Three-, Rather Than Two-Dimensional Burst Generation for Spontaneous Saccadic Eye Movements
The torsional displacements out of Listing’s plane were made in a random direction and amplitude, since no strong contribution of neither initial eye position in Listing’s plane, nor of the saccade displacement vector was obtained. Panel IB shows such a violation of Listing’s law during a spontaneous eye movement. In contrast, the ‘plant’ model assumes that the saccade burst generator remains essentially two-dimensional down to the level of the oculomotor neurons, by encoding eye velocity in Listing’s plane. The torsional reset component is saccadic in nature, since it is initiated in close synchrony with the onset and offset of the horizontal and vertical components of a saccadic eye movement. The more typical behaviour is that, after an erroneous torsional displacement, the eye remains stable at the new position, sometimes for periods exceeding 800 ms, until the next saccade resets it. It is important to note, that all torsional movements were elicited in the absence of either vestibular or otolith stimulation.