Questions about the brain

a neuroscience listicle

The Disintegration of the Persistence of Memory by Salvador Dalí

Even with all the recent advances in technology—cars that can drive themselves and chatbots that are ~indistinguishable from humans—I still think the brain is the coolest thing ever.

I also think it’s good to write down your open questions. The questions you ask are a window into your mind: they contain an endless matrix of preconceptions and assumptions; they reflect what you think is interesting and valuable. They form a time capsule that your future self can look back on with nostalgia (and embarrassment).

Every now and then the world is changed by a question no one had dared to ask before. Newton asked why an apple falls perpendicularly to the ground rather than sideways or upwards, and out came his universal theory of gravitation.¹ Einstein asked what it would be like to ride a beam of light, and revolutionized our understanding of space and time, which later helped us build satellites and land on the moon. The next great scientific revolution is hidden under a question we haven’t bothered to ask yet.

Some people say there are no bad questions. I think there are bad questions, but it’s only by asking the bad questions that you can start to figure out what the good questions are. And it’s only by writing your questions down, by speculating and discussing them with others, that you’re forced to think more deeply about them. You have to be willing to entertain questions that seem silly or obvious to discover something new.

In the spirit of indulging our curiosity—of asking questions both good and bad—here are some questions I have about the brain, paired with commentary and speculation.

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Q: How come even when we are not doing anything that requires thought/mental effort, our mind goes on continuously thinking about things, wandering in random directions, instead of just sitting silent? How come we have to work to get our mind to quiet down?

I’m so used to the concept of a wandering mind that I never bothered to ask this question until I saw it in a book. One proposed answer is that mind-wandering helps us with memory consolidation—e.g. we replay recent experiences in order to “sharpen” them and differentiate them from other memories, helping to make each individual memory more discrete.

Another way I tend to think about this is that mind-wandering is our subconscious’s way of “bubbling up” memories/thoughts it deems important to our conscious awareness. And the subconscious tends to do this more often when we are actively suppressing some part of ourselves, e.g. we are resisting an emotion we don’t want to feel. The more tension we hold, the harder it is to keep our mind still.

Q: Among those of us who read a lot of novels, how come we never confuse events that actually happened in our real life versus events we “experienced” by reading them in a vivid story? As entertainment gets more immersive (e.g. mixed reality) are we likely to make this kind of mistake more often?

When I posed this question to a friend, he pointed out that some of us do confuse things that happened to us with things we heard about, especially for distant memories. But I still think it’s significant that this confusion doesn’t happen more often. Here’s one answer for why: when we directly experience something, it’s a much richer experience than having a story recounted to us, and this makes it fairly easy to distinguish things that actually happened from things we read about.

Contrary to this view though, there was an interesting case study of a patient who, after a brain injury, could still remember all his memories, but no longer felt like they were his memories. They just felt like stories that he had read or heard about from someone else. As his brain gradually recovered, individual memories would start to feel like “his own” again, one by one. This suggests that it might not just be the content of the memory (e.g. its vividness) that tells us whether it was our own, but some additional “I-was-there-at-the-time” property that our brain attaches to it, which can be selectively removed.

Q: How come we are never surprised by the non-sequiturs in our dreams (and in our trains of thought)?

You’ll be dreaming of being in line at the airport and the person behind the counter will be exchanging your passport for ice cream, and on your left a gorilla will be taking away your baggage, and all of this will seem totally unremarkable. How come we never stop and go “wait, I wouldn’t expect there to be a zoo inside an airport”?

I have a similar question for when we’re awake: why are we never surprised by the absurd non-sequiturs in our trains of thought? You’ll be eating chicken nuggets, thinking about which TV show to watch, and suddenly remember that you forgot to respond to your friend’s message three days ago, and it will not appear at all strange to you that this thought entered your mind out of nowhere.

A friend who’s a neuroscientist pointed out that the answer here might have to do with working memory. When a thought to respond to your friend’s text enters your mind unprompted, it pretty much displaces whatever else was there, so you momentarily forget that you were trying to figure out what show to watch. (And then you only remember again once you notice the TV in front of you.) Note that working memory is extremely limited.

When dreaming, we have even less working memory, which leaves us with less room to make the observation that there is no legible connection between what happened one second ago and what’s happening now. Another general property of dreams is that your higher-level cognitive faculties (system 2?) are less functional, so you can’t think critically about what’s in front of you.

Q: Why is it that as we learn vastly more facts over our lifetime, the “lookup time” for any specific fact or memory remains the same?

Although we forget many things, the facts we do remember are all still ~instantly accessible, even when there is ten times as much of them as there was before.

I was surprised to discover that we have not found any upper limit on the capacity of our long-term memory. Which makes it even stranger that lookup is still so fast. Like if you imagine a database on a computer, the more entries that database has, the harder it will be to search for the specific item you want from the database. Of course, this all depends on the implementation of the database—there are ways to organize it ahead of time to make lookup really fast. Which then just begs the question of how the brain organizes information to make retrieval efficient.

Q: How does the brain “compute”?

An individual neuron can fire signals every millisecond at most, and it takes at least 10ms for one neuron to transmit a signal to an adjacent neuron, and often longer.² How does the brain do things like register a unified experience of the world within 500ms, which is something computers still can’t do with processors that are a million times faster?³ The brain seems to be making up for this with enormous parallelism: while your laptop might have about a dozen CPU cores, the brain has a hundred billion neurons.

Of course, this question makes the assumption that the only kind of “computation” the brain does is at the level of connections between neurons, which I’m not sure is true. For example, there is good evidence that neurons are not the only kind of cell in the brain that communicates with electrical signals—astrocytes do too. Also, once you read more about the inner workings of neurons you realize that an individual neuron is doing many things that could conceivably be described as “computation.”

Q: How come the brain, composed of a hundred billion neurons all distributed at different points in space, and thus needing many milliseconds to propagate signals to each other, is able to generate the experience of a single, unified, momentary “now”?

This is a version of the binding problem. Despite reading several books and papers on consciousness I don’t have a good idea of where to start with this one, but Andrés Gómez-Emilsson is converging on an interesting answer in this video and paper.

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Thanks to Amanuel for notes on an earlier draft.

1. Whether Newton’s insight was actually sparked at the moment that he saw an apple fall isn’t a totally settled historical question but I enjoyed this quote from wikipedia:

“why should that apple always descend perpendicularly to the ground”, thought he to him self: occasion'd by the fall of an apple, as he sat in a comtemplative mood: “why should it not go sideways, or upwards? but constantly to the earths centre? assuredly, the reason is, that the earth draws it. there must be a drawing power in matter. & the sum of the drawing power in the matter of the earth must be in the earths center, not in any side of the earth. therefore dos this apple fall perpendicularly, or toward the center. if matter thus draws matter; it must be in proportion of its quantity. therefore the apple draws the earth, as well as the earth draws the apple.”

2. In chapter 5 of Neuroscience: Exploring the Brain: “Fast forms of synaptic transmission last from about 10–100 msec…Slower forms of synaptic transmission may last from hundreds of milliseconds to minutes.” (p. 122)

3. If we think of the “processing speed” of a neuron as 1Khz (1000 action potentials per second), this is a million times slower than a CPU that clocks at 1GHz.