(4) Finally, we assessed whether the angular extent of the VMA (i.e., upper vs. lower visual fields) differed under monocular and binocular viewing conditions. Linear mixed-effects models included angular distance, visual field (upper vs. lower), and viewing conditions (monocular vs. binocular) as predictors and participants as random effect.
Figure 11 shows SF threshold (
Figure 11c) and SF cutoff (
Figure 11d) estimates plotted separately for upper and lower visual field locations as a function of the angular distance from the VM under either monocular or binocular viewing conditions. First, as expected, SF estimates linearly increased as the angular distance from the VM increased for both SF threshold (
Figure 11c; intercept:
t(188) = 25.61,
p < 0.001, CI [.6688, .7804]; angular distance
t(188) = 7.78,
p < 0.001, CI [.0011, .0019]) and SF cutoff (
Figure 11d; intercept:
t(188) = 41.40,
p < 0.001, CI [.7968, .8765]; angular distance:
t(188) = 15.58,
p < 0.001, CI [.0015, .0019]). As in
Figures 7c,d, SF estimates were significantly higher in the lower than in the upper VF for both SF estimates (SF threshold:
t(188) = 3.95,
p < 0.001, CI [.0176, .0526]; SF cutoff:
t(188) = 2.91,
p = 0.004, CI [.0102, .0533]), with the effects of upper-lower VF interacting with angular distance (SF threshold:
t(188) = 3.82,
p < 0.001, CI [.0002, .0006]; SF cutoff:
t(188) = 2.14,
p = 0.034, CI [.00003, .0006]). All participants showed a steeper linear slope with angular distance in the upper VF than in the lower VF for both SF threshold (
Figure 12a) and SF cutoff (
Figure 12b) estimates. Finally, viewing condition was associated with a binocular advantage (SF threshold:
t(188) = 3.14,
p = 0.002, CI [.0060, .0261]; SF cutoff:
t(188) = 2.95,
p = 0.004, CI [.0059, .0298]) but did not interact with either visual field or angular distance (
p values > 0.1 and CIs including 0). This binocular advantage in acuity was observed in all participants, for both SF threshold (
Figure 12c) and SF cutoff (
Figure 12d) estimates. Thus, although visual acuity was overall lower under monocular viewing condition, we observed a similar linear decrease in the upper-lower asymmetry (i.e., VMA) with angular distance under binocular and monocular viewing conditions (see
Figures 11c,d for the corresponding linear equations).