Abstract Rapid shifts of the point of visual fixation be- tween objects that lie in different directions and at differ- ent depths require disjunctive eye movements. We tested whether the saccadic component of such movements is equal for both eyes (Hering’s law) or is unequal. We compared the saccadic pulses of abducting and adduct- ing movements when horizontal gaze was shifted from a distant to a near target aligned on the visual axis of one eye (Müller paradigm) in ten normal subjects. We simi- larly compared horizontal saccades made between two distant targets lying in the same field of movement as during the Müller paradigm tests, and between targets ly- ing symmetrically on either side of the midline, at near side of the midline, at near or far. We measured the ratio of the amplitude of the movements of each eye in corre- sponding directions due to the saccadic component, as well as corresponding ratios of peak velocity and peak acceleration. In response to a Müller test paradigm re- quiring about 17° of vergence, the change in position of the unaligned eye was typically twice the size of the cor- responding movement of the aligned eye. The ratio of peak velocities for the unaligned/aligned eyes was about 1.5, which was greater than for saccades made between distant targets. The ratio of peak acceleration for un- aligned/aligned eyes was about 1.0 during shifts from near to far and about 1.3 for shifts from far to near, these values being similar to corresponding ratios for saccades between distant targets. These measurements of peak ac- celeration indicate that the saccadic pulses sent to each eye during the Müller paradigm are more equal than would be deduced by comparing the changes in eye posi- tion. We retested five subjects to compare directly the peak acceleration of saccades made during the Müller paradigm with similar-sized “conjugate” saccades made between targets at optical infinity. Saccades made during the Müller paradigm were significant slower (P<0.005) than similar-sized conjugate saccades; this indicated that the different-sized movements during Müller paradigm are not simply due differences in saccadic pulse size but are also influenced by the concurrent vergence move- ment. A model for saccade-vergence interactions, which incorporates equal saccadic pulses for each eye, and dif- fering contributions from convergence and divergence, accounts for many of these findings. Key words Eye movements · Saccades · Vergence · Hering’s law Introduction Under natural conditions we frequently shift our point of visual fixation between objects that lie in different direc- tions and at different depths in the visual environment. These shifts of gaze require a combination of saccadic and vergence eye movements (Collewijn et al. 1995; Enright 1998). An interesting case occurs when gaze is shifted between two objects that are both aligned on the visual axis of one eye, one located near to it and the oth- er far from it (Fig. 1A). This experimental stimulus is similar to the paradigm used by Johannes Müller (1843). Under these circumstances, only the nonaligned eye is required to move, although both do so; however, the S. Ramat Dipartimento di Informatica e Sistemistica, University of Pavia, Pavia, Italy V.E. Das · J.T. Somers · R.J. Leigh ( ✉ ) Department of Neurology, Department of Veterans Affairs Medical Center and University Hospitals, Case Western Reserve University, Cleveland, OH 44106, USA e-mail: rjl4@po.cwru.edu Tel.: +1-216-4213224 Fax: +1-216-4213040 V.E. Das · R.J. Leigh Department of Biomedical Engineering, Department of Veterans Affairs Medical Center and University Hospitals, Case Western Reserve University, Cleveland, OH 44106, USA R.J. Leigh Department of Neuroscience, Department of Veterans Affairs Medical Center and University Hospitals, Case Western Reserve University, Cleveland, OH 44106, USA Exp Brain Res (1999) 129:500–510 © Springer-Verlag 1999 RESEARCH ARTICLE Stefano Ramat · Vallabh E. Das · Jeffrey T. Somers R. John Leigh Tests of two hypotheses to account for different-sized saccades during disjunctive gaze shifts Received: 31 December 1998 / Accepted: 14 July 1999