Model of the Control of Saccades by Superior Colliculus and Cerebellum

Abstract

Experimental evidence indicates that the superior colliculus (SC) is important but neither necessary nor sufficient to produce accurate saccadic eye movements. Furthermore both clinical and experimental evidence points to the cerebellum as an indispensable component of the saccadic system. Accordingly, we have devised a new model of the saccadic system in which the characteristics of saccades are determined by the cooperation of two pathways, one through the SC and the other through the cerebellum. Both pathways are influenced by feedback information: the feedback determines the decay of activity for collicular neurons and the timing of the activation for cerebellar neurons. We have modeled three types of cells (burst, buildup, and fixation neurons) found in the intermediate layers of the superior colliculus. We propose that, from the point of view of motor execution, the burst neurons and the buildup neurons are not functionally distinct with both providing a directional drive to the brain stem circuitry. The fixation neurons determine the onset of the saccade by disfacilitating the omnipause neurons in the brain stem. Excluding noise-related variations, the ratio of the horizontal to the vertical components of the collicular drive is fixed throughout the saccade (i.e., its direction is fixed); the duration of the drive is such that it always would produce hypermetric movements. The cerebellum plays three roles: first, it provides an additional directional drive, which improves the acceleration of the eyes; second, it keeps track of the progress of the saccade toward the target; and third, it ends the saccade by choking off the collicular drive. The drive provided by the cerebellum can be adjusted in direction to exert a directional control over the saccadic trajectory. We propose here a control mechanism that incorporates a spatial displacement integrator in the cerebellum; under such conditions, we show that a partial directional control arises automatically. Our scheme preserves the advantages of several previous models of the saccadic system (e.g., the lack of a spatial-to-temporal transformation between the SC and the brain stem; the use of efference copy feedback to control the saccade), without incurring many of their drawbacks, and it accounts for a large amount of experimental data.

A neural mechanism for coordinate transformation predicts pre-saccadic remapping

“…We do not explicitly model the processes of saccade target selection and saccade initiation, although the retinocentric field or the transformation field (when read out along the retinocentric dimension) could provide the input for this. In our previous work, we have provided detailed models of saccade planning that are compatible with our current architecture (Kopecz and Schöner 1995;Wilimzig et al 2006; see also Quaia et al 1999;Trappenberg et al 2001). Here, we only represent the result of this process by applying an input to the appropriate location in the saccade field for every gaze shift required in the simulations.…”

Discharge of Monkey Nucleus Reticularis Tegmenti Pontis Neurons Changes During Saccade Adaptation

“…For example, in some models, a pontine nucleus, either the NRTP (Dean et al 1994) or the dorsolateral pontine nucleus (Schweighofer et al 1996), simply serves as a passive relay for a saccadic command to the OMV. In another popular model (Quaia et al 1999), there is no pontine relay nucleus at all. All models assume that adapted signals will show up only at the cerebellum or downstream from it.…”

“…Much is known about the neuronal substrate for the motor learning of reflex behaviors, such as the blink reflex and the VOR (e.g., Boyden et al 2004;Raymond et al 1996). In both, motor learning seems to be distributed between sites in the cerebellum and in the more direct reflex pathways.…”

Towards Computational Modelling of Neural Multimodal Integration Based on the Superior Colliculus Concept

“…In the deep layers of the SC integration takes place among auditory, visual and somatosensory stimuli. Very few types of neurons, such as burst, build-up and fixation neurons are responsible for this behaviour [4,10]. By studying these neurons and their firing rates, integration can be successfully explored.…”

“…The enhancement is increasing the relevance of a particular modality stimulus based on the influence of other modalities while depression is decreasing the relevance of a particular modality, in particular if the stimuli disagree. Several authors have attempted to examine multimodal integration in the Superior Colliculus [2,4,5,10,27,37,51,55]. Different approaches have been suggested including biological, probabilistic and computational neural network approaches.…”