By investigating social cognition, researchers aim to understand the processes underlying the perceptions and memories of social stimuli, the effects of social factors on how the brain processes this information, and the behavioural consequences of these processes. Such phenomena are extremely difficult to investigate; although studies of patients with frontal or parietal lobe damage enable neurologists to make inferences about the functioning of the social brain – it is well established that frontal lobe damage affects social interactions, and recent research shows that damage to the ventromedial prefrontal cortex affects the ability to make moral decisions – the neural mechanisms of social cognition are still very poorly understood.
A new study, published this week in the open access journal PLoS One, goes some way towards answering an important question about the brain mechanisms underlying social cognition: how do individual neurons modulate their activity in response to social context? The work, by researchers from the RIKEN Brain Science Institute in Japan, provides some evidence that the activity of individual neurons is correlated with adaptive social behaviours. The Japanese group used a new technique called multi- dimensional recording to correlate the social interactions of macaque monkeys (Macaca fuscata) with the activity of neurons in the left parietal lobe. They found that the neuronal activity was modulated in a conflict-dependent manner, suggesting that cells in the parietal lobe are involved in encoding information about social interactions.
The new technique involved the surgical implantation of twelve microelectrodes into the left parietal cortices of two adult male Japanese macaques (Macaca fuscata). The electrodes were implanted just anterior to the intraparietal sulcus, to record the activity of motion-responsive cells. The animals were then fitted with motion capture suits, with ten reflective markers located on various parts of the animals’ bodies. Both monkeys were restrained in chairs that were positioned around a small square table. During the course of a number of trials, items of food were placed on different areas of the table, and a change in the seating arrangement brought the two monkeys into close proximity to each other, so that interaction and social conflict was introduced into the situation. Video cameras were used to record the animals’ movements during the experiment, and the footage was analyzed so that specific movements could be correlated with the activity of neurons in the left parietal lobe.
Positions of the two monkeys, M1 and M2, around the table during three trials. The circles show the positions in which items of food were placed; they are also pie charts representing each monkey’s success ratio for retrieving the food item from that position on the table.
In one trial, the monkeys were seated opposite each other (position A in the figure above), and items of food were placed in four different locations around the table. Both monkeys took the food when it was within reach, with no hesitation. Although the monkeys faced each other, they behaved as if the other was not there; in other words, there was no apparent interaction or conflict. In the second trial, the seating arrangement was changed so that the monkeys were closer together (position B). If a piece of food was placed in a position where only one of the animals could reach it, the monkey retrieved the food. But when the food was placed in a corner where it could be reached by both monkeys, one of them (designated as M2) showed very little inclination to retrieve it, and the other monkey (M1) took the food 97% of the time. While M1 continued to behave as if the other animal wasn’t there, M2 sneakily watched while resisting retrieving the food with its left hand.
Finally, the seating arrangement was changed a second time (position C), and although it was very similar to the arrangement in the second trial, the monkeys behaved differently. As in the other trials, each monkey quickly retrieved a food item if it was placed in a position where only that animal could reach it. However, when food items were placed within reach of both monkeys, M1 was much more successful in retrieving it than M2. In these trials, it was observed that M1 looked at M2 frequently, in a threatening or intimidating way, particularly when M2 succeeded in retrieving the food placed on the table. M2, meanwhile, watched the behaviour of M1 closely, looking for an opportunity to seize the food. It is possible that the asymmetries in food retrieval in trails B and C occurred because both animals were right handed; thus, M1 had an advantage over M2 in trial B. But this doesn’t explain why M1 was more successful in retrieving the food during trial C; from the behaviours observed in that trial, the researchers determined that M1 was the dominant monkey, and M2 the subordinate one.
It was found that some of the parietal neurons were activated when the monkeys moved their right arm. Subsets of cells fit the definition of mirror neurons: some cells also fired when one monkey passively watched its competitor move its left arm, and others fired when movements of the right arm were observed. In order to determine if there was any correlation between the social interactions and activity of parietal neurons, the researchers first looked at the motion footage they had filmed, and extracted episodes during which only one monkey moved one of its arms. The basal activity of the cells was determined by extracting control periods during which neither of the monkeys moved their arms. Then, a comparison of the activity of cells during performance of the same movements over different trials showed that the activity of motion-responsive cells was modulated in a conflict-dependent manner. The responses of neurons that fired robustly to movements of the monkeys’ own right arms during non-conflict situations were diminished during the same movements in conflict situations, and instead became responsive to other actions, even though the visual and motor aspects of the trials remained almost completely unchanged.
Like humans, macaques and other monkey species live in highly complex heirarchical societies. The authors conclude that the parietal lobe neurons they investigated may constitute part of a social cognition network whose activity is dependent upon social context. The function of these cells is unclear, but they researchers suggest that they may be involved in the extraction of another’s body and movements from the environment, the encoding of the meaning of a movement, the differentiation of the self from others inhabiting the same spatial location, or the retrieval of knowledge about social heirarchy. Neuronal populations in this network are adapted to a particular social context, and acquire new functional properties when they recruit new cells during a novel situation. Because the social environment is in a constant state of flux, this functional reorganization of the social brain network would be a continuous process.
Fujii, N., et al. (2007). Dynamic social adaptation of motion-related neurons in the primate parietal cortex. PLoS ONE 2 (4): e397. doi: 10.1371/journal.pone.0000397. [Full text]