A nonvisual eye tracker calibration method for video-based tracking.

Video-based eye trackers have enabled major advancements in our understanding of eye movements through their ease of use and their non-invasiveness. One necessity to obtain accurate eye recordings using video-based trackers is calibration. The aim of the current study was to determine the feasibility and reliability of alternative calibration methods for scenarios in which the standard visual-calibration is not possible. Fourteen participants were tested using the EyeLink 1000 Plus video-based eye tracker, and each completed the following 5-point calibration methods: 1) standard visual-target calibration, 2) described calibration where participants were provided with verbal instructions about where to direct their eyes (without vision of the screen), 3) proprioceptive calibration where participants were asked to look at their hidden finger, 4) replacement calibration, where the visual calibration was performed by 3 different people; the calibrators were temporary substitutes for the participants. Following calibration, participants performed a simple visually-guided saccade task to 16 randomly presented targets on a grid. We found that precision errors were comparable across the alternative calibration methods. In terms of accuracy, compared to the standard calibration, non-visual calibration methods (described and proprioception) led to significantly larger errors, whilst the replacement calibration method had much smaller errors. In conditions where calibration is not possible, for example when testing blind or visually impaired people who are unable to foveate the calibration targets, we suggest that using a single stand-in to perform the calibration is a simple and easy alternative calibration method, which should only cause a minimal decrease in accuracy.

Contrasting effects of exogenous cueing on saccades and reaches.

Previous studies have shown that eye and arm movements tend to be intrinsically coupled in their behavior. There is, however, no consensus on whether planning of eye and arm movements is based on shared or independent representations. One way to gain insight into these processes is to compare how exogenous attentional modulation influences the temporal and spatial characteristics of the eye and the arm during single or combined movements. Thirteen participants (M = 22.8 years old, SD = 1.5) performed single or combined movements to an eccentric target. A behaviorally irrelevant cue flashed just before the target at different locations. There was no effect of the cue on the saccade or reach amplitudes, whether they were performed alone or together. We found no differences in overall reaction times (RTs) between single and combined movements. With respect to the effect of the cue, both saccades and reaches followed a similar pattern with the shortest RTs when the cue was closest to the target, which we propose reflects effector-independent processes. Compared to when no cue was presented before the target, saccade RTs were generally inhibited by the irrelevant cue with increasing cue-target distance. In contrast, reach RTs showed strong facilitation at the target location and less facilitation at farther distances. We propose that this reflects the presence of effector-dependent processes. The similarities and differences in RTs between the saccades and reaches are consistent with effector-dependent and -independent processes working in parallel.