I am a political centrist. In today's polarized political climate, this means that almost all the politically opinionated people I speak with are opposed to centrism. We seem unpopular everywhere.
The preprint of a summary of measures of brains in 5,216 donors to the UK's Biobank program was released on bioRxiv this past week. As a summary, they found that male brains were slightly larger in most structures, on the average, than female brains. This should not be surprising given that human males are on average slightly larger than human females. After controlling for average brain volume, though, there was still frequently a 0.7 to 0.8 Cohen's D value effect size of biological gender on many brain measures, with male brains usually larger on such measures, except, as has been noted before, female brains had a slightly thicker cortex and a larger corpus callosum.
The chart above is to show the degree of overlap for such differences in an ideal normal distribution case. The charts below reflect the actual current study under review's data. It's important to note that such size differences do not generally influence social aspects of gender. For example, persons who identify themselves more with their opposite biological sex (the trans-gendered) have scans that reflect on average measures that reflect their biological sex, not their psycho-social preference for gender. A further point: variances between individuals were larger than variance between men and women, reinforcing that we need to understand a mutiplicity of each individual's own personal qualities in order to better understand them and their abilities, much more than we need to rely on a single fact such as gender. As the paper says, "Overall, for every brain measure that showed even large sex differences, there was always overlap between males and females (p. 8)."
Sex differences in the adult human brain: Evidence from 5,216 UK Biobank participants
Stuart J. Ritchie1,2*, Simon R. Cox1,2, Xueyi Shen3, Michael V. Lombardo4,5, Lianne M. Reus6, Clara Alloza3, Matthew A. Harris2,3, Helen L. Alderson7, Stuart Hunter8, Emma Neilson3, David C. M. Liewald1,2, Bonnie Auyeung1, Heather C. Whalley3, Stephen M. Lawrie3, Catharine R. Gale2,9, Mark E. Bastin2,10,11, Andrew M. McIntosh2,3, Ian J. Deary1,2
bioRxiv preprint first posted online Apr. 4, 2017; doi: http://dx.doi.org/10.1101/123729.
SEX DIFFERENCES IN THE HUMAN BRAIN
Sex differences in human brain structure and function are of substantial scientific interest because of sex-differential susceptibility to psychiatric disorders [1,2,3] and because of the potential to explain sex differences in psychological traits . Males are known to have larger brain volumes, though the patterns of differences across brain subregions have typically only been examined in small, inconsistent studies . In addition, despite common findings of greater male variability in traits like intelligence , personality , and physical performance , variance differences in the brain have received little attention. Here we report the largest single-sample study of structural and functional sex differences in the human brain to date (2,750 female and 2,466 male participants aged 44-77 years). Males had higher cortical and sub-cortical volumes, cortical surface areas, and white matter diffusion directionality; females had thicker cortices and higher white matter tract complexity. Considerable overlap between the distributions for males and females was common, and subregional differences were smaller after accounting for global differences. There was generally greater male variance across structural measures. The modestly higher male score on two cognitive tests was partly mediated via structural differences. Functional connectome organization showed stronger connectivity for males in unimodal sensorimotor cortices, and stronger connectivity for females in the default mode network. This large-scale characterisation of neurobiological sex differences provides a foundation for attempts to understand the causes of sex differences in brain structure and function, and their associated psychological and psychiatric consequences.
In the study abstract below, Kitamura and others show that long-term memory traces in the outer cerebral cortex of the mouse are laid down early in the experience to be remembered. Prior to this study the prevailing opinion was that neuronal traces for short-term memories are initially created only in the hippocampus and transferred later to long term cortex. The study below shows that initial weak traces of long term memory are created simultaneously with the strong short term memory traces in the hippocampal short term memory region, and that, over a couple weeks, the cortical area traces become stronger and more active and the corresponding hippocampal areas fade in their activity.
This suggests that prior theories were too simplistic, perhaps because they tended to model memory after human information storage, such as writing or memory disk, where information is generally laid down once and remains the same strength thereafter unless erased. Modeling the brain after our technology metaphors can be misleading.
Engrams and circuits crucial for systems consolidation of a memory
Takashi Kitamura1,*, Sachie K. Ogawa1,*, Dheeraj S. Roy1,*, Teruhiro Okuyama1, Mark D. Morrissey1, Lillian M. Smith1, Roger L. Redondo1,2,†, Susumu Tonegawa1,2,‡
Science 07 Apr 2017
Vol. 356, Issue 6333, pp. 73-78
The network of memory consolidation
Memories are thought to be formed in the hippocampus and later moved to the neocortex for long-term storage. However, little is known about the mechanisms that underlie the formation and maturation of neocortical memories and their interaction with the hippocampal network. Kitamura et al. discovered that at the onset of learning, neurons for contextual fear memory are quickly produced in the prefrontal cortex. This process depends on the activity of afferents from both the hippocampus and the amygdala. Over time, the prefrontal neurons consolidate their role in memory expression. In contrast, the hippocampal neurons slowly lose this function.
Episodic memories initially require rapid synaptic plasticity within the hippocampus for their formation and are gradually consolidated in neocortical networks for permanent storage. However, the engrams and circuits that support neocortical memory consolidation have thus far been unknown. We found that neocortical prefrontal memory engram cells, which are critical for remote contextual fear memory, were rapidly generated during initial learning through inputs from both the hippocampal–entorhinal cortex network and the basolateral amygdala. After their generation, the prefrontal engram cells, with support from hippocampal memory engram cells, became functionally mature with time. Whereas hippocampal engram cells gradually became silent with time, engram cells in the basolateral amygdala, which were necessary for fear memory, were maintained. Our data provide new insights into the functional reorganization of engrams and circuits underlying systems consolidation of memory.
Yet another travel writer fails to surpass Mark Twain's Letters from Hawaii. This new travelogue could be seen as a sad reminder of how we cannot easily escape our issues if we bring the problematic attitudes with us. I can but quote others on this kind of attitude:
I just want people to take a step back, take a deep breath and actually look at something with a different perspective. But most people will never do that.
"Though we travel the world over to find the beautiful, we must carry it within us or we will find it not."
--Ralph Waldo Emerson
The NYT travel review reviewed above can be found
According to a theory of knowledge called confirmation holism, we believe facts on the basis of their being part of a larger body of knowledge that we believe in total. For example, my belief in Sweden is a part of my knowledge of geography and history as a whole. I would have to doubt much of what I know of the nations and history of the world to disbelieve in Sweden.
One problem that's been advanced against such confirmation holism is the idea that we must have to assign each fact we know a probability, and then, by the laws of probability, don't we calculate the odds of our entirety of holistic conjunction of facts just as the product of the individual probabilities? For example, if we believe each fact of a 500-fact conjunction with p = 0.95, does this make our degree of belief in the 500-conjunct equal to (0.95)^500, or just 0.0000000000073?
But, of course, we DON'T calculate the probabilities of the whole this way! The multiplication rule for the conjunct of two probabilities assumes that those probabilities are independent! . And, in the case of confirmation holism, that is exactly what we would deny.
The Ship of Theseus is a classic thought experiment in Greek philosophy. As related by Plutarch the historian, Theseus' ship was kept as a famous artifact, and, over the years, as its wood decomposed the boards were replaced, until none of that ship was the same wood as the original.
Was that ship still the ship of Theseus after all those replacements? Heraclitus, who said all was change, would likely have said not the same ship, and Plato and Aristotle, with their emphasis on form as what makes things what they are, might have thought otherwise. In a materialistic society, there are direct implications of the story for questions of identity over time--including human identity.
In The God Delusion, Richard Dawkins quotes, with apparent approval:
Steve Grand points out that you and I are more like waves than permanent 'things'. He invites his reader to think . . . ... of an experience from your childhood. Something you remember clearly, something you can see, feel, maybe even smell, as if you were really there. After all, you really were there at the time, weren't you? How else would you remember it? But here is the bombshell: you weren't there. Not a single atom that is in your body today was there when that event took place . . . Matter flows from place to place and momentarily comes together to be you. Whatever you are, therefore, you are not the stuff of which you are made. If that doesn't make the hair stand up on the back of your neck, read it again until it does, because it is important.
This is a quote from Grand's book on supervenience in nature. Though I'm not disagreeing here with such a supervenience thesis, the assertion above that "Not a single atom that is in your body today was there" (when you, now an adult, experienced a past, childhood event) is, in fact, false.
What evidence do Grand and, via Grand, Dawkins, have for such an assertion?
Their outdated, incorrect claim is in fact a philosophical urban legend, where everyone vaguely quotes a paper written over 60 years ago by Dr. Paul Aebersold, then a physicist at Las Alamos with Ernest Lawrence, as part of a review of the then new technologies using radioisotopes made possible by peaceful nuclear technologies. Writing in 1953 in the Annual Report of the Smithsonian Instituion, Aebersold says,
Tracer studies show that the atomic turnover in our bodies is quite rapid and quite complete. For example, in a week or two half of the sodium atoms that are now in our bodies will be replaced by other sodium atoms. The case is similar for hydrogen and phosphorus. Even half of the carbon atoms will be replaced in a month or two. And so the story goes for nearly all the elements. Indeed, it has been shown that in a year approximately 98 percent of the atoms in us now will be replaced by other atoms that we take in in our air, food, and drink.
Aebersold's paper was later to be quoted in 1954 in Time Magazine, whence it passed into the "quoted by everyone facts" category. By the 1960's, it seems to have become embedded in the background notions of the philosophy of biology and identity, perhaps slowly taking on urban legend status over the next few decades.
Let's now update to the twenty-first century. Pulse-chase analysis is a technique used in biology to understand the changes in a cell or tissue over time. The cells are exposed to a labeled substance which it incorporates into cellular metabolism at the time of exposure. This "pulse" is followed by a further exposure of the same type substance, but without the label (the "chaser"). The cell or tissue can later be examined for the tracer at various times after the pulse of label, to see if and where in its metabolic flow the tracer can be located.
In the past 15 years, studies done mostly at the Frisen lab at the Karolinska Intitute in Norway took advantage of a global carbon-14 pulse-chase experiment: the global atmospheric fallout exposure of the world's population alive or in utero at the time of atmospheric testing of atomic weapons by the United States, Russia, and Great Britain from 1945 through 1963 (note that France and China did a few more such tests through 1980). Here's one of the first abstracts:
Cell. 2005 Jul 15;122(1):133-43.
Retrospective birth dating of cells in humans.
Spalding KL1, Bhardwaj RD, Buchholz BA, Druid H, Frisén J.
The generation of cells in the human body has been difficult to study, and our understanding of cell turnover is limited. Testing of nuclear weapons resulted in a dramatic global increase in the levels of the isotope 14C in the atmosphere, followed by an exponential decrease after 1963. We show that the level of 14C in genomic DNA closely parallels atmospheric levels and can be used to establish the time point when the DNA was synthesized and cells were born. We use this strategy to determine the age of cells in the cortex of the adult human brain and show that whereas nonneuronal cells are exchanged, occipital neurons are as old as the individual, supporting the view that postnatal neurogenesis does not take place in this region. Retrospective birth dating is a generally applicable strategy that can be used to measure cell turnover in man under physiological and pathological conditions.
So, here is a chart of the results of many such tests, courtesy of bionumbers.org:
What was wrong with the old urban legend? Note that Aebersold was mostly right (except for that word "complete") in what he wrote: it was extrapolation of what he said that was wrong!
Even in 1953, we knew that the body was not a uniform bag of tissue, and that cells were not mere bags of solute. It turns out that although it's true that over 98% of the body undergoes relatively rapid turnover during life, there are several tissues whose DNA does not turn over nearly as fast, or in some cases turn over at all.
Most of the human body is water, and this water flows in and through us like Heraclitus' river. However, bone turnover is far slower, and some tissues, such as tendons, turn over even less. Brain tissue contains some neurons which have not divided since we were born. DNA repair in those cells is not pervasive enough to turn over much of it during our lifespan.
So, not all the atoms of the human body turn over during life. Much of our DNA lingers.
Does it actually matter to philosophical conclusions about our identity that some small part of us (possibly an ounce) is completely unchanged during our lifetimes? Not if, like most of the world's population, you know that a ship is not just deadwood, and that matter is not all there is to living.
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