SIGMA‐MOVEMENT AND SIGMA‐NYSTAGMUS: A NEW TOOL TO INVESTIGATE THE GAZE‐PURSUIT SYSTEM AND VISUAL‐MOVEMENT PERCEPTION IN MAN AND MONKEY *

The simple observation that started our experiments has an age-old tradition in the history of science. Aristotle (384-322 B.c.) was presumably the first to describe that after looking into the sun or another bright light, a long-lasting afterimage persists and is seen thereafter in the respective line of sight.’ Such an afterimage is, of course, a perfectly stabilized retinal image; however, it can be seen to change its position in the extrapersonal space whenever the eyes move voluntarily. A small afterimage placed within 2” to 3’ around the fovea centralis can be used to elicit smooth-pursuit eye movements in darkness when one tries to fixate it intentionally. During the pursuit movements, the afterimage is seen moving in the direction of the eye movement~.’.~.~ This von Helmholtz afterimage movement illusion was explained by 19th century physiologists as being caused by a central interaction of premotor commands controlling gaze movement and the afferent visual-signal f l o ~ . ’ . ~ . ~ Visual movement perceived with a stimulus pattern that is stationary on the retina but with moving eyes is called efferent movement perception, while the visual movement elicited by a change in position of the retinal stimuli is called afferent movement p e r ~ e p t i o n . ~ . ~ To our knowledge, the first block diagram of the interaction between afferent sensory signals and the corollary discharges of efferent motor signals was published in the general scheme “MerkweltWirkwelt-Koppelung” by J . von Uexkull.’ This model [FIGURE la ) contains the essential properties that later became known as the efference copy hypothean efference copy of the motor commands interacts with the afferent sensory-signal flow; the result of this interaction constitutes the percept and simultaneously the signals driving the pursuit mechanisms. Despite its long existence, the efference copy interaction model rarely has been analyzed quantitatively and no systematic experiments have been performed with nonhuman primates. The afterimage paradigm is difficult to apply in animal experiments. We have therefore, during the last eight years, used another paradigm [FIGURE lb).’4.’5 A row of equally spaced stationary black and white stripes or black dots of the spatial period P, is illuminated stroboscopically at the flash frequency fs [flash duration half-time, 56 pseconds). When the center sis~10.11,1Z.13