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The Journal of Neuroscience, August 15, 2007, 27(33):8757-8770; doi:10.1523/JNEUROSCI.2311-07.2007

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Behavioral/Systems/Cognitive
The Sources of Variability in Saccadic Eye Movements

Robert J. van Beers^1,2

¹Department of Neuroscience, Erasmus MC, 3000 DR Rotterdam, The Netherlands, and ²Department of Physics of Man, Helmholtz Institute, Utrecht University, 3584 CC Utrecht, The Netherlands

Correspondence should be addressed to Robert J. van Beers, Department of Physics of Man, Helmholtz Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands. Email: r.j.vanbeers{at}phys.uu.nl

Our movements are variable, but the origin of this variability is poorly understood. We examined the sources of variability in human saccadic eye movements. In two experiments, we measured the spatiotemporal variability in saccade trajectories as a function of movement direction and amplitude. One of our new observations is that the variability in movement direction is smaller for purely horizontal and vertical saccades than for saccades in oblique directions. We also found that saccade amplitude, duration, and peak velocity are all correlated with one another. To determine the origin of the observed variability, we estimated the noise in motor commands from the observed spatiotemporal variability, while taking into account the variability resulting from uncertainty in localization of the target. This analysis revealed that uncertainty in target localization is the major source of variability in saccade endpoints, whereas noise in the magnitude of the motor commands explains a slightly smaller fraction. In addition, there is temporal variability such that saccades with a longer than average duration have a smaller than average peak velocity. This noise model has a large generality because it correctly predicts the variability in other data sets, which contain saccades starting from very different initial locations. Because the temporal noise most likely originates in movement planning, and the motor command noise in movement execution, we conclude that uncertainty in sensory signals and noise in movement planning and execution all contribute to the variability in saccade trajectories. These results are important for understanding how the brain controls movement.

Key words: saccades; movement; variability; noise; motor command; model

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Received June 27, 2006; revised June 22, 2007; accepted June 23, 2007.

Correspondence should be addressed to Robert J. van Beers, Department of Physics of Man, Helmholtz Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands. Email: r.j.vanbeers{at}phys.uu.nl

This article has been cited by other articles:

				J. Munuera, P. Morel, J.-R. Duhamel, and S. Deneve Optimal Sensorimotor Control in Eye Movement Sequences J. Neurosci., March 11, 2009; 29(10): 3026 - 3035. [Abstract] [Full Text] [PDF]

				J. D. Koehn, E. Roy, and J. J. S. Barton The "Diagonal Effect": a Systematic Error in Oblique Antisaccades J Neurophysiol, August 1, 2008; 100(2): 587 - 597. [Abstract] [Full Text] [PDF]

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