Error Correction in Vergence Eye Movements: Evidence Supporting Hering’s Law

Pure vergence stimuli fall strictly along the midline or “cyclopean” axis and while rare in natural viewing, are easy to create in the laboratory. They are particularly useful for studying the neural control of disparity vergence eye movements. Responses to such stimuli should not contain version components, but two factors can lead to transient version components during a symmetrical vergence response. First, vergence responses often contain saccades (Levi et al., 1987; van Leeuwen et al. 1998; Zee et al. 1992; Collewijn, et al., 1995, Semmlow et al. 2008, 2009; Coubard and Koupola, 2008) and even a small saccade will generate a substantial deviation from the midline. Second, even if no saccades are present, responses to pure vergence stimuli are often asymmetrical because one eye moves faster than the other during the fusion initiating