Prepulse inhibition of the acoustic startle reflex to obtain a psychometric function in mice

The acoustic startle reflex (ASR) is an automated motor response to an unexpected and intense auditory stimulus (Ouagazzal, Reiss, & Romand, 2006). When an audible ‘prepulse’ stimulus is presented before the intense, startle-evoking stimulus (SES); the startle reflex response is reduced and this is known as prepulse inhibition (PPI). The degree of ASR inhibition serves as a measure of the behavioral salience of the prepulse (Carlson & Willott, 1996). This study aimed to obtain a psychometric function from the amount of PPI of the ASR that resulted from varying intensity levels of a prepulse stimulus (PPS). Twelve mice were used for this study and each was tested twice. Six of the mice were of the C57BL/6J background (a common strain often used as a control) and six were wild-type offspring of mice that had a mutation of the ephrin (EphA4) gene (labeled as EphA4+/+(WT)) and were expected to be normal aside from possible early rearing effects from their mutant parents. An accelerometer measured amount of movement associated with the SES with and without the PPS. The PPS randomly varied between 13 different intensities in the range of 25 dB SPL to 75 dB SPL. In addition, there were two control trials of the SES with a PPS of 0 dB SPL and one random trial with no sound at all. Therefore, there were a total of 16 trials which were presented randomly in each of 11 blocks. For each test session, the PPS randomly varied by frequency filter; high-pass (HP) or band-pass (BP). The SES was presented at an intensity of 120 dB SPL for a duration of 15 ms and medium inter-stimulus interval (ISI) of 50 ms was used for all trials. A psychometric function was successfully obtained. There was no significant difference between the twostrains of mice (p=0.15) so data between the groups was pooled. A significant effect (p=0.04) of frequency filter was seen as more PPI was obtained with the HP vs. BP filter. The obtained threshold ranged from 19 dB SPL to 45.8 dB SPL depending on how threshold was defined