During prepulse inhibition trials, the same 125-dB 100-ms burst was preceded (250 ms) by a prepulse stimulus of 85 dB, lasting for 50 ms. Trials including prepulse stimuli were randomly presented with normal startle stimuli, the final total being 25 of each.
The PPI test, in which an acoustic startle response is reduced by a prepulse stimulus, is considered an operational measure of sensorimotor gating [ 16].
There were 7 different types of trials: the no-stimulus trials, trials with the acoustic startle stimulus (40 msec; 120 dB) alone, and trials in which a prepulse stimulus (20 msec; 74, 78, 82, 86, or 90 dB) had onset 100 msec before the onset of the startle stimulus.
An operational measure of sensorimotor gating is made in the 'prepulse inhibition of startle' paradigm (PPI), whereby a subthreshold prepulse stimulus presented around 100 ms before a strong, startle-inducing stimulus, can attenuate the startle response.
The startle inhibition was determined in various conditions: using (1) only a startle stimulus, (2) a startle stimulus preceded by a prepulse, and (3) a startle stimulus preceded by a gap of 50 msec in ongoing noise.
Prepulse stimuli of certain intensities elicited a response, and this response was greater when the prepulse stimulus was a white noise burst versus a tone of the same intensity.
In Experiment 2, the percent of PPI was linearly related to prepulse stimulus intensity for trials using a tone, but was biphasic on trials using a white-noise prepulse stimulus.
Second, we applied exactly this optimal prepulse stimulus for varying durations from 300 ms to 10 s (△=0.75x), followed by a return to 0 mmHg.
Further, the response to the prepulse altered the amount of inhibition and, therefore, confounded the overall measure of PPI at the higher prepulse stimulus intensity levels.
Scopolamine treatment attenuated PPI in both experiments and interacted significantly with the prepulse stimulus intensity in Experiment 1.
A test session began by placing a mouse in a Plexiglas cylinder, and left undisturbed for 10 min. The startle stimulus was a broadband white noise that lasted 40 msec for all trial types and the startle response was recorded during this time period (measuring the response every 1 msec) without the prepulse stimulus.
