It is commonly assumed that saccades are ballistic and stereotypical. Yet, there is structured variability in the motor commands that initiate a saccade to a target. We propose that this variability is partly a reflection of a systematic change in the internal value that the brain associates with a visual stimulus. That is, our brain assigns a value to targets of our eye movements, and this relative value modulates the motor commands that move our eyes. If this variability was uncompensated, that is, if saccades were open-loop, then the variability in the motor commands that initiated the saccade would affect saccade endpoints. In healthy people, however, saccade endpoints are accurate despite the fact that the motor commands that initiate the saccade are variable. We suggest that this is because control of saccades is strongly dependent on internal models through the cerebellum, monitoring the outgoing motor commands and effectively `steering'the saccade by adding motor commands late in the saccade's trajectory. The compensation is effective only if this internal model is calibrated, which links the problem of control with adaptation. Here, we propose a single principle of control for saccades: Expected costs and rewards of a movement are evaluated by the basal ganglia, resulting in an internal value that modulates the motor commands that initiate the saccade. As the motor commands are generated, the cerebellum monitors them and predicts their sensory consequences, producing adjustments that steer the movement to the goal. If successful, our project may produce a major shift in oculomotor research by linking variability in trajectory of saccades with expected rewards and internal models. These concepts have proven fundamental in understanding control of other movements like reaching. As a result, we may be able to produce a single conceptual framework for how the brain controls movements in general.
Our aim is to produce a new, coherent theory of how various brain structures like the basal ganglia and the cerebellum contribute to control of saccades. While cerebellar patients exhibit dysmetria in their saccades, our proposal suggests that dysmetria is not random, but related to the intrinsic value that the brain assigns to the visual stimulus. The role of basal ganglia in control of movements has remained a deep puzzle;our proposal suggests that there may be a link between this structure and the internal value that the brain assigns to a visual stimulus, and that this factor accounts for some of the variability in the motor commands that initiate saccades.
| Shadmehr, Reza (2010) Control of movements and temporal discounting of reward. Curr Opin Neurobiol 20:726-30 |
| Shadmehr, Reza; Orban de Xivry, Jean Jacques; Xu-Wilson, Minnan et al. (2010) Temporal discounting of reward and the cost of time in motor control. J Neurosci 30:10507-16 |
| Xu-Wilson, Minnan; Zee, David S; Shadmehr, Reza (2009) The intrinsic value of visual information affects saccade velocities. Exp Brain Res 196:475-81 |
| Xu-Wilson, Minnan; Chen-Harris, Haiyin; Zee, David S et al. (2009) Cerebellar contributions to adaptive control of saccades in humans. J Neurosci 29:12930-9 |
