Ⅰ. Introduction

There is an increase in number of people age between 10~30s who have insufficient accommodative function because of constantly smartphones and computers usage for a long time.¹⁾ Accommodative insufficiency leads to eye strain, headache, blurred vision, and diplopia.¹⁾ Multifocal Contact lens (MFCL) can be prescribed for patients with presbyopia or accommodation insufficiency to relax from over-accommodation fatigue.^2,3)

MFCL is designed with different focal points that gradually changes from the periphery to the center for good viewing at far, intermediate and near distances.²⁾ The MFCL is optically designed to have different refractive power for each part of the lens that in the eye wearing MFCL, the light rays passing through the different parts of lens are focused on the retina as a clear focused image and blurred unfocused image simultaneously.²⁾ Even in the successful fitting, the clear images and blurred images are focused on the retina simultaneously and transferred to the visual cortex through visual pathway.²⁾ In the visual system, the blurred images could be interpreted as noise stimuli in the brain before sensory fusion.⁴⁾ and which, could be selectively suppressed, or create ghost images.

The visual perception and cognition have been reported to be linked with neuroscience, which is a technology that evaluates signals from the visual cortex. This technology conduct to measure electroencephalogram (EEG) signals from the brain,⁵⁾ which detects electrical activities in the brain using small, metal discs (electrodes) attached to the scalp. Neurons in the brain communicate via electrical impulses and gives functional activities, and these activities show up as wavy lines on the EEG recording.⁶⁾ The evoked potential (EP) of EEG is interpreted as the neuronal response from specific distinct stimulus related with visual perception and initiates perceptual suppression.^6,7)

An event-related potential (ERP) is subtype of EP and measured brain response of a cognitive or motor event.⁸⁾ The positive or negative peaks are observed around 100 ms and 170 ms. The P100 peak occurs around 100 ms and represents initial visual consciousness through sensory fusion.^9,10) The N170 component of ERP is related to the stimulus in the cognition processing occurs around 170 ms and reflects the processing of the configural information of facial features.^10-14) The visual analysis of faces seems to be a highly specific skill in humans, which is probably based on the activation of occipitotemporal regions in visual cortex.¹²⁾ It has been shown that the N170 amplitude to faces depends on the presence of facial features, such as the mouth, the eyes or the nose.¹⁴⁾ Facial expression stimuli are easy for people to distinguish instantly, many researchers had used facial expression stimuli for the visual recognition study.

The P100 can be extracted from the O1 and O2 channels of electrode channels and analyzed it is the positive ERP signal around 80 to 120 ms. The N170 can be extracted from the P7, PO7, P8, and PO8 channels and it is the negative potential value at the periods from 130 to 210 ms after the stimulus is presented.^15,16)

Stimuli are delivered respectively to each retina and retinal images are transferred to the visual cortex through the optic nerve, optic chiasma, lateral geniculate body.¹⁷⁾ In this process, two retinal images can be fused or competed.¹⁸⁾ In MFCL wearing eye, blurred image can be induced on the retina due to the limitations of the design of the MFCL,¹⁹⁾ and blurred retinal images from both eyes can be suppressed or fused to get clear retinal image, or ghosting image can be occurred in the process of visual perception.^20,21)

It is important to investigate whether ghost images does affect binocular vision in the wearing MFCL eyes, that we evaluate the neural activities generated in the visual cortex using face related stimuli in the MFCL wearing eyes.

Ⅱ. Subject and Method

1. Subject

Twenty young myopes subjects (15 females, 5 males, 24.00±0.51 years) with no history of refractive or ocular surgery were participated in this experiment. They were a contact lens wearers who have the corrective visual acuity was better than 1.0, anisometropia <-0.50 D, astigmatism <-0.75 D and normal accommodation function. DAILIES TOTAL 1 (Alcon Co.) was used in this experiment for the single vision contact lens (SVCL) and low add MFCL (Table 1). This study was approved by the study was approved by the Institutional Reviews Board (IRB) of Ilsan Paik Hospital (No. 2018-05-008-002).

2. Measurements

1) Basic Screening

The motor dominant eye for far distance was determined by Hole-in-the-card test. The non-dominant (sensory dominant) eye for near distance was determined with the experiment that add +1.50 D trial lens alternately between the right and left eyes, and the eye that patient feel more cloudy is considered as sensory dominant eye. Pupil size in scotopic, low and high mesopic conditions was measured with Digital Variable Pupillometer (VIPTM-200 Pupillometer, Neur Optics, USA) in a room with background illumination of 35 lx after dark adaptation. The power of SVCL and MFCL was based on the subjective refractive error.

2) Stimuli

Stimuli were given with the facial expression samples on the monitor by E-prime 2 program. The distance between the subject and the stimulus screen was adjusted to 80 cm, and the subjects was allowed to watch stimuli during the experiment. Stimuli were the images of neutral and sad face emotions and presented for 600 ms followed by a blank screen repetitively and regularly with a total of 126 times per cycle (Fig. 1).

3) ERP Recording

The ERP signals were recording by a Neuroscan Synamps 2 amplifier (Compumedics, El paso, TX, USA). There are 64 channels to extended active electrodes. Vertical VEO and horizontal HEO electrodes were using to control blinking and eye movements and the impedance of electrode was maintained below 5 kΩ. The EEG signals from brain were bandpass filtered between 1.0~30.0 Hz and 60.0 Hz noise signal was removed using a notch filter, and continuously digitized at 1,000 Hz. The recorded data was pre-processed, and pre-processing artifacts were eliminated using Scan 4.3 software (Compumedics) and the final data was extracted from the following epochs 200 ms prior to stimulus until 500 ms after stimulus.

4) ERP analysis

The ERP signals were analyzed by program with Curry 7. Channels O1 and O2 were used for analysis of P100, and PO7, P7, PO8 and P8 channels were used for analysis of N170. In order to abstract the peak value of amplitude and latency, we conducted the average peak searching function in the Curry 7. For the P100 amplitude, the maximum positive value was selected in the period of 80 to 140 ms. For the N170 amplitude, the minimum negative value was selected in a period of 150 to 210 ms. Statistical analyzes were performed using SPSS statistics version 21 (IBM, Armonk, NY, USA). Shapiro-Wilk test was conducted to confirm the normality test of this experiment. Paired t-test was practiced to compare the visual cognitive response in the eyes wearing SVCL and low add MFCL. ANOVA test was performed to compare the difference of response between groups, and Bonferoni test is used for post-hoc analysis.

Ⅲ. Results

1. Subjects

Twenty healthy subjects (15 females, 5 males) with an average age of 24.00±0.51 years participated in this experiment. The spherical equivalent refractive error of the subjects was –2.57±2.18 D, and the average of anisometropia was 0.23±0.28 D. The mean of accommodation power was 7.99±1.80 cm and the mean of facility was 13.64±5.53 cpm (Table 2). The participants have right dominant eye except 4 participants. The motor dominant eye and sensory dominant eye were same eye except 1 subjects that the dominant eye was determined based on the Hole-in-the-card test.

2. ERP plot in the eyes wearing SVCL and MFCL

1) The variation of P100 amplitude and latency

In the variation plot of P100, it showed a continuous positive peak from 80 to 120 ms. In the dominant eye, the P100 with MFCL showed similar variation tendency with the SVCL wearing (Fig. 2-A). In the non-dominant eye, the P100 amplitude tended to be slightly higher and P100 latency was slightly longer when wearing MFCL than SVCL wearing (Fig. 2-B). When MFCL was worn in both eyes, the P100 showed similar variation tendency with SVCL wearing (Fig. 2-C).

2) The variation of N170 amplitude and latency

The N170 graph showed a negative peak from 150 to 120 ms. Compared to those wearing SVCL, the amplitude was similar and latency was longer in the dominant and non-dominant eye wearing MFCL (Fig. 3-A & B). However, when MFCL was worn in both eyes, the amplitude and latency showed similar variation tendency with those of SVCL wearing (Fig. 3-C).

3. The P100 in the eye wearing SVCL and low add MFCL

1) The amplitude of P100 in the eye wearing SVCL and MFCL

The P100 amplitude, there was no significant difference in the dominant eye, non-dominant eye and both eyes wearing SVCL, and no difference in the eyes wearing MFCL also (p=0.920, p=0.668, respectively).

In the dominant eye, the P100 amplitude was 8.57±4.80 μV with SVCL, and 8.67±3.58 μV with MFCL, and there was no significant difference between them (p=0.225). In non-dominant eye, the P100 amplitude was 8.42±4.20 μV with SVCL, and that of MFCL was 9.78±4.27 μV, and there was no significant difference between them (p=0.175). In both eyes the P100 amplitude was 8.02±4.03 μV with SVCL and 7.99±3.70 μV with MFCL, and there was no significant difference between them (p=0.956)(Table 3).

2) The latency of P100 in the eye wearing SVCL and low add MFCL

The P100 latency, there was no significant difference in the dominant eye, non-dominant eye and both eyes wearing SVCL, and no difference in the eyes wearing MFCL also (p=0.168, p=0.089, respectively). The latency of P100 reflects the reaction time to stimuli in the visual cortex. The longer latency means delayed reaction time to stimulation in the visual cortex.

In the dominant eye, the P100 latency with MFCL wearing (138.10±12.40 ms) was longer than that of SVCL (131.73±14.58 ms), and there was significant difference between them (p=0.036). In the non-dominant eye, the P100 latency was 138.85±9.60 ms with MFCL and that of SVCL was 137±9.95 ms, and there was no significant difference between them (p=0.237). In the both eyes, the P100 latency was 130.35±16.70 ms with MFCL and 130.70±7.80 ms with SVCL, and there was no significant difference between them (p=0.913)(Table 4).

4. The amplitude and latency of N170 in the eye wearing SVCL and low add MFCL

1) The amplitude of N170 in eye wearing SVCL and low add MFCL

The N170 amplitude, there was no significant difference in the dominant, non-dominant eye and both eyes wearing SVCL, and no difference in the eyes wearing MFCL also (p=0.678, p=0.611, respectively). The N170 represents the area that reflect fusing and recognizing of visual stimuli and that was accepted as high-level recognition and visual perceptual process.

In the dominant eye, the N170 amplitude with MFCL (–3.14±3.44 μV) was weaker than that of SVCL wearing (–4.64±3.14 μV), and there was significant difference between them (p<0.001). In the non-dominant eye, the N170 amplitude with MFCL (–2.46±3.35 μV) was weaker than that of SVCL wearing (–3.56±3.68 μV), and there was significant difference between them (p<0.008). However, in both eyes, the N170 amplitude was – 3.56±3.68 μV with MFCL and –4.15±3.52 μV with SVCL, and there was no significant difference between them (p=0.200)(Table 5).

2) The latency of N170 in eyes wearing SVCL and low add MFCL

The N170 latency, Table 6 showed significant difference in the N170 of the dominant, non-dominant eye and both eyes wearing SVCL and MFCL also. When SVCL wearing, the N170 latency was shortest in the both eyes (172.83±11.72 ms) and longest in the non-dominant eye (186.74± 13.55 ms), that showed significant difference among them (p=0.007). When MFCL wearing the N170 latency of both eyes (172.83±11.72 ms) was significantly shorter than the dominant and non-dominant eye (p=0.014).

In the dominant eye, the N170 latency was 184.90±13.10 ms with MFCL, and that of SVCL was 181.70±15.26 ms, and there was no difference between them (p=0.225). In the non-dominant eye, the N170 latency was 187.16±14.11 ms with MFCL, and that of SVCL was 186.74±13.55 ms, and there was no significant difference between them (p=0.861). And in the both eyes, the N170 latency with MFCL was not different compared with that of SVCL wearing (p=0.249)(Table 6).

Ⅳ. Discussion

The ghosted and blurred images could occur in the MFCL wearing eyes during the contact lens wear, especially in the adaptation period. It is reported in the eyes wearing MFCL, monocular sensory fusion and binocular rivalry could occur between clear and blurred images in the process of visual perception.¹⁹⁾ We assumed there may be a difference in the visual perception and cognition in the dominant, non-dominant and both eyes wearing MFCL compared to those of SVCL wearing. The EP of EEG was used to figure out the neuronal response related with visual perception in the eyes, and the ERP, represents the initial visual and perceptual processes.^7,8,22,23) In order to extract ERP values accurately, measured data was filtered and artifacts were removed by referring the previous research method.²⁰⁾ In this experiment, we investigated visual perception response related to face recognition in the visual cortex when wearing MFCL.

The P100 of ERP is the most constant component and clinically significant information. In the eyes wearing SVCL, there were no significant differences in the P100 amplitude and latency in the dominant, non-dominant eye, and binocular eyes. And there was no difference also in the eyes wearing low add MFCL.

The P100 of ERP is the most constant component and clinically significant information. In the eyes wearing SVCL, there were no significant differences in the P100 amplitude and latency in the dominant, non-dominant eye, and binocular eyes. And there was no difference also in the eyes wearing low add MFCL.

However, in the results of comparing P100 latency in wearing MFCL and SVCL, P100 latency of the dominant eye wearing MFCL was significantly longer than that of the dominant eye wearing SVCL (p=0.036). The Fig. 1 shows the similar result of statistics regarding the P100 latency, the latency of P100 in dominant eye wearing MFCL was delayed compared with SVCL wearing. However, in the non-dominant eye wearing MFCL and SVCL, there was no difference in the P100 latency.

The non-dominant eye could tolerate blurred images well compared with the dominant eye,²³⁾ that the non-dominant eye wearing MFCL could tolerate blurred image easily and showed similar time to accept the blurred image as with SVCL wearing. However, in the dominant eye wearing MFCL, clear and blurred images are simultaneously focused on the retina, the blurred image can be detected as a noise stimulus, that the latency can be changed to longer. Binocularly, there was no difference in P100 latency in the MFCL and SVCL wearing. According to this result, it could be interpreted that the dominant eye wearing MFCL takes more time to accept the blurred image than SVCL wearing, and the ghosted or blurred images formed on the each retina of both eyes can be fused that blurred image did not affect initial visual process in the binocular vision.

The N170 is a component of signals that visual perception, especially face expression stimuli. The N170 amplitudes in the in the dominant, non-dominant eye, and both eyes were not significantly different when wearing MFCL and SVCL also.

However, the N170 amplitudes of MFCL wearing dominant and non-dominant eye were significantly smaller than those of SVCL wearing eyes (p<0.001). And in the both eye, the N170 amplitude of with MFCL wearing was not significantly different with that of SVCL wearing. From these results, we could estimate that visual perception stimuli of the eye wearing MFCL can be smaller than that of wearing SVCL monocularly, but the binocular summation could occur successfully in the brain, similar to wearing SVCL in the binocular vision.

In the SVCL wearing eyes, the N170 latency was the shortest in the both eyes and the longest in the non-dominant eye. However, in the MFCL wearing eyes, the N170 latency was not different between dominant and non-dominant eyes, and which was the longer than that of the both eyes. These results suggest that, when a clear image is formed on the retina, the dominant eye's visual perception is faster than that of the non-dominant eye. However, when a blurred image is formed on the retina, the non-dominant eye, which overcomes the blurred image well, shows a similar time to recognition to that of the dominant eye. In addition, it can be seen that binocular visual recognition is shorter than monocular visual recognition. As a result, we assumed in the MFCL wearing eyes binocular summation could occur successfully in the brain.

This study have some limitations for the addition power of the MFCL and number of subject, that further studies on ERP changes are needed using MFCL with high addition powers and more subject. We expect these results could be used as a cornerstone to understand visual performance and prescription for MFCL fitting.

Ⅴ. Conclusion

This study focused on the visual perception processing that occurs when a clear and blurred image were focused on the retina simultaneously appearing in the MFCL wear. The recognition responses were different in dominant, non-dominant, and both eyes.

Therefore, it is considered that the ghost images that appears wearing MFCL would be overcome through the binocular fusion and would not affect the binocular visual performance.