Tests of two hypotheses to account for different-sized saccades during disjunctive gaze shifts

Abstract

Rapid shifts of the point of visual fixation between objects that lie in different directions and at different 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 adducting 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 similarly compared horizontal saccades made between two distant targets lying in the same field of movement as during the Müller paradigm tests, and between targets lying 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 corresponding directions due to the saccadic component, as well as corresponding ratios of peak velocity and peak acceleration. In response to a Müller test paradigm requiring about 17 degrees of vergence, the change in position of the unaligned eye was typically twice the size of the corresponding 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 unaligned/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 acceleration 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 position. 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 movement. A model for saccade-vergence interactions, which incorporates equal saccadic pulses for each eye, and differing contributions from convergence and divergence, accounts for many of these findings.

Depth cue combination in spontaneous eye movements

“…There is ample, experimental and computational, evidence that saccadic and vergence systems interact, and not necessarily linearly, when directing gaze across different depth planes (Busettini & Mays, 2005a, 2005bChaturvedi & Gisbergen, 1998;Collewijn, Erkelens, & Steinman, 1995, 1997Enright, 1984Enright, , 1986Erkelens, Van der Steen, Steinman, & Collewijn, 1989;Kumar et al, 2006;Ramat, Das, Somers, & Leigh, 1999;Zee, Fitzgibbon, & Optican, 1992). Whether this indicates that vergence accompanying saccades is distinctly different from fixational vergence, because the movements are generated via different visuomotor pathways, remains to be seen.…”

Disconjugate Eye Movements

“…asymmetric convergence, only the inferior oblique muscle contracts in this eye, as demonstrated with MRI; contraction of the same muscle, apart from contractile changes in the lateral and medial rectus muscles, is also seen in the fellow eye, which is directed inward [47]. During rapid gaze shifts along the line of sight of one eye, which calls for asymmetric vergence, the horizontal peak accelerations of the two eyes are similar, despite different position trajectories [67]. This finding suggests equal saccadic pulses for each eye, according to Hering's law, together with an additional vergence signal.…”

Dynamic properties of eye movements in mitochondrial chronic progressive external ophthalmoplegia

“…For normal subjects, peak acceleration of the smallest saccades usually exceeds 10001/s/s, whereas large saccades typically saturate at values of about 35001/s/s. 13 No smooth pursuit or vergence eye movements were evident and the patient used a series of small, slow saccades to track moving targets. In response to sinusoidal head rotations, VOR gain was estimated to be 0.07 horizontally and 0.22 vertically.…”