The Upper Trap
The large trapezius extends from the back of the head, out to the shoulder and down to the lower thoracic vertebrae. It can be thought of as having three sections upper, middle and lower. The barbell shrug seems to be the exercise of choice for this muscle group, yet to fully train the upper portion it requires targeting the upper trap with specialized movements.
When the upper limb is securely stabilized, while holding on to something, only then is the upper trap involved in extending the head.
When you are shrugging or doing a high pull you are holding on to a bar with fixed hands, but when you extend the head there is no resistance, hence no upper trap development.
Training on a 4 or 5 way neck machine develops the upper trap. To isolate one side of the upper trap utilize a one arm seated dumbbell or the Pendulum seated shrug. Grasp one side of the bench and shrug as high as possible. If your upper trap is strong your head will turn to the opposite side of the movement. This is natural as the upper cervical vertebrae move in the opposite direction of the lower segments.