The Power of Interstimulus Interval for the Assessment of Temporal Processing in Rodents

Summary

Temporal processing, a preattentive process, may underlie deficits in higher-level cognitive processes, including attention, commonly observed in neurocognitive disorders. Using prepulse inhibition as an exemplar paradigm, we present a protocol for manipulating interstimulus interval (ISI) to establish the shape of the ISI function to provide an assessment of temporal processing.

Abstract

Temporal processing deficits have been implicated as a potential elemental dimension of higher-level cognitive processes, commonly observed in neurocognitive disorders. Despite the popularization of prepulse inhibition (PPI) in recent years, many current protocols promote using a percent of control measure, thereby precluding the assessment of temporal processing. The present study used cross-modal PPI and gap prepulse inhibition (gap-PPI) to demonstrate the benefits of employing a range of interstimulus intervals (ISIs) to delineate effects of sensory modality, psychostimulant exposure, and age. Assessment of sensory modality, psychostimulant exposure, and age reveals the utility of an approach varying the interstimulus interval (ISI) to establish the shape of the ISI function, including increases (sharper curve inflections) or decreases (flattening of the response amplitude curve) in startle amplitude. Additionally, shifts in peak response inhibition, suggestive of a differential sensitivity to the manipulation of ISI, are often revealed. Thus, the systematic manipulation of ISI affords a critical opportunity to evaluate temporal processing, which may reveal the underlying neural mechanisms involved in neurocognitive disorders.

Introduction

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 disorders^3,4. 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 lifespan^3,4,5,6.

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 frogs^7,8, 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 papers^11,12,13. 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 inhibition^3,5. 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 deficits^5,15. 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.

Protocol

All animal protocols were reviewed and approved by the Animal Care and Use Committee at the University of South Carolina (federal assurance number: D16-00028). 1. Defining Parameters and Calibration of the Startle Apparatus Set up the startle response system (see Table of Materials) according to the manufacturer’s instructions. Enclose the startle platform in a 10 cm-thick double-walled isolation cabinet. </…

Representative Results

A prominent non-monotonic ISI function was observed in cross-modal PPI (Figures 2A, 3A, 4A) and gap-PPI (Figures 2B, 3B, 4B). Baseline startle responses were observed at the 0 and 4000 ms ISIs, included as reference trials within a test session. The importance of the 4000 ms ISI cannot be understated, as it most closely resembles the PPI test trials (i.e., 30, 50, 100, 200 ms ISIs) in that the subject receives both the prepulse and startling stim…

Discussion

The present protocol describes the power of varying ISI for the assessment of temporal processing for studies employing either cross-sectional or longitudinal experimental designs. Examining the effects of sensory modality, psychostimulant exposure, or age on the shape of the ISI function demonstrated its utility in revealing a differential sensitivity to the manipulation of ISI (i.e., shifts in the point of maximal inhibition) or a relative insensitivity to the manipulation of ISI (i.e., sharper inflec…

Materials

SR-Lab Startle Response System	San Diego Instruments
Isolation Cabinet	Industrial Acoustic Company
SR-Lab Startle Calibration System	San Diego Instruments
High-Frequency Loudspeaker	Radio Shack	model #40-1278B
Sound Level Meter	Bruel & Kjaer	model #2203
Perspex Cylinder	San Diego Instruments		Included with the SR-Lab Startle Response System
SR-Lab Startle Response System Software	San Diego Instruments		Included with the SR-Lab Startle Response System
Light Meter	Sper Scientific, Ltd.	model #840006
Airline Regulator	Craftsman	model #16023
SPSS Statistics 24	IBM		Used for Statistical Analyses (Optional)