Object Motion Computation for the Initiation of Smooth Pursuit Eye Movements in Humans

Abstract

. Pursuing an object with smooth eye movements requires an accurate estimate of its two-dimensional (2D) trajectory. This 2D motion computation requires that different local motion measurements are extracted and combined to recover the global object-motion direction and speed. Several combination rules have been proposed such as vector averaging (VA), intersection of constraints (IOC), or 2D feature tracking (2DFT). To examine this computation, we investigated the time course of smooth pursuit eye movements driven by simple objects of different shapes. For type II diamond (where the direction of true object motion is dramatically different from the vector average of the 1-dimensional edge motions, i.e., VA IOC ϭ 2DFT), the ocular tracking is initiated in the vector average direction. Over a period of less than 300 ms, the eye-tracking direction converges on the true object motion. The reduction of the tracking error starts before the closing of the oculomotor loop. For type I diamonds (where the direction of true object motion is identical to the vector average direction, i.e., VA ϭ IOC ϭ 2DFT), there is no such bias. We quantified this effect by calculating the direction error between responses to types I and II and measuring its maximum value and time constant. At low contrast and high speeds, the initial bias in tracking direction is larger and takes longer to converge onto the actual object-motion direction. This effect is attenuated with the introduction of more 2D information to the extent that it was totally obliterated with a texture-filled type II diamond. These results suggest a flexible 2D computation for motion integration, which combines all available one-dimensional (edge) and 2D (feature) motion information to refine the estimate of object-motion direction over time.

Predicting 2D Target Velocity Cannot Help 2D Motion Integration for Smooth Pursuit Initiation

“…1A). This finding replicates and generalizes our results previously obtained in human subjects (Masson and Stone 2002;Wallace et al 2005) with diamond-shaped line-drawings. Small transient tracking drifts, not related to stimulus orientation/direction and subjectspecific, were observed in two subjects during the gap period, either on the vertical component (subject AM) or on both horizontal and vertical velocities (subject AR, see The shaded region corresponds to the gap between fixation offset and motion onset.…”

“…Eye movements were recorded from three observers (1 naïve) using methods described in detail elsewhere (Wallace et al 2005). All procedures followed the Centre National de la Recherche Scientifique (CNRS) guidelines for the use of human subjects.…”

“…Early openloop pursuit responses were estimated by computing the average eye velocity over a second 40-ms time window, centered at 160 ms after target onset. The late open-loop response was also estimated over a 40-ms window centered at 200 ms after target onset (Wallace et al 2005). Moreover, horizontal and vertical eye accelerations were computed during the open-loop phases by differentiating eye velocities.…”

“…The initial pursuit direction is biased toward the direction of 1D motion signals (or their average), orthogonal to the edge(s), eventually resulting in a direction error with respect to the global, 2D direction of motion. In the latter case, the tracking direction starts to be slowly corrected before the closing of the visuooculomotor loop, i.e., before any negative feedback signal becomes available (Lisberger and Westbrook 1985), so that ϳ150 ms after pursuit onset, pursuit matches the actual 2D object trajectory (Born et al 2006;Masson and Stone 2002;Wallace et al 2005).…”

“…A striking aspect of the initial bias for pursuit is that it is highly reproducible and seems to be immune to cognitive influence such as shape cueing (Wallace et al 2005). However, when the object is transiently blanked during steady-state tracking, its reappearance does not elicit the transient error observed after first target appearance (Masson and Stone 2002).…”