Stimulating the prefrontal cortex

(Warning: neuroscience weeds)

This is an interesting study getting attention on social media: Does the Prefrontal Cortex Play an Essential Role in Consciousness? Insights from Intracranial Electrical Stimulation of the Human Brain. Ned Block is one of the authors. (Warning: paywalled, but you might have luck here.)

The study looks at data from epileptic patients who had a net of electrodes implanted across various portions of their cortex. The electrodes are used to stimulate cortical regions to figure out where drug resistant seizures are coming from.

From the abstract. (Note: I took the liberty of breaking this into paragraphs.)

A central debate in philosophy and neuroscience pertains to whether PFC activity plays an essential role in the neural basis of consciousness. Neuroimaging and electrophysiology studies have revealed that the contents of conscious perceptual experience can be successfully decoded from PFC activity, but these findings might be confounded by postperceptual cognitive processes, such as thinking, reasoning, and decision-making, that are not necessary for consciousness.

To clarify the involvement of the PFC in consciousness, we present a synthesis of research that has used intracranial electrical stimulation (iES) for the causal modulation of neural activity in the human PFC. This research provides compelling evidence that iES of only certain prefrontal regions (i.e., orbitofrontal cortex and anterior cingulate cortex) reliably perturbs conscious experience. Conversely, stimulation of anterolateral prefrontal sites, often considered crucial in higher-order and global workspace theories of consciousness, seldom elicits any reportable alterations in consciousness.

Furthermore, the wide variety of iES-elicited effects in the PFC (e.g., emotions, thoughts, and olfactory and visual hallucinations) exhibits no clear relation to the immediate environment. Therefore, there is no evidence for the kinds of alterations in ongoing perceptual experience that would be predicted by higher-order or global workspace theories.

Nevertheless, effects in the orbitofrontal and anterior cingulate cortices suggest a specific role for these PFC subregions in supporting emotional aspects of conscious experience.

Overall, this evidence presents a challenge for higher-order and global workspace theories, which commonly point to the PFC as the basis for conscious perception based on correlative and possibly confounded information.

If you’re not familiar with the anatomical references, stimulating the upper and front portions of the prefrontal cortex had limited or no effect on conscious experience. However, stimulating the lower regions did have effects on emotional (affective) experiences. The paper can make for pretty dry reading, but it does have some very interesting diagrams, particularly on page 8.

The authors may have a point that this puts pressure on at least some HOTTs (higher order thought theories), particularly the ones that put the full conscious perception in the PFC. HOTTs posit that for anything to be conscious, there must be a higher order representation or thought about the first or lower order sensory representation. These higher order structures are thought to reside in the PFC, particularly including the regions which when stimulated lead to few or no reported effects. (Although I expect HOTT advocates to mount a response.)

However, I think the conclusions about GWT (global workspace theory) are hasty, and may involve an incorrect understanding of this class of theories. It begins with this statement.

It is important to distinguish between the neural constitution of consciousness and contingent global enabling factors of consciousness (Boly et al., 2017). For example, blood flow enables conscious processing but does not constitute it (but see Moore and Cao, 2008). Therefore, advocates of first-order recurrent activation can allow that PFC activity may control arousal, thus enabling sensory cortices to be activated, yet nonetheless argue that PFC activity is not constitutive of consciousness. Similarly, proponents of prefrontal involvement can allow that, without sensory input, there would be no perceptual consciousness, yet argue that only PFC activity plays a constitutive role. Throughout this Viewpoints article, we are concerned with the neural constitution of consciousness, not the enabling conditions.

This is a problematic standard for GWT, because GWT sees the entire thalamo-cortical system as being constitutive of consciousness, as well as seeing it all, along with numerous subcortical regions, as enabling. Put another way, under GWT, there is no particular region of the brain which is conscious. Consciousness is a system wide effect. For something to be conscious, it must be widely broadcast in the thalamo-cortical system and reacted to by most, or at least a substantial portion of the various subsystems. Just about any region in this system can be one of the sources of the broadcasted content in any particular instant, but no region is always conscious.

It is true that Dehaene’s GNW (global neuronal workspace) theory has historically seen a special role for the PFC, but I think this is more about its role in broadcasting content than where consciousness takes place. It’s worth noting that GNW also sees the parietal regions as part of that broadcast hub. But as noted in the last couple of studies I’ve shared, this role for the PFC as historically described looks increasingly unlikely.

The results of this study actually seem in accord with the study that identified various subcortical locations for the global workspace broadcast hubs. As the authors of that study noted, their findings don’t contradict Dehaene and Changeux’s overall model so much as fine tune it. The other study which identified possible stages of workspace, proposing the concept of a playground, was seen by Dehaene as confirmative, if again fine tuning.

All of which is to say, the data presented in this paper doesn’t seem, to me, to put much pressure on GWTs, except in terms of particular aspects of global neuronal workspace theory, aspects which are already having to be adjusted. It does give possible insights into which regions may be more connected to the broadcast hubs, and that in and of itself is very interesting.

More broadly, I think the authors’ conclusions show how much they’re operating under the theater of the mind paradigm and so attempting to find that theater, to find the presentation, the movie. (Admittedly, many HOTT advocates are in a similar mode, albeit seeing the movie in a very different place.) I’m not sure whether GWT is the answer. Even if it is true it’s likely only part of the overall answer. But even if it isn’t, I think looking for the presentation will be unproductive.

But maybe someone will find evidence tomorrow and prove me wrong. What do you think? Am I overlooking what the data in this paper means for GWT?

11 thoughts on “Stimulating the prefrontal cortex

  1. This struck me.

    “confounded by postperceptual cognitive processes, such as thinking, reasoning, and decision-making, that are not necessary for consciousness”

    So “thinking, reasoning, and decision-making” are not a part of human consciousness?

    “Not necessary” but a part of human consciousness nonetheless?

    I think I understand what is being said but I think oscillating neurons, possibly via EM fields, generate consciousness but different anatomical structures generate it with different forms and contents.

    But I need to read the actual paper.

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    1. Yeah. Their view of consciousness is something pre-cognitive. As I’ve said before, I think they’re looking for a version of it that doesn’t exist. There’s no sharp dividing line between the preconscious and actual conscious content, except for what we can remember and report after the fact.

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  2. Put another way, under GWT, there is no particular region of the brain which is conscious. Consciousness is a system wide effect.

    That is how I understand GWT as well. Of course there is particular interest in the locations (if they are specific) of the “broadcast stations”, but in Block’s terms the broadcast stations are “enabling conditions”.

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    1. Exactly. Of course, under GWT, if someone did mange to stimulate one of those actual broadcast hubs, it should have effects on consciousness, but that wouldn’t mean those regions were where the conscious experience was happening, just that they’d have the connectivity to have the wide ranging effects that are consciousness.

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    2. I guess the question becomes whether GWT is testable or falsifiable if there are no identifiable regions of the brain where it resides. Is there anything that could be read on an MRI or through EEG? Or some other sort of test?

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      1. On the one hand, there is plenty of evidence for the dynamics that GWT cites. Dehaene discusses reams of it and I’ve shared numerous studies that go over it. On the other hand, there will always be the question whether that, in and of itself, is consciousness. Many will insist those dynamics only pertain to attention and cognition.

        It’s easier if we talk in terms of capabilities, such as when sensory processing becomes reportable, where it’s obvious there are multiple paths and no single finish line. But that doesn’t fit well with the theater metaphor so many people insist it isn’t the answer. Consciousness is as much a matter of philosophy (as in definition) as science.

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  3. BTW, the abundant evidence that electrical stimulation of various parts of the brain alters consciousness most obviously suggests that consciousness is something bioelectrical in nature even if it is not electromagnetic. I note that many tests of this sort are stimulating the surface of the cortex which is exactly where the apical dendrites of L5 pyramidal neurons terminate. The difference in the negative electrical charge at the surface of the cortex and positive charges in lower parts of the neuron are a key aspect in Pockett’s theory. See diagram here.

    https://broadspeculations.com/2019/12/01/em-fields-and-consciousness/

    Note that stimulating the surface would exactly result in alteration of the negative charges at the surface of the cortex.

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    1. BTW again I have been vaguely playing with the idea that consciousness in fact does occur primarily at the surface of the cortex (or surface of other structures). Some deeper structures may be involved but may in effect have “surfaces” themselves as part of the boundary layers with the other layers. The amount of folding in the human brain effectively provides more surface area.

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      1. It’s long been understood that nervous systems had an electrical nature. Interestingly, that was once a bitterly contested proposition. There were people in the early 1800s who were adamant that electricity had nothing whatsoever to do with nerve functionality. They’d probably only be slightly mollified by our current knowledge that it’s electrochemical in nature. Which seems odd to me, since electricity is essentially tamed lightning, and that seems like as good a medium for an animating force as any.

        Since the cortex is essentially a big sheet, just one crumpled up to fit, the surface thing doesn’t seem far off the mark. But I think it’s more about the amount of neural substrate, and the interconnectivity that architecture allows it to have between itself and subcortical regions, rather than the surface aspect in and of itself, that makes it what it is. But then my take tends to be pretty consistently more mechanistic than yours. 🙂

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