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Using mechanical soothing to comfort crying babies?

Posted by fvanderhorst on 30 Jul 2020 at 12:54 GMT

Using mechanical soothing to comfort crying babies?

Maartje Luijk1, Rianne Kok1, Lenny van Rosmalen2, Frank van der Horst1

1Department of Psychology, Education, and Child Studies, Erasmus University Rotterdam, Rotterdam, the Netherlands
2Centre for Child and Family Studies, Leiden University, the Netherlands

*Corresponding author Rianne Kok

In this paper the authors respond to a study by Möller, De Vente and Rodenburg (2019) [1] in which the calming effect of ‘mechanical soothing’ was compared to that of parental soothing. We question the conclusions drawn by Möller et al. about mechanical soothing as an appropriate alternative to parental soothing. We argue that mechanical soothing is undesirable as it is insensitive to the child’s cues and the longterm effects on child development are unclear.

Keywords: infant crying; mechanical soothing; sensitivity

Babies cry. It is one of the few ways (next to looking cute) they have to elicit the care they need to survive the vulnerable period in which they are completely dependent upon adult care [2]. Caregivers are in turn provoked by the intense sound of crying. From an evolutionary perspective, it makes sense to calm a crying baby rapidly as not to attract the unwelcome attention of wild animals. But even in modern society, adults react strongly to crying: they perform worse on cognitive tests when they listen to cry sounds compared to when they listen to other, similarly loud sounds [3], and infant crying may be a risk factor for child maltreatment [4]. In typically developing infants, the amount of crying rises from birth onwards, peaks around the age of six weeks, and then diminishes [5]. Importantly, crying in early infancy can occur in spite of excellent parental care [6] and does not usually predict long-term problems [7].

In the study of Möller, De Vente and Rodenburg (2019) [1], the calming effect of ‘mechanical soothing’ was compared to that of parental soothing. To assess this, the soothing abilities of a ‘smart crib’ [a device developed to calm infants using swaddling, shushing via white noise, and swinging] were compared to the soothing abilities of a parent instructed to soothe according to a strict protocol [infant swaddled by experimenter, then held by mother horizontally and facing away from her]. Infant distress was triggered by eliciting the Moro reflex and placing the infant two meters away from its mother without eye contact or vocalization. Assessments of fussiness and heart rate showed a calming response in infants during both parental and mechanical soothing. When heart rate variability was assessed, no calming response was observed. When the strength of the calming response was compared between parental and mechanical soothing, the calming response was stronger for mechanical soothing when assessed through heart rate, and stronger for parental soothing when assessed through heart rate variability and fussiness. From this, the authors conclude that both soothing techniques result in a calming response, but that ‘it remains unclear whether parental or mechanical soothing is more effective for calming infants’ (p. 11). The study evokes many questions, but our main concern is two-fold. Firstly, we question whether it is justified to conclude that mechanical soothing can be considered an alternative to parental soothing, based on these results. It appears that the study only uses mechanical soothing, either by a machine (the smart crib), or by mothers instructed to behave like a machine. Secondly, we argue that mechanical soothing is undesirable; we argue that it is insensitive by definition and that it may have dire consequences for child development.

Sensitive responsiveness and dyadic regulation
Parents can choose from a range of different comforting techniques dependent on the type and level of distress of the baby, and their personal preferences. The ability to gear a response to the signals of the baby is a key feature of maternal sensitive responsiveness [8]: the ability to perceive infant signals, to interpret them correctly, and to respond to them in a prompt, appropriate, and contingent manner. The principal function of maternal sensitive responsiveness is to provide a safe haven for infants in the context of distress [8], and to help regulate the child’s emotions [9,10]. When an infant’s signal of discomfort is followed by a sensitive and comforting response by the parent, the child learns that they are able to elicit support in their environment whenever needed and that emotional states are manageable and transient. This process of dyadic regulation is important for the formation of the attachment relationship [11], and later self-regulatory skills [12]. Because infants have limited capacity to self-regulate, sensitive calming and comforting is crucial in establishing this pattern of dyadic regulation in parent-child interactions [9]. In the experimental design by Möller et al. [1], mothers’ sensitivity was artificially restricted, by instructing the parents to soothe their baby according to a protocolised procedure, instead of adapting their behaviour to its signals. The effect of soothing in the parent condition can therefore not be compared to an actual sensitive response of parents to infant crying.

The protocolized procedure in which the infant was swaddled by the experimenter, then held by mother horizontally, and facing away from her, actually resembles insensitive caregiving more than sensitive caregiving. For example, lack of responsiveness to crying or lack of modulation of comforting to the level of crying, is one of the indicators of insensitive parenting [8], and mistimed movements of parents, approaching the infant with their head averted, and becoming motionless while their infant is crying, are indicators of disoriented and frightening parenting behaviour [13], which has been related to disorganized attachment [14]. The classical Still Face Paradigm [15], an experimental procedure in which parents are instructed to be unresponsive to the child’s signals, has robustly demonstrated an increase in distress in infants when they are confronted with an unexpectedly unresponsive parent [16].

Thus, although the parental condition in Möller et al. [1] can be understood as an experimentally sound comparison for the mechanical condition, it is not representative for sensitive comforting and soothing by a parent. Treating the infant in a strange and unfamiliar way, different from what the infant is used to, can hardly be classed as “parental soothing”. Whether or not mechanical soothing is an appropriate alternative for sensitive caregiving can therefore not be concluded.

Historical perspective on mechanical soothing
Parents have many ways of comforting a crying infant, like increasing proximity to the parent (which allows for feeling the parental heartbeat and adjusting the body temperature), singing, talking, offering the nipple or a pacifier, using rhythmic movements (like swinging or patting), making shushing sounds, swaddling, carrying, or massaging. These practices differ across cultures [17], and there is no single approach that always works, nor is there an approach that works best for most infants [18]. Möller et al. [1] turn to mechanical soothing as an alternative to soothing by parents. If we consider previous research, however, it is easy to find plenty of examples of the potentially devastating consequences of mechanical soothing of infants and children.

Using a smart crib to calm a crying baby bears a worrying resemblance to the (in)famous experiments of American psychologist Harry Harlow. Harlow [19] separated baby rhesus monkeys from their mothers within hours after birth and placed them with two surrogate mothers. The first was a ‘wire mother’, hardly more than a wire frame, the second was a ‘cloth mother’, made of wire but covered with terry cloth. In the experiments, Harlow wanted to see whether it made any difference which of the ‘mothers’ provided milk. He reported that the baby monkeys spent the largest part of their days on the cloth mother, independent of which mother provided milk. Initially, Harlow suggested that real mothers could possibly be replaced by artificial mothers. Later in their development, however, these monkeys proved socially very incompetent [20]. Harlow concluded that love, warmth, and contact comfort are of crucial importance for healthy socio-emotional development of young infants and children.

With these results, Harlow gave a new impetus to the debate about the care for children in hospitals and institutions that started in the 1930s and lasted until the 1970s [21]. In this debate, studies by Bakwin, Spitz, Bowlby and others, slowly but surely, convinced scientists and laymen that the physical and emotional separation from a familiar environment was detrimental to the child’s well-being and that distant and detached care in institutions did not meet their emotional needs. Harlow’s studies are now generally seen as corroborating Bowlby’s attachment theory [22]. If anything, Harlow’s studies showed that Bakwin, Spitz, Bowlby, and others’ concerns about the cold and distant care for children were justified.

In recent years, Harlow’s conclusions have been confirmed by many studies [e.g., 23]. For healthy development, babies need a loving and available caretaker. Mechanical soothing by a smart crib or strictly protocolized caretaking resembles surrogacy parenting. Mechanical soothing can offer care, but not love and warmth. A machine, or machine-like behaviours by a parent, can calm the baby, and help to have some time to yourself as a parent. Taking good care of oneself is undoubtedly important, but the effects of mechanical care for the development of children are far from clear.

The caring for and raising of young children is no easy task. Parents may feel stressed and overburdened by (excessive) crying. Yet children need their parents for comfort and although mechanical soothing may seem an attractive alternative for parental love, we strongly question the desirability of such devices. There are some [24] who doubt whether we should search for interventions to stop infant crying at all. Möller et al. [1] nevertheless take on the challenge but draw conclusions about comforting by parents based on an experimental paradigm in which parents were unable to sensitively respond to their infant. The conclusion of Möller et al. [1] should therefore realistically be restricted to stating that it remains unclear which is more effective for calming infants: mechanically soothing by parents or mechanically soothing by machine.


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No competing interests declared.

Reply to 'Using mechanical soothing to comfort crying babies?'

elinemoller replied to fvanderhorst on 30 Jul 2020 at 15:13 GMT

Reply to ‘Using mechanical soothing to comfort crying babies?

Eline Möller123, Wieke de Vente23, Roos Rodenburg2345

1 iHUB, alliance of youth care, mental health care, and educational organizations, Amsterdam, The Netherlands
2 Research Institute of Child Development and Education, University of Amsterdam, Amsterdam, The Netherlands
3 Research Priority Area Yield, University of Amsterdam, Amsterdam, The Netherlands
4 Youth Health Service, Public Health Service (GGD) Hollands Noorden, Alkmaar, The Netherlands
5 Stichting Epilepsie Instellingen Nederland (SEIN), Heemstede, The Netherlands

*Corresponding author: Eline Möller

We appreciate the initiative of Maartje Luijk, Rianne Kok, Lenny van Rosmalen and Frank van der Horst for a scientific dialogue [1] regarding our manuscript ‘Infant crying and the calming response: Parental versus mechanical soothing using swaddling, sound, and movement’ that was published a year ago in PLOS ONE on April 24, 2019 [2].

The aim of our paper was to investigate whether swaddling, sound, and movement would evoke an immediate calming response (CR) when parents soothed their infants under six months of age and when a smart crib soothed the infants using the same stimuli. The basis for our experiment was derived from the concept of the prolonged intra-uterine effect [3-5], which relies on infants’ need for a continuation of intra-uterine motor and sensory stimuli [6]. More specifically, rich and nourishing sensory stimuli are thought to evoke a CR in infants when they are in the womb [4]. In our study, consisting of three phases of each two minutes, after a baseline in which the infant sat on the mothers’ lap, infant fussiness was induced by eliciting infants’ Moro reflex and subsequently placing the infant supine out of sight of the parent. During this procedure, parents were instructed to refrain from any verbal and non-verbal contact with the infant. Using a counterbalanced procedure, the swaddled infant was then soothed by the mother and the crib. Infant fussiness was observed and coded and infant heart rate and heart rate variability were measured. The CR was operationalized as decreased fussiness and HR, and increased HRV during soothing compared to lying supine. Infant fussiness and HR were lower during parental soothing and being soothed by the crib compared to the supine phases. Infant HRV tended to be higher during parental soothing than during supine, but did not significantly differ between mechanical soothing and supine. For HR, infants’ CR was stronger in the crib than in the parent condition, whereas for HRV, infants’ CR was stronger in the parent condition. For fussiness, infants’ CR tended to be stronger in the parent condition. Thus, we found clear support for a CR in response to swaddling, sound, and movement, regardless of means of soothing (parent versus crib).

Luijk et al. [1] raise two main concerns with our study. First, they argue that mechanical soothing cannot be considered as an alternative to parental soothing. Second, they argue that mechanical soothing is insensitive and therefore undesirable. In this letter, we explain why these concerns are unfounded and even rabble-rousing.

Before going into their main concerns about our paper, we first need to address the central problem, that is excessive crying. The authors stated that crying does not usually predict long-term problems, which is true in many cases, but which is far from true in case of excessive crying, the context sketched in our study. As excessive crying is reported by about 5-19% of parents [7], we can conclude that the problem of excessive crying is rather common. As reviewed in our study, there is a growing body of research showing us the potential detrimental consequences of excessive crying varying from short term problems such as cessation of breastfeeding [8], impaired infant sleep [9], parental chronic sleep deprivation [10], parental postpartum depression [11], and shaken baby syndrome [12], to the long-term risk of regulatory and behavioral problems in later childhood, such as externalizing problems and ADHD [13,14]. By ignoring the literature that clearly shows the negative consequences of excessive crying on the short and long term, the authors misjudge the gravity of the problem of excessive crying and as such parents’ need for new, improved responsive soothing and sleep methods. In addition, it should be noted that crying does not peak at six weeks of age, but rather is a plateauing phenomenon from birth on which gradually diminishes around 8-9 weeks of age [15].

The authors incorrectly state that we conclude that mechanical soothing is an appropriate alternative to parental soothing. Why this is stated by the authors is unclear as we wrote in our discussion [2]: “During the night, aiming to prolong infant sleep and to decrease the number of night wakings, the smart crib would not replace the parent, but would replace a conventional crib. Parents would, however, respond to the needs of their baby similarly to when using a normal crib” (p. 11). We therefore stress again that the goal of our study was to investigate whether infants would be soothed by swaddling, sound, and movement, not whether mechanical soothing could replace parental soothing. In the context of excessive crying, assistance of soothing by a smart crib would give exhausted parents the indispensable rest, such as during the night when infant and parent both can catch several hours of sleep. As we also wrote in our paper [2] “following Kurth’s system model on infant crying and maternal fatigue, infants who are easily soothed allow their parents to recover. Well-rested parents are better able to take care of their infants and to help them regulate themselves, resulting in more successful soothing and positive parent-child interactions” (p. 2). In contrast, exhausted parents are less able to take care of their infant, thereby increasing the risk of dyadic dysregulation. Thus, by improving the sleep of both parents and child, a smart crib may even help with the process of dyadic regulation between parent and child.

Relatedly, the authors state that because parental soothing was protocolized, the effect of soothing in the parent condition cannot be compared to an actual sensitive response of parents to infant crying. Moreover, the authors even claim that soothing the infant with its face turned away from the parent would be insensitive caregiving. The authors’ comparison of the parental soothing condition in which parents do respond to the infant’s crying, with situations such as the Still Face Paradigm [16] in which the parent is unresponsive to the infant’s signals while facing the infant, thereby calling the soothing in our study an example of insensitive, disoriented, and frightening parenting is abject. Although parents were indeed instructed to soothe their baby according to a protocolized procedure, which was necessary to be able to compare the parental soothing to the mechanical soothing, the way the parent soothed the infant consisted of soothing methods used all over the world: swaddling the baby, and then shushing and swinging the baby [17,18]. In the parental soothing condition, we chose to face the infant away from the parent as pilots showed that when infants’ heart rate belt touched the parent, infants’ ECG signal was sometimes lost, leading to serious data loss. Another reason for facing the infant away from the parent was that we wanted to code infants’ facial expressions as an index of infant fussiness. Soothing the baby facing away from the parent is, however, also a normal soothing position, called the reversed breastfeeding position [4]. In this way, parents are also able to massage the tummy of the infant in the case of colic. Some infants even get more distressed when they are soothed facing the mother, as they smell her breast milk [4]. Parents from all over the world also use other soothing positions, such as the football hold in which the infant is lying on its tummy on the parental arm, infants are carried on the parent’s back, or are held upright over the shoulder. These positions are all lovingly ways of holding the infant which do not include direct infant-parental facial interaction. We assume that Luijk et al. [1] would not claim these methods as insensitive as well, because of the absence of facial contact. As such we do wonder the purpose of Luijk et al. of framing our experimental procedure as insensitive. Thus, although the soothing of the infant by the parent was protocolized for research purposes, we reject the notion that holding the swaddled baby in the parent’s arms, while making shushing sounds and gently rocking the baby would be insensitive. Of note, parents all over the world have naturally occurring rituals with their infants, such as sleep rituals in which the baby is rocked in for that parent-infant dyad typical movements or rhythms [19].

Moreover, Luijk et al. [1] suddenly argue that we would have instructed parents to behave like a machine, which baffled us. What we actually did was to instruct parents to make soothing sounds while gently rocking their swaddled baby. Swaddling, shushing, and rocking or jiggling are soothing methods used by parents from all over the world for centuries. These methods are bundled into The Happiest Baby method [4], which also includes the infants’ sucking needs. Karp, pediatrician and child developmental specialist, was struck by the number of babies seen for shaken baby syndrome due to excessive crying [12]. Karp was determined to investigate how to help parents with new, sensitive and developmentally appropriate methods for soothing their baby, as to ultimately help parents and infants with bonding and providing loving care. This is what is referred to as the cuddle cure [4]. Besides that several studies have demonstrated positive effects of the Happiest Baby method on crying [20-21] and sleep [20,22,23], parents have strongly embraced the Happiest Baby method for soothing their infants [24,25]. Depicting parents as behaving machine-like using elements of the Happiest Baby method is, in our opinion, a disregard of parents’ needs and intention to soothe the baby. The implication would be that millions of parents around the world would “soothe” their baby coldly and without affection. Taking into account that parents have used rhythmic motion, soothing sounds and swaddling since ancient times, being prevalent in cultures all over the world [17,18], we strongly condemn the notion of Luijk et al. [1] that in doing so parents would act machine-like while using these stimuli. We rather consider offering the infant sensory rich stimuli as responsive, sensitive and age-appropriate ways of soothing and caring for young infants in the context of excessive crying and sleep.

Another aspect that deserves attention is the idea of Luijk and colleagues [1] that the infant would be deprived by the mother or parent while being in the smart crib as they refer to Harlow’s experiment of rhesus monkeys and the forced separation from their mothers [26], mentioning that our experiment has a worrying resemblance to Harlow’s infamous experiment. Recall that Harlow separated the monkeys for two years. In our experiment, infants were separated from their mothers for two minutes per phase and the whole experiment took 12 minutes to conduct. As such, our study is no different from other experimental parent-infant studies [27,28]. Short, that is about two minutes, moments of separation between infant and mother are common in normal daily life, in which infants and their mothers are reunited throughout the day repeatedly.

Furthermore, we wonder how familiar Luijk et al. [1] are with the functioning of the smart crib. Once the infant starts to show signs of tiredness, the infant can be put asleep into the smart crib, rather than a conventional crib, next to the parental bed (for instance during the night when parents will also go to sleep). It may be that after being cuddled by the parent or after following a sensitive sleep ritual, which is normal for parents to do, the infant needs some extra jiggling and soothing sounds to fall asleep, which is provided by the smart crib. If the infant is fussy or cries for more than three minutes, the crib stops the gentle rocking and white noise to inform the parent that the infant requires still some parental care. This procedure, putting the infant in bed, waiting for three minutes to give the infant a chance to fall asleep, and then checking whether the infant needs some additional care is comparable with the commonly provided advice to wait a short period of time to provide the infant the chance to fall asleep and regulating this process by itself [29]. However, infants that have not made the first biobehavioral shift may have difficulties with such a self-regulative process in the context of sleep [30]. Thus, if parents feel exhausted or suffer from postpartum depression, the crib may act as a co-regulator in the context of sleep, providing both infant and parents the chance to sleep. The crib was developed using third wave sleep science [31] to help parents respond sensitively to their child, not to ignore the child or use cry it out methods or using first wave behavioristic cycles of feed-play-sleep. Imagine the infant would wake during the night, it may be that the soft rocking and soothing sounds produced by the smart crib help infants to make the transfer to the next sleep cycle by themselves. Viewed from the perspective of attachment, if the infant would be in need of more sleep, the crib responds timely and appropriately to the infant’s need for sleep. If the infant would have other needs than continuation of sleep (for example a feeding, diaper change), the infant would not be soothed and the crib would stop after three minutes (the parent may also respond earlier). So, comparable to using a conventional crib, using the smart crib requires the parent to respond just as appropriately and responsively with love and warmth to the needs of the infant. It is thus a painful conclusion of Luijk et al. [1] that use of the crib would deprive the infant from the parents.

The idea arises as well from Luijk et al. [1] that parents won’t be capable of proper use of the crib, misusing the crib as a device to be used all the time. We consider this a serious disregard of the parental desire to be available for their infants, to cuddle them, to feed them, and to love them. In addition, the use of the smart crib can be weaned, simultaneously with the gradually diminishing intra-uterine effect at the end of the fourth trimester [4,5]: the smart crib’s weaning feature prepares the infant for an easy transition to a conventional crib. When the weaning mode is on, the smart crib will still produce white noise sounds, but no motion. Usually, within two weeks infants get used to sleeping in a still bed and are ready to transfer to a conventional crib. The white noise can still be used, but if parents wish to end it, they are advised to gradually put the volume down every night. At the same time, parents can help boost infants’ sleep by using several responsive techniques, such as the wake-sleep technique and building a clear sleep routine [32].

Furthermore, Luijk et al. [1] point to the devastating consequences of mechanical soothing of infants. It would have been worthwhile if the authors had provided examples of this, such as the recent recall of baby rockers due to reports of infant deaths [33]. The crib that was used in our study was developed in collaboration with Massachusetts Institute of Technology (MIT) not only to restore sleep in infants and parents, but also to prevent SIDS [34]. Still, it remains the responsibility of each parent to address the needs and safety of their children and the responsibility of infant youth care professionals to assist parents with safety issues [35], including the safe use of infant care seats and not letting your infant sleep in the car seat, the use of pacifiers and not dipping them in honey [36], or the use of a focal breastfeed [37], and not using sweet bed time drinks to fill up tummies, to name a few.

We consider that, taken from the parental perspective, not examining Happiest Baby or the smart crib as methods for soothing and sleep would be a missed opportunity. That is, not only pediatricians are educated about using the Happiest Baby for soothing [38,39], parents have embraced Happiest Baby [24,25], and up to date infants from the US, Canada, Australia, and Europe have slept over 100 million hours in the smart crib (H. Karp, personal communication, July 17, 2020). Understanding the science underlying infant calming using sensory stimuli is therefore of utmost significance. As researchers, we therefore refer to our call for future studies into sensory stimuli for soothing in the home setting, especially in the context of parental exhaustion and postpartum depression [2]. Its importance can be underlined with the model of Kurth et al. [10]. Bear in mind that Luijk et al. [1] raised concern about the still face as we addressed above. Following Kurth’s model in which the parent is not able to soothe, is sleep deprived, feels exhausted thereby eventually triggering postpartum depression, the parent and infant may express less reciprocal positive and more neutral affect [40]. Exhausted parents, struggling with depression may thus be less likely to tune in with their infant sensitively and responsively. This is exactly what we mentioned in the introduction of our experiment: “Parent and infant may then end up in a vicious circle, in which the baby and the parent bring each other out of balance time after time. A solution to break this vicious circle or to prevent parental exhaustion may lie in mechanical soothing” (p. 2). One of the mothers who participated in our pilot study into effects of the smart crib in the home situation [41] remarked while evaluating her experience with the smart crib: if I had not been able to let my infant sleep in the smart crib, I would have become a robot myself. So, besides the scientific significance of studying sensory stimuli as responsive soothing methods, we want to stress that we should not underestimate parents’ needs for loving solutions and the fact that parents find their ways into finding soothing methods themselves. In this light, the notion of Luijk et al. [1] that there is doubt about whether researchers should continue with their quest for mechanical interventions that help soothing the infant is questionable.

Lastly, we feel urged to comment on the tenor of the authors’ reply. It gives rise to the idea that we conducted a study using dark and malicious methods (i.e. “Using a smart crib to calm a crying baby bears a worrying resemblance to the (in)famous experiments of American psychologist Harry Harlow”). Although we appreciate that the authors informed us of their letter before beforehand, we regret the tone of voice Luijk et al. [1] used to reach out to us, as it does not testify to an elegant scientific discourse. Based on the description of our experiment, the research into the Happiest Baby method [20-23], books about The Happiest Baby or soothing techniques [4,33,38,39,42,43], and the widely available information about the smart crib, no such allegations should have been made. The authors could also have invited us to discuss the experiment, parental soothing and the crib, but instead they wrote an intimidating reply to our experiment a year after publication. Luijk et al. recently published a Dutch position paper [44] that calls for a broader spectrum of sensitive interventions for sleep in young children. We are determined to contribute to this spectrum by examining sensory stimuli used in parental presence to lovingly and developmentally appropriately soothe infants. While we are doing that, we encourage Luijk and colleagues to go ahead and ask the parents for their needs-and their babies’.

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No competing interests declared.