Postural and facial asymmetries of the startle response in 5-month-old infants

The study aimed to analyze the lateralization of the postural and facial startle components, providing an analysis of the relation between these two response constituents. Thirteen 5-month-old infants were involved in a new non-invasive whole body acoustic startle protocol. Six brief acoustic noise bursts were presented to elicit the startles. Using AIMMSS (Dondi, Agnoli, Vacca, Franchin, & Scatturin, 2005; Scatturin & Dondi, in prep.), a computerized instrument for non-invasive startle measuring, an analysis of the intensity and latency of the whole body motor response (postural component) was realized in the right and left body sides. Using the EBS Scale (Essex et al., 2003) and the Baby FACS (Oster, in press), we analyzed the intensity and latency of the right and left sides of the facial reflex component. The comparisons between the right and left body measurements showed no asymmetry in the startle intensity for both response components. On the contrary, the analyses regarding the latency showed a right response bias in the whole body startle, but not in the facial component. Finally, correlations between the measurements of the postural and facial components showed a positive association, in particular for the intensity parameter. The right latency postural bias could be explained by the presence of spinal asymmetries. In addition, the positive association between the postural and facial startle components on the right and left body sides showed that these two components are probably activated bilaterally in a similar way by the Nucleus reticularis pontis caudalis.