What we talk about when we talk about heritability

There’s some behavioral evolution stuff I want to talk about soon.  This is great, because it’s what I studied in college and I will be able to contribute something beyond wikipedia synthesis.  In order to really dig into this, I have to explain how we measure heritability.

Heritability“, or h^2, is a very specific measurement in biology.  It measures how much  variation of a particular trait in a particular population is due to genetic variation, relative to the total variation.  It is not a synonym for “genetically determined.”  For example, having two arms is barely heritable, because there’s almost no variation in the number of arms people have.  Of people that have less than two arms, most of them lost them to environmental accidents, not genetics.  There’s no gene for “one arm”, although developmental accidents may have some genetic basis.  Biological sex isn’t very heritable either.  A person’s parents’ sex has very little predictive power on the person’s own sex.    Despite this, number of arms and biological sex is very, very genetically determined

Additionally, a heritability measurement is only valid for the population it was measured in.  In a population in which every individual has exact the same environment will demonstrate much higher heritability for every trait than a population with a varied environment.

Now here is the really weird thing: how sensitive a trait expression is to environmental variation can be influenced by genetics.  High Sensitivity was originally discovered in people who were having severely maladaptive responses to normal stimuli, and was assumed to have a uniform negative effect on people who carried it.  Newer research indicates that the effect is more complicated:  Highly Sensitive People with really good environments do better that Low Sensitive People in those environments.  HSPs in bad environments do worse that LSPs.  Whatever genes cause the trait we call High Sensitivity make a person more sensitive to their environment.  Ellis et al call this biological sensitivity to context.

Lee et al  investigated a particular HSP- associated allele, DRD2 Taq1A, and found that mothers with the gene were harsher parents during a recession than mothers without the gene.  Moreover, they found worsening economic conditions led to worse parenting in everyone, but the effect was more pronounced for mothers with the T allele as opposed to the CC allele.  So a child’s outcome is being affected by their own genetically-driven response to their parents, their parents’ genetically-driven response to the environment, and the actual environment.

I thought what depressed me here was that highly sensitive parents are more likely to have highly sensitive children, which will magnify the negative effects of the recession on children.  Then I read a study showing that maternal psychological trauma was associated with higher disorganization in infants, but only if they had a high-SPS associated allele.*

High Sensitivity is going to come up a lot, so remember that.  But also remember that when journalists talk about heritability, they do not mean what they think they mean.

*The same mutation, DRD4 7-repeat polymorphism, is associated with ADHD.

Sensory input and economies.

Expanding on my metaphor from yesterday, let’s posit three different types of sensory inputs/equations: Trivial (sensory input that is not necessarily effortless to process, but not taxing), Unsolvable (puts you into overload), and Solvable (requires noticeable amounts of processing power).  Unsolvable equations represent either completely  new data, or complex combinations of problems that would be Trivial on their own, but combine in difficult ways.

More concretely:  let’s say you have two sensations, A and B.  Either one on its own is a Taxing problem, but together they’re Unsolvable.  If you manage to move A into the Trivial category (by repeated exposure and processing), that moves A+B into Solvable.  And when you have solved it, you have A, B, and A+B in the trivial category, which will come in handy when you see A+B+C later.  In this way, very small differences in initial processing capabilities can compound into vastly different levels of ability.  Going into overload isn’t just painful, it cuts you off from learning the things that could prevent it next time.

This suggests a huge payoff to investing problems that are right on the border between Trivial and Solvable.  Not only do you move that one thing off your plate, and move some other problems from Unsolvable into Solvable, you create an environment where you can eventually solve the other components of that previously Unsolvable problem.  

[The following paragraphs are free-association at best]

For some reason I’m reminded of Jane Jacob’s The Economy of Cities, in which she postulates that most economic growth follows the following pattern:

  1. I’m doing a thing.
  2. Hurray, I invented a slightly more efficient way to do that thing.
  3. Wait, I could use that new invention to do this other thing.  There’s no demand for it yet because no one knows they want it, but there will be.
  4. Welp, I invited a whole new sector of the economy.

Tim Harford implicitly talked about this in Adapt.  The best economics have several interlocking industries with moderate overlap.  Similar enough that innovations in one can help enough, but dissimilar enough to cause them to approach problems in different ways.  Adapt as a whole is about trying a lot of things quickly, with the expectation that most of them will fail.  One important component of that is minimizing the cost of failure, and one important component of that is recognizing failure quickly.  Which sounds like a sensory problem.

Like I said, this is free association.    I know I’m on to something, but I’m not sure what yet.

I figured it out.

One thing that has frustrated me as I researched SPS has been that I couldn’t connect the macro with the micro.  I know damn well what the macro pattern is (too much noise -> everything is terrible, and also I bump into walls a lot), and I’ve been learning about the micro pattern (auditory, visual, or touch stimulus leads to over-activation of certain areas of the brain), but how did overactivation lead to irritability?    The Threads of Autism (which I’ve since finished) went on and on about teaching people to “organize” sensory input, but it only described the effects of this in macro language (people become more relaxed, more resilient, able to thrive under higher levels of stimulus), but not what that actually looked like in the brain.  Much less why pronating and supinating my wrists in time with my breathing or tapping along my facial nerves was going to accomplish this.

I think I’ve figured it out.  Before I tell you, I want to make it very clear that this is my own personal metaphor, and not something that’s been tested scientifically.  I’m not even sure what it would mean to test it.  But it makes sense to me.

Your/your body’s ultimate goal is to figure out what it should be doing at any given second, and especially if it should be running away from or trying to breed with something.  In order to do that, it tries to work backwards from the sensory input to derive a model of what is actually happening*.  It can then decide how it wants to respond to that thing that is happening.  For example, if you’re wandering through the jungle and hear a twig snap, you would like to know if it’s a delicious herbivore, a hungry tiger, or a human being, and if so, is it from  your tribe or the one you’re at war with.  Past sensory experience is really helpful in this interpretation.  For example, if you’ve heard lots of tigers walking in the jungle, you can pattern match the current sounds against the ones you remember and see how close they are.

But what if it were more basic than that?  We think we just know when and where and how someone is touching us, or how we’re oriented in space, but that is actually something you have to learn.  We don’t notice because it’s mostly done when when we’re very very young, and because it’s done in parts of our brain that we’re not  consciously aware of.  But human brains are actually very plastic, and we devote an extraordinary amount of time and energy to learning how to translate “nerve 43b is firing” to “something happened on my left ring finger.”

My metaphor is as follows:  people with SPS either don’t have the same bank of experiences to pattern match against, or are worse at matching.  So given the same amount of sensory input, it takes them a lot more energy to correctly model the source.  It’s sort of like simplifying a mathematical equation.  If you have something awful with lots of terms you can solve it by hand, but it’s error prone and time consuming.  If you take that same equation and simplify it by removing terms that cancel and grouping like terms together, that same equation can be trivial.

I think the point of the sensory integration exercises is to build up either the database of experiences and/or your skill at simplifying equations.  You can’t give a person every single experience, but you can teach them “this is what stimulus from the trigeminal nerve feels like deep inside your brain.”  This makes it marginally easier to identify, or at least not freak out about, novel stimulus.  It’s like teaching someone about a 3-4-5 triangle.  They’ll not only recognize other 3-4-5 triangles faster, but eventually 6-8-10 and 4.5-6-7.5 as well.

When I started in computational biology, I was all about complex computer simulations.  By my senior year of college, I’d learned to appreciate mathematical models.  They left out details, but that was what let you see the general patterns.  If I’m right, I’m about to undergo the same process for basic sensory data.  And I know I can do it.

*Exception: reflexes

Possibly fictitious diseases of the endolymphatic sac

Last week I asked: what could go wrong with the vestibular system.  I was hoping to have a more satisfying answer by now, but I haven’t.  So here are some random wanderings.

The scientifically suspect book my scientifically suspect first sensory integration therapist gave me suggests “underinflation of the endolymphatic sac”, without anything so droll as a definition of the endolymphatic sac.  No problem, I have the internet.  Wikipedia’s entry on the subject is… weak.  I originally misread it as saying the endolymphatic sac was connected to only the saccule (one of the two linear-motion detecting mini-organs).   The Pictorial Guide To Cochlear Fluids sets me straight on this: all of the various vestibular systems are interconnected, and they all connect to the endolymphatic sac.

An underinflated endolymphatic sac would imply an insufficient amount of endolymph- either because you’re not producing enough, you’re somehow leaking it, or you need extra for some reason  (extraordinarly large semicircular canals?  I don’t know).  The various vestibular structures can’t have their fluid levels too tightly coupled, because that would ruin their specificity, but they are connected via endolymph ducts.  It would (famous last words) make sense if they all had some ideal fluid level, and overflow went into the endolymphatic sac.*

Dr. Internet has pointed me to many descriptions of how an excess of endolymph is bad for you.  And it’s not hard to imagine how a deficit could hurt you.  But is it possible to have enough for the vestibular system, and yet the endolymphatic sac is undesirably empty?  Unscientific Book suggests that because the sac is directly touch with brain fluid, information is reported to the brain through it, and that an underinflated sac can’t do this.  I can’t prove that’s not true, but it strikes me as unlikely to be a large effect.  If I had more faith in the book I’d poke around more, but for now I’m going to leave it.

*Last week I implied the ortoliths (linear motion detecting systems) contained only gel, not endolymph.  In my defense, I thought it was true.  I was wrong.


The magic of placebos

People say placebo effect like they mean “it didn’t work” or “they made it up”; It puts me in mind of Bill’s comment in True Blood: “No offense Sookie, but humans are shockingly susceptible to just about every form of thought manipulation.”The thing is, the placebo effect isn’t in our heads. It’s chemically measurable- through an increase in dopamine levels when told you were going to get an an analgesic, through an increase in basophil levels when injected with homeopathic (i.e. nonexistent) levels of histamine*, and through a changes in ghrelin levels when given different expectations about the calorie content of a shake. The original placebo effect- a decrease in pain when told a sugar pill was a pain medication-doesn’t work if you introduce an opiate blocker. My psych 101 professor said women could gain about half a cup size over six months through hypnosis. And I have seen multiple doctors that spotted things others didn’t, which led to greatly improved quality of life, that also believe in homeopathy.Which leads me to conclude that human brain is just an astonishingly powerful device that hasn’t yet figured out how to properly harness itself. Yet.*Extra interesting because most allergy tests use a saline injection as a negative control. Last time I had it done they told me which one was the control. I wonder what happens if they don’t.

Optimal hammering time

Last month I watched Home, a documentary about a charity offering homes to poor people (maybe just poor single mothers?) at a significant discount.  It focuses specifically on one woman who applied for help and the case worker assigned to her.   Watching it, I was struck by how much the case worker defined her goal as getting this woman this house, rather than helping her, or giving the house to the person to whom it would do the most good.   I thought it was a case of cargo cult, another friend described it as a cultural fixation on helping the poor by making them middle class rather than making being poor bearable.  Either way, it seemed to me like an example of misapplied charity.

Last week I started training to volunteer for a crisis hotline.  One of the things they drill into us is that most callers have a lot of problems we can’t solve.  We have very few tools:  occasionally we make referrals if they have certain  specific issues (e.g.  we’ll offer LGBTQ kids the number for the Trevor Project, or suggest they call 211 to get referrals to programs that could help their material problems), but mostly we listen.  That is what we do.  We are to apply that one tool as best we can.  If it helps, great.  If not, we end the conversation anyway.  There’s a weird tension between “Anything is a crisis if it feels like one to you.  We’re here to listen to anyone, any time, for any reason.”  and “Some people are just black holes, cut them off after 45 minutes.  But they can call back tomorrow.”

The only way I can justify this is by thinking “We have one tool.  It’s impossible to know if this tool is what this person needs.  Even when it is, there are diminishing returns to using the tool.  After 45 minutes, the marginal returns to further use are 0.  Therefore, treating everyone as receptive at minute 0 and no one as receptive at minute 45 is the optimal use of our time.”

I still think the case worker in the movie was pushing her tool too hard, and not listening when the person she was nominally trying to help brought up very reasonable concerns.  But I’m a lot more sympathetic to the myopia now.

The vestibular system

The library took away The Second Brain and there’s a long wait to get it back, so we’ll be putting digestive issues aside for a moment  and talking more about sensory stuff.  While I have a variety of issues that are in retrospect sensory, the driving complaint that drove me to seek treatment was misophonia.  You know how nails on a chalkboard or a crying baby creates a kind of aural pain you can’t block out?  Misophonia is having that reaction to sounds normally considered safe.   http://misophonia-meerkat.tumblr.com/ is full of people are driven to thoughts of self harm or even suicide by very common sounds, like breathing, low bass, and typing.   Luckily, I’m not nearly that bad.  But I also find it almost impossible to concentrate when people are talking, and work in an open office.

I have a new SI therapist (who I have yet to compare to a Guantanamo Bay interrogator, which makes her much better than the last one) who says my problem lies in my vestibular system, so let’s do some recon on that.  The vestibular system has two parts: a series of canals that track rotational acceleration, and two sacs that track acceleration in a single direction.  I wrote several paragraphs explaining exactly how these work, but ultimately I don’t think I added much over wikipedia, so I’m just going to summarize.  The canals and sacs are constructed differently but operate on the same physical principle:  if you have a rigid container holding a liquid, the atoms in the solid share momentum/inertia with each other but not the atoms in the liquid.  That’s why you can spill water by moving a glass very quickly, even if it’s perfectly level the whole time.  The canals and sacs detect this somewhat differently- the canals are filled with an electrically charged fluid (endolymph) that interacts with electrically sensitive hairs when you spin, which triggers a signal to the relevant nerve.  You have three canals, each of which detects rotation in a different plane.  Do not bother reading the descriptions of which plane each covers, they do not make any sense.  Instead, read this article on the axes of rotation for airplanes.  The theory is equivalent and the pilots have much better diagrams.

The sacs have a number of stones or crystals sitting in a gel, and nervous system is triggered when the stone hits the sac wall, bending the hairs on it.   Both sacs provide information on both horizontal and verticle movement, but the uticle is more sensitive to vertical movement and the saccule more sensitive to horizontal.

Note that both of these track changes in movement, not movement itself.  In order to track speed, your brain needs to keep track of your existing state and then calculate the change indicated by the vestibular organs.  It then automagically triggers the necessary changes in posture and eye movement to keep you upright and looking at your target.  That is why you can read when shaking your head, but not when someone moves your book.

Motion sickness is caused by a disagreement between your vestibular system and your eyes about how fast you are moving, which means my trick of looking at a fixed point in the vehicle was exactly wrong.  What I need to do is look outside so my eyes track motion.

At this point, I have two questions:  what can go wrong in these systems, and how to do they relate to misophonia, which is a hearing thing, not a balance thing?