ABSTRACT
It is now well established that Listing’s law holds not only during fixation, but also during pursuit and saccades (for review, see chapters by Vilis and Misslisch, this volume). Implementing the law during these movements requires that the angular velocity axis tilts out of Listing’s plane by half the eccentricity of the trajectory. Now that this has been established, it becomes important to understand how neural and biomechanical mechanisms enable this behaviour. Current theories have assigned different roles to each of these factors (Tweed and Vilis, 1990; Tweed et al., 1994; Schnabolk and Raphan, 1994; Demer et al., 1995). An equally important further issue concerns not so much the causation of this movement strategy but its putative biological function. Most earlier studies on Listing’s law have concentrated on kinematic principles and have not yet yielded a compelling conclusion as to the functional meaning of this behaviour. It may be argued that studying conditions where the law does not hold, or is modified, may yield insight into these basic questions. In this vein, the present contribution will concentrate on Listing’s law in binocular vision. Earlier, we have studied the dynamical properties of the torsional changes during pure vergence movements in normal subjects in order to see to what extent horizontal and torsional vergence are dynamically coupled. This work (Minken and Van Gisbergen, 1994, 1996), as well as experiments by other groups (Mok et al., 1992; Van Rijn and Van den Berg, 1993) showed that the implementation of Listing’s law is vergence dependent which causes Listing’s plane to be rotated temporally in near vision. These findings render it unlikely that Listing’s law has a pure motor function such as minimizing muscular effort (Tweed, 1994). An alternative is that Listing’s law subserves some sensory purpose. However, Melis and Van Gisbergen (1995), who found it impossible to adapt Listing’s plane by intrasaccadic rotation of an extensive frontal-plane scene in a large number of trials, concluded that a purely visual purpose of Listing’s law is also unlikely. A recent theoretical analysis by Tweed (1994) proposes that Listing’s law and its modification in binocular vision may be a compromise strategy seeking to optimize binocular image correspondence while simultaneously minimizing muscle effort. Against this background, we have made 3D eye movement recordings in a condition where the binocular coordination of eye movements is severely disturbed. The data were obtained 108from an adult strabismus patient who lacks stereoscopic vision entirely and has the ability of voluntarily alternating fixation between his preferred right eye and his left eye. As we will show, his Listing planes, despite extensive eye muscle surgery at early age and stereoblindness, appear remarkably normal in many respects and show signs of binocular coupling. These data will be compared to those of normal control subjects and will be briefly discussed in the framework of current theories on the implementation of Listing’s law and its functional significance.
