Component stretching in fast and slow oblique saccades in the human.

We have studied the dynamics of human saccades along various cardinal (horizontal and vertical) and oblique directions in two different experimental paradigms yielding fast and slow saccades, respectively. We found that the saturation of vectorial peak velocity with amplitude, which is already well known from earlier studies on fast saccades, was equally pronounced in slow saccades. In both paradigms, the saturation level had a quite similar strong dependence on saccade direction. We found that the peak-velocity/amplitude relationships (main-sequences) of fast saccades in different directions were not simply scaled versions of one another. Whereas peak velocity in small saccades showed less anisotropy, different rates of saturation with amplitude in different directions (expressed in the angular constant parameter) caused the bundle of main-sequence curves to fan out at larger amplitudes. This property is reflected in a straight-line relationship between the angular constant and the asymptotic peak velocity parameters of the main-sequence. The possible contribution of neural control signals and plant properties is discussed. We also studied the main-sequences of oblique saccade components and found clear evidence for component stretching in both paradigms which increased as the saccade vector turned away from the cardinal direction under study. We conclude that the factors determining component stretching probably reside in the final pathway common for both saccade types. These experimental findings, revealing several common features in fast and slow saccades, were compared with quantitative predictions for the dynamics of oblique saccades, made from two existing two-dimensional models, predicting that the dynamic properties of components depend upon the direction of the saccade vector.