and feeding into the classical MNS and SIN,
respectively—with different functions is the most
plausible model for the organization of high-level
world-processing in the primate brain.
Visual analysis of interactions is a computationally daunting problem: Each interaction
generates a complex spatiotemporal flow pattern, each interaction category consists of many
different such patterns, and even the smallest
change to a pattern can change an interaction’s
748 19 MAY 2017 • VOL 356 ISSUE 6339 sciencemag.org SCIENCE
Fig. 4. Multiple social networks in the primate brain. (A) PCA of each area
as a function of its mean activity to each condition of the main paradigm.
Colors and dashed contours indicate each brain area’s identity. Areas that
are defined independently of the main task (object patches, face patches,
object patches, or MNS) tend to cluster separately. A gradient of similarity
mainly along the first principal component (PC1) goes from the object
patches to the MNS to the body patches to the SIN and the ESIN.
(B) Correlation distance of the object patches (green), face patches (yellow),
and body patches (magenta) to the MNS (from left to right; blue arrow) and to
the ESIN (from bottom to top; black arrow) based on their mean activity to
the conditions of the task. Object and body patches are more closely related
to the MNS, whereas face patches are more closely related to the ESIN.
(C) Anatomical distances between areas displayed on a flat map of the monkey
brain, provided for comparison with the PCA showed in (A), demonstrate that
STS areas that appear interleaved anatomically are demixed functionally in
response to the task conditions. (D) PCAs and dendrograms representing each
condition (color code and legends to the right) as a function of its mean activity
in each patch of the group. Spatial arrangements and tree clustering show a
progressive separation of the physical interactions and social interactions
respectively moving from the object patches (left) to the ESIN (right).