Last but Not Least

Our eyes move in a continual stream of very rapid saccadic rotations, punctuated by brief periods of stationary fixation that typically last for about one third of a second. We are usually unaware of these movements, and indeed we cannot observe our own saccadic eye movements however hard we try. Here we employ a familiar principle to allow us to catch a glimpse for the first time of our own eyes in motion. Our inability to see our own eye movements was recognised by Erdmann and Dodge in 1898 thanks to a fortuitous discovery while observing, with the aid of a mirror, the eye movements of subjects reading text. The saccadic relocations of gaze were easy to spot when the experimenters observed the eyes of subjects. However, Erdmann and Dodge were intrigued when they `̀ chanced on the observation that when the head was held perfectly still we could never catch our own eye moving in a mirror. One may watch one's eyes as closely as possible, even with the aid of a concave reflector, whether one looks from one eye to the other, or from some more distant object to one's own eyes, the eyes may be seen now in one position and now in another, but never in motion'' (Dodge 1900, page 456). The readers of this article can repeat Erdmann and Dodge's demonstration themselves by looking into a mirror at their own eyes; looking from one eye to the next it is not possible to see the eyes movingöthey appear always to be stationary. The reasons that we cannot see our own eye movements were not appreciated fully at the time of Erdmann and Dodge's report. However, more recently a wealth of research has uncovered details of mechanisms involved in suppressing the visual pathways during saccadic eye movements (eg Burr et al 1994; Matin 1974; Volkmann 1976). A combination of this active suppression, of motion blur, and of the motion effect (Nakayama 1981) means that we cannot perceive visual stimuli during saccades. How then can we circumvent these visual mechanisms in order to allow us to catch a glimpse of our eyes in motion? The principle that we employ is based upon that first used by Pulfrich (1922) in his Pendulum effect, also known as the Pulfrich stereophenomenon (see Morgan and Thompson 1974). Pulfrich found that, when observing a pendulum swinging back and forth in a plane perpendicular to the observer's line of gaze, if a dark filter was used to cover one eye and the other eye was left uncovered, after a few moments the pendulum appeared to move in an elliptical arc. This effect arises from a temporal asynchrony in the visual pathways for the two eyes, introduced by the dark filter. The filter causes slight dark-adaptation in the darkened eye, which results in both retinal and cortical delays in the signalling pathways. Conversely, the other eye remains fully light-adapted and is not delayed. Because temporal synchrony is used as a signal to identify position in depth, the artificial asynchrony introduced by the filter results in a perceived mislocation in depth of the pendulum. It is induced temporal asynchrony in the visual signalling pathways that we might be able to exploit in order to observe our own eye movements. Saccadic suppression mechanisms must be coordinated temporally with the saccadic movements, such that the suppression occurs during the movement, but does not encroach significantly into the surrounding fixations. There has been some recent debate as to the source of saccadic suppression. Many researchers support the notion that suppression is of central origin, arising from an efference copy or corollary discharge of the eye movement command (eg Burr et al 1982; Ross et al 2001). However, Castet and colleagues Last but not least Perception, 2002, volume 31, pages 1403 ^ 1406