Acoustic startle modification as a tool for evaluating auditory function of the mouse: Progress, pitfalls, and potential

Abstract

Acoustic startle response (ASR) modification procedures, especially prepulse inhibition (PPI), are increasingly used as behavioral measures of auditory processing and sensorimotor gating in rodents due to their perceived ease of implementation and short testing times. In practice, ASR and PPI procedures are extremely variable across animals, experimental setups, and studies, and the interpretation of results is subject to numerous caveats and confounding influences. We review considerations for modification of the ASR using acoustic stimuli, and we compare the sensitivity of PPI procedures to more traditional operant psychoacoustic techniques. We also discuss non-auditory variables that must be considered. We conclude that ASR and PPI measures cannot substitute for traditional operant techniques due to their low sensitivity. Additionally, a substantial amount of pilot testing must be performed to properly optimize an ASR modification experiment, negating any time benefit over operant conditioning. Nevertheless, there are some circumstances where ASR measures may be the only option for assessing auditory behavior, such as when testing mouse strains with early-onset hearing loss or learning impairments.

Figure 1

Summary of primary and modulatory acoustic startle response (ASR) circuits with presumed or confirmed primary neurotransmitters. See abbreviation list.

Figure 2

Typical ASR modification stimulus paradigms. A) Background sounds such as noise can potentiate or inhibit the response to a startle-eliciting stimulus (SES), depending on the background sound level. B) Traditional experiments present noise or tone non-startling prepulses ~2–100 ms before a loud, brief SES. Short lead times (<20 ms) between the prepulse and SES result in facilitation of the ASR. Longer lead times inhibit the ASR. These stimuli are often presented in the presence of low level background noise to mask extraneous sounds. C) Offsets and onsets of otherwise continuous background noise can be used to assess temporal processing. The offset-only conditions are typically presented with very short lead times between the noise offset and the SES to inhibit the ASR. D) Silent gaps that include a noise offset followed by an onset are typically presented at longer lead times. The amount of gap-induced inhibition of the ASR varies the duration of the gap, the decay time, the noise bandwidth, and the lead time between the gap and the SES.

Figure 3

Lead time, frequency, and level effects on ASR. A) Tone prepulses presented at very short lead times (<10ms) produce prepulse facilitation (PPF). Longer lead times (50–200ms) produce prepulse inhibition (PPI). Lead times of >200ms produce mixed effects. Data replotted from Willott and Carlson (1995), CBA/CaJ strain. B) Effects of prepulse level on PPI. The amount of PPI increases as prepulse level increases, shown here for strains tested using SR-lab systems and similar stimulus conditions in two reports. Data replotted from Paylor and Crawley (1997) and Bullock et al. (1997).

Figure 4

Strain differences in PPI across laboratories. (A) Comparison of seven strains tested using different experimental conditions and apparatuses in two laboratories. (B) Comparison of six strains tested using somewhat similar stimulus conditions and similar apparatuses in two laboratories. Symbols are the same in 4 A and B. (C) Comparison of PPI in C57BL/6J mice tested using very similar stimulus conditions and apparatuses in two laboratories. (D) Comparison of PPI in DBA/2J mice tested using very similar stimulus conditions and apparatuses in two laboratories. Data replotted from: A-Willott et al. (1994), A, B-Paylor and Crawley 1997, B, C, D-Bullock et al. (1997), C, D Tarantino et al. (2000).

Figure 5

Age effects on the ASR amplitude in different strains. The startle-eliciting stimulus varied across studies and are denoted for each curve. ASR is normalized to baseline at 1 month of age because different units were reported across studies. Data replotted from: Ison et al. (1998), Willott and Turner (1999), Yun et al. (2006), Ison et al. (1997).

Figure 6

Tone PPI changes differently and depends on prepulse frequency with age in two strains of mice with early-onset age-related hearing loss: A) DBA/2J, and B) C57BL/6J. C) Noise offset PPI also changes differently in three strains with different progressions of age-related hearing loss. Data replotted from Willott et al. (1994) and Ison et al. (1998).

Figure 7

A) Short term habituation across the first ten trials within a test session varies across strains. Data from Bullock et al. 1997; Pilz et al. 2004; Simons-Weidenmaier 2006; Typlt et al. 2013a. B) Long term habituation across test days also varies with strain. Data from Plappert and Pilz (2005); Pilz et al. (2004); Typlt et al. 2013b.

Figure 8

Comparing operant conditioning and PPI sensitivity using signal-detection measures. A) Discrimination of intensity increments. Data from Behrens and Klump (2015). B) Sound location discrimination; Data from Behrens and Klump (2016); C) Gap detection. Data replotted from Ison et al. (2005); Radziwon et al. (2009).