Left side cradling of an appetitive doll is associated with higher heart rate variability and attenuated startle in nulliparous females Susanne E. Suter a, ⁎, Harriet J. Huggenberger a , Steffen Richter b , Terry D. Blumenthal c , Hartmut Schachinger a,b a sesam — Swiss Etiological Study of Adjustment and Mental Health — National Centre of Competence in Research, Institute of Psychology, University of Basel, Birmannsgasse 8, Postfach, CH-4009 Basel, Switzerland b Clinical Physiology, FB I — Psychobiology, University of Trier, Johanniterufer 15, D-54290 Trier, Germany c Department of Psychology, Wake Forest University, Winston Salem, NC, USA abstract article info Article history: Received 6 May 2009 Received in revised form 6 July 2009 Accepted 10 July 2009 Available online 17 July 2009 Keywords: Acoustic startle reflex Heart rate variability Lateralisation Care giver–infant relationship Cradling behaviour Cradling represents a unique type of mother–infant interaction. A bias towards left side cradling of infants and baby-like dolls has been demonstrated in human females, irrespective of handedness. One explanation for this behavioural bias involves right hemispheric specialisation of decoding visual, acoustic, and tactile emotional signals of left perceptual field origin. This implies that emotional signals from the child could have a greater impact on the caregiver's affective state when originating in the left than right perceptual fields. This may represent a key reinforcing mechanism responsible for left side cradling, but this has never been tested. In the present study, sixty-two never-pregnant female volunteers held an appetitive baby-like doll on the left or the right arm while reflexive startle eye blinks to binaural acoustic noise probes, as well as heart rate variability (HRV), were assessed. During left side cradling startle eye blink magnitude was attenuated, and low frequency HRV was decreased while high-frequency HRV was significantly increased. Attenuation of startle occurs in positive affective contexts, and high-frequency HRV is a reliable marker of vagal activity known to increase in appetitive, non-stressful contexts. Thus, our results suggest that appetitive infant signals have more positive effects on affective processes of the female caregiver when presented in her left perceptual fields. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The mother–infant relationship is a complex and unique human bond with many distinct characteristics, one of which is cradling. Cradling has elicited intense research interest over the last 30 years, and studies have shown that 70–80% of mothers exhibit a left side preference for cradling their babies (Bogren, 1984; De Chateau, 1991; Salk, 1973). This phenomenon was also observed in non-human primates (Hopkins, 2004; Manning and Chamberlain, 1990), and in nulliparous female students cradling a doll (Huggenberger et al., 2009; Suter et al., 2007). Several possible explanations for this bias have been subjected to research, such as the heartbeat hypothesis which suggests that the bias originates in the soothing effect of the maternal heartbeat (Salk, 1973). However, strong empirical support for a single explanation has not emerged. One theory to explain this phenomenon focuses on mother–child communication, especially of emotional signals. When a mother cradles her infant on the left side the infants' face is located in the left visual field of the mother. Stimuli positioned in the lateral visual field are primarily processed in the contralateral hemisphere. In a left- cradling position the visual information of the infant is mainly processed in the right hemisphere, which is specialized for decoding of visual emotional signals such as facial expressions (Basu and Mandal, 2004; Leventhal and Tomarken, 1986). Moreover, in a left side cradling position, sounds of the infant are perceived preferentially by the left ear and transmitted largely to the right hemisphere, which has been described as being superior to the left hemisphere in processing emotional auditory input (Sieratzki et al., 2002; Sieratzki and Woll, 1996). A similar right hemispheric advantage may be present for tactile perception, since it has been suggested that right hemispheric lesions cause a stronger decrement of tactual perceptions, and that most parts of the left side of the body tend to be more sensitive than the right side (Weinstein, 1962, 1968). All of the above described effects may be relevant for a left perceptual field advantage to more easily detect changes in the infant state, for example a change in the infant's motor behaviour, when the infant is located at the left side of the mothers' body. Taken together, cradling in the left perceptual fields of several sensory systems might result in more extensive processing of the child's affective signals by the mother. As such, left side cradling would allow for a better monitoring of the infant's emotional expressions (Manning and Chamberlain, 1991). International Journal of Psychophysiology 74 (2009) 53–57 ⁎ Corresponding author. Tel.: +41 61 267 02 59. E-mail address: susanne.suter@unibas.ch (S.E. Suter). 0167-8760/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.ijpsycho.2009.07.002 Contents lists available at ScienceDirect International Journal of Psychophysiology journal homepage: www.elsevier.com/locate/ijpsycho