May the Force be against you:

Better sensitivity to speed changes opposite to gravity

Beyond seemingly lower-level features such as color and motion, visual perception also recovers properties more commonly associated with higher-level thought, as when an upwardly accelerating object is seen not just as moving, but moreover as self-propelled, and resisting the force of gravity. Seeing an object accelerate causes us to perceive physical forces, but the reverse may also be true — such that the orientation of an object’s speed changes with respect to gravity may in turn determine whether those speed changes are noticed in the first place. Across six experiments, we found that observers are more sensitive to objects’ accelerations when they move upward (when those accelerations are opposite to gravity) and less sensitive to their accelerations when they move downward (when those accelerations are consistent with gravity). Moreover, observers were more sensitive to objects’ decelerations when they moved downward (when those decelerations appeared as ‘braking’ against gravity), and less sensitive to their decelerations when they moved upward (when those decelerations were consistent with gravity). We conclude that the perception of physical forces is not just an outcome of visual processing, but also determines the perception of other, seemingly lower-level, features of how objects move.

Demonstrations

Experiment 1 — Acceleration Detection (Upward vs. Downward)

In an initial experiment, observers viewed animations featuring single moving objects. After each animation, they reported whether the object accelerated, or remained at a constant speed. We predicted that observers would be more sensitive to the acceleration of upward-moving objects (i.e. when the speed change was opposite to gravity) compared to downward-moving objects (i.e. when the speed change was consistent with gravity).

Observers were better at detecting acceleration for Upward-moving objects than for Downward-moving objects. These results indicate that we are more sensitive to an object’s acceleration when it is opposite to the force of gravity, compared to the same acceleration when it is consistent with the force of gravity.

Click/tap the bars to view example displays

Experiment 2 — Acceleration Detection (Upward vs. Horizontal)

The results of the previous experiment suggest that we are more sensitive to acceleration when it is opposite to normal gravitational acceleration. However, there is an alternate interpretation of these results, which is that they do not reflect greater sensitivity detecting upward acceleration, but rather lower sensitivity to downward acceleration. To determine whether the previous effect was driven by an advantage for seeing acceleration opposite to gravity, and not just by worse performance detecting acceleration consistent with gravity, in Experiment 2 we compared Upward acceleration detection to a new baseline: Horizontal acceleration detection.

Observers were better at detecting acceleration for Upward-moving objects than for Horizontally-moving objects. These results indicate that the results of Experiment 1 (greater sensitivity to Upward vs. Downward acceleration) are driven, at least in part, by an advantage for detecting speed changes opposite to typical gravitational speed changes.

Click/tap the bars to view example displays

Experiment 3 — Acceleration Detection (Downward vs. Horizontal)

Just out of curiosity, we next tested whether another part of the effect in Experiment 1 might have been driven by a disadvantage for gravity-consistent acceleration, by comparing detection of Downward acceleration to detection of Horizontal acceleration.

Observers were worse at detecting acceleration for Downward-moving objects than for Horizontally-moving objects. These results indicate that the results of Experiment 1 (greater sensitivity to Upward vs. Downward acceleration) are not driven by a disadvantage for gravity-consistent acceleration.

Click/tap the bars to view example displays

Experiment 4 — Deceleration Detection (Upward vs. Downward)

The hypothesis that observers are more sensitive to speed changes that oppose gravity makes the reverse prediction for the detection of deceleration. If an upward-moving object decelerates, then this deceleration is attributable to the force of gravity, and observers should be relatively insensitive to this. By contrast, if a downward-moving object decelerates, then this deceleration may be attributed to a ‘braking’ force resisting gravity, in which case observers should be more sensitive to this. To test this prediction, we next ran a deceleration detection experiment, which was analogous to Experiment 1, except that now observers detected decelerations instead of accelerations.

Observers were better at detecting decelerations for Downward-moving objects than for Upward-moving objects. These results indicate that we are more sensitive to an object’s deceleration when it is opposite to the force of gravity, compared to the same deceleration when it is consistent with gravitational acceleration.

Click/tap the bars to view example displays

Experiment 5 — Deceleration Detection (Downward vs. Horizontal)

To determine whether the previous effect was in part driven by an advantage for seeing deceleration opposite to gravity, in Experiment 5 we compared deceleration detection in a Downward moving condition (which was identical to the Downward moving condition in the previous experiment) to deceleration detection in a new Horizontal baseline condition.

Observers were better at detecting deceleration for Downward-moving objects than for Horizontally-moving objects. These results indicate that the results of Experiment 4 (greater sensitivity to Downward vs. Upward deceleration) are driven, at least in part, by an advantage for detecting speed changes that are opposite to typical gravitational speed changes.

Click/tap the bars to view example displays

Experiment 6 — Deceleration Detection (Upward vs. Horizontal)

Just out of curiosity, we next tested whether another part of the effect in Experiment 4 might have been driven by a disadvantage for gravity-consistent deceleration, by comparing detection of Upward deceleration to detection of Horizontal deceleration.

Observers were worse at detecting deceleration for Upward-moving objects than for Horizontally-moving objects. These results indicate that the results of Experiment 4 (greater sensitivity to Downward vs. Upward deceleration) are not driven by a disadvantage for gravity-consistent deceleration.

Click/tap the bars to view example displays
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Downward Accelerating Object
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Downward Constant Speed Object (Slow)
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Downward Constant Speed Object (Fast)
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Upward Accelerating Object
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Upward Constant Speed Object (Slow)
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Upward Constant Speed Object (Fast)
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Downward Decelerating Object
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Downward Constant Speed Object (Slow)
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Downward Constant Speed Object (Fast)
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Upward Decelerating Object
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Upward Constant Speed Object (Slow)
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Upward Constant Speed Object (Fast)
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Horizontal Accelerating Object
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Horizontal Constant Speed Object (Slow)
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Horizontal Constant Speed Object (Fast)
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Horizontal Decelerating Object
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Horizontal Constant Speed Object (Slow)
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Horizontal Constant Speed Object (Fast)
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