Auditory-Processing Malleability Focus on Language and Music

Auditory processing forms the basis of humans’ ability to engage in complex behaviors such as understanding spoken language or playing a musical instrument. Auditory processing is not a rigid, encapsulated process; rather, it interacts intimately with other neural systems and is affected by experience, environmental influences, and active training. Auditory processing is related to language and cognitive function, and impaired auditory processing negatively affects the quality of life of many people. Recent studies suggest that the malleability of the auditory system may be used to study the interaction between sensory and cognitive processes and to enhance human well-being. KEYWORDS—perceptual learning; plasticity; training Auditory processing refers to the broad range of sensory and perceptual skills used to extract meaningful information from sound. Traditionally, the initial stages of auditory processing were attributed to a passive system automatically encoding the physical properties of sound in a bottom-up hierarchical manner (that is, from peripheral to more central structures). Here, we discuss evidence to the contrary: Not only are most stages of auditory processing susceptible to change resulting from either longor short-term experiences, many of these changes are mediated in a top-down fashion (that is, in a manner consistent with the influence of higher-level cognitive factors such as attention, memory and context), allowing even low levels of the auditory system to encode sound in a context-specific manner. This dynamic processing is achieved through the intricate anatomical and functional connections between auditory and other brain areas and between cortical and subcortical areas within the auditory system. THE EFFECTS OF LONG-TERM EXPERIENCE ON AUDITORY PROCESSING Language Experience A striking example of the effects of experience on auditory processing is that even though human babies are born with the ability to discriminate all possible speech sounds, this ability is constrained by learning their native language, such that older infants can discriminate only sounds from their own language. Kuhl (2004) suggested that language learning results in the infants’ brains becoming committed to patterns of a specific language, thus facilitating further learning of that language. An outcome of this process is reduced sensitivity to the sounds of other languages. Experience with one’s native language shapes not only speech perception but auditory processing in general. Thus, native speakers of Mandarin (in which pitch provides meaningful information) were better at processing pitch contours even in a nonlinguistic context, compared to native speakers of English (Bent, Bradlow, & Wright, 2006). At the physiological level, Mandarin speakers show more robust encoding of the pitch content of Mandarin sounds at cortical and subcortical levels of their auditory system, suggesting that language experience fundamentally changes the neural circuitry of the auditory pathway (Krishnan, Xu, Gandour, & Cariani, 2005). Musical Experience Striking differences in auditory brain function between musicians and nonmusicians are observed. Not only do musicians’ brains respond more strongly to the sound of the instrument they play in comparison to other instruments, they also show stronger responses to simple, artificial tones (Peretz & Zatorre, 2005). Furthermore, as shown in Figure 1, musicians’ brains manifest a more robust and faithful encoding of the pitch information contained in speech sounds in subcortical levels of the auditory pathway (Wong, Skoe, Russo, Dees, & Kraus, 2007). These findings suggest that, similar to linguistic experience, intensive music experience affects auditory processing in general. An alternative explanation is that individuals with better auditory function may be more likely to engage in music training Address correspondence to Karen Banai, Northwestern University, 2240 Campus Drive, Evanston, IL 60208; e-mail: k-banai@ northwestern.edu. CURRENT DIRECTIONS IN PSYCHOLOGICAL SCIENCE Volume 16—Number 2 105 Copyright r 2007 Association for Psychological Science to begin with, but recent studies support the first view. Although the brain responses of children about to start music lessons did not differ from those of a control group, after a year of training, researchers did find differences between the two groups in response to violin sounds (Fujioka, Ross, Kakigi, Pantev, & Trainor, 2006). Furthermore, longitudinal data indicate that music training in children results in improved verbal memory (but not visual memory) compared with children who have no musical training (Ho, Cheung, & Chan, 2003). Reorganization Following Sensory Loss and Injury Auditory processing changes not only to respond to the auditory environment but also to compensate for visual loss. Provided that the loss happened early in life, visual brain areas can become activated by auditory and tactile stimuli (Neville & Bavelier, 2002). Furthermore, congenitally blind adults are better than sighted controls at detecting sounds occurring at peripheral (as opposed to central) locations in space (Roder et al., 1999). The effects of sensory loss are thus not uniform across the auditory system or across developmental periods; some aspects of auditory processing are more malleable than others. Taken together, these effects demonstrate that auditory processing is dynamic and can be altered according to context. The effects of sensory loss described above are related to natural experiences; the exact causes of observed effects are hard to decipher. In the following sections, we describe evidence for reorganization and plasticity obtained through controlled studies in animal models. This evidence shows that specific neural loss and environmental inputs affect wide areas of the auditory system and are therefore expected to be important in accounting for sensory and cognitive conditions accompanying these circumstances. Consequently, these studies may help inform effective rehabilitation strategies. Hearing loss from damage to the inner ear (induced by noise exposure or age) leads to physiological consequences that extend throughout the auditory system. When a region of the cochlea sensitive to a particular frequency is severed, the representation of this frequency in the auditory cortex is also altered; representations of neighboring frequencies in the cochlea replace that of the missing frequency. This means that even in adult animals, higher levels of the auditory system will change their function in response to the type of information received from the auditory periphery. When information related to certain B ra in st em e nc od in g (S tim ul us -t oR es po ns e