On Snake Walking

(from reference 1)


Human beings transition between from one style of gait to another as they transition from walking to running (see figure, above). The act of walking is fundamentally one of transferring weight from one limb to another, while running is primarily an act of maintaing inertia. Furthermore, these two gaits apparently have their origin in the minimization of energy costs associated with moving at a particular speed¹.



(from reference 2)


Interestingly, it seems that snakes do not make any such transition in their locomotive behavior. A study appearing in the Proceedings of the National Academy of Sciences has used theoretical modeling, friction measurements, and slithering-observations to demonstrate that snakes simply "speed-walk" at high speeds ². Although they have no limbs, and thus no "gait" to speak of, at slow speeds, they do move about by transferring weight from one part of their body to another. In contrast to other animals, however, they do the same thing, only faster, at high speeds. It is unclear why speedwalking is not an undue energetic costs for these animals, perhaps there is simply no less-costly way to move about. Further work will be needed to more completely understand the energetics of snake locomotion.

References:
1. Srinivasan M, Ruina A. (2006) Computer optimization of a minimal biped model discovers walking and running. Nature. 439(7072): 72-75.
2. Hu DL, Nirody J, Scott T, Shelley MJ. (2009) The mechanics of slithering locomotion. Proc Natl Acad Sci U S A. [Epub ahead of print]
PMID: 19506255 [PubMed - as supplied by publisher]

On The Tree-for-the-Forest in Autism

One commonly reported feature of autism-spectrum-disorder (ASD) is the tendency to favor details over whole-object properties. That is, to notice the forest and not the trees. A study appearing in the journal Vision Research quantifies this effect experimentally¹.

The authors of this study relied on a concept known as "visual crowding." This term refers to a commonly experienced phenomenon in which objects that are spaced closely together are more difficult to individually attend to or resolve. For example, some have invoked this idea to explain why it is difficult to pick individual faces out of a crowd. It is important to note, that there is a spatial-scale, a threshold, associated with visual crowding, such that objects of a given size must be spaced within some distance limit to be considered within the crowding limit (although some objects are so large that they are immune to such effects).

from reference 1

Interestingly, the authors found that children with ASD had much lower thresholds for visual crowding than those without the disorder (see figure, above). That is: those with ASD were able to resolve and report the properties of more densely packed objects than those without ASD. Furthermore, children with ASD out-performed non-ASD children in the employed task within the crowding limit (as defined by the threshold for non-ASD children) while underperforming outside this limit.

Such a finding suggests structural irregularities in the visual-corticies of these children; while this is nothing special in and of itself, there are many different cortical areas which are affected by ASD, which leads to the intriguing possibility (suggested by many) that the disorder might be a generalized structural deficit of the cerebral cortex.

References:
1. Baldassi S, Pei F, Megna N, Recupero G, Viespoli M, Igliozzi R, Tancredi R, Muratori F, Cioni G, Search superiority in autism within, but not outside the crowding regime, Vision Research, In Press, DOI: 10.1016/j.visres.2009.06.007.

On Leanness

Although many acknowledge that some people are inherently (perhaps genetically) leaner than others, it remains unclear what the biological basis for the body's "set-point" might be. A study appearing in the open-access journal PLoS One suggests one possible factor¹.


from reference 1

Both rat and human data were collected in this work, which concludes that "the lean phenotype is characterized by high endurance capacity and high activity and may stem from altered skeletal muscle energetics." These researchers gathered data from a population of people who they categorized as non-exercisers (less than 1 hour per week of activity exceeding 4 METS). They subjected these individuals to a treadmill test to determine their endurance (as assessed by oxygen consumption during exercise) and kept track of their average daily activity over a period of 10 days; finding that there was a significant relationship between endurance and leanness, as well as average daily activity and leanness. Furthermore, they found that there was no significant difference in the amount of food consumed by lean versus non-lean rats, and fascinatingly, that the skeletal muscle tissue of lean rats has significantly higher levels of the enzyme PEPCK-C.

Of course, it is not surprising that those individuals with higher daily activity are leaner in general, rather, this study is suggesting that there may be a fundamental, biological reason why certain individuals are more active: they simply have a greater capacity for activity. Indeed, if one fatigues more easily, it wouldn't be surprising if they were less active; it is also conceivable that reduced activity could feed-back on behavior in the sense that an individual with lower endurance might progressively reduce the amount of physical activity they engage in so as to reserve energy for other tasks. This is especially true in contemporary society, where mental activity is often the basis for work and play.

It is unclear as yet, however, whether the differential muscle-properties found in rats extend to humans; further work will be required to clarify what the molecular-biological basis for increased human endurance might be.

References
1. Novak CM, Escande C, Gerber SM, Chini EN, Zhang M, et al. (2009) Endurance Capacity, Not Body Size, Determines Physical Activity Levels: Role of Skeletal Muscle PEPCK. PLoS ONE 4(6): e5869. doi:10.1371/journal.pone.0005869

On Feedback

One of the most fascinating questions in neuroscience, is how "high-level" cognitive properties of mind like attention feed back on and affect our biology. For example, it is known that video-game players have better visual acuity than non-video game players¹. Another example of this phenomenon can be found in the result (described below) from Lee et. al., published in the Journal of Neuroscience².


from reference 2

The authors of this study found differences in the responses of the auditory brainstems of musicians as compared to non-musicians. Specifically, these two groups (musicians and non-musicians) were presented with pairs of consonant and dissonant tones; it was found that musicians showed larger response magnitudes to certain components of consonant tones than did non-musicians (see figure, above).

The hypothesized reason for this difference is that a musician's heightened attention to consonant tones (and their makeup or properties) leads to changes in his or her neurobiology, such that the neurons of the auditory brainstem eventually respond more strongly to certain aspects of these auditory signals. This is especially fascinating because the area of the brain that was measured was not the cortex (usually associated with consciousness and "high-level" cognitive activity), but the brainstem (the area of the brain that is evolutionarily much older; bearing greater resemblance to animal brains ).

How the conscious act of focusing on one aspect of a stimulus can lead to an enhancement of the responses of brain-regions devoted to their representation is an open question, and one with wide-ranging implications. Further research will be required to understand its basis.

References:
1. Green CS, Bavelier D. (2007) Action-video-game experience alters the spatial resolution of vision. Psychol Sci. 18(1):88-94. PMID: 17362383
2. Lee KM, Skoe E, Kraus N, Ashley R. (2009) Selective subcortical enhancement of musical intervals in musicians.
J Neurosci. 29(18):5832-40. PMID: 19420250

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.

On Finding Yourself

The hippocampus and associated structures such as the entorhinal cortex, have long been known to play an extremely important role in navigation and memory formation (as previously discussed in this forum: 1, 2). For example, the hippocampus is enlarged in London taxicab drivers, who presumably employ it heavily for navigating around the city, and individuals with damage to this area are unable to form new memories at all, though they can recall past experiences with no loss of fidelity.

The entorhinal cortex feeds into the hippocampus, and it seems to be far more specialized for navigation purposes. There are cells in this area that seem to encode the direction that an animal's head is pointing. There is another varietal, referred to as the grid-cell, whose response-properties are illustrated below.

from reference 2

Grid-cells have the intriguing property of responding vigorously in a regular array of spatial locations. If an experimenter puts an animal in a small confined space, the regularity of these responses are evident after a brief period of exploration by the animal. On the left, you see the black trace of a rat's position as it wanders around this enclosure, with red traces representing locations where the response of a single neuron under consideration was strongest. In the middle, you can see a rasterized representation of this information, and on the right, a "cross-correlation" of the middle plot, showing the regularity of the responses.

Much of the pioneering work on the hippocampus and entorhinal cortex has come from the lab of Edvard and May-Britt Moser, a married pair of neuroscientists working at the Kavli Institute for Systems Neuroscience at the Norwegian University of Science and Technology. They have recently discovered another, infrequently occurring, class of cell in the entorhinal cortex, the border cell. Predicted from theoretical considerations in the year 2000 by Neil Burgess, these cells respond preferentially when the animal is placed near a border, of a certain orientation, of an environment (these can be walls, or drop-offs; see image, below).

from reference 1

Panel A shows that the response is independent of the size of the space, and that if the space is expanded, so too does the area over which the neuron responds (right). Panel B shows that if a new separation is inserted into the space, the cell begins to respond strongly to the boundary. Panel C shows that the response properties persist even after the walls are removed, leaving only a drop-off). Panel D shows that the orientation specific quality of the responses are independent of the shape of the room. If a landmark, in the form of a marker on one of the walls, is employed, the responses rotate along with the landmark (see below). Which is impressive, but also expected and necessary for these cells to function efficiently as part of a navigational system.



It is still a mystery how these cells produce these responses. It must be the case that a computational transformation of a variety of sensory and motor information must contribute to the computation of border location. These cells represent some of the few that have such clearly defined properties. That is to say, in many other parts of the brain, single neurons contribute only a small part of the over-all response to a stimulus, and it is thus surprising to find single cells devoted to the entire border of a space. Another impressive and enigmatic feature of the responses of these cells is the fact that they must rapidly re-compute and respond to the borders of a novel place. This sort of short-time-scale neuronal plasticity is of great interest to neuroscientists everywhere.

Understanding the properties of these cells, and their role in navigation and memory formation will be a great and rewarding challenge, one that I'm sure the Mosers are up to.

References:
1. Solstad T, Boccara CN, Kropff E, Moser MB, Moser EI. Representation of geometric borders in the entorhinal cortex. Science 322: 1865-1868, 2008.
2. Hafting T, Fyhn M, Molden S, Moser MB, Moser EI. Microstructure of a spatial map in the entorhinal cortex. Nature 436: 801-806, 2005.

On the Life Cycle of Stars

In honor of the 400th anniversary of Galileo Galilei's (and humankind's) first observations with a telescope, 2009 has been declared the International Year of Astronomy. I thus thought it only appropriate to devote at least one post to the heavens.


"Although stars are frequently assumed to be constant and unchanging features of the firmament, they are in fact evolving dynamic systems. New stars condense out of gaseous nebulae, and old stars evolve through planetary nebulae and supernovae into white dwarfs, neutron stars and black holes. These processes—star formation and evolution—are critical to understanding many features of the Universe, including the evolution of galaxies, the dispersal of chemical elements and the distribution and energetics of gas.

Some of the [Hubble Space Telescope's] HST's most lasting (and beautiful) contributions to stellar astronomy have been its studies of star-forming regions like the Orion nebula [see figure, above]. In these regions, luminous massive stars ionize the gas cloud from which they coalesced, causing the cloud to glow brightly in various emission lines. The HST's earliest observations of the Orion nebula revealed that it was peppered with a remarkable population of young stars surrounded by dense disks of gas and dust. These disks are undoubtedly remnants of the late accretion phase during which the stars condensed. Although the presence of such disks had been inferred from theory and from observations with the Very Large Array, the HST's superior image resolution revealed the first true pictures of the disks' structures and physical properties."¹

References:
1. Dalcanton JJ. 18 years of science with the Hubble Space Telescope. Nature 457: 41-50, 2009.