Archive for the ‘Visual cortex’ Category

03.23.09 [#082] Yogurt Reach
Image by Jeezny via Flickr

Pity the poor brain.  What a job it has!  Did you know that just to reach into a refrigerator and grab a glass of milk, involves at least 50 or so key muscles in the hand, arm and shoulder which can, in principle, lead to over 1,000,000,000,000,000 possible combinations of muscle contractions?  Just so you know, this is 1,000 times MORE contraction possibilities than there are neurons in the brain (only a mere 1,000,000,000,000 neurons).  I’m sorry brain, I’ll keep my hands out of the fridge, I promise!

To accomplish this computational feat, Rodolfo R. Llinas and Sisir Roy in their paper entitled, “The ‘prediction imperative’ as the basis for self-awareness” [doi:10.1098/rstb.2008.0309] suggest that brain has evolved a number of strategies.

For starters, the authors point out that the brain can lower the computational workload of controlling motor output by sending motor control signals in a non-continuous and pulsatile fashion.

“We see that the underlying nature of movement is not smooth and continuous as our voluntary movements overtly appear; rather, the execution of movement is a discontinuous series of muscle twitches, the periodicity of which is highly regular.”

This computational strategy has the added benefit of making it easier to bind and synchronize motor-movement signals with a constant flow of sensory input:

“a periodic control system may allow for input and output to be bound in time; in other words, this type of control system might enhance the ability of sensory inputs and descending motor command/controls to be integrated within the functioning motor apparatus as a whole.“

Another strategy is the use of memory for the purposes of prediction (actually, their paper is part of a special theme issue from the Philosophical Transactions of the Royal Society B entitled, Predictions in the brain: using our past to prepare for the future).  The authors describe the way in which neural circuits in the body and brain are inherently good at learning and storing information which makes them very good at using that information for making predictions and pre-prepared plans for what to do with expected incoming sensory inputs.  These neural mechanisms may also help reduce computational loads associated with moving and coordinating the body.  Interestingly, the authors note,

“while prediction is localized in the CNS, it is a distributed function and does not have a single location within the brain. What is the repository of predictive function? The answer lies in what we call the self, i.e. the self is the centralization of the predictive imperative.  The self is not born out of the realm of consciousness—only the noticing of it is (i.e. self-awareness).”  Here’s a link to Llinas’ book on where the “self” resides.

Lastly, the authors suggest that the genome might encode certain structural and functional aspects of neural development that create a bias for certain types of computation and prime neural networks with a Bayesian type of prior knowledge.  Their idea is akin to an organism being “experience expectant” rather than a pure blank slate that has to learn every stimulus-response contingency by trial-and-error.  To support their notion of the role of the genome, the authors cite a 2003 study from the Yonas Lab on the development of depth perception.  Another related study is covered here.

Methinks that genetic variants might someday be understood in terms of how they bias computational processes.  Something to shoot for in the decades to come!

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Binocular Smile
Image by cobalt123 via Flickr

Is the human brain a blank slate? or a pre-programmed machine that is ready to take the S.A.T.s right out of the box? Obviously neither, or both as it were. Some have gingerly waded into the nature vs. nuture debate and suggested that the human brain comes pre-wired to receive certain experiences – experience expectant – and thus acknowledge the importance of natural selection in shaping an organism via heritable factors but also the need to be able to use the brain to learn from experience and adapt on the fly.

In their paper entitled, “Nature versus Nurture in Ventral Visual Cortex: A Functional Magnetic Resonance Imaging Study of Twins [DOI:10.1523/JNEUROSCI.4001-07.2007] Thad Polk and colleagues provide a wonderful example of this.  The team suggested that the brain (visual system) should be somewhat innately (genomically if you will) prepared to process visual stimuli such as faces and objects, but not so for stimuli such as pseudo words.  They proposed to test the role of the genome by comparing patterns of brain activity in identical vs. fraternal twins.  If the brain activity patterns were very similar for identical twins, and less so for fraternal twins, then it is likely that the genome plays some role in the generation of brain (at least with respect to blood flow) responses to such stimuli. The team used fMRI to assess 13 pairs of identical twins and 11 pairs of fraternal twins for their brain responses to pictures of faces, houses, chairs and non-word strings on letters as well as control “scrambled” images that were comparable in visuo-spatial frequency.

Interestingly, the team found that for faces and houses, there were significant identical vs. fraternal differences in the “activation maps” of the twins but no such differences for chairs and pseudowords.  Thus it seems that the genome plays a role in the way the brain processes faces and houses (or perhaps faces and places in general), but not so much for items that are not found (or weren’t found by our evolutionary ancestors) in a natural setting.

I’m surprised by the chair result … although perhaps being a couch potato is something evolution does not select for.

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Comparison of zygote development in monozygoti...Image via Wikipedia Twin studies are oft used to gauge the role of the genome in behavioral science. A recent report, “Nature versus Nurture in Ventral Visual Cortex: A Functional Magnetic Resonance Imaging Study of Twins” by Polk et al., (DOI) shows that brain activity during early stages of visual processing is more similar in twins vs. unrelated subjects across several object categories such as faces, houses, pseudowords and a control category consisting of -ok- chairs? When the brain activity of identical vs. fraternal twins was examined, the activity associated with faces showed the greatest difference in similarity of activity compared to other categories. Its always fun to speculate about why the genome might weigh-in more heavily when it comes to face processing – certainly an important skill for our primate order.

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Lysergic acid diethylamideImage via Wikipedia Without a doubt, one of the low points of any marriage comes when you have to select a new paint colors. To avoid unnecessary strain, I usually just go along to get along, but Mother Nature allows no easy escape from this inevitable moment in our life cycle. After a third trip to the paint store, I found myself literally, up the wall, painting another test patch in a dark upper corner. Whilst brushing away, I was reminded of a lecture by V. S. Ramachandran who happened upon a colorblind subject who reported subtle differences in the colors of certain digits. In their article, “We also observed one case in which we believe cross activation enables a colorblind synesthete to see numbers tinged with hues he otherwise cannot perceive; charmingly, he refers to these as “Martian colors.” Although his retinal color receptors cannot process certain wavelengths, we suggest that his brain color area is working just fine and being cross-activated when he sees numbers.“Jay Gingrich and colleagues report (DOI) that the serotonin 2A receptors mediate the “synesthesia-like” effects of psychoactive hallucinogens such as LSD specifically via pertussis toxin-sensitive heterotrimeric G(i/o) proteins and src. Now, I’m a fan of genetic conflict hypotheses of all sorts, and perfectly willing to acknowledge that Mother Nature has stacked the deck against my Y-chromosome in many ways, but as my wife complained, yet again, that the new color was not, “the color in her head”, I began to wonder about natural mechanisms of synesthesia and the natural history of HTR2A and Mother Nature’s often dark sense of humor.

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