The Trust Game

OR, Trust and Fairness in Science.

Tomlin D, Kayali MA, King-Casas B, Anen C, Camerer CF, Quartz SR, Montague PR. Agent-specific responses in the cingulate cortex during economic exchanges. Science. 2006 May 19;312(5776):1047-50.

Interactions with other responsive agents lie at the core of all social exchange. During a social exchange with a partner, one fundamental variable that must be computed correctly is who gets credit for a shared outcome; this assignment is crucial for deciding on an optimal level of cooperation that avoids simple exploitation. We carried out an iterated, two-person economic exchange and made simultaneous hemodynamic measurements from each player's brain. These joint measurements revealed agent-specific responses in the social domain ("me" and "not me") arranged in a systematic spatial pattern along the cingulate cortex. This systematic response pattern did not depend on metrical aspects of the exchange, and it disappeared completely in the absence of a responding partner.

OK, so in this figure we have "me" and "not me" brain responses with a different spatial distribution from those seen in the "Mental as Anything" study by Mitchell et al. Granted, making judgments about people similar or dissimilar to one's self (Mitchell et al., 2006) is different from "ascribing agency" in an economic game (Tomlin et al., 2006).


The Mitchell et al. paper concludes thusly,

As such, prejudice may arise in part because perceivers assume that outgroup members’ mental states do not correspond to their own and, accordingly, mentalize in a non-self-referential way about the minds of people from different groups. Without a self-referential basis for mentalizing about outgroup members, perceivers may rely heavily on precomputed judgments—such as stereotypes—to make mental state inferences about very dissimilar others. This view suggests that a critical strategy for reducing prejudice may be to breach arbitrary boundaries based on social group membership by focusing instead on the shared similarity between oneself and outgroup members.

and here's the Tomlin et al. conclusion:

However, it is reasonably clear that cingulate and paracingulate cortices contribute to normal social cognition and adaptive decision-making. The results of this paper add the important possibility that many other variables in the social domain may be arranged in such a systematic fashion through the spatial domain, a phenotype that could be disturbed in afflictions where the capacity to distinguish "me" from "not me" is impaired.

Uh, yeah.

But let's go back to the experiment. I give the authors credit for developing the nifty "hyperscanning" methodology, which involves two subjects who interact with each other while their brains are scanned simulateously (in adjoining or distant magnets). The task ("the trust game") is illustrated below. Brain images are taken at the three critical "reveal" points below:

An obvious confound is that the subject knows what he/she did, but not what the other person did. How did the researchers account for that? They did control experiments in separate groups of subjects.

One question deserves separate consideration: Did the reveal screens generate simple surprise or novelty responses along cingulate that were not related to the social element of the exchange? Although this reasonable interpretation is possible, the control experiments suggest otherwise. The response pattern along the cingulate disappeared in the control experiments where subjects received stimuli that were visually identical to those in the trust game and were composed of novel, reward-related information.

However, it's very surprising(!) that novelty and reward did not activate the cingulate in these control conditions, given the number of studies that have reported otherwise.

For the life of me, I can't figure out what we've learned from this article and why it's imporant. So I'll ask some other questions.

(1) Whose work forms the basis for the article?

These experiments have elicited not only brain responses in previously described theory-of-mind networks (27–29), but also have elicited formerly unreported activations along the cingulate cortex that correlate with the revelation of a social partner's decision (29). Although evoked during an economic exchange with another human, these cingulate activations did not modulate as a function of the fairness of the exchange, nor did they occur in exchanges with computer partners (28).

. . .

Given the previously reported activations in the anterior and posterior portions of the medial cingulate during a social exchange (28), a detailed analysis of the cingulate cortex in each pair of subjects was performed.

27. J Rilling, D Gutman, T Zeh, G Pagnoni, G Berns, C Kilts, Neuron 35, 395 (2002).

28. J. K. Rilling, A. G. Sanfey, J. A. Aronson, L. E. Nystrom, J. D. Cohen, Neuroimage 22, 1694 (2004).

29. A. G. Sanfey, J. K. Rilling, J. A. Aronson, L. E. Nystrom, J. D. Cohen, Science 300, 1755 (2003).

(2) Who's on the Board of Reviewing Editors at Science?

Jonathan D. Cohen
Princeton University


(3) Any publications in common?

Montague PR, King-Casas B, Cohen JD. Imaging Valuation Models in Human Choice. Annu Rev Neurosci. 2006 Apr 20; [Epub ahead of print]

Montague PR, Hyman SE, Cohen JD. Computational roles for dopamine in behavioural control. Nature. 2004 Oct 14;431(7010):760-7. Review.

Montague PR, Berns GS, Cohen JD, McClure SM, Pagnoni G, Dhamala M, Wiest MC, Karpov I, King RD, Apple N, Fisher RE. Hyperscanning: simultaneous fMRI during linked social interactions. Neuroimage. 2002 Aug;16(4):1159-64.

G.S. Berns ("Dr. Dread") is acknowledged "for early discussions and efforts leading to the development of hyperscanning" (but J.D. Cohen is not).


(4) Do you trust that the review process at Science is completely fair??