Article Figures & Data
Figures
- Figure 1.
A, Task procedure. A black disk moved across a gray monitor background from left to right at a constant rate of acceleration. After an initial period during which the target was visible, it moved behind an occluder of a fixed width (13.4°) and then reappeared. Observers had to estimate the time of reappearance (equivalent to TTC) and intercept the target with a rapid pointing movement of their right index finger (red dot). B, Target parameters in experiment 1. Targets moved with a variable initial velocity (vinit) and accelerated or decelerated at a constant rate. The initial and average velocities (vavg) of the targets were related in such a way with acceleration rate that all targets reached the occluder with the same velocity (vfinal) of 20°/s. C, Target parameters in experiment 2. Initial and final target velocities were related in such a way with acceleration rate that all targets had the same vavg of 20°/s.
- Figure 2.
Single trial eye and hand movements from one representative observer. A, B, Two trials with a +8°/s2 accelerating target (A) or −8°/s2 decelerating target (B). Light blue traces indicate smooth pursuit components, dark blue traces represent saccades. Upper panels show the horizontal position of the eye (blue) and target (black) locked to target motion onset. The red x represents the interception position and time. Lower panels show horizontal velocity of the eyes and target over time. Gray area represents the time of target occlusion. C, 3D-hand position trace (green) from the same trial as in A. The 2D interception position on the screen is indicated by the red x and the target position at the time of interception in represented by the black disk. The gray area illustrates the position of the occluder on the screen. Dotted lines in the x-y plane illustrate the upper and bottom edges of the screen.
- Figure 3.
Effect of presentation duration on smooth pursuit and predictive saccades. A, Average de-saccaded smooth pursuit velocities. Squares and error bars at the top show the mean ± 1 SEM of the first catch-up saccade (left) and predictive saccade (right) onsets. Shaded areas indicate occlusion period and dashed lines represent target velocities. B, Normalized pursuit velocity during target presentation. C, Distribution of 2D saccade landing positions. The black line represents the location of target reappearance (i.e., right border of the occluder). D, SLTs. Dots and error bars represent the mean across observers ± 1 SEM.
- Figure 4.
Hand movement paths and interception position. A, Top view of the hand movement paths in the x-z plane. The gray bar illustrates the x-position of the occluder on the screen. Hand movement paths were shifted along the z-axis for better visibility. B, Distribution of 2D interception positions. Histogram shows the distribution of horizontal interception positions. C, Kernel density plots of horizontal interception positions for the different target acceleration condition.
- Figure 5.
Manual interception time. A, Mean TTChand (interception time relative to occlusion onset). B, Mean and individual observers’ constant interception errors. Semi-transparent dots represent individual observers’ median performance. Negative values indicate interceptions that occurred before the target reached the end of the occluder (too early), and positive values indicate interceptions occurring after the target reached the end of the occluder (too late). Error bars represent ± 1 SEM.
- Figure 6.
Trial-by-trial correlation across acceleration conditions and presentation durations between SLT and TTChand. A, Distribution of individual correlation coefficients. B, C, Scatterplot and trend lines of the trial-by-trial correlation for an observer with a strong (B) and for an observer with a weak (C) correlation between SLT and TTChand. Dots represent individual trials. Thin lines represent trend lines for the different acceleration conditions, thick line shows the trend across conditions.
- Figure 7.
Model comparison. A–C, Three competing models of how observers might predict TTC for predictive eye and interceptive hand movements. A, The final velocity model postulates that observers predict TTC based on vfinal (identical for all targets in our design, hence predicting a fixed TTC). B, The average velocity model predicts interception timing based on the average target velocity before occlusion, yielding a negative correlation between veridical and measured TTC. C, The acceleration model suggests that observers use target acceleration for interception and predicts the veridical TTC. D, Comparison of model predictions and measured TTChand data. Dashed line shows the TTC prediction based on the mean hand prediction times. E, Root mean squared errors for the competing models on TTChand. F, Individual and mean hand prediction times. Asterisks represent significant differences (p < 0.05).
- Figure 8.
Results from experiment 2. A, B, The model predictions of the final (A) and average velocity (B) models in experiment 2. C, Comparison of TTChand to the model predictions. E, Comparison of model fits. Asterisks represent significant differences (p < 0.05).
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