a remarkably-specific function has been given to light-sensitive cells in the eye & the brain area to which they send their signals: contagious itching. the basic behavioural finding didn't replicate in another laboratory.  

original articles: Gao et al., 2022 (Cell Reports), Yu et al., 2017 (Science), Liljencrantz et al., 2017 (Science), Barry et al., 2017 (Science), reported in: The New Scientist by Miriam Fauzia on 4th October 2022

this story was in episode 31 #itch #mirror #neurons #contagion #vision

the error bar says

the New Scientist reports that the reflex pathway for contagious itching has been found, suggesting that contagious itching evolved to protect animals from threats.

you are probably already itching as i speak. i had this experience recently when i was editing a chapter in a forthcoming volume about Somatosensory Research Methods. just reading, seeing, hearing or thinking about itch can make you feel itchy & want to scratch.

this is called 'contagious itch'. it can be annoying. a series of papers report that mice show the same behaviour as humans, even when watching videos of other mice scratching.

this week a paper in the journal Cell Reports makes the case that this visually-induced itching response - in mice - depends on specific chemical signals in a specific brain pathway that takes visual signals from the eyes to the brain.

has the neural basis of contagious itch been found?


the error bar can only hope to scratch the surface of this work. it's deeply biochemical, molecular work that is way, way beyond my expertise. but it was by far the most interesting neuroscience story covered by the news in the last two weeks. so i have tried - tried - to read it.

for background, i also read a 2017 paper by the same group in Science magazine. this paper was easier to follow & contained both behavioural & biochemical tests of visually-induced itching - in mice.

the behavioural experiments were done on 7 or 8 mice in different groups. each mouse was paired-up with a demonstrator mouse in an adjacent, transparent box. the experimenters noted how long each mouse looked at the demonstrator mouse & when it scratched itself. one group of demonstrator mice had a chronic itch condition which meant they scratched themselves a lot. the question was: would the observer mice scratch themselves after viewing the demonstrator mice?

yes. a lot. over a 60-minute period, mice watching a very scratchy mouse would imitate them about 5 times each, while only one of the mice watching a non-scratchy mouse gave a single imitatory scratch. & this was not because the mice did not look, or because they didn't, in general, scratch; & it still worked via video-feed rather than in person, so to speak.

then it got complex. a chemical called c-Fos was found more often in the dissected brains of mice that had been 'perfused & processed' - a biological synonym for killed. the brain areas implicated were the suprachiasmatic nucleus, nucleus accumbens, caudate putamen & amygla - urgh - & amygdala - which are all important brain areas involved in lots and lots of things. the suprachiasmatic nucleus was then focused on - this brain area receives visual inputs directly from the eyes, so the researchers assumed that it plays an important role in contagious itch.

one comment from me: despite this paper being in the world's most prestigious, journal, Science, the statistics are incorrect here: only 3 mice were studied in each group, with each mouse contributing 10 slices of brain to the c-Fos analysis; but the statistical test did not take this nested data structure into account - we have 10 brain samples from 3 independent mice, we don't have 30 independent samples, as the test assumed.

then it got more complex. the legend to Figure 3 is 231 words long, but only 137 of those are actual words - the rest is acronyms, symbols & numbers. so i shall summarise the rest of the Science paper by saying: the authors proposed that a chemical called GRP in the SCN is responsible for contagious itch in mice & that its input comes directly from light-sensitive cells in the eye.

now back to the new article from 2022.

the new paper replicates these basic effects - mice scratch themselves more after watching another mouse scratch - & the authors then rule in & out other chemicals, visual pathways & mechanisms that might account for this behaviour. but it's done in so much jargon- & acronym-laden detail that...

...i have given up, dear listeners. perhaps this is how most science journalists feel when confronted with scientific papers so dense & so full of technical terms that they are almost impossible to read, even with a science background.

i shall say only this: if true, this would be a remarkably-specific visual pathway. the claim is that light-sensitive cells in the eye are able to extract the specific visual features of another mouse scratching itself & that this visual scratching signal is sent directly to a part of the brain that processes the information & cues the mouse to scratch itself. that would be remarkable.

but it's probably worth pointing out that a second group of researchers replicated the original 2017 study, in Science, under identical conditions & with larger numbers of mice. they concluded:

"Despite using mice of the same strain, age, sex, & supplier as [the former paper by Yu & colleauges], we found no evidence for itch contagion in mice using either their definition of imitative scratching or other, less stringent, definitions."


detailed & unreplicated


The New Scientist: