It appears some demonstrated experimental effects depend more heavily on personality than previously thought: Serious Problems With Interpreting Rubber Hand “Illusion” Experiments

Serious Problems With Interpreting Rubber Hand “Illusion” Experiments. Warrick Roseboom; Peter Lush. Collabra: Psychology (2022) 8 (1): 32274. https://doi.org/10.1525/collabra.32274

The rubber hand “illusion” (RHI), in which participants report experiences of ownership over a fake hand, appears to demonstrate that subjective ownership over one’s body can be easily disrupted. It was recently shown that existing methods of controlling for suggestion effects in RHI responding are invalid. It was also shown that propensity to agree with RHI ownership statements is correlated with trait phenomenological control (response to imaginative suggestion). There is currently disagreement regarding the extent to which this relationship may cofound interpretation of RHI measures. Here we present the results of simulated experiments to demonstrate that a relationship between trait phenomenological control and RHI responding of the size reported would fundamentally change the way existing RHI results must be interpreted. Using real participant data, each simulated experiment used a sample biased in selection for trait phenomenological control. We find that using experiment samples comprised only of participants higher in trait phenomenological control almost guarantees that an experiment provides evidence consistent with RHI. By contrast, samples comprised of only participants lower in trait phenomenological control find evidence for RHI only around half the time – and of greater concern, evidence specifically for “ownership” experience just 4% of the time. These findings clearly contradict claims that the magnitude of relationship between phenomenological control and RHI responding is a minor concern, demonstrating that the presence of participants higher in trait phenomenological control in a given RHI experiment sample is critical for finding evidence consistent with RHI. Further study and theorising regarding RHI (and related effects) must take into account the role that trait phenomenological control plays in participant experience and responses during RHI experiments.

Keywords: embodiment, phenomenological control, hypnosis, imaginative suggestion, rubber hand illusion

Discussion

Quantitative study of subjective body ownership in the RHI is accomplished using participant ratings of (dis)agreement with a series of subjective statements about referred touch (S1 and S2) and (putatively) ownership (S3). Propensity to agree with these statements has previously been shown to be related to trait phenomenological control – the domain general ability to meet expectancies arising from direct or implicit imaginative suggestion (including demand characteristics; Dienes, Lush, et al., 2020; Lush et al., 2020). It has been claimed that this reported relationship is too small to be of concern (Ehrsson et al., 2021; Fan et al., 2021). To concretely demonstrate the potential influence of this relationship, here we simulated a series of experiments with different degrees of sampling bias, selecting a disproportionate number of participants with higher trait phenomenological control. Given the relationship between subjective agreement and phenomenological control, this induced sampling bias may be analogous to the deliberate exclusion of participants (e.g. Chancel & Ehrsson, 2020; Ehrsson et al., 2005) who don’t report a strong experience of the RHI (up to ~30% of people; Riemer et al., 2019). We show that the frequency with which a RHI experiment will provide evidence consistent with subjective agreement for a RHI is directly related to the proportion of participants in the experiment sample that are higher in phenomenological control. Put more concretely - only experiments run with a large proportion of participants higher in phenomenological control will provide evidence for the RHI. This finding directly contradicts the statement that the reported relationship between RHI responding and trait phenomenological control is too small to be of concern.

Why does selecting participants based on trait phenomenological control affect whether a RHI study will conclude in favour of evidence for RHI or not? It can only be because they are substantially related – reiterating the result reported by Lush et al. (2020). Why is it a problem if the same people who are likely to report agreement with RHI questions also happen to be higher in phenomenological control? To interpret the clearly substantial relationship, we must first consider the nature of the constructs these measures are supposed to represent – body ownership for RHI responding; and propensity to respond to suggestion, implicit or explicit, with imagination for phenomenological control. Phenomenological control is a domain general ability (Dienes, Lush, et al., 2020; Lush et al., 2020), relating to sensory and decision processes over and above those specifically related to experiences of body ownership or multisensory experiences of bodily sensation. This is evident both in the measure of phenomenological control used in Lush et al. (2020; SWASH), which interrogates a variety of possible experiences, not just those related to bodily experience or ownership (e.g. abilities related to amnesia or musical and visual hallucination, in response to suggestion) and in several recent findings: Relationships between trait phenomenological control and anomalous experiences when demand characteristics (and therefore participant hypothesis awareness) have not been adequately controlled have been reported (Lush et al., 2020) for other body-related effects including vicarious pain (reports of pain in response to seeing people in apparently pain-inducing situations) and mirror-touch synaesthesia (reports of felt touch in response to seeing people touched), and also for non-body related experiences like the visually-evoked auditory response (Lush, Dienes, Seth, et al., 2021; reports of sound experienced when watching silent videos). The ability to control experience in response to suggestion is evident across perceptual and decision domains and there is much evidence that imaginative suggestion effects can be experienced as subjectively ‘real’ (see Dienes, Palfi, et al., 2020; Lynn et al., 2020; McConkey, 2008), and that hypnotisability is distinct from social compliance (e.g. see Moore, 1964; Tasso et al., 2020; but see Polczyk & Pasek, 2006). In an explanatory framework, a domain general process subsumes domain specific demonstrations of its operation. Therefore, we are left with limited options for interpretation of the result from Lush et al. (2020):

1. participants who can control their experience across sensory and decision domains through phenomenological control are also controlling their experience in RHI experiments (as suggested by Lush et al., 2020)

2. participants who can control their experience in other domains through phenomenological control happen to be the same participants who can experience the RHI, but for distinct reasons (e.g. pure multisensory integration account)

3. the same participants who can control their experience in other domains don’t control their experience in RHI but respond as though they do for other unspecified reasons – perhaps a response bias induced by social context or pure confabulation just for RHI case

The first interpretation is perhaps the most parsimonious in that it requires only that we reconsider the RHI as an act of phenomenological control in response to suggestion and that it allows for group level report of RHI ownership experience to reflect genuine experience. It is important to note that this position does not rule out a multisensory contribution to the RHI, but rather that another cognitive process is in play. This is an important distinction. If we were to say that there is no problem treating an act of phenomenological control (which is necessarily creative and interpretative) as evidence for a sense of ownership, we might also study visual perception through phenomenological control in response to imaginative suggestion. If people report the experience of “seeing” a unicorn in response to a direct or indirect imaginative suggestion, we would be obliged to take this as evidence for the existence of unicorns on the same basis that RHI researchers claim that reports of the RHI are evidence for a sense of body ownership. Certainly, visual imagery is constrained by the properties of visual processing and previous perceptual experience, as is the RHI likely constrained by the properties of (multi)sensory processing and previous experience. The existence of such constraints is not in contention. But the interpretation of imaginatively suggested experience is not the same as studying the properties of multisensory perception, as is the common interpretation of RHI studies (e.g. Ehrsson et al., 2005). Saying that you are “studying the rubber hand instantiation of the influence of phenomenological control on perception and decision making” is perfectly reasonable. But this statement is not consistent with the broader claims made in most papers regarding RHI where it is stated that the RHI represents a fundamental case study in understanding how subjective body ownership is determined in humans.

The second interpretation, while less parsimonious than the first may still be possible and is not explicitly ruled out by the results presented here or previously. However, given that hypothesis awareness is not controlled for in the existing RHI literature (Lush, 2020; Lush, Seth, et al., 2021; Reader, 2021), evidence for this interpretation cannot depend on that existing literature until the possible influences of phenomenological control are ruled out. There are methods by which phenomenological control effects may be controlled; control conditions which are not confounded by demand characteristics could be developed by a two-step procedure which takes into account both expectancies and differences in the relative difficulty of suggestion effects (see Lush, 2020 for details). This requires a difficult process of matching expectancies and direct imaginative suggestion response across candidate illusion and control measures. Note that while this makes the study of embodiment illusions more challenging than has been the tradition, the combined evidence from many small sample studies in which demand characteristics were uncontrolled has only generated illusory evidence which is of no value to theories of embodiment (except perhaps as a cautionary tale). Similar issues have been dealt with successfully in other fields. For example, the suggested approach has much in common with the use of placebo controls in clinical trials, which, although it adds considerable cost to experiments, has been standard practice for decades (Beecher, 1955).

The third interpretation should be considered in light of evidence that phenomenological control may confound interpretation of other effects. The RHI is one of four distinct effects for which such relationships have now been shown (Lush, Dienes, Seth, et al., 2021; Lush et al., 2020) which involve a range of modalities. Correlation is not causation, of course, and relationships between the RHI and phenomenological control may be attributable to some unknown cause (perhaps some difference in multi-sensory integration mechanisms common to RHI response and phenomenological control - even for cases of phenomenological control that require no multisensory percept). However, given the simplicity of the proposal that demand characteristics can act as implicit imaginative suggestions in measures of experience, and the growing evidence that this occurs in a range of effects, it is a more parsimonious explanation that all of these effects are confounded by phenomenological control than that some other explanation is in play for each individual case.

Note that, whenever expectancies are consistent with experimental aims (e.g. participants are hypothesis aware), any given result may be attributable to hypothesis awareness effects other than phenomenological control (e.g., faking or imagination; see Corneille & Lush, 2021). However, for both the RHI and response to imaginative suggestion, there is extensive evidence that at least some reports reflect genuine experience (see Dienes, Palfi, et al., 2020 for a discussion of this evidence as it relates to phenomenological control). Of course, if phenomenological control scales were to reflect trait differences in faking (for example), this would be an important consideration for interpretation of the RHI (and other reported experiences which correlate with trait phenomenological control).

Researchers familiar with the RHI will be likely to note that these results (and those of Lush et al., 2020) only relate to the subjective statements, while there is extensive evidence from ‘implicit’ measures such as proprioceptive drift, and convergent evidence from neuroimaging and animal studies. As mentioned previously, there are only two aspects linking RHI to phenomenological change – extended report and subjective ratings. All other findings, such as proprioceptive drift or changes in measured blood flow (fMRI) or scalp potential (EEG), are linked to changes in subjective ownership only through broad correspondence or co-occurrence with these subjective reports. If human participants’ ratings on the subjective statements are predominantly related to their trait phenomenological control, then proprioceptive drift (or neuroimaging) results are correlating with phenomenological control, not specifically rubber hand subjective ownership. Again, this is because phenomenological control is a domain general ability (Dienes, Lush, et al., 2020; Lush et al., 2020), relating to sensory and decision processes over and above those specifically related to experiences of body ownership or multisensory experiences of bodily sensation. Note that the asynchronous control measure is also employed for “implicit” RHI measures (e.g., proprioceptive drift), and participants show hypothesis awareness for these effects (Lush, Seth, et al., 2021).

Regarding animal studies (e.g. Fang et al., 2019; Wada et al., 2016), while it may be completely reasonable to posit that, for instance, a rat may have a sense of body ownership, the reverse inference on which such claims rely when referencing RHI is problematic. Because it is not possible to simply ask an animal about their subjective experience, to conclude that an animal has a change in subjective experience of body ownership during, for example, a “rubber tail illusion” paradigm (Wada et al., 2016) we start with a potential relationship in human participants between a behavioural observation and subjective reports (e.g. correlate proprioceptive drift measure in hand with magnitude or time course of change in subjective ownership reports). We then attempt to obtain observations of a similar behaviour in rodents (e.g. something like proprioceptive drift of tail). We must then make the conceptual leap that the observed behavioural phenomena (proprioceptive drift in humans and something like it in rodents) are equivalent. Through this combination of directly observing one relationship related to subjective experience of ownership (subjective ownership responding and proprioceptive drift are related in humans), then observing this single phenomenon in rodents (behaviour with similar properties to proprioceptive drift), that we have assumed to be equivalent to a similar observation in humans (hand proprioceptive drift), we can end up with the reverse inference that the rodent also has change in subjective experience of ownership. Importantly, there is, and can be, no direct evidence to support this claim. It is important to also note that the same issue of reverse inference is present in any study using implicit measures that doesn’t provide evidence for concurrent changes in subjective experience (e.g. Tsakiris & Haggard, 2005). If the foundation statement about human subjective body ownership on which this set of inferences is based is instead reflecting some other ability (e.g. to control experience in response to suggestion – phenomenological control), then it is clear that all subsequent inferences are questionable. This is not to say that nothing can be discovered about multisensory perception of the body using neuroimaging or animal models. Our claim in no way invalidates behaviourist approaches that seek to map body perception to neural function through observed behaviour. However, any instances wherein these findings are related to subjective ownership through their connection with RHI-like paradigms are clearly problematic if the original basis for claims of changes in subjective body ownership during rubber hand exposure are not being correctly interpreted.

It is sometimes claimed (e.g. Ehrsson et al., 2021; Tsakiris, 2010) that RHI effects reflect a combination of top-down and bottom-up processes. However, these top-down effects are believed to be distinct from demand characteristic effects and RHI response is considered to be “resistant to suggestions, thoughts, and high level conceptual knowledge” (Ehrsson et al., 2021). Only a few researchers have stated theoretical positions consistent with the theory that demand characteristics play an important role in these effects (Alsmith, 2015; Dieguez, 2018). An influential theory proposes that top-down effects arise when internal representations of body image are matched against incoming sensory signals (Tsakiris, 2010; though see Chancel & Ehrsson, 2020 for a counter-argument that apparently top-down effects are attributable to multisensory congruency). However, given that demand characteristics have not been controlled in existing RHI studies, it is premature to consider theoretical proposals of top-down effects which are not related to demand characteristics.

In sum, agreement with three different subjective statements is the standard basis for linking all aspects of RHI phenomena with body ownership. Propensity to agree with these reports is strongly related to the domain general ability for phenomenological control. Experiments wishing to link RHI experience specifically with subjective body ownership must take this relationship into account or their results cannot be taken to indicate general properties of human body ownership.

Exposure to Extremely Partisan News from the Other Political Side Shows Scarce Boomerang Effects

Exposure to Extremely Partisan News from the Other Political Side Shows Scarce Boomerang Effects. Andreu Casas, Ericka Menchen-Trevino & Magdalena Wojcieszak. Political Behavior, Feb 3 2022. https://link.springer.com/article/10.1007/s11109-021-09769-9

Abstract: A narrow information diet may be partly to blame for the growing political divides in the United States, suggesting exposure to dissimilar views as a remedy. These efforts, however, could be counterproductive, exacerbating attitude and affective polarization. Yet findings on whether such boomerang effect exists are mixed and the consequences of dissimilar exposure on other important outcomes remain unexplored. To contribute to this debate, we rely on a preregistered longitudinal experimental design combining participants’ survey self-reports and their behavioral browsing data, in which one should observe boomerang effects. We incentivized liberals to read political articles on extreme conservative outlets (Breitbart, The American Spectator, and The Blaze) and conservatives to read extreme left-leaning sites (Mother Jones, Democracy Now, and The Nation). We maximize ecological validity by embedding the treatment in a larger project that tracks over time changes in online exposure and attitudes. We explored the effects on attitude and affective polarization, as well as on perceptions of the political system, support for democratic principles, and personal well-being. Overall we find little evidence of boomerang effects.

Discussion

In the US, political understanding is needed more than ever. To achieve this elusive goal, scholars and practitioners encourage exposure to dissimilar political views, with the hope that encountering views that challenge one’s beliefs will minimize extremity and interparty hostility. Although some scholars caution against this approach, suggesting that cross-cutting exposure can increase polarization, the evidence of such boomerang effects is mixed and quite limited in scope.

We set out to contribute solving this debate with an innovative experimental design combining incentivized over time exposure to extreme news domains from across the aisle (Breitbart, The American Spectator, and The Blaze for liberals; and Mother Jones, Democracy Now, and The Nation for conservatives), pre-, post-, and intermediate surveys, trace data on actual online exposure, and participants’s open-ended reactions to the outlets tested. Although this design is counterfactual (after all, most liberals are unlikely to regularly visit Breitbart), it was well suited to detecting boomerang effects if these are in fact a likely outcome of cross-cutting exposure. The design also allowed us to test whether the studied exposure impacts broader societal outcomes and individual well-being, and also for whom these effects emerge.

In short, despite the over-time nature of the treatment (i.e., 12 days), accounting for intended treatment effects as well as the levels of compliance, and testing attitude polarization on a range of salient issues and affective polarization with several indicators and toward various out-groups, we show that cross-cutting exposure is unlikely to intensify political conflict or have any substantive effects on the societal and individual outcomes tested. People did not radicalize their issue attitudes nor their feelings towards the out-party and the supporters of the opposing ideology. Although we did find that people became slightly more negative toward those holding opposing views on a few policies (e.g., climate change and immigration), these effects were minor (< 0.2 standard deviation changes) and did not hold when accounting for multiple comparisons and false discovery rate. Furthermore, although observers fear that strong partisans are most likely to radicalize and drive political conflict, we do not find pronounced heterogeneous effects.

Similarly, our treatment did little to influence participants’ perceptions of the political system, in terms of their support for compromise, attributing malevolent intentions to the outparty, or seeing the polity as polarized. It also did not shift their support for key democratic principles, such as freedom of speech or freedom of the press. Relatedly, extreme cross-cutting exposure did not worsen participants’ well-being.

The findings are a great contribution to the existing literature on the potential negative effects of exposure to counter-attitudinal information. Contrary to some evidence, which finds exposure to opposing views to exacerbate polarization (Levendusky, 2013; Bail et al., 2018; Garrett et al., 2014), and in line with other recent work (Guess & Coppock, 2018; Guess et al., 2021; Levy, 2021), we conclude that boomerang effect are the exception rather than the norm. Extending past work by incorporating people’s evaluations of the outlets and articles (based on short surveys and also open-ended thoughts and emotions), this consistent lack of boomerang effects may be due to people’s largely neutral or even positive reactions to the outlets and their content. We wanted to test the effect of a counterfactual and selected these 6 sites because they are considered far left and far right in most classifications of news media ideology (Robertson et al., 2018; Eady et al., 2019). Nevertheless, despite representing the extreme of each ideological side,Footnote18 and despite being vilified by one’s partisan group, our participants sometimes valued the information they consumed therein. In addition, this study also makes a relevant contribution to the growing body of work that uses trace data to study people’s attitudes and behavior (Stier et al., 2020; Guess, 2021; Guess et al., 2021; Wojcieszak et al., 2021b). Rather than relying on a forced exposure experiment that shows people mock sites with counter-attitudinal articles, we incentivized exposure, accounted for compliance, and exposed them to real articles that actually appeared in news outlets of the opposing ideology. At a time where key stakeholders such as social media companies (Farr and dorsey, 2018; Wood & Ethan, 2020), news organizations (Goodman & Chen, 2010), and governments (Rendall, 2015; Commission, 2013) are designing policies to reduce polarization, we believe that the findings reported here can help inform the decision-making process moving forward.

Measuring Gratification From and Consequences of Likes: Gratification from receiving likes was positively associated with risky behaviors used to enhance fame, tendency to use social media’s black market, and problematic Internet use

Measuring Gratification From and Consequences of Likes: The Potential for Maladaptive Social Media Behavior. Reza Shabahang, Zahra Ghaemi, Mara S. Aruguete, Maryam Saeedi, Hyejin Shim, and Seyedeh Farnaz Sedighian. Journal of Media Psychology, Feb 2 2022. https://doi.org/10.1027/1864-1105/a000328

Abstract: Abstract. Paralinguistic digital affordances (PDAs; e.g., likes) are sought out by social media users and serve an important function of enhancing social reputation in online contexts. Nonetheless, there has been no standardized measure for evaluating gratification from receiving PDAs. This study provides a brief, validated self-report questionnaire on PDA gratification. The results of factor analysis verified the three-factor structure (i.e., emotional, status, and social gratifications) of the questionnaire. Internal consistency was established using inter-item correlation, corrected item-total correlation, and Cronbach’s alpha. Gratification from receiving PDAs was positively associated with risky behaviors used to enhance fame, tendency to use social media’s black market, and problematic Internet use. The findings provide preliminary evidence that gratification from receiving PDAs may increase the likelihood of maladaptive fame-seeking behaviors in social media users. The Gratification From Receiving PDAs Questionnaire appears to be a promising measure that may offer new insights into the motivations involved in social media use.

In Sweden, men with limited resources are considered less attractive; male financial resources are not seen as a bonus, but rather a prerequisite; sexual exchange theory is then a useful theory

Ngaosuvan, Leonard and Holmberg, Linus Carl and Saleh Al-Basri, Naila and Elshani, Rebecca, Sexual Economics in Swedish Dating: Pity Poor Men. SSRN, Jan 29 2022: http://dx.doi.org/10.2139/ssrn.4020856

Abstract: Sexual exchange theory (SET) is a controversial theory describing heterosexual partner selection in terms of economic market factors. This paper explores SET empirically in Sweden, one of the most financially equal nations in the world. Experiment 1, a vignette study with four dating profiles, tested whether access to resources increases male attractiveness. Experiment 2, a vignette study measured how justifiable men’s disappointment was, depending on financial courtship investments in a failed courtship attempt. The results of Experiment 1 indicated that, even in Sweden, men with limited resources are considered less attractive. Male financial resources are not seen as a bonus, but rather a prerequisite. In Experiment 2, participants felt that it was not justifiable to be disappointed for men who were ‘cheap’ in courtship. These results indicate that SET is a useful theory, even in a relatively gender-equal society.

Keywords: Sexual Economics, Gender equality, Partner Selection, Attraction, Courtship, Sexual Exchange Theory.

Psychedelics: Medical risks are often minimal, and that many – albeit not all – of the persistent negative perceptions of psychological risks are unsupported by the currently available scientific evidence

Adverse effects of psychedelics: From anecdotes and misinformation to systematic science. Anne K Schlag et al. Journal of Psychopharmacology, February 2, 2022. https://doi.org/10.1177/02698811211069100

Abstract

Background: Despite an increasing body of research highlighting their efficacy to treat a broad range of medical conditions, psychedelic drugs remain a controversial issue among the public and politicians, tainted by previous stigmatisation and perceptions of risk and danger.

Objective: This narrative review examines the evidence for potential harms of the classic psychedelics by separating anecdotes and misinformation from systematic research.

Methods: Taking a high-level perspective, we address both psychological and psychiatric risks, such as abuse liability and potential for dependence, as well as medical harms, including toxicity and overdose. We explore the evidence base for these adverse effects to elucidate which of these harms are based largely on anecdotes versus those that stand up to current scientific scrutiny.

Results: Our review shows that medical risks are often minimal, and that many – albeit not all – of the persistent negative perceptions of psychological risks are unsupported by the currently available scientific evidence, with the majority of reported adverse effects not being observed in a regulated and/or medical context.

Conclusions: This highlights the importance for clinicians and therapists to keep to the highest safety and ethical standards. It is imperative not to be overzealous and to ensure balanced media reporting to avoid future controversies, so that much needed research can continue.

Keywords: Psilocybin, dimethyltryptamine, ayahuasca, mescaline, d-lysergic acid diethylamide, abuse liability/dependence, hallucinogen persistent perception disorder, toxicity, hypertension

Conclusion

Some of the perceived harms of psychedelics – for example, that they lead to addiction and are neurotoxic – are largely refuted by research of the past decades. Other risks, such as the risks of psychotic episodes or overdose, are rare and only reported in individual cases, but these risks still need to be minimised by careful patient selection and preparation. The past decade of research and clinical experience has increasingly demonstrated how psychedelics can be used safely under medical supervision, and safe use guidelines are progressively well defined (e.g. Griffiths et al., 2006).

Regulatory and legal hurdles of getting psychedelic medicines proven as mainstream medicines are still substantial, so overcoming historic misperceptions is vital. The past decade has seen an increasing focus on research on the therapeutic applications of psychedelics – a direct benefit for the public, which is positively represented in current media (Aday et al., 2019). A recent YouGov study (2017) indicates that public perceptions in the United States becoming more positive, with the majority (63%) being open to medical treatment with psychedelics if faced with a pertinent medical condition, and a UK YouGov survey (2021) corroborates these results.

These changes in public interest are in line with the recent regulatory changes in the United States and Canada. Collectively, these changes in public perception and regulation suggest that the stigma surrounding psychedelics may be beginning to dissipate, and that society is moving away from previous negative narratives to a more scientific, evidence-based approach to risks and benefits of psychedelics as medicine.

Political Resources and Online Political Hostility: How and Why Hostility Is More Prevalent Among the Resourceful

Rasmussen, Stig H. R., Mathias Osmundsen, and Michael Bang Petersen. 2022. “Political Resources and Online Political Hostility: How and Why Hostility Is More Prevalent Among the Resourceful.” PsyArXiv. February 2. psyarxiv.com/tp93r

Abstract: Toxicity and hostility permeate political debates on social media, but who is responsible? Canonical theories of political engagement equate political resources with being a “model democratic citizen.” In contrast, we develop the theoretical argument that in the current polarized political climate, those same resources come to motivate hostile engagement. Combining two years of survey and behavioral Twitter data, we provide empirical support for a link between political resources and online political hostility. This link is especially pronounced among citizens high in affective polarization -- a trait held by many resourceful citizens in current US society. Concerningly, resourceful but hateful social media users do not simply cater to fringe audiences. Rather, they dominate online debates by tweeting more frequently, having more friends and followers, and by occupying powerful positions in their online networks. The present findings thus shed important light on the causes and consequences of online political hostility.

Human adult neurogenesis seems unlikely: Found signatures of adult neurogenesis in mouse, pig, and macaque dentate gyrus, but not in humans, adding to a growing body of evidence that this process is likely lost in humans

Transcriptomic taxonomy and neurogenic trajectories of adult human, macaque, and pig hippocampal and entorhinal cell. Daniel Franjic et al. Neuron, Volume 110, Issue 3, February 2 2022, Pages 452-469.e14. https://doi.org/10.1016/j.neuron.2021.10.036

Highlights

• Single-nucleus RNA-seq of adult hippocampal-entorhinal cells in human, monkey, and pig

• Transcriptomic signatures of adult neurogenesis in mouse, pig, and monkey but not human

• Excitatory neuron diversification delineates transitions from 3- to 6-layered cortex

• METTL7B defines subregion-specific excitatory neurons and astrocytes in primates

Summary: The hippocampal-entorhinal system supports cognitive functions, has lifelong neurogenic capabilities in many species, and is selectively vulnerable to Alzheimer’s disease. To investigate neurogenic potential and cellular diversity, we profiled single-nucleus transcriptomes in five hippocampal-entorhinal subregions in humans, macaques, and pigs. Integrated cross-species analysis revealed robust transcriptomic and histologic signatures of neurogenesis in the adult mouse, pig, and macaque but not humans. Doublecortin (DCX), a widely accepted marker of newly generated granule cells, was detected in diverse human neurons, but it did not define immature neuron populations. To explore species differences in cellular diversity and implications for disease, we characterized subregion-specific, transcriptomically defined cell types and transitional changes from the three-layered archicortex to the six-layered neocortex. Notably, METTL7B defined subregion-specific excitatory neurons and astrocytes in primates, associated with endoplasmic reticulum and lipid droplet proteins, including Alzheimer’s disease-related proteins. This resource reveals cell-type- and species-specific properties shaping hippocampal-entorhinal neurogenesis and function.

Discussion

We report an extensive single-cell transcriptomics analysis of several anatomically defined cell populations in the adult human, macaque, and pig hippocampal-entorhinal system. Our findings reveal fundamental species differences in adult hippocampal neurogenesis and delineate the molecular diversity of the cytoarchitectural transition from the allo- to the neocortex. These results also outline genes that are selectively enriched in certain species and cell types that may have a role in the specific biology and/or pathology of the hippocampal-entorhinal system

Unlike recent studies that mostly rely on one or two key markers (e.g., progenitors [nestin]; neuroblasts and immature granule cells [DCX]) (Boldrini et al., 2018; Moreno-Jiménez et al., 2019; Tobin et al., 2019), single-cell RNA-seq studies are more comprehensive because they leverage combinatorial gene expression profiles to identify cell populations more robustly (Hochgerner et al., 2018). This approach also allows cross-species analysis, amplifying rare signals within a single species that may be masked when analyzed separately. Our cross-species analysis allowed identification of the neurogenic lineage in the mouse, pig, and macaque, which was virtually absent in the human. We only detected one cell with the transcriptomics profile characteristic of nIPCs and one with putative neuroblast profile among 32,067 granule cells (0.003%) in our adult human DG samples, a proportion considerably lower than the expected 0.09%–3.8% neuroblasts according to previous DCX immunostaining or 14C incorporation studies of the adult human HIP (see Table S2 for data and relevant studies).

The same analytic strategy detected much higher proportions of neuroblasts in the other analyzed species (mouse, 6.6%; pig, 55.6%; macaque, 2.0%; Figure 2B; Table S3). These proportions were higher than those estimated previously based on progenitor proliferation and identification of neuroblasts markers such as DCX (Table S2), suggesting that more studies are needed to fine-tune detection of these neurogenic populations. However, this apparently lax detection protocol confirms that our parameters are unlikely to have missed any appreciable neuroblast populations among the large pool of surveyed human DG granule cells, even if they might exhibit an ambiguous profile.

Alternative confounding of our cross-species integrative analysis from possible human-specific transcriptomics changes was ruled out when human UMAP layouts did not include any clustering of neurogenic cells adjacent to the mature granule cell cluster. Likewise, the possibility that human neuroblasts exist in our samples but that their transcriptomics profile differs from other species and blends with related cell populations is lessened by findings that all neurogenic lineages preceding mature granule cells were absent in human DG samples (Tables S2 and S3).

We also extended our findings to existing snRNA-seq data of the adult human HIP. We reappraised the identity of a recently reported neural progenitor cluster (Ayhan et al., 2021) marked by LPAR1, a gene reported to mark mouse DG neural progenitors (Walker et al., 2016; Hochgerner et al., 2018). Our analyses indicated that this cluster actually represented doublets formed by oligodendrocytes and granule cells (Figure S3S). In addition, reanalysis of the pioneer HIP data (Habib et al., 2017) by Sorrells et al. (2021) showed that the cell cluster labeled as neural stem cells was actually characteristic of ependymal cells.

Analysis of DCX transcripts in all the analyzed species showed expression in mature neurons, mostly InNs, and in glial cells, indicating that DCX expression is not exclusive to DG neuroblasts (Figures 3A and 3B). This pattern is in agreement with the reanalysis of the data from Habib et al. (2017) (Sorrells et al., 2021). All transcriptomics analyses performed so far suggest a lack of neurogenic cell populations in the adult human DG.

At the protein level, DCX was, with a few exceptions (Figure S3L), present exclusively in DG cells resembling neuroblasts and immature granule cells in all analyzed non-human species. Also, cells with immature morphology could be detected in other areas, such as the EC of the macaque or the pyriform cortex of the mouse, as described previously (Gómez-Climent et al., 2008; Zhang et al., 2009). In humans, there is intense controversy regarding DCX immunostaining in the human DG, with some reports showing negative results (Dennis et al., 2016; Cipriani et al., 2018; Sorrells et al., 2018, 2021) and others describing DCX-IL cells (Knoth et al., 2010; Epp et al., 2013; Boldrini et al., 2018; Le Maître et al., 2018; Moreno-Jiménez et al., 2019, 2021; Tobin et al., 2019). We detected clear DCX-IL cells in the amygdala and occasionally in the EC, but we could not find DCX-IL cells resembling neuroblasts in the DG in the same tissue sections. These inconsistencies in detecting DCX-IL cells in the adult human DG cannot be fully attributed to postmortem denaturation and degradation of DCX protein because DCX-IL cells were clearly detected in samples with prolonged PMIs (Figures S3D, S3E, and S3I–S3L). Moreno-Jiménez et al. (2019) reported an intensive protocol for antigen retrieval as a necessary step to label DCX cells in the human DG. However, they reported no positive cells in the EC, a relatively common finding in our study (Figure S3D) and another (Sorrells et al., 2021) using conventional antigen retrieval. Because our analysis did not reveal neuroblasts at the RNA or protein level (using diverse antigen retrieval methods), the question remains what those previously reported cells could be. Apart from underappreciated non-specific and off-target effects (Sorrells et al., 2021), those studies could label mature granule cells and InNs that might contain low levels of DCX protein that was detected specially after multi-step antigen retrieval. In support of this hypothesis is the fact that the faintly immunolabeled cells we detected mostly in the vicinity of the granule cell layer, exhibited the morphology of mature InNs, and some co-labeled with antibodies against GAD1, a marker of InNs (Figures 3E and S3M–S3Q). This faint staining is far from the strong staining and well-defined morphology of somata and dendrites revealed in the EC and in the amygdala (Figures S3D and S3E) and is similar to the light DCX immunostaining reported previously (Seki et al., 2019). Thus, our conclusion is that DCX protein might be expressed at very low levels in InNs or in some mature granule cells that can be lightly immunolabeled under normal antigen retrieval but can show more intense and widespread staining under more elaborate tissue treatment and stringent conditions of antigen retrieval. In fact, Figure 2I from Moreno-Jiménez et al. (2019) shows that around 75% of DCX-IL cells were colocalized with NeuN (RBFOX3, 75%), a marker of mature granule cells, and 91% of DCX-IL cells were also positive for Prospero homeobox1 (PROX1), a transcription factor expressed by granule cells that is also expressed by InNs generated in the caudal ganglionic eminence (Ma et al., 2013; Laclef and Métin, 2018), supporting the possibility that most DCX-IL cells might actually be mature granule cells or InNs.

Regarding RNA analysis, although the PMI for humans was longer than for other analyzed species, human brains were kept at 4°C for most of the PMI period, whereas the pigs used as controls for PMI were kept at room temperature. This warm PMI will likely exacerbate the postmortem effects, but those conditions were not an obstacle to detect the neurogenic pathway in this species. It could be argued that the neurogenic pathway in the human DG is not detected because our snRNA-seq strategy might preferentially exclude neurogenic cells in humans. However, it seems extremely unlikely that it will affect all cell types in the neurogenic lineage, from progenitors to neuroblasts, and only in human. Overall, the most parsimonious interpretation of the combined results from our RNA transcript analysis and the DCX protein study is that, contrary to the other analyzed mammals, ongoing baseline neurogenesis does not occur or is extremely rare in the adult human DG.

Similar species-related and cell-specific transcriptomics profiling that characterizes neurogenic potential also outlines the transition from allocortical to neocortical domains in the hippocampal-entorhinal system and shows that ExNs are the main drivers of the differences between subfields (Figure 4), which evidences a richer complement of ExNs than traditional descriptions based on cytoarchitecture. Our analysis provides a primer to further study these populations and characterize the possible implications for hippocampal-entorhinal physiology. These data refine our understanding of the evolution of the allo-, meso-, and neocortex. The transcriptomics signatures we developed strongly suggest homology between the mammalian allocortex and specifically deep layers of the EC and neocortex.

Among the genes contributing to the layer transition, we identified METTL7B to be important in hippocampus physiology and function. We found that METTL7B, equipped with methyltransferase activity, interacts with important AD-related proteins (e.g., APP, LRP1, RTN3, and RTN4). Importantly, our results suggest that these functional interactions in a subset of ExNs and astrocytes seem to be phylogenetically specific to Old World monkeys and apes (parvorder Catarrhini), species that show more marked signs of pathology related to aging, such as AD, than other species (Perez et al., 2013; Finch and Austad, 2015; Edler et al., 2017; Paspalas et al., 2018). Overall, our analyses provide multiple vignettes of how this resource can be used to identify cell types and genes that might be functionally relevant for the biology of the hippocampus, allowing inter-species comparisons.