Prepulse inhibition and facilitation of the postauricular reflex, a vestigial remnant of pinna startle

Abstract

If the postauricular reflex (PAR) is to be used effectively in studies of emotion and attention, its sensitivity to basic modulatory effects such as prepulse inhibition and facilitation must be determined. Two experiments were carried out with healthy young adults to assess the effects of transient and sustained visual prestimuli on the pinna‐flexion response to trains of startle probes. In the first experiment, participants passively viewed a small white square. It was displayed from 1,000 ms prior to onset of a train of noise bursts until the end of that train. Relative to no‐prepulse control trials, PAR amplitude was inhibited, possibly due to the withdrawal of attentional resources from the auditory modality. In the second experiment, participants performed a visual oddball task in which irrelevant trains of startle probes followed most briefly displayed task stimuli (checkerboards). Prepulse inhibition was observed when a transient stimulus preceded the first probe at a lead time of 100 ms. Amplitude facilitation was observed at longer lead times. In addition to documenting the existence of prepulse inhibition and facilitation, the data suggest that the PAR is not elicited by visual stimuli, that temporal expectancy does not influence its amplitude or latency, and that this vestigial microreflex is resistant to habituation. Results are interpreted in light of a recent theory that the human PAR is a highly degraded pinna startle, in which the reflex arc no longer includes the startle center (nucleus reticularis pontis caudalis).

Measuring Pavlovian appetitive conditioning in humans with the postauricular reflex

“…Of note, the postauricular reflex has also been shown to be resistant to habituation (Hackley et al, 2017), which contrasts with the startle eyeblink reflex (e.g., Bradley, Lang, & Cuthbert, 1993; Grillon & Baas, 2003; Hackley et al, 2017; Rimpel, Geyer, & Hopf, 1982) and SCR (e.g., Bradley et al, 1993; Hare, Wood, Britain, & Shadman, 1970) that are both sensitive to habituation, and is thus less affected by repetitive stimulus presentations, as is the case in human conditioning paradigms. In sum, the fact that we observed differential appetitive conditioning at the psychophysiological level with the postauricular reflex suggests that it provides a sensitive psychophysiological measure of human appetitive conditioning, probably even more sensitive than both the startle eyeblink reflex and SCR.…”

Vestibular Evoked Myographic Correlation

“…Potentially problematic, however, would be shortterm temporal effects such as those found for the post-auricular reflex. For example, in experiments with pairs (or longer series) of clicks, the second click elicited a smaller post-auricular response than the first click (Fox et al 1989;Hackley et al 2017). While no corresponding findings were obtained for the inion response (Fox et al 1989), VEMP experiments with pairs of tone bursts (Welgampola and Colebatch 2001) also showed an amplitude reduction for the second response, apart from one experimental condition for which a paradoxical amplitude increase was reported.…”

Evidence for a vestigial pinna‐orienting system in humans

“…Whether the eyeblink response to sudden intense illumination should be considered a component of startle or, alternatively, of the dazzle reflex (along with pupil constriction) remains controversial (Burke & Hackley, 1997;Graham, 1979;Landis & Hunt, 1939, p. 47;Yeomans et al, 2002). Regardless, available evidence indicates that the PAR can only be elicited by acoustic stimuli (Fox, Peyton, & Ragi, 1989;Hackley, Underwood, Ren, & Valle-Incl an, 2015).…”