The effect of the Ebbinghaus visual illusion on the performance of a continuous rhythmic perceptual-motor task requiring speed and accuracy with the dominant and non-dominant hand
Authors: Ghazizadeh Habib , Doustan Mohammadreza , Boushehri Seyedeh Nahid Shetab 
Published: August 2024

Objective
Over the course of several years, numerous researchers have conducted studies in the realm of movement to examine the impact of the visual field, particularly the Ebbinghaus visual illusion, on perception and action. Certain studies have presented evidence that challenges the notion of a distinct separation between the visual stream and motor performance, demonstrating that the Ebbinghaus visual illusion does indeed influence motor performance, particularly in discrete tasks. Limited research has been conducted regarding the impact of this particular visual illusion on the execution of continuous tasks. The objective of this current study was to examine the influence of Ebbinghaus' visual illusion on the execution of a continuous rhythmic perceptual-motor task, which necessitates both speed and accuracy.
Methods
This study employed a semi-experimental and fundamental approach. A total of 60 students, aged between 19 and 24, who were studying at Shahid Chamran University of Ahvaz, were selected as participants using the available sampling method. The participants were instructed to perform a bilateral tapping task under 24 different conditions. These conditions included two levels of time difficulty (with a rhythm of 250 and 350 thousandths of a second) and six different illusion states (NI-NI, SI-SI, MI-MI, MI-SI, NI-SI, MI-NI ) with both their dominant and non-dominant hands. This study utilized a speed-accuracy trade-off measuring device accompanied by an auditory metronome rhythm to complete the assigned task. The researchers assessed and compared the accuracy of hitting the targets, the dispersion of hits in relation to the target's center, and the timing errors in task execution. The data were subjected to statistical analysis using inferential statistics methods, specifically compound analysis of variance with repeated measures, as well as Bonferroni, Friedman, and Kruskal-Wallis follow-up tests. Data analysis was conducted using SPSS version 22 software, with a significance level set at P ≤ 0.05.
Findings:
The study revealed that the specific variant of the Ebbinghaus visual illusion significantly influenced the accuracy of participants in a rhythmic tapping task, both with their dominant and non-dominant hands. The percentage of correct hits varied across different types of visual illusions. Furthermore, participants using their non-dominant hand at a slower pace demonstrated greater precision in hitting targets that had a higher prevalence of shrinking illusions than magnifying illusions. This phenomenon pertains to a reduction in speed during tasks executed with the non-dominant hand. Furthermore, the dispersion of hits at lower speeds exceeded that at higher speeds. However, the dispersion of hits around the central area of the targets was more pronounced in most illusory conditions at higher speeds compared to lower speeds. Additionally, the timing error observed when performing with the dominant hand was lower than that with the non-dominant hand, and this discrepancy was also lower at lower speeds compared to higher speeds. Notably, the Ebbinghaus illusion led to a significant enhancement in movement timing in the dominant hand. Conclusion: Given the impact of different types of visual illusions on motor execution, there is reason to question the hypothesis that the visual stream is separated during the performance of a continuous visual-motor task. Additionally, the results support Fits's law, which suggests a trade-off between speed and accuracy in motor tasks. However, the notion proposed by Asai that visual feedback is specific to the execution of the dominant hand was not supported by the data.