Swept away today by reading about Capgras' syndrome: the unshakable delusion that familiar people around you have been replaced by imposters. Not only because of "how could our brains lead us to think that?!" but also because of its insights into tensions between brain/mind-based causal explanations in psychiatry. This is going to be a long post, but deserves the characters, I think.

The syndrome was first described by Dr. Capgras in 1923. Following the death of her twin boys in 1906, a woman declared that her husband had been murdered and replaced by one among a rotating set of 80 imposters. She wanted to divorce the double and requested a separation from the courts. Capgras speculated that his patient had a disturbance in her ability to recognize familiar people. He also noted that she did not have a distortion in her ability to see or remember who is familiar to her, but that normal sense of familiarity is accompanied by strangeness. You can read a translation of the original paper here:
https://journals.sagepub.com/doi/abs/10.1177/0957154X9400501709

Capgras is rare and when it happens, is most often associated with Schizophrenia, bipolar disorder and dementia. Over the past century, many theories about what causes Capgras syndrome have emerged. They reflect the tension between "organic" (biological) causes and "functional" causes that are not biological. As recently as 1983, Robert Berson reasoned that Capgras could not be organic because it was such a specific delusion. Instead, he reasoned:

In Capgras’ syndrome, I would suggest, pathological splitting has occurred: Internalized object representations are split into "good" consciously acknowledged and "bad" unconscious images. The images persist until an even in interpersonal relationships triggers and important affective change. Previously repressed feelings enter awareness, the "bad" images surface, and the patient declares that a significant other is not who he seems to be but a double.
https://doi.org/10.1176/ajp.140.8.969

You probably won't be surprised to hear that a pile on ensued. They were all very polite about it, though.

Sir: I would like to heartily congratulate Robert J. Berson on his excellent article ... However, I must vehemently disagree ....
https://doi.org/10.1176/ajp.141.4.615-a

In 2017, Michael Fox's group at Harvard used a network mapping approach to identify where the lesions in 17 patients were located. They found that they overlap with brain regions involved in 1) familiarity perception and 2) belief evaluation. In other words, yeah - organic.
paper: https://doi.org/10.1093/brain/aww288
Video of one Capgras patient: https://oup.silverchair-cdn.com/oup/backfile/Content_public/Journal/brain/140/2/10.1093_brain_aww288/6/brain_140_2_497_s1.zip?Expires=1683368818&Signature=bdmW-6ZuKdN~XCCxG6pBZOIjncyX9tCkcjn88tCMv0ifrkz6CsN-uS~iP18hAnUjLRx5VbSb89~d3yUsYkggaBco48IBaTJ1LNdJYPmqAmbdvMP2Gb~4PeNNbxqBYGqRZszD2wtIDwm69QDnFApE9LDcPhsyc45CDNVRiJ7dRU-c~9BPJdmY~V5IhwA4HU9uE5dEdQITRVzDq6MBzGfp1T7jCE2J~iG~U4mp~WDaIj2dJoJsWbctS1AqipTBJM1Uj-U9XT2JRdBJzFzPVSmPUNjqu66uHMCSp7BHeL4skabV8Jin1H2JkdHPdWswudIe5LbTDQQSsMdhX--LU-wJvQ__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA

Still an open question: what exactly is happening in those brain areas that leads to imposter illusions?! Here's a fascinating news and views that puts it in context: https://doi.org/10.1093/brain/aww323

Notably, there's a lot of great work going on both to investigate the nuts-and-bolts of how face familiarity works:
https://pubmed.ncbi.nlm.nih.gov/34210891/
https://www.biorxiv.org/content/10.1101/2021.03.12.435023v2.full
As well as belief evaluation, e.g.:
https://pubmed.ncbi.nlm.nih.gov/22131418/
While I always love the nuts-and-bolts stuff (and have a soft spot in my heart for familiarity in particular), linking this work to Capgras makes it even more fascinating to me.