Amblyaudia is a deficit in binaural integration, a process that begins with excitatory and inhibitory activation in the medial and lateral superior olivary complexes of the auditory brainstem (Tollin, 2003). Interaural timing and intensity differences are used to detect coincident signals and to prioritize neural codes for transmission. Binaural integration is assessed by simultaneous presentations of temporally aligned competing signals to the two ears during dichotic listening tests. Most listeners produce an ear advantage in the ear that is contralateral to the listener’s language-dominant cortical hemisphere. Amblyaudia is characterized by larger than normal ear advantage during dichotic testing.
Several neural mechanisms for amblyaudia have been proposed including abnormal suppression of neural activity in the non-dominant ear by input to the dominant ear (Popescu & Polley, 2010), reduced inhibition in sub-lenticular auditory pathways (Schmithorst, Holland & Plante, 2011), abnormal cellular development in the medial geniculate body of the auditory thalamus (Galaburda, Menard, & Rosen, 1994), and poor interhemispheric transfer of information through the corpus callosum (Westerhausen, Grüner, Specht, & Hugdahl, 2010).
Amblyaudia is diagnosed following a consistent pattern of abnormal responses on at least two dichotic tests. When clinically assessed, 47% of 141 children were diagnosed with amblyaudia (Moncrieff, Keith, Abramson, & Swann, 2016). With objective testing, children with amblyaudia demonstrated no left-right ear correlation in cortical event-related potentials (Moncrieff, et al., 2004) and shorter latencies in the right ear and smaller amplitudes in the left ear for the Pa response of the MLR (Moncrieff, et al., in preparation).
Amblyaudia responds to a treatment similar to occlusion or penalization used for amblyopia. In early studies, Auditory Rehabilitation for Interaural Asymmetry (ARIA) resulted in significant improvement in non-dominant ear dichotic scores following 4 weeks of treatment (Moncrieff & Wertz, 2008). Children with amblyaudia also improved in listening comprehension, word recognition and oral reading whereas normal controls did not. A change in protocol from 3 sessions per week to 1 session per week produced similar improvements in binaural integration and allowed for higher rates of enrollment during the school year (Russo, Snyder & Moncrieff, 2014).
At several clinical sites, 133 children were enrolled in four one-hour ARIA training sessions (Moncrieff, et al., 2017). Children diagnosed with amblyaudia showed significant improvements in dichotic listening non-dominant ear performance and reductions in interaural asymmetry at the fourth training session and again at least 3 months after completion with no significant differences in performance between the two times, demonstrating that improvements from ARIA do not regress.
Levels of brain activity were measured with fMRI in 9 children diagnosed with amblyaudia before and after enrollment in ARIA. Greater left than right hemisphere bilateral activation was noted during silence (noise only from the scanner), diotic listening (same word in both ears) and dichotic (competing words in the two ears). Subtraction methods demonstrated that dichotic listening resulted in dramatically increased activity in the left hemisphere that significantly decreased in temporal and parietal regions following ARIA treatment (Moncrieff & Schmithorst, in preparation).
Many children with amblyaudia also have difficulties when listening to words spoken together with background babble. To assess whether ARIA training generalizes to other auditory processing skills, pre- and post-ARIA measures of words-in-noise performance were compared and significant improvements were noted in 15 children diagnosed with amblyaudia (Moncrieff, AAA 2018).
Current standards of diagnosis for APD have been seriously criticized for lack of homogeneity among multiple symptoms associated with the disorder (AAA, 2010; ASHA, 2005) and the use of sensitized speech tests (Cacace & McFarland, 1998) with poor validity (Moore, et al., 2013) that lead to wildly different results across clinical settings (Wilson & Arnott, 2013). A failure to represent a uniform pattern of results makes APD a poor clinical entity whereas amblyaudia does represent a uniform pattern of behavioral results. Several researchers have reported variable results from using the CHAPS as a reference standard for the diagnosis of APD (Allen & Allan, 2014; Dawes, et al., 2007; Ferguson, et al., 2010; Iliadou & Bamiou, 2012). A recent study in 77 children seen for an APD assessment noted that CHAPS scores were significantly different between children diagnosed with amblyaudia and normal controls for the quiet, ideal, and memory conditions on the questionnaire (Moncrieff & Vermiglio, 2017).
Amblyaudia is a clinical entity that represents asymmetric performance on tests of binaural integration that improves under treatment through targeted training exercises with ARIA. Future studies will help elaborate the neurophysiologic underpinnings of amblyaudia and help to disambiguate it from other auditory processing weaknesses.
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