Part Of: Neuroanatomy sequence
Followup To: The Thalamocortical Plasma Globe
Content Summary: 1100 words, 11 min read

Cortical Area & The Obstetric Dilemma

Last time, we learned that the brain is organized like a plasma globe: a sphere within a sphere. Today, we’ll be exploring a technique for reasoning about the cerebral cortex, or “outer sphere”. A few things you should know about this organ:

• It weighs about a pound.
• Stretched flat, it would cover an area of about 2.5 square feet.
• It is about six millimeters thick.
• It houses 20 billion neurons.

Does your neocortex have the most neurons? No, that title goes to the cerebellum, whose 100 billion neurons coordinates complex movements. But your brain must do a lot more than motor fine-tuning. Your brain perceives its environment, identifies objects, sets goals, makes decisions, feels emotions, and experiences consciousness. Where does your brain perform these tasks?  Primarily in the neocortex! Loosely speaking, the outer sphere does much of the “heavy lifting” for your brain.

Why is the human brain so wrinkled? After all, not all species have brains with this shape:

Consider the following evolutionary pressures, together known as the obstetric dilemma.

• A bigger neocortex does more work, and homo sapiens make its living (ecologically speaking) by intelligence. Thus, natural selection will select towards increased cortical area.
• Compared to other animals, human childbirth is a uniquely dangerous affair. Why? Brain size. Thus, natural selection will select away from increased brain volume.

As any microchip designer can tell you, wrinkles are a way to increase surface area, while holding volume constant!

Flattening The Lobes

And now, a short story. Cognitive neuroscience is rife with Latin terms for cortical areas. There are hundreds of them: “anterior cingulate cortex”, “fusiform gyrus”, “temporo-parietal junction”, etc. Over the past few years, as I consumed more of the field, I had slowly acclimated to hypotheses regarding the functions and interrelations for such areas. But given my lack of robust anatomical intuitions, these Latin names were just linguistic markers; I lacked an appreciation for geography.

While 3D models of different locations were mildly helpful, the pieces really fell into place when I discovered cortical flat maps.

While the neocortex is like a sheet, in some ways it is more accurate to imagine two sheets. Most people know that the brain has two hemispheres, but fewer know that these hemisphere’s are not (directly) connected? The two halves of your brain do talk to one another, but via a subterranean tunnel known as the corpus callosum.

Okay, here it is: a flat map of the human brain.

The brain is often divided into four to six different sections, or lobes. My map colors each lobe, illustrating the relationships between 3D brain and 2D map.

Here are my five lobes:

1. Occipital (red, back of the brain). This lobe is primarily responsible for vision.
2. Parietal (yellow, top of the brain). This lobe is primarily responsible for touch.
3. Temporal (green, sides of the brain). This lobe is primarily responsible for concepts.
4. Frontal Lobe (blue, front of the brain). This lobe is primarily responsible for personality.
5. Limbic Lobe (purple, underneath the brain). This lobe is primarily responsible for emotion.

Flat maps are an underappreciated resource. But I’ll return to this point some other day. In the meantime, I should mention that these flat maps are imprecise elaborations of the flat maps created by the Gallant Lab. That is, my creations strike me a bit like this:

I am planning to ultimately construct more precise flat maps using tools like Freesurfer.  But in the meantime, I’ll stick with my quick-and-easy 16th century cartographic approach. 🙂 

Primary vs Association Cortex

Let’s turn now to perception. In Tunneling Into The Soup, we discussed how sense organs (e.g., the rods and cones in your eye) translate the physical reality outside your body into neuron-compatible signals that your brain consumes. To make this intuitively compelling, I employed the following metaphor:

Your eye transmits data to your brain via information highways known as the optic nerves. The traffic of these highways – sense data – captures only a subset of physical reality; call this subset an umwelt. Such highways drain their contents onto landing sites on the cortex known as primary areas.

Aristotle is wrong: you have more than five senses. Balance and body alignment are senses in just the same way as sight, hearing, touch, taste, and smell. I am currently aware of nine different types of sense, or sensory modalities.  But of these, three sense modalities are particularly important (and consume more space!) in the human brain. They are:

1. Body sense-data. This area includes touch, and other senses associated with body sensation and position.
2. Visual sense-data. Nerves that leave your eyes deliver their data here.
3. Audio sense-data. Nerves that leave your ears deliver their data here.

These three primary areas of your cortex are marked in dark green:

Non-primary areas of the cortex (areas that are not dumping grounds) are called association areas. Association cortex is associated with two distinct functions:

1. Non-modal computations like goal generation.
2. Multi-modal computations such as “hand-eye coordination”.

Evolutionary Considerations

How does the human brain compare to that of other species? Consider again the mouse brain. It is obviously smaller than the human variety, but is the ratio of primary-vs-association cortex preserved?

It turns out that the answer is no. Because primary sensory processing is more immediately useful to survival, the neocortex of the mouse is actually dominated (>50%) by primary areas.

This biological fact reminds me of the (amusing) Expanding Earth conspiracy theory. In contrast to plate tectonics, the Expanding Earth claim is that continent size has remained constant, whereas the oceans have been expanding. Despite being complete bunk, it does provide a colorful metaphor to our genetic distinctions from the mouse. Our species has invested more heavily in association cortex (oceans) than primary cortex (continents).

Have humans invested equally in every part of the association cortex? No: the hominid line shows pronounced (6x) gains in the prefrontal cortex, and comparatively restrained growth elsewhere. We thus have reason to believe that most uniquely human behaviors must be supported by the prefrontal cortex. To be continued.

Takeaways

• The outer sphere of your neural plasma globe – your neocortex – is responsible for nearly all of your higher thought.
• Your neocortex covers an area of about 2.5 feet. Flat maps of the neocortex allow us to intuitively visualize the brain.
• Your neocortex contains primary (direct sensory input) and association (abstract computations) areas.
• Humans have more association areas, especially in the prefrontal cortex. This hints at a basis for our unique abilities.

Part Of: Neuroanatomy sequence
Content Summary: 400 words, 4 min read

Today, we embark on a (very) brief tour inside your head!

How Neurons Work

What is a brain? A brain is a collection of 120 billion neurons.

What is a neuron? A neuron is simply a cell. Since you are a eukaryote, all cells in your body – neuron cells, skin cells, etc – have a lot in common, including:

• cell body (environmental barrier)
• nucleus (genome storage)
• mitochondrion (energy production)
• endoplasmic reticulum (protein production)

In addition to these shared features, specialized tendrils (dendrites and axons) set neurons apart, giving them their web-like shape.

Neurons are useless individually. But when chained together, they may transmit pulses of electrochemical energy known as spike trains. input dendrites receive such signals and – sometimes! – fire, pushing the outgoing signal through the output axon. The output tendrils of a neuron connect with the input tendrils of other neurons; these connections are called synapses. 

In this way, neurons mediate a relationship between input and output signals. What other kinds of things do this? Mathematical devices known as functions, and electrical devices known as transistors.

Gray Matter vs White Matter

Let’s zoom out to consider the entire brain. A brain has two symmetric halves (the hemispheres) connected by a bridge (the corpus callosum). Here’s what it looks like from the inside:

Certain parts of the brain are gray, and others are white. Why? What happens when you put these under a microscope?

• Gray matter turn out to be dense clumps of neuronal cells.
• White matter aren’t made of whole neurons at all, but axons coated in a fatty substance (myelin). These tissues accelerates neural signals.

If white matter doesn’t process information, why is there so much of it?

The Neural Plasma Globe

To answer this question, we turn to the gray matter. Where does it live? In only two places: the wrinkly surface of the brain (the cerebral cortex), and in an evolutionarily ancient part of the brain (the thalamus, and neighboring midbrain structures). Think: spheres within spheres. 

We can see now why there is so much white matter. White matter comprises information highways which transport information to and from different areas of the cerebral cortex. Some of these highways directly connect cortical regions, but much travels through the thalamic “central hub”. We will call such highways thalamocortical radiations.

With tractography (a sister technology to the MRI scan), we can now directly visualize such radiations. There is something almost poetic about these highways… you could say that the brain is like a plasma globe.

 

 

Next time, we’ll explore some implications of this metaphor.

Part Of: Attention sequence
Content Summary: 800 words, 8 min read

No one denies attention is intimately related to consciousness.  But how much do you know about attention? Does it feel like a synonym for consciousness?

Today, we will be learning about some interesting software suspended above your perceptual systems: the attentional spotlight.

Foveal Vision and Saccades

To understand visual attention, I first need to tell you about vision.

Your eyes contain rods & cones. Rods detect light intensity, cones detect light color. 

Rods and cones are not distributed evenly. Cones live right behind your pupils, rods live everywhere else.

As this graph makes clear, there are two different types of vision:

• Foveal Vision, located at the center of your field of vision. This system collects detailed information about an object, including the color.
• Peripheral Vision, located at, well, the periphery. This system computes the “gist” of a scene, and detects movement.

Why is there a hole in the above pictures? This blind spot is due to your optic nerve intruding on your retina. Natural selection can sometimes design itself into a corner. Animals inhabiting different evolutionary lineages, such as octopi, do not suffer this problem.

Foveal vision is very narrow. In order to get more detail, your eyes have to move. Specifically, your foveal spotlight needs to travel from point to point, slowly adding detail to your understanding of a scene.

Such eye movements are known as saccades.  Saccades usually operate subconsciously.  With modern imaging technology, we can graph precisely how your eyes move over an image. Consider, for example, a saccade trace over the following portrait:

Why are saccades drawn to the eyes? Let me answer that question a bit later.

Let’s return to the question of (foveal) color vision. I’ve shown you two distinct features in this system:

• Foveal Spotlight: only objects in the very center of your vision are in clear focus.
• Blind Spot: you are literally blind in one particular spot in your field of vision. (If you like, you can prove this to yourself).

Crucially, neither the spotlight nor the blind spot enter conscious awareness:

This is a bit unsettling. We literally see less than we think. Somewhere between perception and the Mental Movie, our brains inject an illusion of transparency.  Such “false wholeness” effects should give us pause.

Overt vs Covert Attention

We can now explain visual attention! 🙂 Suppose I give you the above picture and say, “pay attention to the shape of her nose”. How do you respond?

Your eyes would most likely trace a saccade until your foveal spotlight was centered above her nose. A detailed image of her nose would then probably appear in your Mental Movie. Are we done? Is this an adequate description of the biology of attention?

Before declaring victory, consider sound. Imagine deciding to pay attention to the cello section of an orchestra. Is there an acoustic equivalent of a saccade, through which your body can amplify this particular frequency? Absolutely not. (Tilting your head won’t work, because that addresses your ability to detect directional sound).

Another counterexample comes from proprioception, the sense of the orientation of your limbs (as reported by nerves in your muscles). Here too, there is no way your body to “zoom in” on any limb. Yet it is possible to hold specific body parts in the center of your conscious experience.

Thus, attention doesn’t require assistance from mechanisms like saccades. The attentional spotlight is thoroughly independent. We can call unassisted cases of attention (e.g., sound) covert attention, and “enhanced” versions of attention (e.g., vision) overt attention.

This approach is confirmed by a careful study of visual experience. The foveal spotlight and the attentional spotlight can come apart. While these two spotlights often converge, sometimes they attentional spotlight will broadcast the contents of peripheral vision (as any driver can attest). Inattentional blindness is another striking example of the two spotlights diverging: even surprising stuff – like a gorilla – can escape detection, despite their placement within the foveal spotlight.

We can now improve our understanding of overt attention. In vision, overt attention moves the foveal spotlight (“green flashlight”) so that it follows the attentional spotlight (“orange flashlight”). In this ways, your eyes seek to “enhance” your conscious experience by feeding it more detail.

Takeaways

• Foveal and peripheral vision create detailed and coarse descriptions, respectively.
• Your vision system have two flaws: a blind spot, and a narrow foveal spotlight. 
• Attention is also a spotlight, delivering a subset of experience to the Mental Movie.
• Often, the attentional spotlight moves on its own (covert attention).
• Sometimes, the attentional spotlight is amplified by e.g., saccades (overt attention).
• Inattentional blindness shows how the foveal spotlight separates from the attentional spotlight.

Next time, we will examine how visual attention relates to consciousness.

Core Sequence

• Binding Problem: The Language of Consciousness
• An Introduction To Phase Locking
• Consciousness as a Learning Device

Neurobiology

• Two Cybernetic Loops
• ERTAS: The Engine of Consciousness
• Reentrant Loop Topology

Selfhood Sequence

• An Introduction to Confabulation

Attention sequence

• An Introduction To The Attentional Spotlight
• Attention As Gatekeeper
• Winner Take All: How Meat Decides
• Salience Maps: The Auction For Awareness

Sleep sequence

• Glymphatic System: Why We Sleep

Philosophy of Consciousness:

• Qualia: Can Consciousness Be Explained?
• Perceptual Objects: Implications for AI and Philosophy
• Philosophy of Mind sequence [2 posts]

Older Content

• Baars: The Conscious Access Hypothesis
• [Review]  Jonathan Evans – In Two Minds: Dual Processes and Beyond
• [Quotes] Alan Baddeley: Working Memory, Thought, And Action

Part Of: Demystifying Consciousness sequence
Content Summary: 1000 words, 10 min read

Debates about consciousness are as old as Western Civilization itself. Here, we survey 2000 years of intellectual history in 1000 words. 🙂 Wish me luck!

Movie and Subject

We begin with what consciousness is not.

• Consciousness is not mind. Consciousness is part of mind, but mind including many other algorithms which operate outside of consciousness.
• Consciousness is not conscience. Our moral circuitry does it computations elsewhere.

Let me instead put forward a metaphor. Consciousness feels like the movies. More specifically, it comprises:

1. The Mental Movie. What is the content of the movie? It includes data captured by your eyes, ears, and other senses.
2. The Mental Subject. Who watches the movie? Only one person, with your goals and your memories – you!

On this view, to explain consciousness one must explain the origins, mechanics, and output of both Movie and Subject.

It would be easy to get such a theory wrong. If we aren’t careful, our metaphor for consciousness might start to look like this:

But this literal interpretation (the Cartesian Theater) is nonsensical: if some person truly was watching the Mental Movie, where does its consciousness come from? 

Thus, on pain of infinite regression, a theory of consciousness must ensure that the Subject is itself unconscious.

Phenomenal Consciousness

But how could scientists possibly hope to produce such a theory? After all, science is in the business of explaining publicly accessible data. But consciousness is private!

Let’s be specific. Recall your experiences of color. Every photon has a wavelength. Now, suppose you see a flashlight producing light of 700 nanometers. Two things happen:

1. You experiences the distinctive color of red.
2. You can truthfully report that “that light beam is red”.

Take a moment to recall how this second ability comes from. In infancy, red light presents itself frequently. The infant brain slowly consolidates these redundant sensations into a single concept RED within semantic memory. RED also contains facts about apples, strawberries, and the sunset. Later, as Word Comprehension software comes online, the word “red” (and its accompanying audio signature) are bound into RED to enable communicating with other people.

But let’s imagine your friend has some kind of strange mental disorder which “inverts” all the colors of her mental movie. The experience of red to her feels like the experience of violet to us. How would she tell us about her condition?

If you take a moment to chew on this hypothetical, a truly frightening outcome comes into view. Your friend could never learn she has a disorder. In childhood, she would acquire the exact same facts about 700 nm light, and learn the same word to denote this sense data. She would be in full agreement with her peers when they say, for example, “that apple is red”. She would pass any color test. And yet, something unsettling remains…

Concepts like RED are studied by computer scientists all the time. But, at least in our thought experiment, the experience of red feels private, even untestable.

The experience of red is known as a quale, and learning more about qualia is one of the central occupations of philosophers of mind. The subject is put most forcefully in David Chalmer’s Conscious Mind, where he coins the phrase The Hard Problem. Indeed, it is hard to see how someone could go about even beginning to construct an solution.

Let’s call this experiential view of consciousness, phenomenal consciousness.

Access Consciousness

Let me now introduce another key figure in philosophy of mind: the philosophical zombie (p-zombie). Imagine one of your friends wakes up and, while behaving exactly as they would have before. If asked, your friend would still claim to feel the same etc, but his experience would be… well, he would have NO experiences.

Now, is such a thing possible? To answer, it helps to distinguish between two kinds of possibility:

• Nomological Possibility: is X possible in our universe with our universe, our idiosyncratic particle soup?
• Metaphysical Possibility: could X be possible in some universe with some completely alien laws of physics?

If p-zombies are nomologically possible, if a pill for behavior-neutral quale removal could be synthesized, then perhaps we pay ought to give credence to solipsism (“what if I am the only conscious being in the whole world”) and substance dualism (“what if my soul just floats over my body, choosing to reflect its states by magic”).

But, while p-zombies may be metaphysically possible, they are not nomologically possible. Consciousness is an adaptation: neurons and qualia are causally interwoven, and you cannot remove consciousness without crippling an organism. 

The remainder of this sequence will focus on making good on the last sentence. In the meantime, let’s give this functional interpretation of consciousness a name: access consciousness.

Retiring Armchair Philosophy

To sum up, we have identified two different views of consciousness:

1. Phenomenal consciousness, which focuses on experience.
2. Access consciousness, which focuses on causal signature.

While philosophical debate surrounding the former has consumed millenia, scientific research into the latter is only a few centuries old. Plainly stated, the science of consciousness has been making huge strides in recent decades, and I intend to share its results.

It is my view that conceptual analysis – armchair philosophy – can only get you so far. The empirically informed will inherit the earth. We live in the age of the neural correlates of consciousness, an age where polymaths weave together seemingly disparate theories into architectures, which tower above the speculations of their ancestors. 

This sequence presents a solution to access consciousness. Its cognitive structure will provide tools for reasoning about phenomenal consciousness.Like most of my writing, its contents are not uniquely my own. In fact, this sequence’s main purpose is to sketch an emerging consensus.

Until next time.