What makes for a good scientific recipe?
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When you cook frozen pizza in the oven, there are usually instructions on the back of the box: whether or not to remove any packaging, what temperature you should bake it at, how long you bake it for, which rack it goes on, etc. With these clear instructions, everyone who makes the same pizza will generally get the same result. Now imagine if the instructions were very vague, stating things like “bake for 5 minutes to 1 hour” or “bake at either 100°F or 400°F”. If this was the case, then each person who makes the same pizza might get a different outcome. Some people will burn the pizza while others will undercook it. Similarly, in research, it is important to have consistent, validated experimental protocols to measure any outcomes of interest. Otherwise, studies that are conducted on the same topic might get drastically different results simply because their protocols varied too much from each other. More detailed protocols that consider many variables result in more reliable data, just like how recipes with more details are easier to follow and have less room for error.
A paper by El-Cheikh et al. (2023) evaluated a new protocol (i.e., “recipe”) for measuring prepulse inhibition (PPI). First, what exactly is PPI? Imagine that you are walking on the street and suddenly you hear a fire truck’s siren close by; you would likely react to this loud noise by tensing up and flinching away from the sound. This is called the startle response, which occurs involuntarily in response to loud, sudden sounds. However, if you first heard a quieter siren (e.g. a fire truck that was further away) that did not startle you, and then the louder siren afterwards, your startle response to the second, louder siren would be decreased. This phenomenon is known as prepulse inhibition, or PPI. In other words, when a quieter, non-startling sound precedes a louder, startling sound, your reaction to the startling sound is decreased compared to if you heard the startling sound only. PPI is important for preventing sensory overload and contributes to your ability to filter through sensory information from the environment. Accurately measuring PPI is important because altered PPI is often observed in neurological disorders, such as schizophrenia and obsessive-compulsive disorder, where patients will still startle strongly in response to a loud sound even if it’s preceded by a quieter one.
Traditional methods to measure PPI have been inconsistent between studies, with protocols that include two prepulses, one startle pulse, and two interstimulus intervals (i.e., the amount of time between the prepulse and startle pulse). El-Cheikh et al. (2023) evaluated a new method that was published in Miller et al. (2021) that could potentially be used as a consistent method for measuring PPI moving forward. The protocol used by El-Cheikh et al. (2023) involved three prepulse levels of 75 dB, 80 dB, or 85 dB; five startle pulse levels of 80 dB, 90 dB, 100 dB, 110 dB, or 120 dB; and one interstimulus interval of 100 ms. Additionally, this entire protocol was repeated 10 times across 5 days. The following key points were concluded about this novel method:
The authors confirmed that the new method shows deficits in PPI in a rat model that is known to have disrupted PPI as measured through traditional methods.
PPI data cannot be assumed to be distributed in a consistent manner, so a type of statistical analysis called “non-parametric statistics” should be used to analyze it.
“Parametric statistics” assume that data follows a particular pattern, whereas “non-parametric statistics” do not have that assumption and so it is more flexible for data that does not follow a specific distribution.
The new method allows more detailed metrics to be obtained from PPI behavioural experiments, from which conclusions can be drawn about the brain regions that may be affected in disorders with PPI deficits.
For the rat model used in this study specifically, the authors concluded that disrupted PPI reflected alterations in higher-order brain structures such as the auditory cortex, rather than lower-level structures such as the brainstem.
Overall, this paper provides a thorough evaluation of a new protocol to evaluate prepulse inhibition (PPI), a sensory processing mechanism that is often disrupted in neuropsychiatric disorders. It is important to validate scientific methods so that they are consistent between studies and so that outcomes of interest can be accurately obtained. After all, no one wants to eat burnt pizza.
Original article:
El-Cheikh Mohamad A, Möhrle D, Haddad FL, Rose A, Allman BL, Schmid S. Assessing the Cntnap2 knockout rat prepulse inhibition deficit through prepulse scaling of the baseline startle response curve. Transl Psychiatry. 2023 Oct 18;13(1):321. doi: 10.1038/s41398-023-02629-6. PMID: 37852987; PMCID: PMC10584930.
Works cited
Miller EA, Kastner DB, Grzybowski MN, Dwinell MR, Geurts AM, Frank LM. Robust and replicable measurement for prepulse inhibition of the acoustic startle response. Mol Psychiatry. 2021 Jun;26(6):1909-1927. doi: 10.1038/s41380-020-0703-y. Epub 2020 Mar 6. PMID: 32144356; PMCID: PMC7483293.