Spatial contextual cues that help predict how a target will accelerate can be used to guide interception

Abstract

Objects in one's environment do not always move at a constant velocity but often accelerate or decelerate. People are very poor at visually judging acceleration and normally make systematic errors when trying to intercept accelerating objects. If the acceleration is perpendicular to the direction of motion, it gives rise to a curved path. Can spatial contextual cues help one predict such accelerations and thereby help interception? To answer this question, we asked participants to hit a target that moved as if it were attached to a rolling disk, like a valve (target) on a bicycle wheel (disk) moves when cycling: constantly accelerating toward the wheel's center. On half the trials, the disk was visible such that participants could use the spatial relations between the target and the rolling disk to guide their interception. On the other half, the disk was not visible, so participants had no help in predicting the target's complicated pattern of accelerations and decelerations. Importantly, the target's path was the same in both cases. Participants hit more targets when the disk was visible than when it was invisible, even when using a strategy that can compensate for neglecting acceleration. We conclude that spatial contextual cues that help predict the target's accelerations can help intercept it.

Figure 2.

Fraction of targets hit when the disk was invisible (red) and when the disk was visible (blue). The data of individual participants are shown by black points connected by a line. Almost all participants hit more targets when the disk was visible.

Figure 3.

(A) Three examples of the phase-alignment procedure. The tapping position on a given trial is indicated by a blue dot within the black target presented at its location at the time of the tap. This tap and target position on every trial were rotated to align the areas indicated by the black rectangles in the orientation indicated by the gray rectangles to allow averaging across trials. (B) Mean tapping locations of individual participants when the disk was invisible (red dots) or visible (blue dots). The position of the target at the time of the tap is shown in black, with its previously occupied positions during 100 ms shown in shades of gray. The yellow line shows how the target center would have moved if it had continued to move in the same direction from 100 ms before the tap. The purple curve shows how the target center moved during the last 100 ms before the tap. Any systematic difference between the red and blue tapping points indicates that visibility of the disk influences where participants aim.

Figure 4.

Possible interception strategies. (A) The positions of the target with respect to the center of the disk (phase) at the time of the tap for three participants. The lines show the vector averages of the corresponding positions, so that their lengths indicate the consistency of phase. From left to right, these participants showed a low, middle, and high consistency of phase. The direction of the line was used to determine the deviation of the average direction from that at which the target has its lowest velocity. The participant whose data are shown in the central panel usually tapped when the target was moving fast while the participant whose data are shown on the right usually tapped when the target's velocity was low. The lines connecting the data from these three participants are presented in bold in B and C. The numbers are the lengths and orientations of the lines. (B) The consistent phase strategy: The fraction of targets hit increases with the consistency of the phase in the target's motion at the time of the tap. Each participant is represented by a pair of dots connected by a line. The numbers show the correlation coefficient and the p value associated with a comparison with there being no correlation. (C) The low-velocity strategy: The fraction of targets hit tends to decrease with the deviation of the average phase from the phase at which the target's velocity was at its lowest.