The brainstem response has the capacity to reveal auditory pathway deficits in a non-invasive and passive manner, which has engendered its long history of clinical use even in difficult-to-test populations. Although source-filter cues are simultaneously conveyed in the acoustic stream of speech, remarkably, they can be readily transcribed as both discrete components and as a whole by the auditory brainstem ( Johnson, Nicol, Zecker, & Kraus, 2007 Kraus & Nicol, 2005). Broadly speaking, linguistic content-vowels and consonants-is transmitted by particular filter shapes, whereas nonlinguistic information such as pitch and voice intonation, relies largely on characteristics of the source. The formants are conveyed through manipulations of the filter and provide cues about onsets and offsets of sounds. The vocal tract resonates and the resulting resonant frequencies are known as formants. Everything else-vocal tract, oral cavity, tongue, lips and jaw-comprises the filter. The lowest frequency of a periodic signal, such as speech sounds, is known as the fundamental frequency (F 0) and it is the rate of the vocal fold vibrations. In this model, the vibration of the vocal folds represents the sound source. The literature on speech production provides a useful dichotomy to describe the acoustics of speech ( Fant, 1960 Ladefoged, 2001 Titze, 1994), the source-filter model. Successful communication relies on being able to both produce and process speech sounds in a meaningful manner. Further, children with ASD have particular difficulties processing speech in background noise, as demonstrated by high speech perception thresholds and poor temporal resolution and frequency selectivity ( Alcantara, Weisblatt, Moore, & Bolton, 2004). Both receptive and expressive deficits can occur and both have been attributed, at least in part, to abnormalities of auditory processing ( Siegal & Blades, 2003). An individual with ASD will usually not engage in typical reciprocal communication ( Rapin & Dunn, 2003 Tager-Flusberg & Caronna, 2007). When speech is present, it is often slow to develop, echolalic, stereotypic, emotionless or excessively literal ( Boucher, 2003 Shriberg et al., 2001 Siegal & Blades, 2003). In severe cases, children with ASD are non-verbal. Impairment in the social and communicative use of language is a hallmark of ASD. Because it is both passively-elicited and malleable, the speech-evoked brainstem response may serve as a clinical tool to assess auditory processing as well as the effects of auditory training in the ASD population.Īutism spectrum disorders (ASD) is a cluster of disorders that includes autism, Asperger syndrome, and Pervasive Developmental Disorder Not Otherwise Specified (PDD-NOS) ( Siegal & Blades, 2003 Tager-Flusberg & Caronna, 2007). These data support the idea that abnormalities in the brainstem processing of speech contribute to the language impairment in ASD. Neural synchrony in noise was significantly related to measures of core and receptive language ability. They also exhibited reduced magnitude and fidelity of speech-evoked responses and inordinate degradation of responses by background noise in comparison to typically developing controls. Children with ASD exhibited deficits in both the neural synchrony (timing) and phase locking (frequency encoding) of speech sounds, despite normal click-evoked brainstem responses. We measured brainstem responses to the syllable /da/, in quiet and background noise, in children with and without ASD. Unlike cortical encoding of sounds, brainstem representation preserves stimulus features with a degree of fidelity that enables a direct link between acoustic components of the speech syllable (e.g., onsets) to specific aspects of neural encoding (e.g., waves V and A). Little is known about the processing and transcription of speech sounds at earlier (brainstem) levels or about how background noise may impact this transcription process. The origin of the deficit is poorly understood although deficiencies in auditory processing have been detected in both perception and cortical encoding of speech sounds. Language impairment is a hallmark of autism spectrum disorders (ASD).
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