Abstract
Brain circuits controlling eye movements are widely distributed and complex. The etiology of irrepressible square wave saccades is not fully understood and is likely different for different neuropathologies. In a previous study, spontaneously occurring irrepressible saccades were noted after a cerebrovascular accident that damaged the rostral superior colliculus (SC) and its commissure in a Rhesus monkey. Here, we tracked and quantified the development of similar symptoms in a Rhesus monkey caused by a lesion in the rostromedial SC and its commissure. We documented the changes in these saccadic intrusions while the monkey attempted fixation of a target on three consecutive days post-onset. On the first day, eye jerk amplitude was ~10 degrees and the direction was ~30 degrees above the left horizontal meridian. On the second day, the amplitude decreased to 6.5 degrees and the direction shifted towards vertical, ~20 degrees to the left of the vertical meridian. Size, but not direction, of the eye jerks continued to decrease until intrusions dissipated within one month. Histological examination after ~6 months from the first appearance of the intrusions revealed a lesion in the commissure of the SC. Results from this and the previous study confirm the involvement of the commissure of the SC as the common target for triggering this neuropathy. Our data suggest that commissural fibers play an important role in maintaining normal visual stability. Interrupting the commissure between the two superior colliculi causes saccadic intrusions in the form of irrepressible jerking of the eyes, probably by disrupting inhibitory signals transmitted through the commissure. Furthermore, disappearance of the symptoms suggests that inhibitory fields within the SC are plastic and can expand, possibly via inputs from inter-collicular and nigrotectal pathways.
Highlights
We observe the world by making successive fixations disrupted by periods of eye movements
At the first observation of fixation deficits, no doublet eye jerks were made, but we observed a number of staircase saccades
Over the course of the following three days, the number of doublet eye jerks increased until day 3, when we found nearly equal numbers of staircase saccades and doublet eye jerks
Summary
We observe the world by making successive fixations disrupted by periods of eye movements. During the periods of fixation, we gather and integrate relevant visual information about our environment and decide where to look next. Square wave eye jerks observed in the majority of subjects showed small (0.5–3 degrees) saccades, which disrupt fixation and are followed by a second saccade, about 200–500 ms later that return eye gaze to the target. These eye jerks (normal instability) can occur at low rates (far fewer than 12 per minute), in any direction, but are usually present in the horizontal direction. These movements happen infrequently and there remain questions regarding their role and necessity during fixation
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