Which is the most evolutionarily advanced part of the human brain?

This is a potentially controversial issue, since there is no consensus yet on the evolution of the brain, beyond a very coarse-grained chronology. Broadly speaking, neocortical areas are new, hence the term “neo-cortex”. But among cortical areas, there is still some disagreement about which areas emerged most recently in primates.

Based on what we know about development in the womb, along with structural findings, my labmates, who are neuroanatomists, suggest that the “eulaminate” areas — the ones that have sharply defined layers — may be the most recent, evolutionarily, compared to the “agranular” and “dysgranular” cortices, which have less sharply defined layers. These less sharply defined areas are also labeled as “limbic”.

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If serotonin deficiency isn’t the cause of depression, then why do SSRIs work?

Acetaminophen (a.k.a paracetamol) relieves some types of headache. But this does not mean that these headaches are caused by acetaminophen deficiency. The brain doesn’t even produce acetaminophen.

The point of this analogy is to make clear that a medicine can work even if it is not acting on the cause of the symptom. In many cases a medicine can work even when the cause of the symptom is completely unknown.

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Why is the brain so sensitive to early life experiences?

I was asked this question on Quora:

From an evolutionary standpoint, why would the early years of brain development be paramount in determining life-long neurological patterns, when those patterns can often be detrimental to long-term success in life?

Good question. We can restate it as follows:

Why would natural selection allow animals to be so sensitive to negative early experience?

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How many minds can one organism have? (Answer: One.)

I was asked this question on Quora:

Do the two halves of the brain have wills of their own? Can they dislike each other or fight?

Here’s my answer:

It’s very difficult to define what exactly the will is in neuroscientific terms. It seems best to reserve this word for the whole organism, rather than some part of it.

Some philosophers would say that attributing will to some sub-component of an organism is an example of the “mereological fallacy”. It’s like saying the stomach eats, or the brain thinks, or the legs walk. We use these kinds of phrases as a kind of poetic shorthand, but only a complete organism can be said to eat, think, or walk.

In the case of the two hemispheres, we also know that the left brain right brain story — that one is “rational” and the other “holistic/artistic” — is wildly misguided. Some neural processes are lateralized, but most normal tasks that humans perform require close integration and communication between the hemispheres.

But we do have to make sense of a common experience — being “in two minds” about something. Most people know what it is like to be in a conflicted state — multiple goals or biases seem to be tugging at us. Clearly decision-making involves a sort of “parliamant” in the brain, in which multiple vested interests vie to enact legislation that suits them. 🙂

But the parliament metaphor should not be taken too seriously. There is little to be gained in anthropomorphizing neurons or groups of neurons. Neural ensembles might sometimes seem to behave as if they have a will, but that idea will not really help us understand decision-making, or the subjective feeling of having a will.

So brain areas don’t have likes or dislikes — organisms do, and brain areas mediate the processes by which these likes and disliked become manifest.

For more on the problems with anthropomorphizing neural processes, see these two essays I wrote:

Persons all the way down: On viewing the scientific conception of the self from the inside out | 3 Quarks Daily (This essay is partly a gentle critique of the Pixar movie Inside Out.)

Me and My Brain: What the “Double-Subject Fallacy” reveals about contemporary conceptions of the Self | 3 Quarks Daily (This essay explores the tendency of people, including neuroscientists, to think of the brain is a separate agent from the person as a whole.)

I admit that it is often fun to anthropomorphize neurons, which is what I do in the essay below. I paint a picture of a neural city and a neural economy, complete with start-ups and investors. 🙂

Be Careful What You Wish For: Some Wild Speculation on Goodhart’s Law and its Manifestations in the Brain | 3 Quarks Daily


Further reading

Yohan John’s answer to Is the left brain and right brain concept a myth, or is it true?

Yohan John’s answer to What happens to consciousness when the brain’s two cerebral hemispheres are disconnected?

A Clockwork Orange? (A brief musing on the concept of a neural “code”)

I was asked this question on Quora:

Are there many layers of neural codes from the human retina to the optic nerve and the optic nerve to the brain, or are they essentially same signals relayed?

Here’s how I responded:

Here’s a question: in a system composed of clockwork, is there a “code”?

I ask this because I find that the “code” metaphor is often misleading when thinking about biology. Codes are composed of symbols. But it is not clear that neurons communicate using symbols.

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