Overall, we confirmed the ubiquitous nature of serial dependence, even in a VR environment. Every condition of all three experiments revealed strong (up to 6 degrees-7 degrees in certain conditions) positive serial biases towards orientations from preceding trials. These amplitudes are larger than the ones observed in previous studies (except in Fischer & Whitney, where they observed amplitudes up to approximately 8 degrees). Moreover, previous studies have observed repulsive biases when the orientation difference between the previous and current object was large (
Bliss, Sun, & D'Esposito, 2017;
Fritsche et al., 2017;
Samaha, Switzky, & Postle, 2019). However, in our experiments, the width of the DoG curves was large such that there were no repulsive effects at higher orientation differences.
One potential reason for the strength of serial dependence found here is the higher uncertainty introduced by the third dimension (i.e. depth). For example, in all three experiments, we observed stronger positive serial dependence when the object was rotated in-depth which may have increased the stimulus uncertainty in orientation estimates, leading to higher positive serial dependence. This would be consistent with previous studies showing a positive relationship between stimulus uncertainty and serial dependence biases (
Cicchini et al., 2018;
van Bergen & Jehee, 2019;
Ceylan et al., 2021,
Kondo, Murai, & Whitney, 2022).
Other factors may have contributed to uncertainty in our experiments. Our stimuli included familiar real-life objects whose shape properties are far more complex than simple bars or oriented Gabors. For example, even though our objects had a clear longitudinal axis with a specific orientation, the borders of these objects do not constitute straight contours, but instead include undulations. This could lead to noisier orientation encoding, compared to the orientation signals caused by a straight line. Furthermore, the lower resolution of VR head-mounted displays, compared to high-resolution screens used to show two-dimensional stimuli, could add even more noise to the encoding process. Another factor was that a line or an oriented Gabor repeats itself every 180 degrees, however, our objects were not symmetric across their midpoint, technically making them repeat their orientations every 360 degrees. However, we do not think that this should have a large effect, because the response bar was symmetric across its midpoint, so participants reported their estimates along a 180 degrees orientation space, as in previous serial dependence studies. It is, however, difficult to pin down the exact reasons for the large DoG curves we observed, among these display and stimuli differences. Similarly, these differences may also be responsible for not observing any repulsive biases when the orientation difference was large between the current and the previous object. Such repulsive biases are generally attributed to adaptation effects (
Pascucci et al., 2019), and our display does not seem to give way to such effects.
This relationship between stimulus uncertainty and serial dependence may potentially explain the effects observed in all three experiments, especially effects of rotation type on serial dependence biases. DoG amplitude was positively correlated with response scatter, suggesting that the uncertainty was responsible for the large serial biases observed here. In conditions where stimulus uncertainty (as inferred from task performance) was presumably higher (such as when the object was rotated in depth in
Experiments 2 and
3), the correlation disappeared and individual differences in serial biases emerged. This becomes apparent when the response scatter in the fronto-parallel conditions of all three experiments is compared (i.e. compare
Figure 4A,
6C,
8A). The separation between the two rotation type conditions (the blue and the red data points in these plots) is very clear for
Experiments 2 and
3, which did not yield a positive correlation for when the object was rotated in depth. However, the response scatter was overall lower in
Experiment 1, which might have allowed this significant positive correlation to emerge for both rotation types. However, these results should be interpreted with caution because our experiments were not designed to manipulate or measure the effect of stimulus uncertainty. Even though these results do not allow us to make strong conclusions about the effect of stimulus uncertainty, they point out that manipulating uncertainty by introducing depth (via stereoscopic disparity) could be an interesting direction for future studies.
Further comparison between experiments may uncover the factors that influence serial biases, but it also raises questions. In
Experiment 1, the effect of rotation type was only observed when the object was 12 m-away, whereas in
Experiment 3 it was only observed when the object was 4 m-away. This indicates that the crucial factor determining the DoG amplitudes was the retinal size of the object (as opposed to its distance), because the retinal size of the 12 m-away object in
Experiment 1 was equal to the retinal size of the 4 m-away object in
Experiment 3. We observed a similar pattern of results in these two conditions across
Experiments 1 and
3, and what was common between them was the retinal size of the object. However, we also observed an effect of distance on DoG amplitudes in
Experiment 1 when the object was rotated in the frontoparallel plane. This effect was independent of the retinal size of the object. In addition to that, we should note that, in
Experiment 2, even though the swords were three times larger than the toothbrushes, this difference did not affect serial dependence amplitude. To sum up, we independently observed the effect of retinal size and object distance on serial dependence separately, but not in all experimental conditions. In principle, it is possible that serial dependence occurs before stereo depth cues, such as those created by the rendered distance in VR, have their effect. Our results cannot answer this, however. Future studies could disentangle effects of stimulus uncertainty and distance to understand whether depth cues (such as stereo-disparity) influence serial biases independent of stimulus uncertainty. This would, in turn, determine whether the bias originates from retinal or perceived size.