Autism spectrum disorder (ASD) is a developmental disability associated with social, communication and behavioral challenges (Allen, 1988; Wing, 1997; APA, 2013; CDC.gov, 2022). Approximately 1 in 44 children are diagnosed with ASD and this rate is 4 times higher in males compared to females (CDC.gov; 2022). While subjects with ASD may exhibit a variety of signs and symptoms, most have some degree of hearing dysfunction (Greenspan and Wieder, 1997; Tomchek and Dunn, 2007; Bolton et al., 2012; reviewed in Smith et al., 2019; Mansour et al., 2021a) which ranges from deafness to hypersensitivity to sounds (Roper et al., 2003; Alcántara et al., 2004; Khalfa et al., 2004; Szelag et al., 2004; Teder-Sälejärvi et al., 2005; Gravel et al., 2006; Tharpe et al., 2006; Russo et al., 2009). The auditory brainstem response (ABR) has provided insight into auditory brainstem dysfunction in ASD. Specifically, numerous studies of ABRs in subjects with ASD have revealed longer latency responses (Ornitz, 1969; Student and Sohmer, 1978; Rosenblum et al., 1980; Sohmer, 1982; Tanguay et al., 1982; Gillberg et al., 1983; Sersen et al., 1990; Thivierge et al., 1990; Wong and Wong, 1991; Maziade et al., 2000; Kwon et al., 2007; Roth et al., 2012; Azouz et al., 2014; Tas¸ et al., 2017; Miron et al., 2018, 2021; Ramezani et al., 2019; Delgado et al., 2021) and smaller amplitudes in waves I through V (Ornitz et al., 1972; Gillberg et al., 1983; Martineau et al., 1987, 1992; Klin, 1993). The binaural interaction component is calculated from binaural recordings of the ABR and is a metric to assess spatial processing (Dobie and Berlin, 1979; Levine, 1981; Kelly-Ballweber and Dobie, 1984). Consistent with the aforementioned hearing deficits, subjects with ASD have significantly lower amplitudes and longer latency binaural interaction peaks (ElMoazen et al., 2020). These ABR changes in ASD have been attributed to the immaturity of brainstem circuits (Li et al., 2020). Furthermore, these hearing deficits are supported by postmortem studies demonstrating consistent auditory brainstem hypoplasia and dysmorphology in ASD (Kulesza and Mangunay, 2008; Kulesza et al., 2011; Lukose et al., 2015; Mansour and Kulesza, 2020). Indeed, hearing difficulties have been proposed as one of the cardinal features of ASD (Osterling and Dawson, 1994).

In utero exposure to the antiepileptic drug valproic acid (VPA) is associated with a significant increase in the risk of an ASD diagnosis in human subjects (Moore et al. 2000; Williams et al. 2001; Rasalam et al. 2005; Koren et al. 2006; Bromley et al. 2013; Christensen et al. 2013). Therefore, timed in utero exposure to VPA is a biologically relevant and validated animal model of ASD (rodents: Rodier et al., 1996; Mabunga et al., 2015; primates: Zhao et al., 2019). Consistent with the hypoplasia and dysmorphology in the human auditory brainstem in ASD, VPA-exposed rats have significantly fewer neurons in their auditory brainstem and thalamus (human: Kulesza and Mangunay, 2008; Kulesza et al., 2011; Lukose et al., 2015; Mansour and Kulesza, 2021a; rodent: Lukose et al., 2011; Zimmerman et al., 2018; Mansour et al., 2019; Mansour and Kulesza, 2021b). Further, VPA-exposed animals have abnormal tonotopic maps, hyperactivation of brainstem centers in response to pure tones (Dubiel and Kulesza, 2016) as well as reduced ascending projections from the superior olivary complex (SOC) and ventral cochlear nucleus (VCN) to the inferior colliculus and medial geniculate (Zimmerman et al., 2020; Mansour et al., 2021b). So, not only are there fewer neurons in the SOC and VCN in VPA-exposed animals, fewer of these surviving neurons are making the appropriate ascending projections. Despite these structural and connectivity differences in VPA-exposed animals, ABRs have not yet been examined in this animal model. Based on previous findings in humans with ASD and VPA-exposed rodents, we hypothesized that VPA-exposed animals would have longer latency and lower amplitude responses to broad-band click stimuli and further, that these changes would persist throughout their lifespan. We examined this hypothesis by recording ABRs in a cohort of rats at postnatal day 22 (both ears) and then examined monaural responses in longitudinal cohorts from P28 to P360.