Development of adaptive sensorimotor control in infant sitting posture

Highlights

• Infants demonstrate an inverted-U pattern for visual-postural coupling.

• Sensory re-weighting in postural control exists a few months after sitting onset.

• Postural development involves improved self-motion control and perceptual ability.

Abstract

A reliable and adaptive relationship between action and perception is necessary for postural control. Our understanding of how this adaptive sensorimotor control develops during infancy is very limited. This study examines the dynamic visual–postural relationship during early development. Twenty healthy infants were divided into 4 developmental groups (each n = 5): sitting onset, standing alone, walking onset, and 1-year post-walking. During the experiment, the infant sat independently in a virtual moving-room in which anterior-posterior oscillations of visual motion were presented using a sum-of-sines technique with five input frequencies (from 0.12 to 1.24 Hz). Infants were tested in five conditions that varied in the amplitude of visual motion (from 0 to 8.64 cm). Gain and phase responses of infants’ postural sway were analyzed. Our results showed that infants, from a few months post-sitting to 1 year post-walking, were able to control their sitting posture in response to various frequency and amplitude properties of the visual motion. Infants showed an adult-like inverted-U pattern for the frequency response to visual inputs with the highest gain at 0.52 and 0.76 Hz. As the visual motion amplitude increased, the gain response decreased. For the phase response, an adult-like frequency-dependent pattern was observed in all amplitude conditions for the experienced walkers. Newly sitting infants, however, showed variable postural behavior and did not systemically respond to the visual stimulus. Our results suggest that visual–postural entrainment and sensory re-weighting are fundamental processes that are present after a few months post sitting. Sensorimotor refinement during early postural development may result from the interactions of improved self-motion control and enhanced perceptual abilities.

Introduction

Postural control is an important motor skill acquired during early development. To control the multi-segmented body in various conditions, a reliable and adaptive relationship between action and perception is necessary. Evidence of adaptive sensorimotor control in posture has been shown in adults [1], [2] and in children [3], [4], [5]. Little is known about how this type of control develops during infancy.

Young adults entrain their standing posture to the frequency properties of vision and somatosensory information [1], [6], [7]. The frequency response of this sensory-postural relationship demonstrates an inverted-U pattern with the greatest in-phase entrainment near 0.2 Hz and weaker and out-of-phase entrainment as the stimulus frequency decreases or increases [6], [7]. Furthermore, adults’ postural sway is proportional to the stimulus amplitude within a range. When the amplitude exceeds a certain value, postural response decreases [2], [8]. This change in the postural response to a change in the sensory stimuli indicates sensory re-weighting, a critical component of the adaptive sensorimotor control [1], [2], [9]. When a source of sensory information is unreliable, the postural system needs to attenuate this source of information and increase reliance on another modality. This sensory re-weighting process has been shown in children as young as 4 years of age [3] but has not been investigated in very young children or infants.

Newborn infants show directionally appropriate and velocity-scaled head response to visual flow information [10], suggesting that the visual–postural relationship may be a fundamental component of the sensorimotor system. A few studies have investigated infants’ postural responses to changes in the frequency of visual stimuli [11], [12], [13]. However, the results were conflicting. While some studies showed that infants’ postural entrainment increased as the frequency of the visual motion increased [11], [12], one study reported a linear decline in sway coherence as a function of stimulus frequency [13]. Furthermore, the developmental changes in infants’ visual–postural coupling were inconsistent between the studies [11], [12]. These conflicting results may be due to differences in the postural measures as well as the range of sensory properties employed in the studies. Since children were shown to respond to a wider frequency range of visual stimuli compared to adults [5], a sufficient range of sensory properties is required to better characterize the sensorimotor relationships in infants’ postural control.

In the present study, we sought to systematically examine the dynamic visual–postural relationship in infants as they develop postural control. Specifically, we examined infants’ ability to adapt their sitting posture to different properties of the visual signal, i.e., frequency and amplitude. Incorporating a wide range of input frequency and amplitude, we addressed the following research questions: (1) does the adult patter (inverted-U) of visual–postural coupling exist in early infancy? (2) is sensory re-weighting a fundamental process of postural control present in young infants? Using a cross-sectional design, we also examined how the adaptive visual–postural relationship may differ by infants’ advances and experience in postural development from sitting to standing and finally to walking.