This study aimed to investigate the impact of the visual environments that manipulate gaze behavior on cyclists’ dynamic balance control. Therefore, we employed a laser fixation point and a monochromatic path to assess changes in eye movements and cycling performance. The results showed that the amplitude at the peak frequency of eye movements was significantly lower in the laser condition than in the control condition. Additionally, both cycling distance and cycling duration were shorter in the laser condition compared to the control condition. These results suggest that fixation on a laser point projected onto the path suppresses the eye movement amplitude and impairs balance control during cycling.

The amplitude at the peak frequency of eye movements was significantly lower in the laser condition than in both the control and monochrome conditions. This result indicated that the amplitude of eye movements was suppressed under the laser condition, as we hypothesized. During locomotion, it was reported that using a heads-up display to present a fixation point in the forward field of view suppressed eye movement (Moore et al. 2008). It was also known that fixating on a point suppressed the amplitude of OKR in experiments using visual stimuli (Pola et al. 1992). In our study, there was no difference in the peak frequency of eye movement, which appeared to be consistent with previous studies indicating that fixation at a point did not affect the frequency of OKR (Pola et al. 1992). Although fixation on laser did not alter the peak frequency of eye movements, it led to a reduction in their amplitude, suggesting a diminished stabilizing function of OKN on the visual environment. Furthermore, the SD of head tilt angle showed no difference among the conditions. The mean values were 3.18 deg in the control condition and 2.37 deg in the laser condition, both of which are close to the value reported in a previous study using straight and narrow path cycling (2.51 ± 0.81 deg; Kojima and Kokubu 2024), indicating that head movement was also small in the present study. These findings suggest that fixation on the laser suppressed eye movements rather than head movements, thereby altering the visual environment. In summary, the visual environment designed to manipulate gaze behavior using a fixation point projected on the path at a fixed distance suppressed the amplitude of OKN, and this may have altered the velocity vector of retinal flow.

The cycling distance and duration in the laser condition were significantly shorter than those in the control condition. These results suggest the possibility that dynamic balance during cycling was deteriorated by fixation, as hypothesized. Previous studies have reported that compensatory eye movements, such as OKR, are predominantly observed during cycling on a straight and narrow path, whereas travel fixation, in which eye movements are suppressed, is rarely observed (Vansteenkiste et al. 2013; Kojima and Kokubu 2024). Compensatory eye movement contributes to the suppression of retinal slip and the stabilization of the fovea (Lappi 2016; Matthis et al. 2022; Muller et al. 2023). Suppression of these eye movements may disrupt visual information and negatively affect motor control. Interestingly, our results differed from those of previous studies on static balance control. In tasks evaluating static balance, fixation on a point in front has been shown to reduce body sway (Guerraz et al. 2000; Laurens et al. 2010). For example, fixation on a distant point has also been reported to reduce both the distance and velocity of the center of pressure (Lee and Lishman 1975). This stabilizing effect of fixation is attributed to the contribution of extraocular muscle signals associated with eye movements (Jahn et al. 2002; Strupp et al. 2003; Glasauer 2005). These different results may be attributed to the position of the fixation point and the presence or absence of movement in the participants. In our study, the fixation point was set on the ground, and the participants rode a bicycle and controlled their balance using their entire body. Additionally, cycling speed was not significantly different among conditions. The mean cycling speed observed in all conditions was approximately 1.72 m/s (range: 0.96–2.37 m/s), which is below the 4.3 m/s required to maintain independent stability (Meijaard et al. 2007). This indicates that the cyclists were required to maintain dynamic balance during cycling. During self-motion, retinal flow arises from a combination of optic flow generated by movement through the environment and flow resulting from eye movements. Our findings partially support the idea that compensatory eye movements, which stabilize the visual field, are necessary for dynamic balance control that relies on complex retinal flow during more dynamic self-motion.

An alternative explanation for the observed decrease in cycling stability due to fixation can be discussed from the perspective of attentional demands. The cycling task employed in this study was relatively challenging, as reflected by the mean cycling distance of 12.26 ± 4.11 m in the control condition. Although cycling is often considered an automated motor skill, it is likely that the task imposed a considerable cognitive load on participants. According to the attentional capacity model (Kahneman 1973), allocating attentional resources to maintain fixation on a specific point may have interfered with the attention necessary to effectively perform the cycling task. This notion is supported by evidence from walking studies, where even automated locomotion requires attentional resources, and dual-task interference negatively affects walking performance, especially in older adults with impaired balance (Woollacott and Shumway-Cook 2002; Beurskens and Bock 2012). Furthermore, the fixation point was fixed at a constant distance ahead, which may also have caused the observed destabilization during cycling. Instructing participants to fixate on the laser point likely caused their attention to be narrowly focused on a nearby location. In contrast, it is well documented that directing attention toward an external, more distant point can enhance balance control (Park et al. 2015), with improvements increasing as the attentional focus distance grows (McNevin et al. 2003). Therefore, in the laser fixation condition, attentional focus may have been constrained to a nearer distance compared to the control condition, possibly resulting in decreased cycling stability.

We also attempted to suppress eye movements without constraining the gaze position by using a monochrome cycling path. However, the amplitude at the peak frequency of eye movements and the peak frequency did not differ between the control and monochrome conditions. The results suggest that the monochrome path did not suppress eye movements. The reason why eye movements occurred even under monochrome conditions might be the outdoor experimental setting; this was probably due to the lack of control over the texture of the surrounding ground. It has been reported that eye movements are not only triggered by objects captured in the central retina but also by the stimulus in the peripheral retina, leading to a high OKN gain at stimulus velocities under 30 deg/s (Howard and Ohmi 1984). In our study, since cycling was performed at low speeds, stimuli from the peripheral retina may have contributed to the induction of OKN. Therefore, the visual environment designed to manipulate the gaze using the narrow path of a monochromatic surface did not suppress OKN induction. The fact that the amplitude of OKN was not suppressed under the monochrome condition means that we could not completely rule out the possibility that the decrease in cycling distance and duration under the laser condition was due to fixation of the gaze position. The fixation distance used in the laser condition in this study was based on the average value from a previous study on cycling through a straight narrow path (Kojima and Kokubu 2024). In the previous study, the gaze angle was − 22.2 deg, and the eye height during cycling was 1.60 m, suggesting that the participants were focusing on a path approximately 3.92 m ahead. Therefore, in this study, we set the fixation point at a fixed distance of 4 m ahead for all participants. This approach was intended to minimize the possibility that constraining the gaze position would reduce the stability of cycling. However, future experiments are needed to clearly distinguish the effects of eye movements suppression and gaze position.

This task was deliberately chosen to model real-world situations that place heightened demands on balance control such as navigating poorly maintained roads, wet or slippery surfaces, or riding close to vehicular traffic. Under these physically challenging conditions, cyclists tend to direct their gaze more toward the road surface (Vansteenkiste et al. 2013, 2014). For instance, Vansteenkiste et al. (2014) found that cyclists riding on unpaved paths spent approximately 63% of the time looking at the ground, compared to only 8% looking straight ahead. This gaze behavior is consistent with our task setup and supports its ecological relevance. Our findings have important practical applications. For example, looking at a smartphone screen while cycling, walking, or riding a motorcycle can increase lateral deviation and pose a safety risk. It would be wise to avoid looking at electronic device screens, especially for navigation, in situations with crowded, narrow, or unpaved road conditions. Our findings suggest natural gaze behavior is likely to be an important factor for cycling safely.

The present study has three limitations. First, it remains unclear whether the suppression of eye movements directly caused the decrease in cycling stability. To investigate this potential causal relationship, future studies could involve participants cycling under natural viewing conditions, while simultaneously recording gaze data and cycling stability indicators such as lateral deviation, steering variability, and trajectory. By conducting time-series analyses to determine whether travel fixations　(i.e., gaze behaviors characterized by suppressed eye movements) precede instances of instability, it may be possible to infer a causal link between eye movement suppression and decreased balance control. Second, the study lacks a monochrome condition using a laser. In this experiment, the number of conditions was limited to three in order to prevent participants’ fatigue. This comparison would help clarify whether the differences in head tilt angle observed between the laser and monochrome conditions are primarily driven by gaze fixation or by the visual texture of the path. Third, not only eye movements but also gaze position were manipulated in the laser condition. Thus, the manipulated gaze position may have impaired balance control. In this experiment, the gaze position of the laser was determined based on previous studies about cycling on a straight and narrow path. Future research should investigate the impact of different gaze positions, such as 2–6 m ahead, on balance control.