Slow-velocity asymmetrical convergence: a decisive failure of "Hering's law".

With targets aligned in the midsagittal plane, six of seven subjects tested were often able to make smooth symmetrical convergence movements in which no detectable saccade occurred during the initial 300 msec of the eye movement (12-95% of their trials). With targets located in a plane parallel to, but appreciably to one side of the midsagittal plane, those same six subjects were also often able to make smooth, slow, "saccade-free" asymmetrical convergence movements that were appropriate in magnitude and velocity to the target location (ratio of excursions about 2 to 1). Vergence movements are thus more versatile than can be accounted for by a single generator of binocularly symmetrical input to the eye muscles (Hering's Law of Equal Innervation). The occurrence of "saccade-free" asymmetrical convergence suggests instead that during binocular viewing, each eye can respond independently to that eye's view of the target, resulting in binocularly simultaneous slow-velocity nasalward eye movements--which will represent symmetrical or asymmetrical convergence depending on the arrangement of the targets. A similar interpretation is also apparently demanded by recent data on the initiation of disjunctive smooth-pursuit movements in the monkey [King & Zhou (1995) Vision Research, 35, 3389-3400].