August 17, 2007

A very sociable brain

Brain activity in primates is directly influenced by social context

image of dominant and submissive monkeys

Figure 1: Computer graphic reconstructed from monkey behavioral data. The experimenter is about to place food on the table. The dominant monkey (left) is watching the submissive monkey (right) but the submissive monkey does not look back due to social suppression.

Copyright © Naotaka Fujii 2007

Human society puts heavy demands on the brain. Neurons must adapt rapidly to contextual changes in the social environment. Researchers at the RIKEN Brain Science Institute in Wako are gaining insight into this ‘social brain function’ by observing Japanese macaque monkeys1.

“We can understand how our brain recognizes and adapts to social environments by expanding research in primates,” says cognitive neuroscientist Naotaka Fujii, “because the social behavior of monkeys is very similar to that of human children.”

Fujii’s team placed two male monkeys around a square table in chairs that restricted their necks and lower bodies but arms and head free to move. Food was placed in random positions on the table, and the monkeys’ arm and head movements were closely monitored using a 3D motion capture system. Individual neuron activity was measured by inserting electrodes into the parietal cortex, a region of the brain that processes sensory information to determine the locations and movements of objects. In total, 174 neurons were isolated of which 91 were related to specific actions.

When the monkeys were seated opposite one another, they could only reach half the table each, so could not compete with one another. They effectively behaved as if no other monkey was present, and most activated neurons were associated with the monkeys’ own right-arm motion.

However, when the monkeys were seated on adjacent edges of the table, they shared a corner and could fight over food items that were placed there. One monkey appeared to be dominant, taking the food about 90% of the time. The dominant monkey frequently watched the submissive monkey, and became aggressive on the rare occasions that the submissive monkey managed to grab the food (Fig. 1). “When the monkeys are placed in a conflict situation, they tend to be more active to get the reward, even when they are not hungry,” says Fujii.

Brain activity changed dramatically when this social conflict was introduced. The researchers saw decreased activity in neurons related to the monkeys’ own right-arm movements, and increased activity in neurons related to other stimuli, such as the actions of their competitor’s arms.

“This is the first evidence that neurons are manipulating activity depending on social context at a single cell level,” says Fujii. “However, the parietal cortex may not be the only structure that implements the function. We have to expand recording areas to obtain an entire view of social brain functions.”


  1. Fujii, N., Hihara, S. & Iriki, A. Dynamic social adaptation of motion-related neurons in primate parietal cortex. PLoS ONE [online] 4, e397 (2007) (doi: 10.1371/journal.pone.0000397). doi: 10.1371/journal.pone.0000397 (Link)