Do Psychedelics Turn Off the Left Hemisphere?

Some thoughts on recent findings... also, Iain McGilchrist is on substack.

One of my favorite authors and thinkers is now on substack. Dr.

Iain McGilchrist

’s first book The Master and His Emissary (M&E) changed the way I think about pretty much everything. McGilchrist’s claim was that the left and right cerebral hemisphere have different ways of perceiving the world and that historical movements in Western culture represent the perspective of one hemisphere or the other. While the left hemisphere has a narrow, detailed, and precise mode of attention, the right hemisphere’s mode of attention is more broad and open. McGilchrist claimed, for example, that romanticism was more associated with the right hemisphere while modern Western culture is dangerously tilted towards the left hemisphere. This may seem like a strange claim, and I’m not even sure it’s right, but I do know that McGilchrist brings together a tremendous amount of evidence and argument that makes for interesting reading regardless of whether his main idea is correct. Everyone, including skeptics, will learn something by reading M&E.

His appearance on substack reminded me of a finding that I noticed a few years ago but never ended up writing about. While looking through some neuroscience research on classic psychedelics (i.e., serotonin 2A receptor agonists like psilocybin, LSD, and mescaline), I noticed that some papers reported (or at least implied) hemispheric differences in brain activity under the influence of psychedelics. It’s well-established that classic psychedelics tend to reduce blood flow in the brain, but most papers using fMRI to assess brain activity under psychedelics don’t report hemispheric differences. The few papers that do report hemispheric differences show that blood flow is more reduced in the left hemisphere than the right hemisphere under the influence of psychedelics (e.g., Lewis et al., 2017). At least one of these papers didn’t analyze any data related to hemispheric differences, but they did publish images which show hemispheric difference in blood flow (Carhart-Harris et al., 2012). The images below, published by Carhart-Harris and colleagues (2012), show that areas of reduced blood flow are mostly found in the left cerebral hemisphere.

Neuroscientists like to invert their images, so the right side of the brains above represents the left hemisphere. These cross-sections show that the majority of the area with reduced blood flow (70-80% by my estimation) is in the left hemisphere.

One paper looking at brain activity under the influence of Ayahuasca showed an atypical increase in blood flow which was more apparent in the right hemisphere (Riba et al., 2006). They found that “Increased blood perfusion was observed bilaterally in the anterior insula, with greater intensity in the right hemisphere, and in the anterior cingulate/frontomedial cortex of the right hemisphere…” (p. 93). Their images also show that, although there is some increase of blood flow in the left hemisphere, the majority occurs in the right (these images are not inverted).

Another paper didn’t look at blood flow per se, but at the “diversity” of executive network nodes (Lebedev et al., 2015). In this context, “diversity” refers to the extent to which the region of interest is connected to the regions around it. A region with more restricted connections has low diversity while a region with more diffuse connections has high diversity. These researchers found that low diversity in three regions of interest predicted ego-dissolution under psychedelics. All three regions were in the left hemisphere, meaning that ego dissolution under psychedelics is associated with reduced connectivity in the left hemisphere.

All of these papers indicate that the left hemisphere is less active during a psychedelic trip than the right hemisphere. If confirmed, this is an important finding that is currently not receiving much attention by neuroscientists. A recent review article does make the case that psychedelics work by releasing the right hemisphere from inhibition by the left (which is consistent with reduced blood flow in the left hemisphere), but so far this article has been cited zero times (Levin, 2024). The author hasn’t responded on researchgate or to emails requesting a PDF, so I haven’t been able to read it in full. A research program studying lateralized effects of psychedelics would provide insight into both the therapeutic potential of psychedelics and brain functioning in general.

I have discussed some relevant hemispheric differences at length elsewhere (e.g., here), so I won’t go into detail about them here. In short, the fact that the right hemisphere is more associated with the sense of embodiment, with a broad/open kind of attention, and with making distant rather than obvious connections (i.e., divergent thinking) are all good theoretical reasons to believe that the right hemisphere would be more dominant during a psychedelic trip.

Instead of rehashing the relevant hemispheric differences, I want to end by pointing out the similarities between Jill Bolte-Taylor’s left hemisphere stroke and a psychedelic trip. Bolte-Taylor was a neuroscientist who had an unexpected left hemisphere stroke and documented her experiences. While obviously anecdotal, Bolte-Taylor’s experience provides initial evidence for similarities between the experience of a left hemisphere stroke and the psychedelic experience, providing another line of evidence indicating that psychedelics may work by reducing activity in the left hemisphere.

Jill Bolte-Taylor

Jill Bolte-Taylor described her experience of a left hemisphere stroke as a world where logic, language, and rigid boundaries faded away, leaving only a boundless sense of connection and unity with the universe. This is not so far from descriptions of a so-called “mystical experience” under the influence of psychedelics. In particular, there were two major similarities between her experience and a psychedelic experience that I want to point out: ego dissolution and the sense of “Oceanic Boundlessness”.

As mentioned above, ego dissolution, or ceasing to identify with one’s self, is a common occurrence under high doses of psychedelics. During her left hemisphere stroke, Jill Bolte-Taylor described her own version of this phenomenon:

Without a language center telling me: “I am Dr. Jill Bolte Taylor. I am a neuroanatomist. I live at this address and can be reached at this phone number,” I felt no obligation to being her anymore. It was truly a bizarre shift in perception, but without her emotional circuitry reminding me of her likes and dislikes, or her ego center reminding me about her patterns of critical judgment, I didn’t think like her anymore. From a practical perspective, considering the amount of biological damage, being her again wasn’t even an option! In my mind, in my new perspective, that Dr. Jill Bolte Taylor died that morning and no longer existed. (pp. 67-68)

This is clearly ego-dissolution.

One of the questionnaires given by researchers studying the effects of psilocybin contains a subsection called “Oceanic Boundlessness” and is described as "a sensation of 'eternity', a feeling as of something limitless, unbounded, a "feeling of an indissoluble bond, of being one with the external world as a whole" (quoted from here). These feelings are commonly described by subjects reflecting on a psychedelic experience. In reflecting on her stroke, Bolte-Taylor said this:

Although the ego center of our language center prefers defining our self as individual and solid, most of us are aware that we are made up of trillions of cells, gallons of water, and ultimately everything about us exists in a constant and dynamic state of activity. My left hemisphere had been trained to perceive myself as a solid, separate from others. Now, released from that restrictive circuitry, my right hemisphere relished in its attachment to the eternal flow. I was no longer isolated and alone. My soul was as big as the universe and frolicked with glee in a boundless sea. (p. 69)

If it is the case, as recently argued by Adam Levin, that psychedelics work in part by releasing the right hemisphere from inhibition by the left, then the similarities between Bolte-Taylor’s left hemisphere stroke and the psychedelic experience are easily explained. In both cases, the right hemisphere “mode” came to the fore because the left hemisphere was no longer able to inhibit it.

The empirical research, theoretical considerations, and real-world example above all point to the idea that psychedelics function at least in part by decreasing the activity of the left hemisphere or increasing the relative activity of the right. Hopefully some well-funded scientists will recognize the potential importance of this hypothesis and begin a systematic research program into lateralization under psychedelics.