Eye tracking white sclera (limbus tracking). It presents another challenge for vertical eye movements due to obstruction of eyelids. Eye-tracking setups vary greatly: some are head-mounted, some require the head to be stable (for example, with a chin rest), and some function remotely and automatically track the head during motion. Most use a sampling rate of at least 30 Hz. Although 50/60 Hz is more common, today many video-based eye trackers run at 240, 350 or even 1000/1250 Hz, speeds needed in order to capture fixational eye movements or correctly measure saccade dynamics. Eye movements are typically divided into fixations and saccades
The Eye of the Heron in "The Eye of the Heron" are themes common to much of Le Guin's fiction and include the social constructions of gender, interactions between individuals from different societies, intra-actions within societies, and contact with that which is perceived as alien or other. The novel also explores different forms of social and political organization by juxtaposing pacifist anarchism with violent oligarchy. The characters' metaphorical internal journeys are reflected in literal external journeys throughout the plot. When asked, in a 1995 interview, what role the feminist movement had played in her writing, Le Guin situated "The Eye of the Heron" in the