# Effect of Stimulus Regularities on Eye Movement Characteristics
## 1 Introduction  
Humans detect and extract the temporal and spatial regularities in the environment through a process called statistical learning. While much research has been focused on distributional regularities (e.g., frequency of occurrence) and transitional probabilities (e.g., order of appearance), higher-order regularities remain underexplored [1]. Moreover, in typical studies on statistical learning, the acquired knowledge of the statistical regularities is not examined in its development but is evaluated in a subsequent test phase. One way to explore the learning process is by using non-observational tasks that require a response from the participants. This approach is used in studies applying either the Serial Reaction Time (SRT) task [2] or the Alternating Serial Reaction Time task (e.g., [2]). In both tasks, the stimulus location and order are manipulated, thus varying the predictability of the targets. The participants respond manually, and the response time change in the experimental trials is a measure of the learning success. In a recent study, [3] used eye movement records to evaluate the temporal dynamics of the learning process by examining the occurrence of anticipatory eye movements during the experiment. This study is one of the few studies that evaluate the changes in eye movement characteristics across trials to characterize the ongoing process of learning. However, the eye movement characteristics may provide richer information than those evaluated by the predictive gaze landings on targets with variable predictivity. Eye movements can provide insight into the unobservable processes of visual selection, decision making, and uncertainty reduction associated with learning. It is generally believed that statistical learning occurs without awareness (e.g., [4]), i.e., it involves implicit memory. [PERSON] and co-authors [5] explored whether gaze behavior changes depending on the involvement of explicit or implicit memory in a visual search task. The study's results showed that the location of the first fixation depended on explicit memory, whereas the search efficiency evaluated by the eye scan path improved due tounconscious memory. The study also showed that the effects of the two memory types were independent. Other authors (e.g., [6; 7]) suggested that eye movements were not needed for implicit learning of spatial configurations. [PERSON] and co-authors [8] provided evidence for the mutual connection between eye movement patterns and the acquired knowledge about the statistical structure of the scenes. Their study explored the interaction between eye movement characteristics and the learning process without a well-defined task. However, natural vision is an active process, and movements of the eyes allow continuous comparison of the current visual input with prior experience to guide our future behavior (e.g., make decisions, navigate [9], and use the acquired knowledge from a scene in future situations and conditions). Eye movements are not random but reflect both bottom-up and top-down processes. The bottom-up processes are associated with the salience of the image features like edge distribution, contrast, color, size, and other low-level features. In the present study, we aimed to evaluate whether the eye movement behavior reflects the process of learning the high-order regularities in image characteristics when the salience of the stimuli does not differ significantly. Hence, the differences in eye movement characteristics primarily reflect top-down attentional control. We manipulated the spatial regularities in visual displays, varying the repetition frequency, the co-occurrence of chunks, and the similarity of patterns, aiming to evaluate the interaction between the accumulated knowledge and the gaze behavior.