String-pulling by the common marmoset

Coordinated hand movements used to grasp and manipulate objects are crucial for many daily activities, such as tying shoelaces or opening jars. Recently, the string-pulling task - which involves cyclically reaching, grasping and pulling a string - has been used to study coordinated hand movements in rodents and humans. Here we characterize how adult common marmosets perform the string-pulling task and describe changes in performance across the lifespan. Marmosets (n=15, 7 female) performed a string-pulling task for a food reward using an instrumented apparatus attached to their home-cage. Movement kinematics were acquired using markerless video tracking and we assessed individual hand movements and bimanual coordination using standard metrics. Marmosets oriented their gaze towards the string above their hands and readily performed the task regardless of sex or age. The task required very little training and animals routinely engaged in multiple pulling trials per session despite not being under water or food control. All marmosets showed consistent pulling speed and similar hand movements regardless of age. Adult marmosets exhibited a clear hand effect, performing straighter and faster movements with their right hand despite showing idiosyncratic hand preference according to a traditional food retrieval assay. Hand effects were also evident for younger animals but seemed attenuated in the older animals. In terms of bimanual coordination, all adult marmosets demonstrated an alternating movement pattern for vertical hand positions. Two younger and two older marmosets exhibited idiosyncratic coordination patterns even after substantial experience. In general, younger and older animals exhibited higher variability in bimanual coordination than did adults.