Dr. David Linden, Ph.D., (@david_j_linden) professor of neuroscience at Johns Hopkins University School of Medicine and the author of many popular books about the brain. Check out his latest book, Unique: The New Science Of Human Individuality.
In this episode of Huberman Lab, Andrew Huberman and David Linden discuss what accounts for differences across individual people, the connection between mind and body and the impact on our health, the link between inflammation and depression, and more related to psychology and human development.
Host: Andrew Huberman (@hubermanlab)
Scientists only recently identified the nerve endings responsible for sexual sensation in genitals
The sexual sensation may be experience-dependent
Much of science seeks to find general principles, ignoring individual variation
  • For example, the olfactory system has about 400 receptors; DNA is highly variable from individual to individual – there are odors some love, others hate; some can detect, others can’t
People are highly individual and contextual when it comes to odor
  • There are only a few odors that are innately aversive – other odors are culturally adapted and learned
There is no purely objective world – all senses are based on experience, expectation, and the situation at the moment
Uniqueness in visual perception: kids who don’t get outside much are much more likely to be myopic (nearsighted) when they grow up than kids who go outside
Heat-tolerant people have more of a certain type of saltwater sweat gland innervated
Evolutionary change is slow but phenomena set by life experience can change within a generation
Perfect pitch has low heritability – but – ear training at a young age will enhance the likelihood of developing perfect pitch
Very few traits are entirely predictable based on heritability
  • Most traits are a combination of heritable and social
Genes code for proteins, not traits; genes in the body are rarely solely responsible for one thing
  • For example, there’s a gene that determines whether your ear wax will be wet or dry (with 100% heritability) but it also has many other functions in the body
Speech accent is 0% heritable – it’s entirely dependent on the speech of your peers, not family
Height is about 85% heritable with the rest attributed to random variation, nutrition, health
IQ tests administered by trained psychologists aren’t perfect but are predictive of later success
In developed countries and places not burdened by war or famine, IQ test scores are 60-70% heritable
  • In people who are poor, even in developed countries, IQ tests are less heritable
Good schooling correlates with good medical care, less traumatizing neighborhoods, parents in the home, etc.
Heritability is estimated by comparing identical twins against fraternal twins but with a major assumption that kids raised in the same family have the same environment
  • A more powerful way to estimate heritability is to look at twins reared apart
 Personality traits tend to be about 50% heritable and 50% from random development of the body and nervous system (nothing to do with family)
“The genome, all your DNA…don’t make a blueprint for making your body and brain…rather, it’s a rather vague recipe.” – Dr. David Linden
The current evidence of epigenetic inheritance of trauma is skim – there isn’t a biological mechanism and much of the theory comes from research that’s done after the fact
  • You’re probably inheriting stress and trauma socially, not necessarily biologically
Children of women in 1918 who got the flu (in the 1918, pandemic) while pregnant are shorter
  • Incidence of schizophrenia, autism (though not called autism at the time) also went up
First-trimester hypothesis: if a pregnant woman gets the flu in the first trimester of pregnancy, there’s a higher incidence of schizophrenia and autism
  • This is possible because interleukin 17 causes disordered development of layers of the cortex, which is anecdotally seen posthumously in autism
We don’t know the effects of COVID and other illnesses on children in utero
The cerebellum (AKA mini-brain) is involved in balance, timing, cognition
The cerebellum is one of the most complex regions of the brain
Fun fact: when we read, we’re speaking it in the brain just not in the region that controls speech
Cerebellum is involved in motor coordination, but its very close proximity to critical regions allows it broadly to function as the region of the brain responsible for trying to predict and react to a future event, or the next thing that’s going to happen
  • Damage to the cerebellum causes some clumsiness and poor coordination
The cerebellum evolved to function in non-motor capacities (though we previously understood it in a coordination capacity)
“I have a lot of problems with nature versus nurture as an expression.” – Dr. David Linden
Nature is meant to mean heritability and nurture is intended to mean your environment and how you were raised but “nurture” is too narrow
“Nurture” could more practically be replaced with “experience” in the broadest sense – the foods eaten by the mother in utero, bacterial population of the gut, etc. starting from the earliest development
Nature and nurture are not working in opposition; experience and heredity act in both complementary and opposing ways  
The nervous system extends into the body and the body sends neural signals back to the brain
“The idea that our thoughts can influence our body and our bodily state can influence our thoughts is fortunately just not understood but seems to be accepted and appreciated.” – Dr. Andrew Huberman
Two ways to get signals to the body: (1) interoceptive neurons – neural signals in the body; (2) hormonal chemicals
Research in immune signaling molecules (cytokines, signaling hormones of the immune system) in the brain is emerging and will be highly fruitful
Inflammation and depression theories: (1) inflammation sensing neurons send electrical signals to the brain; (2) immune signaling cytokine molecules produced at the site of inflammation travel to the brain
  • Depression is not easily trackable via pharmaceutical intervention because only 1/3 are helped by traditional SSRIs; there is some evidence that people suffering from treatment-resistant depression get relief by reducing inflammation
Relief from depression has to do with neuroplasticity
Plasticity in the brain can be intrinsic, synaptic, or morphological  
“Exercise is about as good an antidepressant as SSRIs and the side effects are only good side effects as opposed to SSRIs.” – Dr. David Linden
There are two potential pathways conveying signals from the brain to the body
Innervated neurons can affect the progression of cancer through their activity – it’s reasonable to hypothesize that certain tumors may respond to meditative practice
  • It’s speculative but biologically supported
Effects of breathwork and deliberate respiration on health are now being widely studied for effects on mental health, physical health, illness
The problem is there has been a lot of woo science in the past, tarnishing the reputation of the very real brain and body connection
You can record many places of the brain and find a signature of the breathing rhythm – it’s reasonable that breathwork has real impacts on the brain and its function
A few years ago, David Linden learned he had a very rare cancer: heart cancer
Lessons: perception of time is malleable – context changes everything, genuinely engaging with own demise is fundamentally difficult because the brain is always trying to predict the future, gratitude for the big things in life (profound love for family)
His negative emotions and sadness about death are more about the idea of leaving loved ones behind and what they will feel
Advice based on this experience: appreciate what you have while you have it
Ben Barres (Nature)
A Neuroscientist Prepares for Death (The Atlantic)
Can a Neuroscientist Fight Cancer With Mere Thought? (The New York Times)
The Autobiography Of A Transgender Scientist by Ben Barres
Dr. Leslie Vosshall
Dr. Catherine Dulac
Dr. Rachel Herz
Dr. Karl Deisseroth
Dr. Gloria Choi
Dr. Irving Zucker’s Publications
Dr. Rena Malik: Improving Sexual & Urological Health in Males and Females
Dr. Oded Rechavi: Genes & the Inheritance of Memories Across Generations
Dr. Eddie Chang: The Science of Learning & Speaking Languages
Dr. Robin Carhart-Harris: The Science of Psychedelics for Mental Health
Dr. Matthew Johnson: Psychedelic Medicine