Component stretching during oblique stimulation-evoked saccades: the role of the superior colliculus.
1. During oblique visually guided saccades, the peak velocity of each component is reduced from what it would be for a purely horizontal or vertical saccade of the same amplitude, and the durations of the components are prolonged. We tested predictions of two competing accounts of the neural basis of this "component stretching" phenomenon. Using a recent experimental approach, we electrically stimulated sites in the monkey superior colliculus (SC) immediately after either vertical or horizontal visually guided saccades. Under these conditions, the amplitude of one component (corresponding to the direction of the preceding movement, horizontal or vertical) of the stimulation-evoked saccade varies systematically, while the amplitude of the other component ("constant-amplitude component") remains essentially constant. These changes in saccade metrics occur even though both the locus and parameters of collicular stimulation are fixed. 2. As with visually guided saccades, the peak velocity of the constant-amplitude component in stimulation-evoked saccades decreased with increasing amplitude of the orthogonal component, while the duration of the constant-amplitude component increased with orthogonal amplitude. The component stretching effect in these stimulation-evoked saccades was qualitatively indistinguishable from component stretching in metrically matched, visually guided movements. Yet, unlike visually guided saccades, the locus of stimulation-induced activity was fixed from one stimulation-evoked saccade to the next. 3. Therefore component stretching in stimulation-evoked oblique saccades does not depend on the locus of activity in the collicular motor map. These results are inconsistent with the predictions of common source models of component stretching, but are consistent with cross-coupling models. We discuss the present results in the context of a new interpretation of collicular interaction with downstream saccadic controllers.