Experiment 2
In the field of motion perception, prior research has predominantly focused on proprioceptive motor adaptation, such as hand tapping, while adaptation to visual motion, such as viewing a motion lattice, has received limited attention. Despite the growing body of literature on motor perception ( Anobile et al., 2021a ; Anobile et al., 2021b ; Sixtus et al., 2023 ; Anobile et al., 2016 ), however, as of now, there has been no research exploring the relationship between visual motion adaptation and the grouping effect. Hence, Experiment 2 in our study fills this research gap by investigating the potential interaction between motion adaptation and grouping strategies during visual motion adaptation. This novel contribution to the literature can enhance our understanding of the underlying mechanisms of motion adaptation and its relationship with grouping strategies.

We collected data from 24 undergraduate students form Guizhou Normal University (15 Female; Mean age = 22.35; SD = 2.33). None of them participated in the experiment 1. All recruited participants had normal or corrected visual acuity, all right-handed and signed an informed consent form.

Stimuli were created and presented using E-prime 3.0. In Experiment 1, proprioceptive motor adaptation of hand tapping was used as the motor adaptation stimulus, while in Experiment 2, visual motion adaptation was used, which involved watching a lattice with fast (3 m/s) or slow (0.5 m/s) movement. A total of 10 blue dots (RGB: 0; 0; 255) were presented, each measuring 0.1° × 0.1°, with random motion direction. Other materials used for stimulation were the same as in Experiment 1.

At the beginning of each trial, a 6-second motor adaptation was performed by watching the lattice with irregular motion at either fast or slow speed, followed by a fixation point “+” displayed for 500 ms. After the disappearance of the fixation point, the stimulation was presented for 250 ms. Participants were then required to estimate the number of stimulations and input the result into the presented input box using a keyboard, followed by pressing “Enter” to proceed to the next trial ( Figure 4 ).

Data analysis was the same as experiment 1.

The results of Experiment 2 revealed that the adaptation index in the no-grouping condition was higher than in the grouping condition ( t = 2.072, p = 0.12, Cohen’s d = 1.353), indicating a stronger adaptation effect under no-grouping condition in visual motion adaptation ( Figure 5A ). Regarding the perceived numerosity, Experiment 2 yielded similar results. Specifically, adaptation to fast visual motion (induced by viewing fast moving dots) led to underestimation of numerosity, led to a 14% reduction in apparent numerosity across all tested numerosities (linear regression slope of 0.85, compared to the baseline slope of 0.99), while adaptation to slow visual motion (induced by viewing slow moving dots) resulted in overestimation, adaptation resulted in a 5% increase in numerosity (linear regression slope of 1.04, compared to the baseline slope of 0.99), which is consistent to previous studies, these results were statistically significant and demonstrate a clear pattern ( Figure 5B ).

As a result of the ANOVA for CVs, we found that the main effect of grouping condition was borderline significant F (1,23) =3.650, p =0.069. The sensory precision under the grouping condition was more accurate. A significant main effect of numerosity also emerged, F (12,12) =35.734, p <0.001***. Similarly, the sensory precision of small numerosities was more accurate than that of large numerosities. And the interaction between group conditions and numerosity was also significant, F (12,12) =3.995, p =0.012*. The follow up simple interaction effect analysis revealed that, there is only a grouping effect in the intermediate numerosities (the grouping condition has more accurate sensing precision than the non-grouping condition), and there is no grouping effect in both the larger and smaller numerosities ( Figure 6 ).

For RTs, only the main effect of numerosity is significant, F (12,12) = 8.294, p < 0.001***. Small numerosities also had shorter reaction times. And the interaction between numerosity and group conditions was borderline significant. F (12,12) = 2.611, p = 0.55.

The findings from Experiment 2 closely mirrored those obtained in Experiment 1, revealing a consistent pattern of results. Specifically, in the condition of adaptation to visual motion, it was consistently observed that after adaptation to fast visual motion ( i.e., viewing the fast-moving lattice), the estimation of the number was consistently underestimated. Conversely, adaptation to slow visual motion ( i.e., viewing the slow-moving lattice) consistently resulted in overestimation of the number. These findings provide robust evidence for the stability of the adaption effect in both proprioceptive motor adaptation and adaptation to visual motion.

In Experiment 2′s visual motion adaptation process, we observed a lower adaptation effect in the grouping condition compared to the no-grouping condition (see Figure 5A ). This result suggests that the impact of visual motion adaptation on groupitizing is smaller than that of proprioceptive motor adaptation. Importantly, prior research emphasizes the positive impact of “groupitizing” on enhancing numerosity perception precision. In contrast, adaptation effects arise from perceptual distortions induced by adaptation, resulting in biases in numerosity estimation. Within the grouping condition, participants employed grouping strategies to actively mitigate perceptual biases induced by visual motion adaptation. As a result, adaptation effects in the grouping condition consistently exhibited a lower impact compared to the no-grouping condition. This not only supports the positive impact of groupitizing on numerosity perception but also contributes to understanding how cognitive processes actively regulate the effects of perceptual adaptation on numerosity estimation. This emphasizes the dynamic interplay between cognitive strategies and visual adaptation, underscoring the role of grouping in fine-tuning the perceptual impact of adaptation.

Additionally, the impact of visual motion adaptation on groupitizing is observed to be smaller compared to proprioceptive motor adaptation, a finding that is further evident in the results of sensory precision. In Experiment 1, the grouping condition did not show a significant advantage over the no-grouping condition in terms of sensory precision, likely due to the effect of adaptation. However, in Experiment 2, the main effect of the grouping condition on sensory precision was borderline significant, indicating that the grouping condition after motion adaptation resulted in higher precision in the estimation task compared to the no-grouping condition. This provides further evidence that the use of a grouping strategy has a greater influence on visual motion adaptation compared to proprioceptive motor adaptation. It is noteworthy that the main effect of grouping condition on sensory precision was only borderline significant in this experiment, which may be attributed to the selection of numerosity range. As demonstrated in Figure 6B , sensory precision under the grouping condition with only intermediate values (9–14) was significantly higher than that under the non-grouping condition, with no significant difference observed between small and large values. These findings suggest that future experiments could explore the relationship between the adaptation effect and the grouping strategy by selecting intermediate values under the adaptation to visual motion condition to further investigate the adaption effect.