VALIDITY OF LISTING LAW DURING FIXATIONS, SACCADES, SMOOTH-PURSUIT EYE-MOVEMENTS, AND BLINKS

In its original formulation, Listing's law referred only to eye positi ons during steady fixation. In recent years, however, several studies have suggested that Listing's law can be extended to the movements of the eyes, including during saccades and smooth pursuit. A major proble m in deciding whether or not Listing's law is obeyed during eye moveme nts is the influence of any spontaneous fluctuations in torsional eye position. To try to settle this question, the three-dimensional positi on of the eyes (around the three axes: horizontal, vertical, and torsi onal) was recorded with dual search coils in five normal subjects duri ng fixations, 20 degrees saccades, blinks, and 20 degrees pursuit move ments with a 20 degrees/s stimulus velocity. Eye movements across a wi de range of horizontal positions were measured at different elevations of gaze during 11 min. Variability (as reflected in the standard devi ation of torsional eye position) was used as a measure of the validity of Listing's law. After linear detrending single trials, each lasting 21.5 s, to remove the effects of drift over minutes, the reduction in the standard deviation of torsional position in tertiary eye position s was 54% assuming a planar and 58% assuming a second-order curved Lis ting's surface. We attributed this long-term fluctuation of the torsio nal signal to slippage of the coil on the eye. The remaining variabili ty was mainly due to short-term fluctuation of eye torsion over second s. The impact of hysteresis, associated with consecutive centrifugal-c entripetal horizontal movements, on the variability of torsional eye p osition appeared negligible. Peak increases in the standard deviation from the fixation baseline after fitting individual Listing's planes f or each trial were 348% during blinks, 141% during saccades, and 72% d uring pursuit movements (median value of five subjects). In conclusion , Listing's law during blinks, saccades, and pursuit is less valid tha n during fixations, which raises doubts about the existence of an inte rnal ''Listing's law operator'' for eye movements. Possibly, central e ye velocity commands do not comply with Listing's law.