Eye-hand coordination during sequential reaching to uncertain targets: the effect of task difficulty, target width, movement amplitude, and task scaling.

When making reciprocal hand movements between two fixed/known targets, Fitts' law states that movement time (MT) is a linear function of the index of difficulty (ID) set by the ratio between movement amplitude (A) and target width (W). Crucially ID also impacts eye-hand coordination. However, because known/fixed targets limit the usefulness of eye movements, and because hand dynamics changes drastically with ID (either continuous or discrete), we reexamined this issue using a variant of the Fitts task in which the next target position was assigned randomly and unveiled only when the ongoing target was reached. Practically, hand and eye movements were recorded in participants (N = 25) who had to successively reach 12 circular targets of width W (0.3, 0.6, or 1.2 cm) separated by an amplitude A (5, 10, or 20 cm), allowing to examine IDs ranging from 2.36 to 6.08 bits. Introducing target uncertainty did not alleviate the linear relationship between MT and ID (R² = 0.99), neither the impact of ID on hand kinematics. Importantly, the influence of ID on eye-hand coordination persisted. Notably, by cross-correlating eye and hand signals, we show a trade-off between its temporal and spatial aspects. Finally, we found that eye-hand coordination was influenced to a larger extent by A than W, making it prone to task scaling effects (differences in AW combinations resulting in similar ID). Altogether these results reinforce the critical role of task difficulty on eye-hand coordination and the need to simultaneously consider its temporal and spatial aspects.