There WAS Plasticity in Brain Sex Difference Research!

JAS-Med 2014 at Baltimore was a fantastic conference. The papers were of really high quality, the talks engaging, and the attendees kind and curious.

As you may have assumed already, I presented parts of my research project on the intersection of adult neurogenesis (ANG) research and the neuroscientific quest for sex/gender differences in brains. As mentioned earlier, it is puzzling to read the standard account of ANG history—or, more broadly, of brain plasticity—and then realize that none of these ideas of malleable brains have had influence on rigid neural and social sex/gender classifications, as Rebecca Jordan-Young, Catherine Vidal, Cordelia Fine et al., and others argue.

I have to admit (and it embarrasses me to say so) that I set out to write the history of the absence of interaction between the two neuroscientific subfields: plasticity research on the one hand and brain sex difference research on the far other hand. What I found, however, calls for a history of the process of eliminating the sex-related origin of the first accepted (re-)discovery of ANG. This process took place within the neuroscientific literature itself. Apparently, stripping ANG of its roots in sexual dimorphism research worked so well and happened so quickly that neither neuroscientists nor neurofeminists seem to have picked up on this.

But how did this happen?

First, you may want to read this pretty useful account of the development of ANG research in the second half of the 20th century: Moheb Costandi’s “Does Your Brain Produce New Cells?” (focus on the section “Fountain of eternal youth?” and the first few paragraphs of “From mice to monkeys and men” and note that MC still doubts that ANG is “real”!).

“Buttercup” by squiddles, CC BY-NC 2.0 

Second, note that Fernando Nottebohm, when (re-)discovering ANG in the 1980s, was researching sexual dimorphisms in song birds. He and his team realized that sex-specific behavior in song learning correlated with sex-specific brain anatomy. In an experiment in the early 1980s, they treated female canaries’ brains with testosterone. This led to both functional and structural changes in the avian brains: the hormone exposure increased the birds’ singing and enlarged specific nuclei in their brains. I am paraphrasing how Steven Goldman and Nottebohm phrased their findings in their 1983 paper: something doubles in size, this is possibly due to the growth of new neurons, and this process also occurs without any testosterone treatment, only less significantly.

We find no discussion of how hormones and exterior influences shape the brain all the way through adulthood or anything the like.

In other words, what Nottebohm and his colleagues found was not any kind of large-scale “brain plasticity” that would be transferrable to human brain sex difference research. Rather, they presented their results as a detected sexually dimorphic, seasonally dependent, and hormonally stimulated size variability of certain brain areas in songbirds. No plasticity of brain sex differences, let alone any finding that would be applicable to other species.

In order to find out why we are missing this link between brain sex difference research and this important episode in ANG research, we have to take a look at the reception of Goldman and Nottebohm’s paper. I did a quick and dirty cited reference survey.

At first glance, out of 549 citations of the paper, only 40 were articles on sexual dimorphism research. Out of the 40 sex difference papers, only 18 deal also with plasticity. That is, the majority of the quoting papers deals with proliferation, precursor cells, cell death, neurogenesis, etc. Only less than 10 % of the citing articles reference the sex research context of Nottebohm’s discovery.

Furthermore, we find:
20 references from papers on human brains, and no overlap with sex dimorphism research in all of these 20 cases,
8 referencing papers on primates, and again not a single overlap with sex,
30 referencing papers on mammals, and still no overlap with sex,
13 referencing papers on vertebrates, two of which also deal with sexual dimorphism research,
and then, finally, there was 1 salamander paper including a discussion of sex differences.

We can see that references to Goldman and Nottebohm’s 1983 paper were pretty much confined to the realm of avian brain research. The ANG part of the paper was more frequently referenced than the results in sexual dimorphism research. And only a very minor part of the referencing papers deals, like the 1983 paper, with sexual dimorphisms and ANG at the same time. It is particularly striking that not a single one out of the 549 citing papers I looked at attempts to make any inference from Nottebohm’s findings to sexual dimorphisms in mammals and primates, let alone in humans.

To summarize very quickly: brain sex difference research and ANG research were initially very closely intertwined, and not only in Nottebohm’s research.

I will keep reflecting on what this episode tells us about the emergence and stabilization of plasticity research as a new subfield within the neuroscientific discipline. But for now, back to the work I get paid for (= grading papers and prepping for class!).

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