**1. Introduction**

Down syndrome (DS) is associated to a genetic perturbation known as trisomy 21 affecting physical, motor, and cognitive functioning. It is the most common genetic cause of intellectual disability. DS vastly affects language processing and development [1–4]. Both language comprehension and production deficits have been described. In particular, growth slopes in comprehension become shallower with age and language production studies demonstrate either delayed or atypical speech patterns (for a review, see [5]), especially in childhood and adolescence (e.g., [6]). Speech production in infants and toddlers revealed mixed results (e.g., [7,8]). Phonological acquisition has been reported to be delayed, showing deviant patterns [9], and comprehension and production of prosody has been shown to be impaired in children with DS and adolescents [10]. Equally important, hearing in individuals with DS is often impaired consequently affecting their language learning (for a review, see [3,11]). Language learning difficulties are evident in the late occurrence of first words/signs which appear between 24–36 months of age, while in typically developing children they usually occur between 12 and 18 months of age. In addition to language processing, DS 26-month-old toddlers show impairment in their social communication abilities (e.g., [12]). Taken together, speech impairment in DS extends to later developmental stages affecting DS individuals' overall communicative skills, and possibly academic success, and general well-being.

Many studies identified the benefit of early parent-implemented intervention in DS children younger than three years of age for their further language skills (for a review, see,

**Citation:** Pejovic, J.; Cruz, M.; Severino, C.; Frota, S. Early Visual Attention Abilities and Audiovisual Speech Processing in 5–7 Month-Old Down Syndrome and Typically Developing Infants. *Brain Sci.* **2021**, *11*, 939. https://doi.org/10.3390/ brainsci11070939

Academic Editor: Margaret B. Pulsifer

Received: 2 June 2021 Accepted: 13 July 2021 Published: 16 July 2021

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e.g., [12]). One of the main aims of these early interventions in DS children is to target abilities that might relate to later language outcomes. For instance, a recent meta-study on joint attention demonstrated that this ability is rather a strength than a weakness in DS population [13]. Developmentally, joint attention refers to a nonverbal skill occurring in social interaction between an infant and a caregiver. Using eye-gaze cues, pointing gestures or vocalizations, attention between the infant and the caregiver is focused/shared to the same object/event, and accompanied by awareness that the attentional focus is shared (e.g., [14]). Joint attention in typically developing populations is related to object learning [15], word learning (e.g., [16,17]; but see other proposals [18,19]), or later language outcomes (e.g., [20]). Similarly, joint attention is relevant for word learning in Down syndrome children as well [21], and it is a strong predictor in DS infants for later expressive and receptive language outcomes [12]. However, this skill emerges chronologically later in DS children (in their second year of life) than in typically developing children (for a review, see, e.g., [22]), suggesting that precursors to joint attention development might be impaired in Down syndrome children. In the present study, we will focus on some of these possible cognitive abilities that might directly or indirectly support further language development in DS population.

To support initial communicative abilities, infants have to learn to take part in nonverbal communication (i.e., joint attention), but they also have to selectively attend to relevant social communicative cues. In particular, they need to attend to faces and communicative gestures, and to process visual communicative cues accompanying the auditory speech signal (i.e., articulatory movements, eyebrows and head movements, gestures, etc.). In adults, attending to these cues facilitates face-to-face communication in noisy conditions (e.g., [23,24]). In infants, visual cues may support phonetic and word learning [25,26], as well as the learning of syntax [27]. Importantly, visual cues are available to infants already in early infancy—by four months of age infants are able to integrate auditory and visual information (e.g., [28–30]). Thus, the ability to attend to visual communicative cues develops early in infancy and is important for language development. Understanding infants' attention to visual communicative cues in atypically developing populations is particularly relevant, especially for infants undergoing speech interventions that are often based on improving communicative abilities.

Studies investigating the ability to process visual communicative cues suggest that DS infants are delayed in comparison to chronologically matched TD infants. For instance, DS infants discriminate between objects and human faces by four months of age, while TD infants do so already by two months of age [31], suggesting impaired ability to detect relevant social communicative cues in DS early development. Further, in a longitudinal study during the first six months of life, TD infants demonstrate a first peak in forming eye contact with their mothers already at one and a halfmonths of age, while DS infants do so around their third month of age [32]. Interestingly, the same study revealed that once DS infants form eye contact, they maintain it longer than the TD group, possibly affecting their ability to shift their gaze towards other objects in their environment that a caregiver is gazing to. A recent study demonstrated that unlike TD toddlers, DS toddlers at 16 months of age (chronologically age matched with a TD group), and at 28 months (mental age matched with the 16-month-old TD group) are not able to detect a mismatch in the audiovisual speech signal [33].

Importantly, attentional (cognitive) impairments in DS infants go beyond the abovementioned impairments in visual speech processing and attention to faces. DS toddlers are slower in disengaging their visual attention from an object they have been engaged to, in comparison to chronologically or mentally-age matched TD infants, as shown by [34]. The same study showed that being faster in visual attention disengagement relates with higher expressive and receptive vocabulary abilities in both TD and DS toddlers. In another study, five-year-old DS children were faster in disengaging than TD children, but similar in how fast they orient (attend) to visual stimuli [35]. Other study yet reported lower performance in DS children from three-six years of age in visual sustained attention [36]. Therefore,

results converge in suggesting that DS children and toddlers' visual attention abilities are impaired in comparison to their TD peers. However, little is known on early visual attention abilities in DS infants, particularly in their first six months of life. Understanding visual attention skills in DS in the first months of life is crucial to understand their reported impairments in early face processing and audiovisual speech processing (e.g., [31,33]), that possibly underlie their impaired language development.

The current study assessed five-to-seven-month-old DS infants and compared them to a chronologically matched TD group in three separate measures of visual attention, audiovisual speech processing, and communication abilities. Our main goal was to establish what the early relations between the three components are and compare them across the DS and TD groups. We hypothesized that DS infants' performance in all the three measures would differ from TD infants' performance. Specifically, we expected that DS infants would show an impairment in visual attention, reflected in slower visual orientation latency, while for the audiovisual task the DS group would attend less to communicative cues than their TD peers. Finally, we expected that the DS group would underperform on measures of communicative abilities in comparison to the TD group.
