Unit quiz: Measuring and perturbing the human brain: what intracranial recording, imaging, stimulation and anaesthesia can license

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Question 1

An intracranial study reports that hippocampal theta power predicts successful recall. The cohort is 12 patients, but 760 of the 900 analysed electrodes come from two patients with extensive hippocampal coverage, and the effect is significant when electrodes are treated as independent observations. What is the main threat to the claim?Pseudoreplication: electrodes are nested within patients, so the effective sample size is closer to 12 than to 900, and the result may reflect the anatomy and seizure history of two individuals rather than a general property of the human hippocampus.The electrode count is large enough that sampling error is negligible; the only remaining concern is whether theta was measured with the right filter settings.

Question 2

A study of 17 participants images brain activity while they watch animated shapes, reports a cluster of 412 voxels in the temporoparietal junction at a corrected threshold, and the press release states that the region is where the brain represents the intentions of others. Which criticism is most warranted?The finding is sound because the cluster survived correction for multiple comparisons, so the only issue is whether the task really engaged mentalising.The cluster establishes that a spatially extended set of voxels responded more in one condition, but it does not establish a selective function for the region, and cluster extent inference with a liberal forming threshold can be invalid when the spatial autocorrelation does not match the assumed shape.

Question 3

Single-pulse transcranial magnetic stimulation over the left dorsolateral prefrontal cortex at 300 ms after stimulus onset slows responses on a working memory task, while stimulation over the vertex and sham stimulation have no effect. The authors conclude that the region stores the memory trace. Which alternative interpretation is strongest?The result shows that activity in the stimulated region is necessary at that moment for efficient performance, which is compatible with a role in manipulating or selecting information rather than in storing it; storage would need evidence that the information is present in the region's activity pattern.Because stimulation disrupted performance and a control site did not, the region must hold the memory representation for the duration of the delay.

Question 4

An open-label series of subthalamic deep brain stimulation in Parkinson disease reports a 45 per cent improvement in motor scores at six months. A later randomised trial with sham stimulation reports a 22 per cent improvement on active stimulation and a 12 per cent improvement on sham. What best explains the discrepancy?The randomised trial under-dosed patients, because open-label studies use higher amplitudes and longer follow-up, so the 45 per cent figure is closer to the true effect.Open-label improvement combines stimulation effects with expectation, attention and the microlesion effect of electrode insertion, all of which the sham-controlled design partly removes, so the unbiased estimate of the stimulation-specific effect is the difference between arms rather than the change from baseline.

Question 5

A patient with a severe brain injury is unresponsive to commands. A processed EEG index reads 45, within the range the manufacturer labels as adequate anaesthesia. A perturbational complexity index obtained with transcranial magnetic stimulation and high-density EEG is below the threshold that separated conscious from unconscious volunteers in the validation study. The family asks whether the patient is conscious. What is the best answer?The complexity index provides probabilistic evidence about the capacity for conscious processing rather than a direct readout of experience, and it should be interpreted alongside repeated behavioural assessment, because unresponsiveness, disconnected consciousness and unconsciousness are different states that can dissociate.The processed EEG index shows an anaesthetic-level value, so the patient is unconscious and the complexity result can be set aside as technical noise.

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