Keywords

1 Introduction

Recently, there has been an increasing demand for performing visual tasks such as computer work and video viewing in the car. In future vehicle development, it is important for passengers to have good visibility of objects even while exposed to car vibrations. It is known that visual acuity, which indicates the ease of seeing objects, decreases due to whole-body vibration [1].

Motivated by this background, to clarify the factors behind the decline in visual acuity, we investigated the relationship between visual acuity and eye movements during vibration. As the result, we showed that the visual acuity during vibration is anticipated by a percentage of visual target’s projection outside fovea centralis [2]. This implies that eye motor control to project a visual target on fovea centralis stably is important for the good visual acuity under whole body vibration such as car riding.

On the other hand, our previous study was on visual acuity during a relatively short period of vibration. Other research showed that long-term exposure to vibration gradually reduces vision acuity [3]. In this research, to evaluate the effects of long-term vibration and visual tasks on the visual acuity, we measured changes in visual acuity before and after playing a video game for long time in the condition of exposure to whole body vibration.

2 Visual Acuity During a Short Period of Vibration

In our previous study [2], we investigated the impact of whole-body vibration on visual acuity using a shaker shown in Fig. 1a. Our findings revealed that the experimental collaborator, who initially had a visual acuity of -1.0 LogMAR (equivalent to 6/4.8 vision), exhibited a visual acuity of 0.0 LogMAR (equivalent to 6/6 vision) under vibrations mimicking car movement with a peak amplitude of 0.05G. Furthermore, the collaborator’s visual acuity was measured at 0.1 LogMAR (equivalent to 6/7.5 vision) under vibrations with a maximum amplitude of 0.1G. We confirmed that the passenger’s visual acuity decreases even under car vibration, consistent with previous research.

To elucidate the mechanism of poor visual acuity under vibration mimicking car movement, we measured the movements of eye, head and visual target under vibration and showed the following relationships between these values and the visual acuity:

  1. (1)

    The reduction in visual acuity due to vibration exposure can be anticipated by a nonlinear function of a retinal slip ratio (Fig. 1b). The retinal slip ratio is defined as a percentage of visual target’s projection outside fovea centralis shown in Fig. 1c.

  2. (2)

    The reduction in visual acuity is independent on the visual acuity in a stationary state.

It can be interpreted that the passenger is no longer able to project the visual target stably on fovea centralis stably under vibration, and the visual target becomes blurred and difficult to see. The eye motor control under vibration is important for good visual acuity in car riding.

On the other hand, our previous study evaluated the visual acuity over a short duration. To make it easier for passengers to see objects during long rides, it is necessary to investigate how long-term vibration and visual tasks affect visual acuity.

Fig. 1.
figure 1

Visual acuity during a short period of vibration mimicking car movement

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3 Experimental Setup for Effects of Long-Term Vibration and Visual Tasks on the Visual Acuity

In this section, we show a new experimental method to investigate how long-term vibration and visual tasks affect visual acuity. Figure 2a shows the new experimental setup in this research. We installed the visual acuity test chart 0.75m and a display for visual task 0.6 m ahead from the experimental collaborator’s eye position. The experimental collaborator, which sits a passenger car seat installed on the floor of shaker having 6 degrees of freedom, can take a vision test before and after performing the visual task.

To investigate the effects of long-term vibration and visual task, the requirement for visual task is that experimental collaborator can focus on performing the task for a long time feeling tired. For this reason, we use a video game “TETORIS” [4] as a visual task.

We use the Early Treatment of Diabetic Retinopathy Study (ETDRS) chart with the Landolt rings arranged in an inverted triangle shown in Fig. 2a as visual acuity test chart. The unit of visual acuity of ETDRS chart is Logarithm of the Minimum Angle of Resolution (logMAR), defined as the common logarithm of viewing angle of the slit of the Landolt ring. An advantage of using LogMAR is that it can be mathematically calculated including addition, average and so on, because the LogMAR is recognized as interval scale [5].

Fig. 2.
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New Experimental setup

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Using the experimental setup, we investigated the effect of long-term vibration and visual task on visual acuity in following three steps.

Step 1. Measurement of visual acuity before visual task

The experimental collaborator sits the passenger car seat installed on the shaker, laying their hands on the knee and their head off the headrest with their eyes close. When the collaborator hears a start signal from the experimenter, the collaborator opens their eyes and answer the directions of the slits of the Landolt rings by voice. The possible answers are only 4 choices such as ‘Upper’, ‘Down’, ‘left’ and ‘right’. We ask them for answering one direction of one Landolt ring in about 1 s even if they are not confident in the answer.

They start to answer the directions of the slits of the Landolt rings corresponding to 0.6 LogMAR from left to right and continue to answer the directions of the lower Landolt rings in the same way when all directions of upper Landolt rings are answered correctly.

When the collaborator answers the wrong direction, we finish 1 trial of the visual acuity test after their answering all remaining slits of Landolt rings on the same line. Using a correct answer rate for the Landolt ring with incorrect answers, we calculate the collaborator’s visual acuity. For example, in the case of the correct answer rate 60% (3 out of 5 rings are correct) for the Landolt ring with 0.3 LogMAR, the collab-orator’s visual acuity is considered 0.34 LogMAR.

As with our previous research [1], we measure the visual acuity before visual task in a stationary state as well as in a vibration-exposed state. Figure 2b shows the translational accelerations in each direction of the floor of the shaker used as vibration input. The translational accelerations are the random noise signals superimposed on sin wave signals at a frequency of 1, 1.1, and 1.4 Hz, filtered through a band pass filter at a frequency of 0 to 5 Hz and have a maximum amplitude of 0.05G. Before the visual task, the visual acuity measurements are taken four times, twice in the stationary state and twice in the vibration-exposed state.

Step 2. Performing the visual task (playing the video game “TETORIS” for a total of 1 h)

After the measurement of visual acuity before visual task of Step 1, to apply loads of the prolonged visual task to the experimental collaborator, we ask them to play the video game “TETORIS” for a total of 1 h.

The collaborator plays “TETORIS” using the display 0.6m ahead from their eye position and the game controller at hand as shown in Fig. 2a. The duration of each playing “TETORIS” is 15 min. The collaborator performs this 4 times, getting out of the shaker and taking a about one-minute break between each playing.

To evaluate the loads of vibration, we use two vibration conditions during playing “TETORIS”: stationary and vibration-exposed. The same data used in Step 1 is utilized for vibration input. The experimental with and without vibration is conducted on different days to avoid interaction between the two.

Step 3. Measurement of visual acuity after visual task

After performing the visual task, we measure the visual acuity in same way of Step 1 and complete this experiment.

The requirement for the experimental collaborator is that the collaborator’s visual acuity is 0.6 LogMAR (equal to 6/24 normal vison) or higher with the naked eye or soft contacts. In this study, four male collaborators (mean age: 36 years) in their 30s to 40s participated. We performed above Step 1, 2, 3 on each collaborator and got A total of 64 samples (2 cases of visual acuity before and after the task, 2 cases of presence or absence of vibration during visual acuity measurement, 2 cases of presence or absence of vibration during visual task, 2 repetitions, 4 collaborators, multi-plication number) of visual acuity data. This study was approved by the Ethics Review Committee of Toyota central R & D Laboratory (No.21B-11).

4 Results and Discussions

Figure 3 shows averages and standard deviations of visual acuity measured under various conditions. Clearly, the followings are shown:

  1. (1)

    There are no significant changes in visual acuity before and after playing the game in the stationary state,

  2. (2)

    After performing the prolonged visual tasks in the vibration-exposed state, the visual acuity in the stationary state remained unchanged, but the visual acuity in the vibration-exposed state decreased.

Fig. 3.
figure 3

Visual acuity under various condition

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Table 1 shows the results of a three-way ANOVA for differences in the visual task, presence or absence of vibration during visual acuity measurement, and between-subject differences. We consider only an interaction between the differences in the visual task and the presence or absence of vibration because the interaction between the differences among experimental collaborators and other factors is less effective. As is clear from Table 1, the visual acuity changes can be expressed by incorporating the differences in the visual task and the presence or absence of vibration.

Table 1. Three-way ANOVA results
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The above results show that the only effect of prolonged visual tasks under vibration is a decrease in visual acuity under vibration. It is thought that the blurring of the projected image on the fovea caused by whole-body vibration results in reduced visual acuity. Therefor it is assumed that among the functions of the eye, the function of stabilizing the eye against vibrations is most likely to deteriorate due to fatigue caused by visual loads. To maintain visibility in the car during long car trips, it is important to suppress head movement. This helps prevent fatigue in the eye stabilization function. The vibration characteristics of the seated head are characterized by high sensitivity to longitudinal and pitch motion. It is increasingly important to design and control vehicles based on these specific human vibration characteristics.

5 Conclusion

In this research, to investigate how long-term vibration and visual tasks affect visual acuity, we studied the changes in visual acuity before and after playing a video game as a visual task under prolonged vibration. The following results were shown:

  1. (1)

    There are no significant changes in visual acuity before and after playing the game in the stationary state,

  2. (2)

    After performing the prolonged visual tasks in the vibrating state, the visual acuity in the stationary state remained unchanged, but the visual acuity in the vibrating state decreased.

From these results, it is assumed that among the functions of the eye, the function of stabilizing the eye against vibrations is most likely to deteriorate due to fatigue caused by visual loads. To maintain visibility in the car during long car trips, it is important to suppress head movement. This helps prevent fatigue in the eye stabilization function.