What Is Eye-Tracking?

Have you ever watched someone's eyes as they read or look at a scene? What do you notice about the eyes’ behaviour from simple observation? There are descriptions of eye-movements, as well as what they might tell us, that go back as far as Aristotle (Wade, 2010). However, this kind of observation only provides a limited understanding of eye-movements, and historically led to the erroneous conclusion that eyes ‘glide over scenes and to alight on objects of interest, which they would fix with unmoved accuracy’ (Wade and Tatler, 2011, p. 37). It was not until the late nineteenth and early twentieth centuries, when early eye-trackers were developed, that we gained insight into the rapid discontinuous nature of eye-movements.

The ‘twitches’ and ‘jerks’ of the eye – in other words their movements – are what we commonly refer to as saccades. The interval between the eyes’ movements, when the eyes ‘stop’, are called fixations. Both are a type of automatic, physiological response, which means that they are not under our conscious control (Rayner et al., 2012). In reading, saccades do not always move the eye forward in a text. About 10–15 per cent of the time, readers move their eyes back (regress) to previously encountered sections of text. These backward movements are referred to as regressions. Regressions can be short or long. Short regressions are usually due to overshooting a target.