LIMBS Lab Researchers Study Humans’ Rhythmic Control Through Haptics

September 5, 2017

Researchers in the Locomotion in Mechanical and Biological Systems (LIMBS) lab at Hopkins are working to understand how humans coordinate rhythmic physical behaviors, such as juggling and tapping out beats with their fingertips. The results of these studies could lead to more agile robots and improved prosthetics.

Leading this effort is Noah Cowan, professor of Mechanical Engineering and director of the LIMBS lab, who is working with two graduate students, Nicole Ortega and Robert Nickl.

“Humans are still far better than our best robotic systems at doing many kinds of manipulation of locomotion in the physical world, and they manage do to so despite worse timekeeping,” Cowan said. “When you’re tapping to a beat and that beat is changing, how do you keep up with the changes in cadence? Does it matter whether or not there’s a rhythm overlapping the beat?”

Ortega’s study focuses on a simple test: Participants tap their fingers on a virtual reality surface while they listen to and follow a rhythm. Ortega is assessing their ability to synchronize their taps with a ticking metronome, and she is modeling how richer cues with more complex rhythms affect their ability to synchronize with audio signals.

Ortega’s study complements research by Nickl on a different task: paddle juggling. As they watch a ball bouncing on a computer screen, study participants attempt to “hit” it with a haptic paddle held in their hand. When they have successfully “hit” the virtual ball, a small force impulse is delivered through the paddle to their hand, providing tactile feedback. Building from a previous LIMBS study suggesting that timing information improves people’s juggling performance by making them more consistent, Nickl is investigating how various types of spatial and temporal cues affect how people coordinate their arm movements.


Because the researchers can alter the timing cues of what the people see in the virtual simulation, they are able to measure their participants’ movements, and they can understand how they are maintaining the rhythm.

“This ability to modify the physics so that it’s not quite right, but close, and see how people adjust gives us insights into what is going on in their brains,” Cowan said.

Whether or not they are conscious of these “tricks,” volunteer participants respond in characteristic ways that reveal how their brain and other parts of their motor systems work.

“It seems we take advantage of those periodic dynamics in the physical world and we synchronize ourselves to those dynamics, whether it’s the external ball that’s bouncing or our own limbs and appendages that help us keep time by helping us use our bodies as clocks,” Cowan said.

LIMBS lab researchers believe that these insights about humans’ ability to perform complex rhythmic tasks could eventually lead to the development of robots with better timekeeping coordination. Cowan, deputy director of the LCSR, continuously works with his lab members to translate their findings about biology to robotics.

“Humans have a natural ability to synchronize their actions to the timing cues that are embedded in a task or an environment.  By analyzing how humans perform simple rhythmic tasks like juggling a ball or synchronizing to a metronome, we can gain insights into how to design controllers for robots that must operate in situations where they must infer and follow cues from the environment, or in environments where one or more sensory modalities like vision, sound, or touch are limited or unavailable,” Nickl said.

This research also has important implications for robotic rehabilitation devices aimed at helping patients recover from illness or injury.

Juggling and tapping experiments conducted by the lab allow researchers to measure human capabilities to sense and act with visual, audio, and haptic feedback.  These measurements can be used to compute a baseline for healthy behavior, which can in turn be used to design assistive technologies or robot-guided therapies.

“As roboticists, we’re ultimately interested in making systems that can interact with humans, and part of the ability to do that is understanding how humans manage timekeeping. You have to time the mechanical behaviors of your robot to make sense with respect to how humans think about the task” Cowan said.

Cowan, an amateur juggler, has always been interested in complex rhythm dynamics. He emphasized that the research done in the LIMBS lab is an ongoing learning process, not necessarily working towards and end goal but continually trying to understand as new questions arise.

“I’m really interested in the basic science. To me, it’s never an end; it’s always what the next door is that you open. We often think of neat new ideas about how to apply what we learned about the biology to robotics systems, but I don’t have a specific one in mind. It’s really just that I’m curious,” Cowan said.