Abstract
This project aimed to explore the neural basis of acoustic startle reflex (ASR) plasticity, namely startle habituation and prepulse inhibition (PPI), and the influence of sex steroid hormones on startle habituation, PPI, and their associated neural substrates. Startle habituation and PPI are two forms of startle plasticity which result in the attenuation of the startle reflex response; however, these two processes reflect different attentional and sensorimotor processing. Moreover, sex differences have been observed in both startle habituation and PPI, with animal and human studies illustrating more rapid habituation and more PPI in males, compared to females. Sex differences in startle modulation can be explored through sex steroid hormones, namely oestrogens, progesterone, and androgens, in the brain, of which these hormone levels will naturally fluctuate, such as during puberty, menstrual cycle, menopause and andropause. In addition, pharmaceutical intervention can affect endogenous sex steroid hormone levels, such as hormonal contraception use or hormonal replacement therapy (HRT). Consequently, the observed sex differences in startle habituation and PPI, may reflect the contribution of sex steroid hormones on its underlying neural circuitry.Neural circuitry underpinning the primary acoustic startle pathway and ASR plasticity has stemmed from animal models which are acoustic-focused. Functional neuroimaging studies in male-only and mixed-sex human studies have corroborated these findings, illustrating cortico-striatal-pallido-thalamic (CSPT) circuitry in PPI, and subcortical, cortical and brainstem activity during startle habituation. Yet existing human functional magnetic resonance imaging (fMRI) studies of ASR, startle habituation and PPI have been challenging due to the loud scanner noise caused by rapid switching of gradient coils in conventional fMRI sequences. In this context, Looping Star, a multi-echo, zero echo time (ZTE), near-silent pulse sequence offers a novel approach to fMRI research. This will encourage the use of auditory startle paradigms in functional neuroimaging studies of startle habituation and PPI, allowing for better translation from animal models of the neurofunctional basis of these forms of ASR plasticity.
This project aimed to demonstrate the applicability of Looping Star with simultaneous electromyography (EMG) to an auditory experimental paradigm to study startle habituation and PPI. Before exploring sex steroid hormone differences on PPI and startle habituation neural circuitry, the project aimed to map the neural correlates of startle habituation and PPI in a healthy mixed-sex adult sample. To explore sex differences observed in PPI and startle habituation, the influence of biological sex and hormonal contraception was investigated. Our goal was to explore whether there were differences in neural activity during startle habituation and PPI between sexes, and between non-hormonal contraceptive users and hormonal contraceptive users.
To aid in the development of the fMRI study, the published EMG study was conducted to systematically explore the effect of task and population parameters on prepulse-induced startle modulation. This would ensure a reliable and valid paradigm was developed for future imaging and drug research. Stimulus onset asynchrony (SOA), and task order were shown to affect prepulse-induced modulation (PPI and prepulse facilitation, PPF). Sex differences in PPI were observed, with males showing more PPI than females. A follow-up of the influence of hormonal contraception on PPI also showed greater PPI in males, compared to females not on hormonal contraception. Consequently, task and population parameters should be considered in the design of future research.
The simultaneous EMG-silent fMRI study was designed based on the EMG study. The auditory startle paradigm consisted of acoustic startle probes to elicit ASR, and repetition of this stimulus was expected to produce habituation effects. PPI trials used SOA based on the EMG study to elicit PPI, but PPF was not examined in the fMRI study as more investigative work was required for robust PPF research. Startle habituation and PPI were analysed separately, and to derive stronger inferences about brain-behaviour correlations, EMG-assessed measures of startle habituation (regression slope) or PPI (percentage) were modelled at the group level as a covariate. In addition, raw ASR amplitude was modelled at the individual level for parametric modulation to better characterise changes in neural activity with expected decrease in ASR during startle habituation and PPI. Hypothesised PPI neural circuitry were derived from the published systematic review of functional neuroimaging research on PPI and PPF. Functional neuroimaging studies of startle habituation studies are few.
Neural activity in thalamic (right), striatal, and insula regions of interest, which decreased in line with startle habituation, was observed across the whole group. Brainstem and thalamic (left) activity were also observed and decreased with more startle habituation, but this was not significant after corrections for multiple comparisons. There were no group differences in startle habituation. Startle habituation neural activity was consistent across sexes and hormonal contraception use, suggesting consistent neural recruitment in our sample. PPI, on the other hand, showed group differences in neural activity, as males and females not on hormonal contraception showed significantly more globus pallidus activity than females on hormonal contraception during PPI trials with SOA 60 ms, compared to pulse-only trials. This finding was not replicated on trials with SOA 120 ms, and this may indicate sensitivity of PPI to SOA when illustrating group differences. In line with the EMG study, PPI scores differed with males and females on hormonal contraception showing greater PPI than females not on hormonal contraception, but these differences were not shown in neural activity. This may result from differences in exploring PPI in a laboratory vs MRI environment.
This project sheds light on sensory information processing mechanisms in healthy populations and can be used to inform clinical studies investigating these processes in clinical disorders with aberrant PPI and startle habituation, such as Parkinson’s disease, Huntington’s disease, and schizophrenia. In addition, sex differences associated with these disorders can be examined in line with the current findings.
| Date of Award | 1 Mar 2024 |
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| Original language | English |
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| Supervisor | Steven Williams (Supervisor) & Veena Kumari (Supervisor) |