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Prepulse inhibition — MRCPsych Paper A MCQ

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ModerateNeurophysiology and neurochemistryPrepulse inhibitionMRCPsych Paper A

A 25-year-old man with schizophrenia undergoes an acoustic startle experiment. Pure-tone audiometry is normal, and he reliably detects the weak test tone. A loud pulse presented alone produces a normal blink electromyographic response. When the weak, non-startling tone is presented 100 milliseconds before the same loud pulse, his blink response is reduced by only 5%, compared with a mean reduction of 40% in healthy controls. Which abnormality best explains this finding?

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Correct answer: E — Impaired sensorimotor gating indexed by prepulse inhibition

The correct answer is E. Prepulse inhibition is the normal attenuation of the startle response when a weak, non-startling stimulus shortly precedes the startling stimulus. It provides an operational measure of sensorimotor gating and is commonly reduced in schizophrenia. Here, the participant detects the prepulse and has a normal response to the pulse alone, but the prepulse produces little suppression of that response. Normal audiometry excludes reduced auditory detection, while the normal pulse-alone response excludes increased baseline startle reactivity. Habituation concerns declining responses across repeated pulse trials rather than suppression by a preceding stimulus. Mismatch negativity measures cortical detection of auditory deviance and is distinct from the startle-gating paradigm.

Reference: San-Martin R, Castro LA, Menezes PR, et al. Meta-Analysis of Sensorimotor Gating Deficits in Patients With Schizophrenia Evaluated by Prepulse Inhibition Test. Schizophrenia Bulletin. 2020;46(6):1482-1497. https://pubmed.ncbi.nlm.nih.gov/32506125/