Hearing loss affects up to three of every 1,000 live births in the United States.1 Recent research has shown that hearing loss from birth has cascading effects on development that extend beyond the auditory system. Deaf children exhibit poorer performance than normal-hearing children on a range of non-auditory skills, including working memory,2 sequence processing,3 visual attention,4 and motor coordination.5,6 In other words, the effects of hearing loss may extend beyond the ear and into general cognitive development. However, research to date has been limited to studies with children. As a result, little is known about the underlying causes of these differences or when they emerge.
To shed light on the effects of auditory experience on early cognitive development, we conducted an experiment comparing the visual habituation between deaf and hearing infants. Visual habituation is an early measure of cognitive processing. Basically, babies will look at a visual stimulus until they have fully encoded it in their cognitive system. They will then become bored and look away. How quickly they habituate reflects how rapidly or efficiently they process and encode visual information. This simple behavioral measure has been shown to predict cognitive abilities, including IQ, later in development.7–9 If deaf and hearing infants show different rates of visual habituation—which has nothing to do with sound or the auditory system at all—this would suggest that hearing loss begins to affect general cognitive development from very early in life. And our findings revealed exactly that: Deaf infants in our experiment took significantly longer to habituate to visual stimuli (Fig. 1) and showed a slower rate of habituation, as indicated by a growth curve analysis.10 These findings were unlikely to be explained by overall attention to the objects since there were no differences in looking times during the first habituation trial. They are also not due to general fatigue in the deaf group, as both groups showed renewed interest in a novel visual item following the habituation test. Therefore, it was not the case that the habituation mechanism was altogether disrupted in the deaf group, but simply that they took longer to process and encode the visual information.
What is the underlying mechanism driving this difference in cognitive processing? One explanation is that early auditory experience is critical for normal development across multiple domains.11,12 This interpretation of our data is supported by evidence of performance differences across a range of nonverbal cognitive measures in children and adults with hearing loss.11–13 In addition, these differences are mitigated following cochlear implantation, suggesting that access to sound makes it possible for deaf children to catch up to their hearing peers.14 Alternatively, another explanation is that language exposure—rather than sound exposure—is the driving force underlying the general deficits observed in deaf children.15 This explanation is supported by evidence that deaf infants born to deaf parents—who are exposed to a native language from birth—achieve typical language and social milestones.16 Another possibility is that deaf infants may take longer to habituate because they process more information about the stimuli rather than simply processing it slower. To dissociate these potential explanations, a next step will be to conduct a follow-up experiment with a group of deaf infants born to deaf parents and include additional tests of visual perception.
IMPLICATIONS FOR EARLY DEVELOPMENT
In sum, cognitive development in infants with hearing loss begins to diverge in the first years of life, prior to advanced language development. Regardless of the underlying mechanism, these findings indicate that we should expect to observe differences across multiple developmental domains in infants with hearing loss beyond the auditory system alone. Our study also highlights the importance of early intervention and suggests potential future directions for early intervention targets that go beyond auditory and speech-language intervention.
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