Without the prism in front of one eye, the red and green crosses on the computer screen were adjusted to the same height by the nine subjects. Accordingly, the angular difference between both eyes was approximately zero (
Fig. 4A, “baseline”). Right after the prism was introduced, the subjects' estimation of relative height of the two crosses became asymmetrical. Differences matched the power of the prism (1.72°) in some subjects but not in others, who appeared to largely ignore the prism (
Fig. 4A, labeled as “no adaptation”). After the prism was worn for 15 minutes, the perceived difference in height was reduced by adaptation in most of the subjects, except for those who had ignored the prism before. Even though not all subjects showed adaptation effects, the averages of the adaptation amplitudes measured in all nine subjects were also significant (
Figure 4B, mean value 0.003° before adaptation, labeled as “no prism, baseline”; and 0.44°, labeled as “prism, adapted [15 min]”; difference significant at
P < 0.05, ANOVA–post hoc LSD). Adaptation was incomplete on average since the perceived height difference between both crosses remained large even after the adaptation phase (mean value 0.86°, labeled as “no adaptation”; and 0.44°, labeled as “prism, adapted [15 min]; difference significant at
P < 0.05). After removal of the prism and another 15 minutes of recovery, the subject adjusted the two crosses again to the same height (
Fig. 4B, labeled as “no prism, recovered”).
There was a strikingly large inter-individual variability as to how the subjects perceived the height differences and how they adapted to it. Amplitudes of adaptation were clearly determined by the initially perceived strength of the prism (
Fig. 5). To find out whether the large intersubject variability could be explained by intersubject variability in vertical fusional eye movements, six of the subjects were measured with the binocular eye tracker.