Eye and hand coarticulation during problem-solving reveals hierarchically organized planning.

During everyday activities, such as preparing a cup of coffee or traveling across cities, we often plan ahead and execute sequences of actions. Yet how such planning unfolds when solutions must be formed from scratch, without external cues or routines, remains unclear. This study examines how participants coordinate gaze and cursor movements while solving path-tracing problems on a grid with multiple targets. By focusing on the action execution phase, we aimed to probe the structure of the gaze-cursor plans that participants used to solve the tasks. Our analysis reveals three main findings. First, consistent with previous studies, participants segment the problem into sequences of gestures, defined as the task execution (gaze and cursor movements) in the time interval between the onset of two consecutive fixations. Typically, within each gesture, gaze focuses on a target and remains fixed until the cursor reaches it, then shifts to the next target. Second, both gaze position, while fixating on the current target, and the kinematics of cursor movement leading up to the current target allow prediction of the next cursor movement's direction, revealing coarticulation in both cursor-cursor and gaze-cursor movements. Third, and most interestingly, the position of the gaze around the current target aligns with the direction of the next saccade, revealing coarticulation between successive gaze fixations. Together, these findings show that participants break the problem into gesture sequences and plan multiple eye and cursor movements in advance to efficiently reach both the current and upcoming gesture targets. This suggests a hierarchical planning strategy, with participants planning ahead at two levels: gesture targets and cursor movements.NEW & NOTEWORTHY Coarticulation, the modulation of an action's execution based on upcoming actions, offers a valuable window into covert planning, typically defined as the goal-directed concatenation of actions. Most prior studies have examined coarticulation in instructed, cued, or routinized sequences. In contrast, we provide a comprehensive characterization of coarticulation in both gaze behavior and hand kinematics (via cursor movements) during a planning task where participants must generate novel solutions from scratch on each trial.