Integration of visual information for saccade production

Ocular tracking of occluded ballistic trajectories: Effects of visual context and of target law of motion

In tracking a moving target, the visual context may provide cues for an observer to interpret the causal nature of the target motion and extract features to which the visual system is weakly sensitive, such as target acceleration. This information could be critical when vision of the target is temporarily impeded, requiring visual motion extrapolation processes. Here we investigated how visual context influences ocular tracking of motion either congruent or not with natural gravity. To this end, 28 subjects tracked computer-simulated ballistic trajectories either perturbed in the descending segment with altered gravity effects (0g/2g) or retaining natural-like motion (1g). Shortly after the perturbation (550 ms), targets disappeared for either 450 or 650 ms and became visible again until landing. Target motion occurred with either quasi-realistic pictorial cues or a uniform background, presented in counterbalanced order. We analyzed saccadic and pursuit movements after 0g and 2g target-motion perturbations and for corresponding intervals of unperturbed 1g trajectories, as well as after corresponding occlusions. Moreover, we considered the eye-to-target distance at target reappearance. Tracking parameters differed significantly between scenarios: With a neutral background, eye movements did not depend consistently on target motion, whereas with pictorial background they showed significant dependence, denoting better tracking of accelerated targets. These results suggest that oculomotor control is tuned to realistic properties of the visual scene.

Switch from ambient to focal processing mode explains the dynamics of free viewing eye movements

Previous studies have reported that humans employ ambient and focal modes of visual exploration while they freely view natural scenes. These two modes have been characterized based on eye movement parameters such as saccade amplitude and fixation duration, but not by any visual features of the viewed scenes. Here we propose a new characterization of eye movements during free viewing based on how eyes are moved from and to objects in a visual scene. We applied this characterization to data obtained from freely-viewing macaque monkeys. We show that the analysis based on this characterization gives a direct indication of a behavioral shift from ambient to focal processing mode along the course of free viewing exploration. We further propose a stochastic model of saccade sequence generation incorporating a switch between the two processing modes, which quantitatively reproduces the behavioral features observed in the data.

Stimulus-driven saccades are characterized by an invariant undershooting bias: no evidence for a range effect

Saccade endpoints are most frequently characterized by an undershooting bias. Notably, however, some evidence suggests that saccades can be made to systematically under- or overshoot a target based on the magnitude of the eccentricities within a given block of trials (i.e., the oculomotor range effect hypothesis). To address that issue, participants completed stimulus-driven saccades in separate blocks of trials (i.e., proximal vs. distal) that entailed an equal number of targets but differed with respect to the magnitude of their eccentricities. In the proximal block, target eccentricities were 3.0°, 5.5°, 8.0°, 10.5° and 13.0°, whereas in the distal block target eccentricities were 10.5°, 13.0°, 15.5°, 18.0° and 20.5°. If the range effect represents a tenable hypothesis, then the magnitude of target eccentricities within each block should selectively influence saccade endpoint bias. More specifically, the eccentricities common to the proximal and distal blocks (i.e., 10.5° and 13.0°) should elicit a systematic under- and overshooting bias, respectively. Results for the proximal and distal blocks showed a reliable undershooting bias across target eccentricities, and a direct comparison of the common eccentricities indicated that the undershooting bias was not modulated between blocks. Moreover, our results show that the presence of online target vision did not influence the undershooting bias. Thus, the present findings provide no support for an oculomotor range effect; rather, results evince the mediation of saccades via a control strategy that minimizes movement time and/or the energy requirements of the response.

Revisiting corrective saccades: role of visual feedback

To clarify the role of visual feedback in the generation of corrective movements after inaccurate primary saccades, we used a visually-triggered saccade task in which we varied how long the target was visible. The target was on for only 100ms (OFF100ms), on until the start of the primary saccade (OFFonset) or on for 2s (ON). We found that the tolerance for the post-saccadic error was small (-2%) with a visual signal (ON) but greater (-6%) without visual feedback (OFF100ms). Saccades with an error of -10%, however, were likely to be followed by corrective saccades regardless of whether or not visual feedback was present. Corrective saccades were generally generated earlier when visual error information was available; their latency was related to the size of the error. The LATER (Linear Approach to Threshold with Ergodic Rate) model analysis also showed a comparable small population of short latency corrective saccades irrespective of the target visibility. Finally, we found, in the absence of visual feedback, the accuracy of corrective saccades across subjects was related to the latency of the primary saccade. Our findings provide new insights into the mechanisms underlying the programming of corrective saccades: (1) the preparation of corrective saccades begins along with the preparation of the primary saccades, (2) the accuracy of corrective saccades depends on the reaction time of the primary saccades and (3) if visual feedback is available after the initiation of the primary saccade, the prepared correction can be updated.