ERTAS: The Engine of Consciousness

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

Existential Mode Generators

In Why We Sleep, we discussed sleep architecture diagrams. These diagrams show clear electrical differences between three existential modes: NREM (“sleeping”), REM (“dreaming”), and Consciousness.

sleep-stages-across-night-3

While EEG excels at providing temporal resolution, it doesn’t provide much spatial information. Where does the brain construct these three modes?

To answer this, neuroscientists cut the brains of cats in half… literally. If you perform a Cerveau Isolé cut (slice above the midbrain), the top half’s electrical signature is NREM. If you do a Midpontine Pre-Trigeminal cut (slice below the midbrain), the top half’s electrical signature is NREM + Consciousness.

Consciousness Ignition- Localizing Circuits (2)

This evidence shows that existential modes are generated by different areas. Specifically:

  • Sleep is induced by the diencephalon.
  • Dreaming is initiated by the metencephalon.
  • Consciousness is ignited by the mesencephon.

Neuroscientists now knew where to look! It was not long before they discovered the machinery that create consciousness, sleeping, and dreaming:

Consciousness Ignition- Mode Localization (2)

We now turn our gaze to the ascending reticular activating system (ARAS).  “Reticular” is a word that means “web-like”, so the name roughly means “web-like ignition switch”.  But before we do so, we need to turn our gaze to the relationship between cortico-thalamic (CT) radiations and consciousness.

Thalamus Anatomy & Function

We have also explained that the purpose of consciousness is to solve the binding problem: gluing together disparate adjectives into coherent nouns:

Objects- Distributed Object Networks (2)

Consciousness creates the coherent objects of working memory by implementing phase binding, where object features are stitched together in distinct frequency bands, not unlike the radio in your car.

Objects- Phase Locking & Wakefulness

We have previously described the thalamus and cortex as dually innervating spheres, not dissimilar to a plasma globe:

Brain- Plasma Globe analogy (2)

And indeed, the nuclei within the thalamus tile the entire cortex:

Consciousness Ignition- Thalamic Architecture

Note, however, that only some thalamic nuclei are specific (project to discrete patches of cortex). Nonspecific thalamic nuclei are also present, including the Intralaminar Nuclei (ILN) and Reticular Nucleus of the Thalamus (RNT).

These nonspecific nuclei are the principal components of the ERTAS system, and plausible candidates for the engine of consciousness.

Damage of specific nuclei produce loss of a particular modality.  In contrast, lesions to nonspecific nuclei produces deep disturbances of consciousness. In fact, recent evidence suggests that such lesions perturb cortico-cortical information transmission.

The ERTAS Hypothesis

The ascending reticular activating system (ARAS) consists of a dense web of nuclei. Indeed, the word “reticular” means “web-like”. Parvizi, Damasio (2001) outline the more significant members of the system:

Consciousness Ignition- Mesencephalon Reticular Formation

These nuclei project to the following three sites:

  1. Reticular Nucleus of the Thalamus (RNT), a sheet that sits on top of the thalamus.
  2. Intralaminar Nuclei (ILN), which are embedded deep within the thalamus.
  3. Basal Forebrain, which receives & distributes several neurochemical systems.

These structures in turn route information flowing to cortex:

Consciousness Ignition- Thalamus ILC NR

The extended reticular-thalamic activating system (ERTAS) hypothesis connects the ARAS system with the phase binding interpretation of the cortico-thalamo-cortical reentrant loop. One hypothesis, adapted from Newman (1999), has three theses:

  • ILN performs phase binding (and thus, the consciousness generator).
  • RNT implements selective attention.
  • Basal Forebrain provides visceral “body-relevant” information.

ras2

More recent research has corroborated the role of the ILN in phase binding, and expanded its scope. Saalmann (2014) notes that the ILN seems to participate in a larger group of higher-order nuclei which each manage information within more constrained parts of cortex. The anterior ILN seems more related to oculomotor processes; the posterior deals with the multimodal integration of different sense data.

One unexpected recent finding has been that lesions of “higher-order nuclei” such as the ILN seem to perturb cortico-cortical information transmission. This underscores the need to understand interactions between the CTC Loop and other reentrant loops.

basal-ganglia-loop-geography-and-hodology-1

The Role of The Claustrum

The claustrum is a tiny sheet of gray matter suspended between thalamus and cortex. However, it receives information from essentially the entire cortex:

Consciousness Ignition- Claustrum Anatomy (2)

Given that the purpose of consciousness is to integrate cortical information, the anatomical position of the claustrum is suggestive.

Recent anatomical evidence has only strengthened the case for claustrum promoting consciousness:

  • Koubeissi et al  (2014) is a case study where they were electrical stimulation of the claustrum induced loss of consciousness (!).
  • Chau et al (2015) announced evidence that correlate claustrum lesions with the duration, but not the frequency, of loss of consciousness.
  • Wang et al (2016) conclusively proved that the claustrum has reciprocal connections everywhere in cortex.
  • Reardon (2017) announced the discovery of a single neuron whose dendrites encircled the entire brain (image credit)

Consciousness Ignition- Claustrum Mega-Neurons

These data are suggestive. However, it will be some time before we know enough to integrate claustrum function within the ERTAS system.

Until next time.

Related Works

  • Chau et al (2015). The effect of claustrum lesions on human consciousness and the recovery of function
  • Crick, Koch (2005). What is the function of the claustrum?
  • Koubeissi et al (2014). Electrical stimulation of a small brain area reversibly disrupts consciousness
  • Newman (1999). Putting the puzzle together: towards a general theory of the neural correlates of consciousness
  • Parvizi, Damasio (2001). Consciousness and the brainstem
  • Reardon (2017). A giant neuron found wrapped around entire mouse brain.
  • Wang et al (2016). Organization of the connections between claustrum and cortex in the mouse

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Two Cybernetic Loops

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

What Is Perception About?

Consider Aristotle’s five senses: vision, hearing, smell, touch, and taste. We know that senses are windows into physical reality. But what aspects of reality do these represent?

Vision and hearing have a special property: despite receptors being located within the body (proximal), they carry information about phenomena outside of the body (distal). They carry information about the world. In contrast, smell, touch, and taste only represent events close to the body; these encode the interaction between body and world.

This distinction is a neural primitive: the brain encodes World and Interaction in extrapersonal and peripersonal space, respectively.

However, there is a significant lacuna within this binary system: none of these concern the body. Body sensation is a crucial “sixth sense”:

twoloops-body-vs-world

Making Sense of Anatomy

We spend a lot of time discussing the nervous system. But the body houses eight other anatomical systems: reproductive, integumentary (skin), muscular, skeletal, endocrine (hormones), digestive (incl. urinary and excretory subsystems), circulatory (incl. immune and lymphatic subsystems), and respiratory.

To regulate these systems, your brain recruits the following peripheral nervous systems:

  1. Somatic, which contains spinal nerves and cranial nerves
  2. Autonomic, incl. the sympathetic “fight/flight” and parasympathetic “rest/ digest” 
  3. Neuroendocrine, incl. the HPA, HPG, HPT, and Neurohypophyseal axes
  4. Enteric, also called the “second brain”, a large mass of digestion-oriented neurons
  5. Neuroenteric, connects enteric nervous system via microbiome-gut-brain axis
  6. Neuroimmune, recently discovered, primarily mediated by glial cells
  7. Glymphatic, recently discovered, which removes metabolites via CSF during sleep
  8. Neurogaseous, recently discovered, mediated by gasotransmission

The CNS must coordinate all of these to respond to sense data and regulate anatomical systems. A complex undertaking. How might we understand such a process?

With the above trichotomy { world, interaction, body }, anatomical and sensory systems can be organized into meaningful categories:

twoloops-two-kinds-of-neural-phenomena-1

The Interlocking Loop Hypothesis posits the existence of two perception-action loops, inhabiting a gradient of abstraction:

  1. The somatic “cold” loop, world- and interaction-oriented, from exteroception to movement.
  2. The visceral “hot” loop, body-oriented, from interoception to body regulation.

Loops As Organizing Principle

Evidence for the Interlocking Loop Hypothesis comes from two anatomical principles of organisation:

First, the Bell Magendie Law is based on the observation that, in all chordates, sensory information is processed at the back of the brain, and behavioral processes are at the front (“posterior perception, anterior action”):

Cybernetics- Posterior Perception, Anterior Action

Second, the Medial Viscera Principle is the observation that visceral processes tend to reside in the center of the brain (medial regions):

two-loops-medial-viscera-principle-4

Thus we can see our loops clustering at different levels of the abstraction hierarchy.

We can also see our loops’ primary site of convergence:

Anatomically, the two loops converge on the basal ganglia, in which both somatic and visceral processes are blended to yield coherent behavior.

two-loops-intersection-at-basal-ganglia

The above quote & image are from Panksepp (1998), Affective Neuroscience.

The Basis of Motivation

Why should our two loops converge on the basal ganglia? The basal ganglia is the substrate of motivation, or “wanting”. It also participates in reinforcement learning, and its mathematical interpretation as Markov Decision Processes (MDPs).

Historically, the reward function in MDPs has proven difficult to interpret biologically; however, this task becomes straightforward on the Interlocking Loop Hypothesis. Of course the cold loop would tune its behavior to promote the hot loop’s efforts to keep the organism alive.

two-loops-motivation

The Basis of Consciousness

In Can Consciousness Be Explained?, I wrote:

Let me 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. (Of course, one must be careful that the Subject is not a homunculus, on pain of recursion!)

The Interlocking Loop hypothesis offers an obvious foothold in the science of consciousness:

  • The world-centric cold loop generates the Mental Movie (“a world appears”). 
  • The body-centric hot loop creates the Subject (“narrative center of gravity”)

Thus, we are no longer surprised that opioid anomalies (a visceral loop instrument) are linked to depersonalization disorders; whereas dopamine (the promoter of somatic behavior) is associated with subjective time dilation effects.

Takeaways

First, we introduced the Interlocking Loop Hypothesis:

  • Some perceptions are about the world, others are about the body.
  • The CNS is a visceral body-centric hot loop, and a somatic world-centric cold loop
  • Bell-Magendie Law: perception for both loops is posterior, action is anterior.
  • Medial Viscera Principle: hot loop is located medially, while cold loop is more lateral.

Then, we examined its implications:

  • Motivation, as generated by the basal ganglia, is loop communication software; it allows the hot loop to influence cold loop behavior.
  • Consciousness has two components: the Mental Movie and Mental Subject. These are supported by cold and hot loops, respectively.

Until next time.

Relevant Materials

  • Northoff & Panksepp (2008). The trans-species concept of self and the subcortical–cortical midline system

 

Attention as Gatekeeper

Part Of: Attention sequence
Followup To: An Introduction to the Attentional Spotlight
Content Summary: 600 words, 6 min read

Global Workspace Theory

The weakest noticeable sound is defined at 0 decibels. Imagine putting somebody into a scanner, and having them listen to two sounds:

  1. A trumpet playing at -5 dB
  2. A trumpet playing at 5 dB

The acoustic difference between the two waveforms are not very different. How similar are the patterns of brain activation?

Attention- GWT

Here we see that subliminal auditory stimuli only activate early perceptual areas. Consciousness brings with it a huge increase in neural activation! Why should this be?

Global Workspace Theory (GWT) posits that consciousness is involved in two mental operations:

  • Binding: perceptual features, distributed across the brain, are bound together into discrete objects
  • Broadcasting: these object networks are broadcast to the rest of cortex, allowing consumer systems to use & modify them.

Attention- GWT Architecture
Three properties of consciousness have long baffled philosophers:

  • Consciousness is small: we can only retain a few (less than 7) objects in our head at one time.
  • Consciousness is serial: we can’t read two books at the same time.
  • Consciousness is flexible: unlike state of the art AI software, human reasoning can effortlessly enter new domains.  

GWT explains these facts. Consciousness is…

  • … small because it is hard to keep global object networks distinct from one another.
  • … serial because it is a singleton: massively parallel modules engage the same centralized resource.
  • … flexible because any consumer system can augment the processing of any perceptual object.

The Role of Attention

Attention is a gatekeeper. Our perceptual systems process myriad sensory events, these must bid for entry into the Global Workspace. The brain contains circuitry that implements this selective process, choosing which perceptual objects to bind & broadcast.  

Attention- Gatekeeper Role

Let’s see if we can use this metaphor to make sense of the sprawling literature on attention.

Consolidating Taxonomies

There are three taxonomies of attention that you’ll find in the literature:

  1. Covert vs overt attention. As discussed in Attentional Spotlight, we can differentiate attending to objects in the periphery, versus saccading to attended targets.
  2. Bottom-up vs top-down attention.  Distinguishes unplanned attention (e.g., to loud noises) vs goal-based attention (e.g., “count the number of times the soccer ball is passed”).
  3. Feature vs spatial attention. Distinguishes attending to a feature (“look for all red things”) vs an object (“look for a red triangle”)

In an influential paper, Peterson & Posner present three attentional networks: functionally independent brain systems which do attention. These are:

  1. Alerting. This network is tightly linked to wakefulness. Startling events induces strong alerting, lounging on a couch less so.
  2. Orienting. These two networks (one dorsal, the other located more ventral) orients the organism to process incoming stimuli.
  3. Executive. This network supports complex task execution, and goal-oriented attention.

Peterson & Posner’s framework allows us to simplify the conceptual landscape:

Attention- Taxonomy Reduction

The Orienting network produces Bottom-Up (“externally-driven”) attention. Its dorsal arm contains mechanisms for covert and overt orienting.

The Executive network produces Top-Down (“internally-generated”) attention. Feature and Object attention are both a form of search template, and as such are constructed here.

An Attentional Organ

In my next post, I’m going to argue that the Dorsal Orienting network is the attentional gateway, full stop. It alone performs selection: a single gateway through which percepts pass into conscious awareness.

On this model, the arousal, ventral orienting, and executive networks play auxiliary roles, modulating our brain’s attentional gateway.

Attention- Architecture Overview

Until next time.

Baars: The Conscious Access Hypothesis, Origins and Recent Evidence

Article Details

Article: The conscious access hypothesis: origins and recent evidence
Author: Bernard J Baars
Published: 01/2014
Citations: 581 (note: as of 03/2014)
Link: Here (note: not a permalink).

Context

In 1988, Bernard Baars authored A Cognitive Theory of Consciousness, which presented his Global Workspace Theory (GWT) of consciousness. In short, he argues that consciousness is caused by global inter-brain sharing of information. This theory does not concern itself much with the construction of phenomenology, and thus does not qualify as a solution to the Hard Problem of Consciousness (which is well explained here).

Methodology

Scientific efforts to understand consciousness evoked vigorous philosophical objections. These were essentially the classic mind-body problems: how does private experience relate to the physical world? … Difficult conceptual questions are routine when the sciences turn to new topics. The traditional scientific response is simply to gather relevant evidence and develop careful theory. Ultimately, philosophical controversies either fade, or they compel changes in science if they have empirical consequences.

I like this quote. While it doesn’t encapsulate my sentiments on the role of philosophy, its call for empirical analysis was long overdue.

You may find yourself asking: how can neuroscience examine consciousness, if consciousness is private to the individual? Baars advocates using an operational definition of conscious awareness: consciousness is the ability to produce a reliable report. An example: suppose I flash a number (0-9) on your monitor, and then ask its value. Say I present the number three for 200 milliseconds. If I ask you what you saw, you would be able to report your conscious experience. But, say I present the same number for 2 milliseconds. If I then ask you what you saw, you would not be able to report the correct value better than a ten-sided die. By this means, I have acquired a variable that represents whether a task is associated with consciousness.

How can we causally distinguish between the effect of consciousness and, say, the effect of low IQ on a given task? Well, most neuroscientific inquiries into consciousness employ a technique Baars refers to as contrastive analysis. This technique involves comparing processes that induce conscious awareness only occasionally. Let’s suppose that, in the above example, 200ms corresponded to 98% correct reports, whereas 2ms corresponded to 3% of subjects being aware of the change consciously. I would then be tempted to “turn the display-time knob” so any one person has a 50% chance of perceiving the number, and then analyzing the differences between the two groups. To see an example of contrastive analysis beyond the above toy model, Baars cites Dehaene et al [1] as an exemplar.

A Philosophical Aside

It is, first, important to distinguish between operational definitions such as the above, and operationalism, which is a more extreme call to operationalize all scientific concepts. While the latter movement is today widely regarded as unhelpful, that doesn’t seem to problematize the desire to operationalize some definitions, such as consciousness or volition.

Let me sketch a problem that will be familiar to any philosophers. The question of philosophical zombie was memorably treated by Descartes: is it possible for a human being behave exactly as one who is conscious, reporting conscious experiences to anyone who may ask, but entirely devoid of an inner life? This metaphysical question has not been satisfactorily resolved. However, let us reframe this question in nomological terms: is consciousness causally linked to the human nervous system? If we provisionally accept the operational definition of consciousness above, we are in position to answer this question with data.

Evidences

The data seems to say yes. Consciousness hugely contributes to the functioning of our nervous system. In this paper, Baars sketches seven lines of evidence that have accumulated since his theory’s inception (1988).

  1. Conscious perception involves more than sensory analysis; it enables access to widespread brain sources, whereas unconscious input processing is limited to sensory regions.
  2. Consciousness enables comprehension of novel information, such as new combinations of words.
  3. Working memory depends on conscious elements, including conscious perception, inner speech, and visual imagery, each mobilizing widespread functions.
  4. Conscious information enables many types of learning, using a variety of different brain mechanisms.
  5. Voluntary control is enabled by conscious goals and perception of results.
  6. Selective attention enables access to conscious contents, and vice versa.
  7. Consciousness enables access to ‘self’: executive interpretation in the brain.

A wealth of data bolsters the above theses; I would point the interested reader to the article.

Baars goes on to claim that his GWT explains the above seven evidences. If GWT is to be overturned, its replacement must do even better.

Mechanisms of Brain Access

So, we see evidence of conscious activity being correlated with full-brain activation. But what mechanisms might produce full-brain activation? Baars identifies several research traditions exploring different (potentially complementary) answers to the question:

  • Dehaene and Changeux have focused on frontal cortex [1]
  • Edelman and Tononi on complexity in re-entrant thalamocortical dynamics [2]
  • Singer and colleagues on gamma synchrony [3]
  • Flohr on NMDA synapses [4]
  • Llinas on a thalamic hub [5]
  • Newman and Baars on thalamocortical distribution from sensory cortex [6]

Thoughts

Baars notes in his article that efforts to integrate research on attention and consciousness are long overdue. I would go a step further. His theory of consciousness also ought to be integrated with:

  • dual-process theory (theoreticians have already correlated System 2 with conscious awareness)
  • working memory (Alan Baddeley is already struggling to integrate his Central Executive with conscious awareness)

References

1. Dehaene, S. et al (2001) Cerebral mechanisms of word masking and unconscious repetition priming.
2. Tononi, G. and Edelmen, G.M. (1998) Consciousness and complexity.
3. Engel, A.K and Singer, W (2001) Temporal binding and the neural correlates of sensory awareness.
4. Flohr, H et al (1998) The role of the NMDA synapse in general anesthesia.
5. Llinas, R et al (1998) The neuronal basis for consciousness.
6. Newman, J and Baars, B.F. (1993) A neural attentional model for access to consciousness: a global workspace perspective.

Metzinger: The Ego Tunnel

With this book, Metzinger furthers an encouraging trend in academia: superstar theoreticians are writing accounts of their work for the layman.

His book is carved into three parts. The first summarizes his theory of consciousness, as rigorously developed in Being No One. The second introduces his theory of self-hood in the context of clinical neuroscience. The third discusses the imminent social conflict that will erupt as the public acquaints itself with the increasingly-surprising results of cognitive science.

Criticisms:

M fails to adequately consider evolutionary mechanisms other than natural selection. Some textual evidence from pg 43: “in principle, consciousness could be a by-product of other traits that paid for themselves, but [its stability] over time suggests that it was adaptive.”

While M excels at presenting cutting-edge research, he often neglects to leave his readers with tools for further research. I kept hoping that he would cite the concepts of “umwelt” and “semiosphere” but he never did. Also, pages 111-113 were stunningly eloquent, but if I had not read the physiological journals beforehand, I would have completely missed the fact that M was describing the theory of pain known as the neuromatrix.

The text is laced with insinuations of consensus. While this is often applicable in surprising ways (scholars agree that thoughts can be inferred from lab equipment), M can cast this authoritative weight inappropriately (his self-less Ego theory is itself immersed in controversy).

Praise:

M’s exceptionally lucid writing style, combined with a compelling bird’s eye view of genuinely pivotal cogsci research, makes this a compelling read. The wealth of illuminating graphics didn’t hurt either.

Three sections stood out as independently valuable. Chapter 2 explores six themes: the One-World Problem, the Now Problem, the Reality Problem, the Ineffability Problem, and the Who Problem. I found this journey to be compelling, and it left me itching to buy M’s magnum opus (Being No One). In addition to this, Chapter 3’s discussion of Out-Of-Body experiences stitched together a fascinating collection of research. Finally, chapter 7 included a well-overdue discussion of the effects of, and viable policy strategies towards, nootropics.

Conclusion:

I warmly recommend this book. A tasty quote to conclude (pg 20):

“The evening sky is colorless. The world is not inhabited by colored objects at all. It is just as your physics teacher in high school told you: Out there, in front of your eyes, there is just an ocean of electromagnetic radiation, a wild and raging mixture of different wavelengths. Most of them are invisible to you and can never become part of your conscious model of reality. What is really happening is that your brain is drilling a tunnel through this inconceivably rich physical environment and in the process painting the tunnel walls in various shades of color. Phenomenal color. Appearance.”