Anatomical Study of Intrahemispheric Association Fibers in the Brains of Capuchin Monkeys (Sapajus sp.)

Abstract

Previous studies suggest that the complexity of fiber connections in the brain plays a key role in the evolutionary process of the primate brain and behaviors. The patterns of brain fiber systems have been studied in detail in many nonhuman primates, but not in Sapajus sp. Behavioral studies indicated that Sapajus sp. (bearded capuchins) show highly cognitive behaviors such as tool use comparable to those in other nonhuman primates. To compare the brain fiber systems in capuchins with those in other nonhuman primates and humans, the intrahemispheric fibers systems in 24 cerebral hemispheres of Sapajus were dissected by a freezing-thawing procedure. Dissection of the hemispheres in lateral view indicated short arcuate fibers, uncinate fasciculus, and inferior longitudinal fasciculus, while that in a medial view indicated short arcuate fibers, the cingulum united with the superior longitudinal fasciculus, and inferior longitudinal fasciculus. The results showed that the fiber systems in Sapajus are comparable to those in rhesus and humans, except for a lack of independent superior longitudinal fasciculus and cingulum in Sapajus.

Figure 1

A simple primate cladogram to indicate the distance of the derivation among primates. Sapajus is a New World Monkey and Macaca mulatta is an Old World Monkey.

Figure 2

Medial aspect of the right hemisphere. (a) 1: united cingulum and superior longitudinal fasciculus; 2: the cingulum fasciculus; 3: the superior longitudinal fasciculus; 4: the inferior longitudinal fasciculus; 5: the short arcuate fibers (in “U”); 7: the body of corpus callosum; 8: the thalamus; 9: the lateral ventricle; 10: the occipital lobe (bar = 1 cm). (b) Fascicles 1, 2, 3, and 4 are highlighted (bar = 1 cm).

Figure 3

Lateral aspect of the right hemisphere. (a) 5: short arcuate bers (in “U”); 10: occipital lobe; 11: temporal lobe (bar = 1 cm). (b) Fascicles 5 are highlighted (bar = 1 cm).

Figure 4

Orbital aspect of the right hemisphere. (a) 6: the uncinate fasciculus; 12: the orbitofrontal region; 13: the optic chiasm (bar = 1 cm). (b) Fascicle 6 is highlighted (bar = 1 cm).

Figure 5

Schematic drawings of the brain of the Sapajus (a), Macaca (b), and Homo (c), indicating the studied fasciculi. 1: united cingulum and superior longitudinal fasciculus; 2: the cingulum fasciculus; 3: the superior longitudinal fasciculus; 4: the inferior longitudinal fasciculus; 6: the uncinate fasciculus.