On The Agency Cortex

As previously reported, the mirror neuron system is a set of cortical circuits that are active both when an individual performs an act (picking up an apple, say) and when he observes that action being performed by another individual. This collection of cells is clearly important for understanding the intentions of others, and perhaps for learning by imitation. Furthermore, there is quite similar sensory activity associated with both active and passive limb movement (movement imposed on one's body).

This presents a problem. How do we attribute self-agency to our actions? This is a particularly important question if one subscribes to the theory (mentioned sporadically in this forum), that free will is not the cause of our actions, but the post-hoc assumption of agency for our actions. Even if one does take the stance that free will is the subjective experience of causing thoughts and acts, this question remains relevant since there must be some neural activity which distinguishes self-caused action from passively experienced action.

Of course, there is a wealth of non-cortical activity which accompanies moving one's arm (signals in the brain-stem and spinal column, in particular) which could provide this signal. However, because of the abstract nature of agency, some have the opinion that there must be a specialized cortical area whose job it is to integrate the diffuse, distributed neural activity associated with a single act and decide whether it was internally generated or externally imposed.

Writing in the pages of the Journal of Neuroscience, Zarinah Agnew and Richard J. S. Wise report that they've found an area of the brain that is a candidate for the job of agency detector, the Parietal Operculum¹.

This work pushes the boundaries of research into the nature of free will. One class of phenomena which motivates the free-will-as-post-hoc theory is the so-called automatisms: actions which are internally generated but feel as though they were caused by an outside agent. A well known example of automatism is the Ouija Board, where it is possible - most likely due to suggestion and the ability to ascribe agency directly to the other "players" - to feel as though one is not moving a planchette. Another is automatic writing, a phenomenon in which an individual composes pieces (in some cases entire novels) without any feeling of agency.

That the experience of will can break down in these ways is a very direct indication (along with a host of others) that our understanding of the phenomenon is minimal, at best. Localizing the brain areas responsible for the feeling of free will is one step towards understanding it.

References:
1. Agnew Z, Wise RJ. Separate areas for mirror responses and agency within the parietal operculum. J Neurosci 28: 12268-12273, 2008.

On The Ecosystem Within (UPDATE)

My last post was concerned with the way mice regulate the set of bacteria which reside in their intestines. Which specific bacteria are present in one's gut is known to be predictive of obesity, but new research suggests that it isn't the bacteria themselves that are important so much as the genes that they carry¹.

Scientists at Washington University in St. Louis studied the bacteria present in the intestines of pairs of twins (a useful methodology for exploring many kinds of similarities amongst individuals with similar genes) and their mothers, expecting to find that those who were obese would have similar species of gut flora (similarly expecting comparable special cross-sections in those who were not obese). Interestingly, they found that the set of bacteria differed widely, but that the core "bacteriome" (the set of all the genes in all the bacteria in a person's gut) was highly conserved across the obese (and separately across the non-obese). They further found that related individuals were more likely to harbor the same set of species.

This is not incredibly surprising. After all, the functional utility - in terms of digestive assistance, molecular synthesis, and nitrogen uptake - of these bacteria is defined by their genes. That is to say, bacteria can only be useful to us and our internal environment in that they are in possession of metabolic pathways that we lack. Furthermore, given the massive number of bacterial species, it is unsurprising that one person gets a specific part of the benefits from species A, while another person obtains that benefit from species B. It is a happy surprise to me that this research is progressing at an increasing pace. I hope it continues as such.

References:
1. Turnbaugh PJ, Hamady M, Yatsunenko T, Cantarel BL, Duncan A, Ley RE, Sogin ML, Jones WJ, Roe BA, Affourtit JP, Egholm M, Henrissat B, Heath AC, Knight R, Gordon JI. A core gut microbiome in obese and lean twins. Nature [Epub ahead of print], 2008.

On The Ecosystem Within

BIOMED. IMAGING UNIT, SOUTHAMPTON GEN. HOSP./SPL


There are more microbial cells than human cells in your body; this is a good thing. Bacteria help us break down foodstuffs by fermentation, synthesize vital molecules, and help us get rid of excess nitrogenous wastes. Furthermore, it has been demonstrated that the kind of bacteria you have in your gut is predictive of obesity: the right bacteria can help keep you thin^{1, 2}. Thus, it is important to have a source of good microbes in your diet, like yogurt or kombucha (a fermented tea drink rich in microbes), especially if you're engaging in activities that tend to kill off these organisms, like drinking heavily or taking antibiotics which don't discriminate between the good & the bad bacteria.

While drugs may not be able to discriminate between good and bad bacteria, our bodies must be able to in order to maintain intestinal homeostasis; how this happens has been unclear, to-date. However, new research demonstrates how one type of "bad bacteria" - the so called gram-negative strains - are selectively targeted by the body's immune system³. These microbes present an excess of a type of molecule on their membranes (peptidoglycans) which the body recognizes. The detection of these molecules causes the body to generate lymphatic tissue that specifically targets these bacteria.

As we come to understand more and more about the relationship between gut flora and health in general, this type of research will prove invaluable because it facilitates our understanding of the body's innate ability to regulate the subset of flora residing within. In other words, there is surely a gradient of immune system function such that some individuals are better able to select which flora to keep and which to oust; understanding how the body achieves this feat will thus widen the scope of western medicine.

References:
1. Turnbaugh PJ, Ley RE, Mahowald MA, Magrini V, Mardis ER, Gordon JI. An obesity-associated gut microbiome with increased capacity for energy harvest. Nature 444: 1027-1031, 2006.
2. Ley RE, Turnbaugh PJ, Klein S, Gordon JI. Microbial ecology: human gut microbes associated with obesity. Nature 444(7122):1022-1023, 2006.
3. Bouskra D, Brézillon C, Bérard M, Werts C, Varona R, Boneca IG, Eberl G. Lymphoid tissue genesis induced by commensals through NOD1 regulates intestinal homeostasis. Nature 456: 507-510, 2008.