First, there are two types of brow wrinkles. Glabellar, or vertical, wrinkles, fall between the eyebrows. They are caused by corrugators, active little muscles located between the brow that exhibit anger or tension. Frontalis, or horizontal wrinkles, fall across the forehead and are caused by contractions in the frontalis muscle located in the central part of the forehead. Whether between the brows or centered in the forehead, it is tension that causes wrinkling.