Neuroplasticity, the capacity of the brain to reorganise itself as a result of experience, is a widely accepted phenomenon studied over the last 40 years. Learning a second language, the spontaneous recovery after a stroke and even learning how to play an instrument are examples of experiences driven by the mechanisms of neuroplasticity. Regarding the auditory system, specifically, several studies have also demonstrated that auditory environmental enrichment can lead to enhanced auditory learning (Engineer et al., 2004; Anderson & Kraus, 2013; Murphy et al., 2015; Jutras et al., 2020). Based on that evidence, auditory neuroplasticity-based interventions, such as auditory training, are currently recommended to manage auditory skills, especially in children with specific auditory-based disorders such as Auditory Processing Disorders (see BSA APD Practice Guidance) or after a cochlear implant (Fu & Galvin, 2007; Dornhoffer et al., 2022).
Auditory training is a set of strategies to develop or improve specific auditory skills (Chermak & Musiek, 2002). It involves actively listening to verbal or non-verbal sounds and responding through exercises with different difficulty levels. Those exercises can be delivered via a computer (computer-based auditory training, CBAT) or in a more traditional way with an audiometer, equipment which controls all the parameters that are supposed to be trained, such as signal-to-noise ratio, frequency discrimination or dichotic listening. Some key factors are directly related to the effectiveness of any auditory training: an appropriate level of difficulty, deficit-specific training, and intensity. The difficulty level must be adaptive to guarantee that the task is challenging enough but not extremely difficult. This level would correspond to around 70% correct responses (Musiek & Schochat, 1998). This number has to be maintained during the sessions through the smooth and gradual change in the parameters (e.g., changing the signal-to-noise ratio from +10 dB to +5 dB as the individual’s performance improves). Note that the auditory training must involve engagement; therefore, it can’t be a passive activity such as just listening to sounds as proposed by some alternative programme commercially available. Deficit-specific training corresponds to the training focused on the aspects that are impaired in the initial assessment. Research has shown that training specific impaired auditory skills such as spatial processing, or temporal pattern leads to the improvement of that particular skill (Graydon et al., 2018; Tomlin & Vandali, 2019). This explains why it is important to have a formal Auditory Processing Assessment before starting any intervention so an individualised, deficit-specific intervention plan can be outlined according to the assessment’s results. The intensity is another essential aspect to be considered; the training must be intense (e.g., at least five days a week) to improve the chances of observing a significant improvement after a few months.
Although there is considerable evidence regarding the benefit of auditory training in improving auditory skills in individuals with APD (Schochat et al., 2010; Murphy et al, 2011; Cameron et al., 2012; Loo et al., 2016; Kumar et al., 2021), there are still ongoing discussions about one specific aspect: the generalization of learning, which means, how far the improvement on the trained task is transferred to untrained tasks in the same or different domains (Murphy et al., 2015). More realistically speaking, the question is: is the improvement observed in the clinic setting also observed in everyday life?
One of the most systematic ways to investigate the extent of learning transfer in clinical studies is by considering outcome measures and classifying them according to distance from the trained domain; the perspective of near- and far-transfer. In figure 1, I propose a diagram of learning transfer from a clinical perspective, considering the impact of training in clinical settings and in everyday life.
Figure 1 – Diagram showing near and far learning transfer
Near transfer is observed from an auditory trained task (on-task learning) to a different auditory task or stimuli. The likely enhancement of auditory performance in trained and untrained auditory tasks benefits children presenting listening difficulties, such as children with auditory processing disorder and cochlear implant users. The improvement in everyday life is also noticeable in complex acoustic situations such as understanding speech in background noise or when mishearing words and following verbal instructions. This improvement has been demonstrated through several studies on children with APD (Jenny, Sloan, 1986; Musiek e Schochat, 1998; Bellis, 1996; Chermak e Musiek, 1992 Schochat et al., 2010; Cameron et al., 2012; Loo et al., 2016; Kumar et al., 2021) and cochlear implant users (Fu & Galvin, 2007; Dornhoffer et al., 2022). The far transfer should be observed through an improvement in top-down skills such as language, attention and executive functions. In this case, the training is supposed to have a broader impact benefiting not only children with different speech/language disorders such as dyslexia, specific language disorder and speesound disorder but also attention disorders such as ADHD. Those studies involving far transfer show mixed results, such as the presence or lack of generalization, and indicate that this specific topic is still controversial (Gillan et al., 2008; Murphy et al, 2011; Halliday et al., 2012; Murphy et al., 2015).
Bringing this more experimental approach to a clinical perspective is critical to understanding more systematically the extent of the training’s impact on different clinical populations and to remain focused on the primary purpose of the training, which is the improvement in daily life rather than in clinical settings. Wright and Zhang have stated that the process of generalization of learning is related to the extent that both trained and untrained tasks share the same neural process modified during training (Wright and Zhang, 2008); thus, it is also important to keep in mind that the more similar the tasks are to daily life the higher the chances of observing a real-life improvement. It is also essential to set realistic expectations regarding the effectiveness of auditory training in children with APD, given that most of them also present with concomitant disorders (e.g., language or reading impairment); thus, currently, research indicates that the training is very likely to improve the impaired specific auditory skills but not necessary abilities such as language, memory or general attention.
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