Fig. 1.

Schematic representation of head unrestrained gaze saccade control schemes. a represents the organization of gaze feedback models. The desired gaze displacement (ΔG) is compared to the actual gaze displacement, and the error causes the superior colliculus (SC) to send a common motor command to the eye and the head (Galiana and Guitton 1992). b represents the feedforward mechanism of Freedman (2001, 2008). The desired gaze displacement is sent to the SC, which sends separate commands to the eye and the head controllers. An inhibition proportional to the head velocity is sent from the head controller to the eye controller (dashed line in panel b) to modulate the maximum eye velocity as a function of the head velocity. c represents the proposed new architecture, based on the concurrent action of three parallel drive pathways head cerebellar pathway CB_H, gaze cerebellar pathway CB_G and collicular pathway, SC. Orange items represent parts of the models with a discharge related to gaze displacement. Blue items represent structures with a discharge related to head movements. Green items represent parts of the models with a discharge related to eye movements. In this figure, circles with a capital sigma (∑) represent summing operators. Arrowheads correspond to excitation and filled circles correspond to inhibition