Early cognitive development relies on the sensory experiences that infants acquire as they explore their environment. Atypical experience in one sensory modality from birth may result in fundamental differences in general cognitive abilities. The primary aim of the current study was to compare visual habituation in infants with profound hearing loss, prior to receiving cochlear implants (CIs), and age-matched peers with typical hearing. Two complementary measures of cognitive function and attention maintenance were assessed: the length of time to habituate to a visual stimulus, and look-away rate during habituation. Findings revealed that deaf infants were slower to habituate to a visual stimulus and demonstrated a lower look-away rate than hearing infants. For deaf infants, habituation measures correlated with language outcomes on standardized assessments before cochlear implantation. These findings are consistent with prior evidence suggesting that habituation and look-away rates reflect efficiency of information processing and may suggest that deaf infants take longer to process visual stimuli relative to the hearing infants. Taken together, these findings are consistent with the hypothesis that hearing loss early in infancy influences aspects of general cognitive functioning.

Funding: This research was supported by grants from the NIDCD ( https://www.nidcd.nih.gov/ ) to C.L.S. (F31 DC010281) and D.M.H. (R01 DC006235). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Copyright: © 2019 Monroy et al. This is an open access article distributed under the terms of the Creative Commons Attribution License , which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Introduction

Infants learn about the world through their multimodal interactions with objects and other people in their environment [1–3]. The atypical functioning of one sensory system may result in widespread effects across multiple sensory modalities and cognitive domains [4,5]. The auditory system, in particular, is thought to play a pivotal role in shaping the cognitive system [6]. As a result, researchers have recently begun to investigate the impact of prelingual hearing loss on cognition from a developmental perspective [7,8].

Several studies have shown that deaf children exhibit poorer performance in multiple nonverbal cognitive skills compared to their hearing peers. These include visual controlled attention (e.g., [9]), sequence processing [10,11], and working memory [12]. Other studies have also reported differences in motor abilities that require cognitive skills, such as in spatial coordination [13] and visual-motor integration skills [14]. Together, these findings suggest that general cognitive abilities differ between deaf and hearing children. However, the underlying sources of these differences are still unknown, as is when the differences emerge during infancy.

Differences between deaf and hearing individuals may initially arise from cortical cross-modal re-organization. In the absence of sound input, the auditory cortex responds to visual and somatosensory input [7,15,16]. Visual cross-modal re-organization has also been demonstrated in children as young as five years of age [17]. Cross-modal plasticity is thought to arise via compensatory mechanisms of the remaining senses following a sensory loss. However, evidence for poorer performance in deaf individuals has contributed to sensory deprivation hypotheses, which stem from the assumption that our sensory systems are intrinsically multimodal [1]. Atypical functioning in one modality will therefore have cascading consequences throughout other sensory modalities. Given the evidence that multimodal sensory processing underlies cognition (e.g., embodied cognition theories), atypical sensory functioning should also affect cognitive abilities.

One example, proposed by Smith and colleagues [18], is the hypothesis that hearing loss causes poorer multimodal sensory integration, which, in turn, causes deficits in visual selective attention and cognitive control. These authors suggest that multisensory integration is essential for the development of attentional skills in each individual sensory modality. In support of this hypothesis, they report data showing that cognitive control and visual selective attention in deaf children improves following cochlear implantation [18]. Another example of a sensory deprivation hypothesis is the auditory scaffolding hypothesis, which emphasizes the importance of auditory input for typical development of cognitive functions [19]. According to this hypothesis, sound provides experience with naturally sequential input that is vital for developing general sequence learning abilities. Sequence learning, in turn, influences a variety of other domain-general cognitive abilities and can have widespread consequences on multiple aspects of development.

An alternate explanation is that deaf children’s performance on cognitive and attentional tasks differs from that of hearing children because of their limited language experiences, rather than general cognitive abilities [20–22]. Early language and communicative experiences are critical for the typical development of social and cognitive skills [23]. However, the majority of deaf infants experience mismatched communication exchanges with their hearing parents—that is, the reciprocal communication pattern in typical parent-infant interactions is disrupted when the infant’s hearing status does not match that of their parent—and therefore experience difficulties in language development [24,25]. As a result, poor performance on attentional tasks could arise from early language delays [21]. This hypothesis is supported by evidence from Deaf children of Deaf parents (DOD)—children who are exposed to native American Sign Language (ASL) from birth and achieve typical social and language milestones [26].

In sum, a growing body of work provides evidence for differences in nonverbal cognitive abilities between deaf and hearing children, but the underlying sources remain unclear. Most of the research described above targeted preschool or school-age children, but no study has yet investigated the effects of hearing loss on cognitive abilities in young infants. As a result, little is known about when these differences between deaf and hearing children emerge.

To address this gap, the current study aimed to compare visual habituation in infants with prelingual hearing loss and infants with typical hearing. Habituation is one of the earliest cognitive processes to emerge in development (for a review, see [27]). Visual habituation reflects a basic form of learning: once a stimulus is fully encoded, the infant habituates to it and demonstrates decreased attention to the stimulus. Researchers have found that the duration of time to habituate in infancy accounts for up to 30 percent of the variance in cognitive ability at older ages [28–31]. This indicates that visual habituation relates to the development of more complex cognitive abilities. In addition, evidence from prior studies suggests that ‘look-away rate’—brief gaze shifts away from the target stimulus during habituation—reflects processing efficiency and attentional control [32]. Higher look-away rates correspond to shorter look durations, and prior research has found that infants who demonstrate shorter looks during habituation are faster and more efficient at encoding information [33].

If hearing loss is associated with differences in the early development of general cognitive abilities, we predicted that deaf infants may require more trials to reach habituation criteria, have slower habituation rates (the amount of decrease in looking time from one trial to the next) and have lower look-away rates than hearing infants. To test these predictions, we compared the number of trials to reach the habituation criteria, growth slopes during habituation, and look-away rate between deaf and hearing infants using a visual habituation-oddity paradigm [34]. For the deaf infants, we also examined potential relations between visual habituation and performance on standard language assessments before and after cochlear implantation. In doing so, we aimed to provide new evidence for the relations between hearing loss and processing of non-auditory stimuli at earlier ages than previously investigated. By testing young infants prior to the onset of advanced language development, we also aimed to shed light on the discussion of whether performance differences in deaf children may be due to domain-general processing or language experiences.