| Our brain monitors motor commands and, through internal feedback, corrects for anticipated errors. Saccades provide a powerful way to test this process because saccades are completed too quickly for sensory feedback to be useful. I first show that motor commands that move our eyes show variability and that this variability is not random noise, but is due to the cognitive state of the subject. Healthy people showed within saccade compensation for this variability with commands that arrived later in the same saccade. However, in people with cerebellar damage, the same variability resulted in dysmetria. This ability to correct for variability in the motor commands that initiated a saccade was a predictor of each subject's ability to learn from endpoint errors, suggesting that maintenance of saccade accuracy in daily life and short-term saccade adaptation have shared neural mechanisms. I then present a novel tool for studying feedback control of saccades: transcranial magnetic stimulation during a saccade perturbed the eye's trajectory, causing a reduction in velocity or an outright pause. This perturbation was corrected within the same saccade with motor commands that compensated and brought the eyes near the target. As this correction occurred even without visual input (in conditions where the target was removed), it appears that the correction is due to an internal feedback process that has an estimate of the current and desired state of the eye. Furthermore our findings emphasize that TMS can have non specific, presumably "startle," effects on motor behavior, and these effects should be taken into account when interpreting the effects of TMS on brain function. |
