The effect of face exploration on postural control in healthy children

Highlights

• The percentage of exploration time during postural measure increases with the age.

• The eyes ROI is more explored than the others ROIs as children grow older.

• Postural control change with age, sad and happy faces increase the surface area of the CoP only.

Abstract

The objective was to explore how face exploration affects postural control in healthy children. The novelty here is that eye movements and posture were simultaneously recorded. Three groups of children participated in the study: 12 children of 7.8 ± 0.5 years old, 13 children of 10.4 ± 0.5 years old and 12 children of 15.7 ± 0.9 years old. Eye movements were recorded by video-oculography and postural stability was recorded by a platform. Children were invited to explore five emotional faces (neutral, happy, sad fear and angry). Analysis of eye movements was done on saccadic latency, percentage of exploration time spent and number of saccades for each specific region of interest (ROI): eyes, nose and mouth. Analysis of posture was made on surface area, sway length and mean velocity of the center of pressures (CoP). Results showed that visual strategies, exploration and postural control develop during childhood and adolescence. Indeed, after nine years-old, children started to look the eyes ROI firstly, then the nose ROI and finally the mouth ROI. The number of saccades decreased with the age of children. The percentage of exploration time spent in eyes ROI was longer than the others ROIs and greater for unpleasant faces (sad, fear and angry) with respect to pleasant emotional face (happy). We found that in front of sad and happy faces the surface area of the CoP was significantly larger compared to other faces (neutral and angry). These results suggest that visual strategies and postural control change during children's development and can be influenced by the emotional face.

Introduction

Many studies have explored the effect of performing a secondary task on postural sway, so called dual-task. Postural stability is not an automatically answer but it depends to environment [1] and different types of secondary tasks may influence postural stability in different ways: either increasing or decreasing postural sway depending on the attentional cost of the task [2]. The U-shaped non-linear interaction model described by Huxhold et al. [3] suggests that the performance of an easy secondary task can shift the focus of attention away from postural control, leading to a better postural stability relative to a single postural task. Olivier et al. [4] using dual task paradigm described that postural control in children decreased when a second cognitive task involved a high level of attention. Indeed, the increase of the difficulty of the secondary task can be responsible for a degradation of postural control. In everyday life postural control always needs to be performed at the same time as other tasks, in other words postural sway is naturally a part of dual or multiple tasks.

Darwin [5] had already proposed a relation between posture and emotional states. In the last decade, some studies have explored the effect of emotion on posture in adult subjects only. Hillman et al. [6] have reported in adults that postural control changes while viewing affective pictures: in front of unpleasant pictures, female increased postural movements in the posterior direction, while males increased postural movements in the anterior direction. In other words, female exhibited increased movement away from unpleasant pictures with respect to males. Furthermore, Azevedo et al. [7] have shown that in front of unpleasant stimuli, adult subjects decreased the surface area of the center of pressure, exhibiting a ‘freezing’ behaviour. They suggested that such behaviour has been reported in humans and other many animal species when faced with threatening stimuli activating neural circuits which promote defensive survival. The study of Ohno et al. [8] in healthy teenagers showed that anxiety state decreased postural stability. They made the hypothesis that the anxiety state involved a less efficiently use of visual inputs due to an increase of pupillary diameter. To our knowledge, no study exists exploring the effects of emotion on postural sway in children by recording at the same time, both eye movements and postural performance.

Several studies have been conducted describing facial emotion exploration in children. Children have a greater attraction for human faces and face processing capability in children changed and improved with aged. Batty et al. [9] showed that face processing depends on the nature of emotion and that there is a developmental aspect concerning face processing of unpleasant and pleasant emotions. The network for processing unpleasant emotion expands earlier than those for pleasant emotions, the latter most likely related to the activity of frontal region of the brain.

Based on such findings facial emotion processing in children could change during childhood and adolescence because the brain activation changed with age. Indeed, Forbes et al. [10] reported that brain activation during face exploration is different depending to the age of young subjects; for instance, pre-adolescents (mean age 11.80 years) and late-adolescents (mean age 12.46 years) showed a different activity in the amygdala and in the ventrolateral prefrontal cortex while they were viewing emotional faces. Moreover, Vuilleumier and Pourtois [11] in a fMRI and ERP study described that emotional face processing implicates an interactive network with several cortical structures (fusiform cortex, amygdala, occipital cortex, superior temporal sulcus, parietal areas). At the same time, it is also well known that eye movement performances are developing with age in relation to brain maturation and that the control of postural sway continues to improve during childhood until the adolescence [12].

In this context, the goal of this study was to record simultaneously postural and eye movement's performance in healthy children during a dual task. More precisely, we wonder to explore in children whether emotional faces exploration could affect postural stability. Based on previous knowledge, our driven hypothesis was that with age increasing, children could improve both postural and emotional faces exploration abilities. Secondly we made the hypothesis that emotional faces processing could also lead to a different saccade and fixation performance, which could affect in a different way postural stability.