Temporal processing deficits have been implicated as a potential underlying neural mechanism for alterations in higher-level cognitive processes commonly observed in neurocognitive disorders. Prepulse inhibition (PPI) of the auditory startle response (ASR) is a translational experimental paradigm commonly used to examine temporal processing deficits, revealing profound alterations in neurocognitive disorders such as schizophrenia1, attention deficit hyperactivity disorder2 and HIV-1 associated neurocognitive disorders3,4. Specifically, assessments of temporal processing in preclinical models of HIV-1 have revealed the generality, relative permanence, and suggested the diagnostic utility of PPI across the majority of the animals' functional lifespan3,4,5,6.
Use of an approach varying interstimulus interval (ISI; i.e., the time between the prepulse and the startle stimulus) in the analysis of reflex modification dates back to Sechenov in 18637. The seminal studies of reflex modification, a measure of sensorimotor gating, employed an approach varying ISI to assess flexor response and audition in frogs7,8, as well as knee-jerk responses in humans9. The first clinical application of the reflex modification procedure assessed visual sensitivity in a man with hysterical blindness10. Over a century after the first reports of reflex modification, the approach of varying ISI was popularized across a series of seminal papers11,12,13. Despite the inherent differences in the seminal studies on reflex modification (i.e., species, experimental procedures, reflexes), they established a temporal relationship that was strikingly similar between species.
Assessment of prepulse inhibition using an approach varying ISI, as detailed in the present protocol, has multiple advantages over the popularized percent of control approach. First, the approach affords an opportunity to establish the shape of the ISI function, including increases (sharper curve inflections) or decreases (flattening of the response amplitude curve)3,15 in startle amplitude, as well as shifts in the peak point of response inhibition3,5. Additionally, when an approach varying ISI is employed, startle response is a relatively stable phenomenon1, suggesting the potential utility of the approach in longitudinal studies examining the progression of neurocognitive deficits5,15. Finally, PPI provides a critical opportunity to understand the underlying neural circuitry involved in neurocognitive disorders16.
In our study, we employed two experimental paradigms (Figure 1), including cross-modal PPI and gap prepulse inhibition (gap-PPI), to evaluate the utility of an approach varying ISI to delineate effects of sensory modality, psychostimulant exposure, and age. The cross-modal PPI experimental paradigm utilizes the presentation of an added stimulus (e.g., tone, light, air puff) as a discrete prestimulus prior to an acoustic startling stimulus. In sharp contrast, in the gap-PPI experimental paradigm, the absence of a background (e.g., removal of background noise, light, or air puff) serves as a discrete prestimulus. Here, we describe both experimental paradigms for the assessment of temporal processing, as well as statistical approaches for the analysis of PPI and gap-PPI. Within the discussion, we compared the conclusions one would draw from the variable ISI approach and the popularized percent of control approach.