Results
Tracking performance across gravity conditions
We measured the dynamic RMSE value (difference between onscreen ball and eye position) every 20ms over the first 500ms, separately for horizontal and vertical components. In the horizontal direction, with ball motion at a constant speed (4 or 16°/s), average performance, RMSE-x gradually worsened between 0 and 200ms. This was because horizontal direction was unpredictable in each trial and the stimulus response, which started around 85-95ms, required a catch-up saccade and so only improved after 200ms (Figure 1A and C, darker grey traces). For RMSE-y in the vertical direction where acceleration acted, the gravity condition was the same within each block of trials so motion could be predicted and anticipated by the participant after some trials. The average response was therefore about 0.5° from onset for the full 500ms (Figure 1B and D darkest grey trace). In contrast, for the antigravity condition, average tracking was worse at ~0.8° from onset gradually improving until 300ms, implying there was an anticipatory antigravity response that was less effective than under gravity (Figure 1B, D mid grey trace). This result is consistent with the previous study (Meso et al., 2020), with worse performance and slower dynamics under AG. The differences are statistically evaluated in the next section. Importantly, the control condition with horizontal gravity was similar to the gravity condition, but with a baseline averaged initial performance of 0.6°, 0.1° worse than the gravity condition (Figure 1A and C lightest grey trace). This suggests there is a performance advantage for the downwards gravity condition that is reduced by 0.1° in the horizontal rightward gravity control condition.