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.