This is the "Neck of Fear". It is very often accompanied by the "Mouth of Fear" of which there are some variations. The above image displays a good example (albeit in lower resolution) of a moderate and closed-mouth fear expression. More pronounced examples of fear will show the mouth opened with some or most of the bottom teeth displayed. The neck display is caused by the contraction of the "Platysma" muscle which is a very thin muscle, just beneath the skin of the neck - which runs between the jaw bone (mandible) and the collar bones (clavicles). Occasionally you will see this neck display with very little changes in the mouth - and thus it is a fantastic tell of fear and apprehensive-fear.