On Prefrontal Guilt

The field of Neuroeconomics has become quite popular in recent years. There are several reasons for this surge in interest; amongst these are: (1) the inevitable intrigue generated by scientific considerations of currency, (2) the utility of studying behaviors contingent on well defined rewards and punishments (losses), and (3) the value of scientifically exploring a human behavior that has been studied and theorized about for hundreds of years (namely by economists and others not specifically interested in the neurological bases of these behaviors).

One particularly rewarding research tactic has been the employment of economic games (a subset of those falling under the heading of game theory, widely associated with the mathematician John Nash). One example of such a game is the following: I (the "house") flip a coin. If it's heads, I pay you a dollar. If it's tails, I flip it again. If it's heads on the second toss, I give you 2 dollars. If it's tails, I flip it again. If it's heads on this second toss, I give you 4 dollars, et cetera. Thus, if you get a head on the nth roll, you receive $2ⁿ. The question is: how much would you be willing to pay initially to be a player in this game? Most people are only willing to pay perhaps $10-20 for the privilege, however, statistically (on average), the earnings in this game are infinite. If you play enough times, you will earn an infinite amount of money. This concept was quoted hundreds of years ago to give credence to the notion that when it comes to estimation of value, we operate far from optimally.

This sort of heuristic - describing performance in a prescribed setting - can be rendered quantitative in such a way that an individual's decision making in a particular game can be used to estimate parameters about their over-all behavior: how risk-averse they are, how benevolent, and even how likely they are to feel guilty.




A paper appearing in the Journal of Neuroscience addresses this last point in the context of relevant brain areas and brain damage. Krajbich et al compared the performance of individuals with certain types of brain damage (along with normal controls) in an economic game. As can be seen in the table above (from their paper), they concluded that those with damage to the prefrontal cortex (PFC) are far less likely to experience feelings of guilt¹.



Those familiar with the story of Phineas Gage may hear the ring of truth in this result. Phineas Gage was a railroad worker responsible for tamping down explosives into holes drilled in pieces of rock, using a long metal rod. During one such episode, the explosives went off, and the rod entered his head below the left eye socket, exiting through the top of the skull and destroying most of his PFC (see image, above). Amazingly, he survived, but with the intriguing effect that his personality changed completely. Before the accident, he was described by his employers as "a great favorite" and "the most efficient and capable foreman in their employ." After the accident, he was so changed that they subsequently "considered the change in his mind so marked that they could not give him his place again." He was furthermore said to be a "braggadocio,and "manifesting but little deference for his fellows, impatient of restraint or advice when it conflicts with his desires, at times pertinaciously obstinate, yet capricious and vacillating.²"

It is likely that the experience of guilt is not the only function of the PFC; it is associated with planning functions and reasoning in general. However, this type of exploration and estimation of the parameters of human behavior is quite novel and powerful, and its utility will only increase as the complexity of our models and understanding of the parameters relevant to the generation of such behaviors increases.

References:
1. Krajbich I, Adolphs R, Tranel D, Denburg NL, Camerer CF. Economic Games Quantify Diminished Sense of Guilt in Patients with Damage to the Prefrontal Cortex. J Neurosci 29: 2188-2192, 2009.
2. Harlow JM. Recovery from the Passage of an Iron Bar through the Head. Pubs Mass Med Soc 2: 327-347, 1868.

On Insect Impersonation

Apparently humans aren't the only creatures that imitate others for social benefits. Writing in science, Barbero et al. report that the pupae of the butterfly Maculinea rebeli may have another tool to use in their ongoing infiltration of the societies of the ant species Myrmica schencki¹.

From the figure above, it is apparent that there is some similarity between the acoustic signals (bottom row) produced by these butterfly pupae (column C) and the ant queens (column A). The result is that ant workers behave around the butterfly pupae as they normally would around the ant queens.

Although it is apparent that these vocalizations are not identical, the authors report that - amongst the behaviors they took into consideration - there was no significant difference in the behaviors elicited from ant workers when comparing presentations of ant queen and butterfly pupa acoustic signals.

Of course the possibility remains that this auditory similarity is incidental and the butterflies have other tactics in their arsenal which are actually responsible for convincing these ants that the butterfly pupae are deserving of the social benefits normally reserved for ant queens. However, other forms of mimicry are common phenomena amongst insects, and it would not be surprising to find that auditory impersonation has evolved as well.

References:
1. Barbero F, Thomas JA, Bonelli S, Balletto E, Schönrogge K. Queen ants make distinctive sounds that are mimicked by a butterfly social parasite. Science 323: 782-785, 2009.

On Spermatozoic-Evolution

I recently listened to an episode of RadioLab concerned with the subject of sperm. It was highly enlightening, as most of their programming is, in my opinion, and it turned me on to one concept in particular that I found of particular interest. In general amongst our close animal relatives, promiscuity is the rule; approximately 3 percent of mammalian species are considered monogamous. One predicted result of this behavioral ubiquity is the specific evolution of sperm, for if male genes are to be carried on, an individual's sperm must compete with the sperm of others inside the female for the right to fertilize her egg(s). In fact, it has been known for some time that evolutionary selection will operate on sperm whenever access to a female's eggs is contested by sperm from more than one male¹. Furthermore, those who speculate about the subject speculate that such competition should yield larger sperm, based on the paired assumptions that larger sperm are faster, and faster sperm are more likely to fertilize an egg.


A study published in the Proceedings of the National Academy of Sciences has shown that female promiscuity does in fact, lead to the evolution of faster sperm in 29 closely related species of cichlid fishes of Lake Tanganyika, Africa. These fish in this lake are of particular interest to evolutionary researchers and theorists because the lake is large enough to constitute several environments - thus it harbors several closely related but distinct species of cichlids - and because of certain "explosive speciation events²," the relationships amongst these species is very well documented.



These researchers scored each species, assigning them a number according to their "sperm competition rank" (see table above). Which strongly predicted the speed of those species sperm (see table, below).




This research is quite intriguing because it represents an example of behavior feeding back on evolution. The effects of behavior on evolution are fascinating because such phenomena must have played a significant role in our own evolution, and continue to be perhaps the most important determinant of our biological fate.

References:
1. Parker GA. Sperm competition and its evolutionary consequences in the insects. Biol Rev 45: 525–567, 1970.
2. Fitzpatrick JL, Montgomerie R, Desjardins JK, Stiver KA, Kolm N, Balshine S. Female promiscuity promotes the evolution of faster sperm in cichlid fishes. Proc Natl Acad Sci U S A 106: 1128-32, 2009.