Effects of Feedback and Aging on Aiming Movements in Virtual Reality

Participants will perform a 3D Fitts' task with their dominant right hand, reaching from a fixed start button to a target positioned along the body's sagittal midline. The target will be placed 32 cm forward (anterior-posterior) and 24 cm upward (vertical) from the start position, yielding a resultant movement distance of 40 cm according to Pythagoras' theorem (a²+b²=c²). This distance will at most correspond to about 80% of maximal forward reach. Participants will be seated comfortably against a backrest in front of a table, with chair and table height as well as apparatus position adjusted individually so that the start button is aligned to an ergonomically optimal desk height, comparable to that used for writing. Movements will be performed using a handheld controller tracked by the built-in system of the head-mounted display (HMD). The HMD will be worn in all conditions: in the real world laboratory (R) condition, participants will view the physical apparatus through passthrough mode while in VR, they will see a visually identical virtual replication. Participants will only use their dominant right hand. Before each aiming movement, the right hand will be rested on the table while pointing at the start button. In the R condition, the start button will be represented by an open frame mounted on the table in front of the participant. The frame outlines a partial spherical contour that indicates the full size of a sphere and is oriented partly toward the participant and partly upward, allowing forward and upward movements toward the target without contacting a solid surface. The target will be a round sphere mounted on a thin vertical rod, into which participants must point without touching its surface. Both start button and target structures will be fabricated from lightweight 3D-printed plastic (2-3 mm thickness) with rounded edges. Target diameters correspond to 4 different values (14.1, 7.1, 3.5, or 1.8 cm), and the start button will be neutral yellow with a medium diameter (3 cm). In the VR condition, the setup is visually and spatially replicated. The start button is represented as a full grey sphere corresponding to the size indicated by the real-world frame, while the target sphere and vertical rod are rendered identically. Unlike the R condition, the full sphere is visible in VR, as there is no physical obstruction preventing participants from passing through it. Task difficulty will be manipulated via target width (14.1, 7.1, 3.5, or 1.8 cm), yielding indexes of difficulty (IDs) of 2.5, 3.5, 4.5, and 5.5 bits. Each trial will begin with the controller tip resting on the start sphere and participants will move as quickly and accurately as possible toward the target. A trial will be considered successful if the controller stays inside the target sphere continuously for 0.5 seconds. There will be four different feedback conditions which are described in detail in the section 'Arms and Interventions'. Participants will be recruited of two age groups: Either young adults aged 18-28 years or healthy older adults aged 65-75 years. Before starting the experiment (i.e., data recording), participants will complete a familiarization phase in all four feedback conditions presented in randomized order. In each condition, they will perform two blocks of 10 trials each at IDs 3 and 5 (distance of 40 cm and target sizes of 10.0 cm and 2.5 cm, respectively), with the order of IDs counterbalanced. This familiarization will comprise a total of 80 familiarization trials and serve to acquaint participants with the apparatus and the controller, VR immersion, and feedback conditions. After each condition, participants will provide a brief motion sickness rating using the Simulator Sickness Questionnaire and have a short break (about 60s, longer if needed). The main experiment will consist of 160 trials per participant (4 feedback conditions × 4 IDs × 10 trials). Trials will be organized into blocks of 40, each comprising four consecutive runs of 10 trials at a single ID (e.g., 10 trials at ID 3, then ID 4, etc.). The order of IDs within each block will be randomized, and the condition order will be counterbalanced across participants using a Latin square design. The motion tracking system of the Meta Quest 3 will be used to capture 3D positional data of the handheld controller throughout each trial (nominal sampling frequency = 120 Hz). Movement onset will be considered to occur when the controller speed exceeds a threshold of 3 cm/s, and movement offset will be defined as the moment the speed drops below 3 cm/s (following target entry for successful trials). These thresholds, used in previous experiments, ensure that only intentional movement periods are captured. Movement time is defined as the time elapsed between movement onset and offset.