The psychology of everyday actions

Without the ability to act, thought is useless. Psychologists have become increasingly interested in the mechanisms underlying physical actions. As mundane as many of our actions may seem, a great deal of fundamental knowledge about the psychological processes people rely on when they accomplish tasks as seemingly simple as grasping a cup is still lacking. Such knowledge is important to not only psychology, but neuroscience, philosophy, robotics, physical rehabilitation, and engineering alike. When people have suffered from a stroke, they have trouble with the planning of actions. A better understanding could inform rehabilitation strategies. In robotics, building autonomous robots has been difficult due in part to the limited knowledge we have on action planning. In philosophy, a great deal of theorizing has concerned how our physical actions relate to our mental experiences. My research projects touch on these issues through carefully controlled experiments.

My general approach is to understand a range of psychological processes through the lens of physical actions. Through this approach, I have contributed knowledge on basic motor control, on cognitive mechanisms for action planning and coordination within and across individuals, on disturbances in action planning in Parkinson’s Disease, on the link between perception and action, and on abstract issues relating to the sense of agency and the tracking of others’ belief states.

Acting alone

One of my research lines focuses on how people perform actions on their own. Two main issues here are what the constraints are that people plan for when they move, and how they string actions together into sequences of meaningful ongoing behavior.

What do people plan for?

My colleagues and I have addressed this question through the study of bimanual object manipulation, in particular where people grasp two objects they manipulate simultaneously. On the one hand, people favor symmetry for their two hands. On the other hand, people plan in ways that allow them to end movements comfortably. By pitting these constraints against each other, we have found that people flexibly prioritize sensitivity to different constraints depending on, for example, the similarity of the target locations, on differences in the mass distributions across objects, and on whether certain physical and mental constraints coincide or compete (van der Wel, Huhn, & Schimpf, in press). Thus, action planning relies on flexible hierarchies, with the importance of each constraint changing dynamically. This characteristic of action planning permits people to behave in consistent yet adaptive ways across different contexts. I have used a similar paradigm to study whole-body coordination during an object transport task (van der Wel & Rosenbaum, 2007). Due to their simplicity, these sorts of paradigms are very amenable to student-centered research.

Psychologists’ interest in object manipulation has recently taken a surge. This is exemplified by our review paper in Psychological Bulletin (Rosenbaum, Chapman, Weigelt, Weiss, & van der Wel, 2012), as well as an article in American Scientist.

Action planning relies on our recent perception-action history

Actions are often embedded in a sequence. A fundamental question is therefore how people plan action sequences. One possibility is that people plan each movement they perform anew. Alternatively, it could be that people maintain and reuse recent movement plans and slightly tweak them for the forthcoming movements. To test this idea, I used a paradigm in which we studied the effect of moving over an obstacle on the preceding and following movements in an action sequence. Participants continued to make excessively high movements above a table after moving over the obstacle. This perseverative effect gradually disappeared while moving away from the obstacle, and also emerged when participants continued moving with their opposite hand after clearing the obstacle. People thus behaved as if they reused aspects of an existing movement plan and made only slight changes between movements. This is noteworthy because it indicates the importance of cognitive efficiency for action planning, even when this comes at a biomechanical cost. In a related study, we found that this perseverative effect could also be invoked by “providing” participants with an appropriate movement through observation. These results provide support for the notion that perceiving and acting are tightly coupled, a topic on which I have co-authored several chapters.

Aside from the academic insight gained from the sequential paradigm just described, this work has applied value for understanding clinical disorders. I used our sequential paradigm to investigate action planning in Parkinson’s Disease (PD). One of the hallmark symptoms of PD is a reduced ability to switch between tasks. In our paradigm, patients showed a healthy perseverative effect when performing the task with one hand, but showed a specific deficit when they needed to transfer an action plan across hands. This finding supports the view that PD patients have difficulties with transferring motor parameters across actions.

The studies just described relied on human motion tracking. I have used this technology for another paradigm to investigate the control of movement rate. We showed that people have a preferred movement rate, and that they engage in strategies to avoid moving below their preferred rate for arm movements. Understanding slowing of movements is important as it occurs in for example stroke patients and PD patients.

Acting in a social world

My research also focuses on planning and coordinating our own actions with those of others, including actions such as carrying furniture, building bridges, playing sports, and dancing together. Due to their pervasiveness, it is likely that our mind has been shaped by (and perhaps designed for) interactions with others. In my second research line, I study the effects of social contexts on action planning and coordination.

In general, my work on understanding how we plan and coordinate actions with other people has applied value in the context of robotics. Roboticists have long tried to design robots that can fluently interact with human beings. Part of the challenge for developing such interactive robots has been the limited knowledge we still have on how people plan and coordinate their actions together.

Planning for others

When we act together, we may plan actions in ways that facilitate our action partners. One could see this when handing over scissors to someone else, but in that case we have been taught not to point the blade towards the action’s recipient. Do people accommodate others’ actions in general? We (van der Wel, Meyer, & Hunnius, 2013) tested this possibility by asking two people to manipulate a novel object together. We found that participants indeed adjusted the way in which they handed over an object to their action partner depending on the subsequent task. We are now investigating how this social ability develops early in life by testing young children.

Coordinating with others in real-time

Many actions people perform together rely on intricate coordination between two or more people. Thus, one issue concerns how we incorporate others’ actions to adjust the timing of our own actions to accomplish joint action goals. In a set of tasks, we found that people use their own motor system to predict their own and others’ timing to coordinate. Interestingly, participants achieved such coordination exclusively through adjustments by the person with the less demanding task only (Vesper, van der Wel, Sebanz, & Knoblich, 2011; 2013).

Another issue concerns how different modalities contribute to joint action coordination. Because people inherently lack full information during such actions (compared to acting alone), they may compensate by seeking information exchange through additional modalities in the service of coordination. To examine this, we (van der Wel, Sebanz, & Knoblich, 2011) had individuals and dyads learn a coordination task in which they moved a pendulum back and forth between two targets by pulling on strings. In this task, participants could create a haptic information exchange by pulling on both strings simultaneously (as this creates a physical link). Dyads learned the task at the same rate as individuals, but used haptic information much more so than when an individual performed the same task bimanually. Dyads may thus differentially tune into different modalities to coordinate successfully with others, which could help them to compensate for lacking complete internal information about joint actions.

Higher-level processes through the lens of action

Philosophers have long been interested in the link between physical actions and our subjective experience of control (sense of agency) over such actions. Very few experiments have addressed how we derive a sense of agency during joint actions. To investigate this issue, we (van der Wel, Sebanz, & Knoblich, 2012) used our string pulling task described above and asked participants to rate their sense of agency. We found that the sense of agency generally links closely to how well people performed. After performing together, people showed a spike in their sense of agency once they performed alone. This spike was not warranted based on objective performance. These kinds of biases provide useful information for theories on action planning and the sense of agency (van der Wel & Knoblich, 2013). Additional experiments are currently under way in my laboratory to further understand the sense of agency for joint actions.

One other line of my work concerns how our own and others’ beliefs influence our actions. In this belief tracking project, I used a computer mouse tracking paradigm and study the mouse movement kinematics to determine how belief representations compete. Although there are limits to using mouse trajectories for inferring discrete versus continuous processing systems (van der Wel et al., 2009), this method offers the advantage of providing an evolution of data points rather than a single data point per trial. With this method, I discovered that people represent their own and another agent’s belief automatically and in parallel, even without any objective reason to do so (van der Wel et al., 2014). I am currently testing how the veridicality of one’s own and others’ beliefs influences overt behavior.

In closing

I started this statement by indicating my interest in studying psychological processes through the lens of physical actions. My work on individual action planning indicates that some of these processes concern the actual planning of actions themselves. Knowledge on this topic has both theoretical and applied value. My work on acting with others indicates that the planning and coordination involved in joint action is remarkably complex. Part of the goal of that line of work is to elucidate just how people achieve such actions successfully. Theoretically, this work forms a revolutionary shift in how one could think about cognition in general. There has been a tradition in cognitive psychology to study the mind by asking people to complete tasks by themselves. Starting at the interpersonal level provides a fundamentally different view on the question of how our minds are shaped. My work on higher-level processes further takes the notion that cognition is both for acting in an individual and in a social world seriously. From this work, it seems that we are truly in it together.