Abstract: An optical device is provided for determining the movement of the head of a user of the device relative to a fixed object that the user is viewing while performing a specific physical task. The optical device includes a pair of spectacles that are worn by the user. A flexible optical altering membrane substantially smaller than the lenses of the spectacles is adhesively secured to each lens of the spectacles in the area of the lens through which the user normally views the object. The optical altering membrane is selectively removable and reattachable to the spectacle lenses. The membrane causes the object to appear to the user to be in a first position when viewed through the optical altering membrane and to "jump" to a second position when viewed through portions of the spectacle lenses not covered by the membrane.