Kinematic specification of object weight
To illustrate this, let's look at the case of lifting an object. A lifted object, like a box, will have mass. Couple this with a gravitational field (like the one on Earth), and we get a downward force that's proportional to the object's mass (i.e., the object's weight). Once picked up, the weight of the object perturbs one's centre of mass such that to maintain balance (while still remaining energetically efficient), an individual will have to lean back proportionally to the object's weight. More specifically, humans typically do this by tilting their torsos backwards while pushing their pelvises forward. This creates an angle between the upper and lower parts of their bodies (this angle is 180° when standing straight without lifting any weight), an angle which varies systematically with object weight. Note how this fits into our definition of the KSD principle -- variation in kinematic information (backwards tilt / trunk-leg angle) is regulated by the causal dynamics (object weight) of the task. To complete the principle, it is hypothesised that this kinematic information then maps 1:1 to the dynamics that caused it in the first place.
But that's not all! The above analysis limits any movement from the lifter. Once the lifter starts to move (e.g., when they initially pick up the object or when they locomote with the object), they will need to handle any reactive and inertial forces that arise. Think about how much more difficult it is to move (or move with) a heavier vs. lighter object. This difficulty arises from the larger forces needed to overcome the resting inertia and/or momentum of the heavier object. As one moves, these forces perturb the body in all sorts of ways. Subsequently, the lifter must continuously adjust their posture to maintain balance, prevent injury, and deal with these forces as they engage in their goal-directed behaviour. To put it simply, for a given object acceleration, the magnitude of postural adjustments specifies the object weight, with heavier objects requiring larger postural adjustments. Linking this back to the KSD principle, variation in how one adjusts their posture as they move is caused by the reactive forces they are subjected to because of the object weight, and this very weight is then uniquely specified by the magnitude of postural adjustments relative to object acceleration.
Kinematic specification of 'invisible' causes
So, kinematic information can specify dynamics involving variables like mass and forces. So far, so good. The bold claim put forth in Runeson & Frykholm (1983) is that dynamics aren't limited only to kinetic variables (i.e., kinematics + mass). Instead, the authors argue that dynamics include any factor that plays a causal role in the manifestation of kinematics. That is, we can extend the scope of the KSD principle to 'invisible' properties, such as intentions and emotions. Take a moment to appreciate the implications of this claim. Unlike theories of social knowing that appeal to the notion of mental representations, properties like intentions and emotions aren't private affairs. They are publicly available as kinematic information, so long as one is sensitive enough to attune to it. So, can individuals actually perceive such properties given access to kinematics?
Experiment 1: Perceiving distance of thrown objects
Runeson & Frykholm (1981) asked participants to perceive the weight of a lifted box. Crucially, retroreflective stickers were placed on the box, giving participants access to the kinematics of the box. Would participants be able to perceive such dynamics if such kinematic information were no longer present? To test this, Runeson & Frykholm (1983) got participants to observe two actors throwing a sandbag to 6 target distances. Here, retroreflective markers were placed only on the actors, with none being attached to the sandbag. This meant that observers only had access to actor, and not thrown-object, kinematics. After each observation, the participants were then asked to report how far they thought the sandbag was being thrown. Remarkably, participants were highly accurate in their reports (see Fig. 1), with their judgements closely corresponding to the actual distances thrown by actors.
Fig. 1 Judged distances of one actor. (Circled curve = mean judgments from observers; thin curve = actual distance thrown by actor; solid curve = standard deviation of judgments)
In this experiment, the dynamics of the task were accurately perceived by the observers, granting support for the KSD principle (i.e., that the dynamics are specified in actor kinematics). Still, it is important to note that it is unclear what exactly was perceived. What dynamic property was perceived? Was it the intention of the actor to throw a certain distance? Or was it the actual distance thrown (intended and actual distance thrown can differ!)? Results showed an insignificant difference between the actual distance and intended distance, so we can't conclude the exact dynamic that was being perceived. Regardless, both these candidate dynamics can be considered 'invisible' causal factors, and this experiment serves as an important proof-of-concept that the KSD principle can apply to this broader definition of dynamics.
In future posts, I'll cover the rest of the experiments (there are 6 in total!) in Runeson & Frykholm (1983). So far, we've looked at how the KSD principle applies to more conventional physical properties like mass and forces. We then broadened its application to the perception of 'invisible' influences of kinematics, and described an experiment demonstrating that people are able to perceive these causes merely from kinematic information. The subsequent experiments go on to show how people can also perceive expectations, deception, and even gender from kinematics. Stay tuned!
References
Runeson, S., & Frykholm, G. (1981). Visual perception of lifted weight. Journal of Experimental Psychology Human Perception & Performance, 7(4), 733–740. https://doi.org/10.1037/0096-1523.7.4.733
Runeson, S., & Frykholm, G. (1983). Kinematic specification of dynamics as an informational basis for person-and-action perception: Expectation, gender recognition, and deceptive intention. Journal of Experimental Psychology General, 112(4), 585–615. https://doi.org/10.1037/0096-3445.112.4.585
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