Fig 1.
Experimental set up and results.
(a) Participants were seated in the centre of the cave. During the experiment, participants made absolute judgements of the distance between their own body and an object (a gift box) appearing in front of them. The positions of the gift box were distributed logarithmically between 5m and 25m. The same distances were presented with the three head inclinations -20°, 0°, and +20°. The gaze was aligned with head inclinations. (b) Left anodal and right cathodal configuration is named ‘L-GVS’. The inverse polarity, namely right anodal and left cathodal configuration, is named ‘R-GVS’. A sham stimulation was also applied placing the electrodes to the left and right side of the neck about 5cm below the GVS electrodes. GVS and sham stimulation were applied delivering a boxcar pulse of 1ma for 3s. (c) Distance errors have been calculated by subtracting the actual distance from the judged distance. Estimation bias in 0° head inclination condition was used as baseline, and all values were corrected by this baseline. Thus, negative values on the ordinate indicate underestimation compared to the horizontal, zero-inclination baseline condition, whereas positive values indicate overestimations. Distance perception varied significantly according head inclination. Specifically, downward distances were underestimated, while upward distances were overestimated, relative to baseline. This pattern of distance illusions is in line with the predictions of the gravity theories. Note that GVS enhances this pattern. (d) Predictions based on linear mixed-effects model. The model containing both fixed and random terms fits well to the actual data.
Fig 2.
Relation between object position and judgement error in individual participants.
See text for explanation. Note that the size and even direction of the relation differs between participants.