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Ziad Hafed; Midbrain superior colliculus motor bursts do not dictate saccade kinematics. Journal of Vision 2021;21(9):2599. doi: https://doi.org/10.1167/jov.21.9.2599.
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The primate superior colliculus (SC) is critical for saccade generation. Its neurons are organized topographically such that the site of neural activation defines the desired size and direction of an executed movement. However, in addition to this spatial code, SC neurons also exhibit a strong motor burst that is tightly synchronized with saccade onset. The functional role of this motor burst, and particularly its stereotypical temporal evolution, has long intrigued oculomotor physiologists. A well-accepted hypothesis is that motor bursts are critical for transforming the SC spatial code into a temporal command for realizing the millisecond-by-millisecond movement kinematics; thus, the site of SC activity dictates saccade dimensions, but the motor bursts define kinematics (e.g. saccade peak velocity). Here, I was motivated by recent observations that SC motor bursts in the upper visual field representation are significantly weaker than bursts in the lower visual field representation (Hafed and Chen, 2016). Moreover, seemingly more lower visual field SC neurons are active for downward saccades than upper visual field neurons are active for upward saccades (due to response field size differences). If SC motor bursts dictate saccade kinematics, then downward saccades should be dramatically different from upward saccades in kinematics. I analyzed thousands of visually-guided, delayed visually-guided, and memory-guided saccades from two monkeys; the saccades ranged in size from microsaccades to approximately 15 deg saccades, and they had different directions. I classified movements as being directed towards upper or lower visual field locations. Even though upper visual field saccades had significantly faster reaction times than lower visual field saccades in all tasks (as expected), kinematics (evidenced by main sequence plots of peak velocity versus amplitude) were very similar. These results suggest that SC motor bursts do not necessarily dictate saccade kinematics, and they motivate future research into the functional role of SC saccade-related activity.
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