Unit quiz: Methods that carry mechanistic claims: tools, recordings, genes and models

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Question 1

A protocol specifies 30 milliwatts of continuous illumination in striatum for 20 seconds and reports a 20 per cent increase in a motivated behaviour. The methods state that no temperature measurement was made. How should a reviewer respond?Require either a temperature estimate or an opsin-free and ion-swapped control, because at that power the reported relationship implies a rise of about 2 degrees Celsius with a rate effect of roughly 15 per cent for a temperature coefficient of 2, which is comparable to the reported behavioural effect.Reject the study outright, because any tissue heating invalidates optogenetic results and the artifact cannot be bounded after data collection.

Question 2

You are asked which single additional condition would most strengthen a chemogenetic necessity claim in which receptor-expressing animals received 10 milligrams per kilogram of the ligand and showed a deficit while vehicle controls did not. Which condition, and why?A lower dose of the ligand in the receptor-expressing animals only, because the point of a chemogenetic experiment is to show that the effect scales with the dose in the animals that carry the receptor.A receptor-free group receiving the same ligand dose, because the systemically administered ligand produces behavioural effects partly through a conversion product with a broad receptor profile, so the comparison that isolates receptor engagement is the one between animals with and without the receptor at that dose.

Question 3

A spike-sorting paper reports that all analysed units had zero interspike-interval violation counts and that the population contains 60 per cent more fast-firing cells than previous papers. What is the most likely explanation to check first?The violation criterion differentially excludes fast-firing units, because chance short intervals accumulate steeply with firing rate, so a zero-violation filter selects against the very population that seems over-represented and the reported composition may reflect the filter rather than the brain.The population contains more fast-firing cells because the new recording method has better electrode impedance and therefore detects more small cells than previous methods.

Question 4

A sales deck states that a polygenic score predicts a cognitive outcome with a correlation of 0.3 and should be used to tailor interventions to individuals. Which response is most accurate?A correlation of 0.3 is a strong effect for a behavioural outcome, so the score is suitable for tailoring interventions provided it is applied with adequate training data.A correlation of 0.3 explains about 9 per cent of the variance, so the score shifts group means but cannot resolve individuals, and the published educational evidence found such prediction inferior to parental socioeconomic factors and uninformative once prior achievement was included.

Question 5

A modelling paper reports that its network predicts neural responses with a correlation of 0.55, chosen as the best of 40 layers, on 50 stimulus presentations, and concludes that the region implements the network's computations. Which revision is best?Report that the network's representations matched the responses at held-out generalisation level, with the selection over layers acknowledged and the precision stated, since a correlation of about 0.55 on that number of presentations carries a margin of roughly a tenth and part of the peak reflects selection among 40 layers.Report that the network explains most of the response variance, since a correlation of 0.55 is a strong fit and the layer selection was principled because layers correspond to stages of processing.

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