1. A head-worn computer, comprising: a camera adapted to capture a plurality of reflected images of an eye of a person wearing the head-worn computer to determine a plurality of positions of the eye, wherein the head-worn computer comprises a see-through display displaying digitally presented content, wherein the camera is positioned to capture the reflected images of the eye through a common optical axis with the digitally presented content in-line with the eye of the user; and a processor adapted to determine a rate of position change statistic describing a rate of positional change of the eye based on the plurality of reflected images, the processor further adapted to remove the digitally presented content from a field of view of the see-through display if the rate of positional change exceeds a predetermined threshold.