A child's (or an adult's) nervous system may detect danger or a threat to life when the child enters a new environment or meets a strange person. Cognitively, there is no reason for them to be frightened. But often, even if they understand this, their bodies betray them. Sometimes this betrayal is private; only they are aware that their hearts are beating fast and contracting with such force that they start to sway. For others, the responses are more overt. They may tremble. Their faces may flush, or perspiration may pour from their hands and forehead. Still others may become pale and dizzy and feel precipitously faint.
Stephen W. Porges, The Polyvagal Theory: Neurophysiological Foundations of Emotions, Attachment, Communication, and Self-regulation
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