Falling into a Rhythm – The Emergence of Periodicity in Discrete Motor Actions

When learning to walk, it has been suggested that humans transition from discrete, aperiodic steps to a periodic gait in order to conserve energy and increase stability. It is possible this same factors apply when humans perform complex movements with their upper limbs. For instance, when a basketball player practices free throws or a tennis player hits many serves in a row, the time between their throws or serves similar becomes more consistent (i.e. periodic) with practice. While this phenomenon is a common, it is not known how or why repeated discrete movements become periodic. More fundamentally, this emergence of periodicity has not yet been quantified. This study aims to demonstrate that when individuals repeatedly perform a discrete action, these actions become as periodic as explicitly rhythmic actions. We further aim to show that this spontaneous emergence of periodicity develops with age. To investigate the emergence of this periodicity, we will ask participants to perform a discrete throwing task and a rhythmic finger tapping task. We predict that as participants practice the discrete throwing task, they will become as periodic as they are in the finger tapping task where they are explicitly instructed to move rhythmically. We also predict that by the end of practice, younger participants will be less periodic compared to older participants. This development of periodicity should mirror decreasing variability in the rhythmic task. Identifying the emergence of periodicity in repeated discrete movements will provide insight into how the central nervous system controls complex, coordinated movements. Falling into a Rhythm – The Emergence of Periodicity in Discrete Motor Actions In a prior experiment, we observed that when participants repeated trials of a throwing task, the time between their throws became more consistent (i.e. periodic) with practice. This was unexpected as participants were only instructed to throw to a target as accurately as possible. No instruction on consistency or timing was given. While it is not known how or why this periodicity develops, spontaneous periodicity in repeated discrete actions is actually a common phenomenon, exemplified in athletics: when a basketball player practices free throws or a tennis player hits many serves in a row, their pacing becomes more periodic and is often described as “falling into a rhythm”. The goal of this study is to investigate the development of periodicity in the repeated performance of a discrete task. To investigate the emergence of this periodicity, we will ask participants to perform a discrete throwing task and a rhythmic finger tapping task. Participants will perform 100 trials of a discrete throwing task, and we will measure the variability of their inter-throw intervals (i.e. time between throws) to quantify periodicity. Next, we will examine how the periodicity in the discrete task compares to the periodicity in a finger tapping task, where participants are explicitly instructed move rhythmically. Lastly, we will investigate if the degree of periodicity in the discrete task varies with age. Aim 1. Demonstrate that when individuals repeatedly perform a discrete action, these actions become as periodic as explicitly rhythmic actions. We predict that in early practice, inter-throw intervals will be more variable than inter-tap intervals within each individual. However by late practice, we predict that individuals will decrease their variability in inter-throw intervals to the same level of variability in their inter-tap intervals. Aim 2. Demonstrate that the spontaneous emergence of periodicity when performing repeated discrete actions develops with age. We predict that by the end of practice, the inter-throw periods of younger participants will be more variable compared older participants. This development of rhythmicity is mirrored in a decrease in variability in the explicitly rhythmic tapping task. Significance. Identifying the emergence of periodicity in repeated discrete movements will provide insight into how the central nervous system learns to controls complex coordinated movements. The emergence of periodicity has been previously studied in locomotion. When younger children learn to walk, their steps are initially discrete and not well-timed. Only after years of practice does locomotion become periodic (1). It has been suggested that this periodicity emerges in order to reduce energy demands and increase stability in the human system (2). Observing emergent periodicity in upper limb movements would suggest that the control objectives that guide learning to walk also guide learning other complex motor skills. In addition, many movement disorders can be characterized by either increased periodicity, such as repetitive stereotypic movements in Autism Spectrum Disorder (3), or by the lack thereof, like arrhythmic Figure 1. Virtual Throwing Task. Figure 2. Finger Tapping Task. gait in cerebellar disorders (4). A measure of the emergence of periodicity could potentially be sensitive enough to detect subtle motor impairment. Such measures are still needed for neurological diseases like Parkinson’s Disease and developmental disorders where early detection is critical. Experimental Design. Participants and Location. Approximately 300-500 participants of ages 5 to 70 years will be recruited at the Museum of Science. Our two experimental set-ups are exhibited in the Living Laboratory, an initiative of the Museum of Science to educate museum visitors about ongoing research. The data collection devices are located next to the Hall of Life. We will approach visitors and invite them to participate in our two experimental tasks. Data Collection of the Discrete Throwing Task. Participants will throw a virtual ball 100 times. The goal of each throw is to hit a virtual target displayed on the screen. The participants first rest their dominant arm on a lever and grasps a wooden ball attached to the distal end (Figure 1). To throw the ball, the participants rotate the lever and trigger the release of the virtual ball by lifting their finger from a pressure sensor on the wooden ball. The angular position of their arm is measured with a digital encoder attached to the lever. The angular position and speed of the arm at the moment of release determine the path of the ball in the virtual environment. The arm position and pressure sensor signals will be analyzed to calculate the variability in inter-throw interval. Data Collection of the Rhythmic Finger Tapping Task. Participants will tap their index fingers on a force sensor at their preferred period for three 60-second trials (Figure 2). As in the throwing task, the force signals from finger contact are analyzed to calculate variability in intertap intervals. Research Opportunities for Undergraduates. I have assembled an interdisciplinary team of 8 undergraduate students from Northeastern to assist with this experiment. Their majors include Biology, Behavioral Neuroscience, Biomedical Physics, Electrical Engineering, and Mathematics. Six of these students are fellow female scientists. The undergraduate team will assist me in setting up the equipment, recruiting museum visitors to participate, and running the experiment. After completing the data collection, I will supervise the undergraduate students to address the two aims. Given the richness of the big dataset, undergraduate students can subsequently also analyze it to answer their own research questions. Community Outreach. The experience of conducting an experiment at the Museum of Science is unique. We expect to collect 300-500 subjects. Besides collecting this big dataset, we also have the opportunity to share our lab’s research with the museum visitors. We can explain how our research impacts the community and inspire young children and teenagers to take an interest in science and engineering.