What must an officer observe about the subject's head during the HGN test?

During the Horizontal Gaze Nystagmus (HGN) test, it is essential for the officer to ensure that the subject's head remains still while their eyes follow the stimulus. This is crucial because movement of the head can interfere with the accuracy of the test results. If the head is moved, it can falsely indicate the presence of nystagmus or reduce the visibility of the eye movements that the officer must evaluate. The HGN test relies on the observation of the eyes’ reaction to the stimulus, so keeping the head steady allows for a clearer assessment of how the eyes track the moving object and whether any nystagmus, which suggests impairment, is present.