Punt, D. | bioRxiv Science

bioRxiv2026

Gaze Direction During Bimanual Reaching Is Associated with Changes in Relative Hand Speed

The control of visually guided bimanual movements is thought to represent a compromise between the drive for synchrony that characterizes many bimanual movements and the competing demands that characterize the constituent unimanual movements. As a result, while visually guided bimanual movements are found to be ostensibly synchronized, they also demonstrate subtle asynchronies. Here, we conducted a fine-grained analysis of bimanual reaching movements focusing on their relative synchrony as movements unfold and the relationship with gaze behavior. Our novel analysis revealed that participants frequently shifted their gaze laterally several times during bimanual reaches and that these gaze shifts were systematically associated with changes in the speed ratio of the two limbs. More specifically, we observed a strong association between gaze shifts and the speed ratio shifting toward the limb on the attended side. This pattern of behavior was seen regardless of hand dominance and target difficulty, suggesting that it may reflect a common visuomotor strategy during bimanual reaching.

neuroscience↗

bioRxiv2025

Time-Dependent Facilitation of Homologous Actions

Unimanual actions can interfere with or facilitate similar actions performed with the opposite hand, especially when in close temporal proximity. Across three sequential button-press experiments, we tested how effector homology - anatomical similarity between fingers - and temporal delays between actions shape these effects. Specifically, we examined whether a priming action altered the reaction time (RT) of a subsequent action. Compared with unimanual RTs, we indexed slowing of the second actions RT as interference, and quickening as facilitation. Priming with homologous actions (e.g., index finger-index finger) produced interference at short intervals ([≤]200ms) but transitioned to facilitation at longer intervals ([≥]400ms). Priming with non-homologous actions (e.g., little finger-index finger) also produced interference at short intervals, but never resulted in facilitation. Critically, these patterns emerged whether the priming actions were performed with the opposite or the same hand, indicating that interference and facilitation do not depend on interhemispheric dynamics. Our results reveal a previously undocumented time-dependent shift from interference to facilitation that is specific to homologous actions, challenging models that explain the interference between parallel actions solely by competitive interhemispheric dynamics or central bottleneck processes. We propose that facilitation and interference flexibly coexist, and are shaped by effector homology and timing. These findings extend current models of bimanual coordination and highlight new opportunities for enhancing motor performance and neurorehabilitation. GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=86 SRC="FIGDIR/small/682693v1_ufig1.gif" ALT="Figure 1"> View larger version (14K): org.highwire.dtl.DTLVardef@20dc18org.highwire.dtl.DTLVardef@151ea4corg.highwire.dtl.DTLVardef@ed2547org.highwire.dtl.DTLVardef@12a9550_HPS_FORMAT_FIGEXP M_FIG A left-hand index finger press primed a subsequent right-hand index (homologous) or right-hand little finger (non-homologous) response at variable delays. For homologous actions (index-index), reaction times (RTs) quickened relative to unimanual single-press baseline for delays [≥]400ms, whereas for non-homologous actions (index-little), RTs slowed and gradually returned to baseline by ~600ms. This dissociation indicates that effector homology and timing dictate whether a preceding action facilitates or interferes with the execution of a subsequent one. C_FIG

neuroscience↗