Cathode-ray tube (CRT) monitors have been used as visual stimulators to elicit pattern-reversal visual evoked potentials (p-VEPs). However, CRT has become less available in the market. As it has been extensively replaced by liquid crystal displays (LCD) as a television monitor and computer monitor, one might imagine that LCD may be good replacement for CRT as a visual stimulator for p-VEPs. But LCDs have an inherent problem as visual stimulators because they take several milliseconds for the crystal molecules to change their alignment to permit the light to pass through the polarizing filter of the LCD [
The recently developed organic electroluminescence (OLED) screen has a faster response time than standard LCD screens [
Fifteen eyes of 9 healthy volunteers who did not have any ocular diseases except for refractive errors were studied. There were nine women whose mean age was
To determine the time delay of each monitor, the luminance change of a single check was measured with a photodiode (S1133, Hamamatsu Photonics Co., Ltd., Hamamatsu, Japan). The photodiode was attached to the upper left corner of one check. The signal was amplified at
In addition, the luminance at the 4 outer corners and one point at the center of the entire checkerboard screen was measured with a luminance meter (CA-100S, Konica Minolta Inc., Osaka, Japan). We confirmed that the variations in the luminance from the center to the periphery were within 20% for each of the monitor which complies with the standards of the ISCEV guidelines (Table
Mean luminance of pattern VEP white and black squares of the checkerboard in each screen.
Screen | Stimulus |
Stimulus |
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CRT | 149, 158 (153) | 3, 3 (3) |
Organic LED | 149, 154 (151) | 3, 3 (3) |
Although the luminance of the OLED screen could be set to be blacker than the other screens, it was set to be equal to that of the CRT screens.
The luminance and contrast of both the CRT and the OLED screens were matched. The contrast between the black and white checks was calculated with the Michelson contrast formula [
The visual stimulus was a black and white checkerboard generated either on a CRT screen (17 inches, 320 × 230 mm, S710, Compaq Computer Co., USA) or on an OLED screen (17 inches, 365.7 × 205.7 mm, PVM-1741, pixel dimensions,
The maximum contrast was 97% and the check size was 0.25 degrees at an observation distance of 70 cm. The reversal rate was 3.0 rev/sec. The resolution of each monitor was 800 × 600 pixels and the vertical frequency was 59.8 Hz.
All recordings were performed under dim room lights of 104 lux and the subjects were preadapted to the room lighting before beginning the recordings. A small black fixation point was placed at the corner of the four central checks of the stimulus pattern and the subjects were instructed to fixate the point or, if the point was not visible, the center of the screen and to try not to blink. The subjects wore their best refractive correction and all recordings were monocular.
The recording electrode was placed on the inion (Oz) and the reference electrode was placed at Fz. The ground electrode was placed on the right earlobe. Signals were amplified 4,000 times (LE-4000, Tomey Corporation, Nagoya, Japan) and bandpass filtered from 1.0 to 100 Hz. The sampling rate was 1.0 kHz and one hundred twenty-eight responses were averaged. The recordings were performed at least twice to determine the repeatability. In addition, the measurements for each subject were performed twice within one week to determine the intermeasurement variability.
The P2 amplitude was measured between the trough of N-75 to the peak of P-100 and the implicit times of N-75 (N1 implicit time) and P-100 (P2 implicit time) between the onset of the trigger and the trough of N-75 or peak of P-100. Student’s
The changes in the luminance are plotted against time in Figure
Changes in the average luminance of a single check of the cathode-ray tube (CRT) screen and the organic electroluminescence (OLED) screen during pattern reversal. There is no luminance change in the overall luminance across the screen because half of the checks are changing in the opposite direction. ((a), (c), and (e)) cathode-ray tube (CRT) screen shows burst of pulses and ((b), (d), and (f)) organic electroluminescence (OLED) screen shows rectangular-shaped luminance change. (a) Luminance changes of a single check from white to black of CRT screen. (c) Luminance changes of a single check from black to white of CRT screen. (e) Averaged luminance changes of the CRT screen. There is no change in the total luminance (
Luminance change of a single check during reversal of black to white. Short and constant delay as a response time was detected during the check reversal of approximately 1.0 ms in the CRT screen and approximately 0.5 ms in the OLED screen.
The luminance changes of the LCD screen (XL2410T, 23.6 inches, 570 × 347.4 mm BENQ Co., Taipei, Taiwan.) are shown in Supplemental Figure 1 (see Supplementary Material available online at
Reproducible VEPs were elicited from the patterns generated on each monitor (Figure
Comparison of p-VEP parameters between two groups.
Amplitude (uV) | Implicit time (ms) | |||||
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N75 | P100 | |||||
Test | Retest | Test | Retest | Test | Retest | |
CRT |
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OLED |
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0.9937 | 0.9883 | 0.1741 | 0.0661 | 0.1718 | 0.3735 |
CI (difference of two groups) | −5.17~5.20 | −4.12~4.16 | −1.70~7.17 | −0.2~4.33 | −1.47~6.27 | −3.1~6.9 |
P-VEP: pattern visual evoked potentials, CRT: cathode-ray tube screen, OLED: organic electroluminescence screen, and CI: confidential interval.
Representative waveform of p-VEP. P-VEP waveforms elicited by CRT (a) and OLED (b) screens.
Comparisons of each parameter between the pattern VEPs (p-VEPs) elicited by CRT and by OLED screens. (a) No significant difference was found between the p-VEP P100 amplitude elicited by the OLED screen and that elicited by the CRT screen. (b) No significant difference was found in the implicit time of N75 elicited by the OLED screen to between the p-VEP elicited by the CRT and OLED screens as a stimulator. No significant difference was observed in the implicit times of N75 between the p-VEPs elicited by the CRT and the OLED screens as a stimulator. ns: not significant.
The ISCEV standard for p-VEPs (2009 update) [
Another solution to minimize the flash effect is to use a LCD screen with a shorter response time. But ERGs could still be elicited when the 2 ms response LCD screen was covered with a diffuser. Thus, we reduced the contrast of the checkerboard pattern to decrease the flash effect as we did for 5 ms LCD screen [
OLED displays have recently been used for digital displays in devices such as mobile phones, handheld games consoles, and personal digital assistances. Due to current difficulties in producing large size OLED screens and their relatively high cost, there are limited number of OLED television screens and computer monitors. But it is expected that they will become more easily available. Their characteristics have been evaluated [
The luminance changes measured with a photosensor were comparable between the OLED and CRT screens with very rapid rise and fall times of the black and white checks (Figure
Recently, the characteristics of an OLED screen (Sony PVM-2541, 24.5 in.; Sony Corporation, Tokyo, Japan) have been precisely measured from the viewpoint of its applicability to visual psychophysics [
Our results showed that the p-VEPs elicited by OLED screens were not significantly different from those elicited by conventional CRT screens. The amplitude of P-100 and implicit times of N-75 and P-100 were almost identical between two waves when the constant input lag was subtracted from the measurements of the p-VEP elicited by the OLED screen.
This study has several limitations. The property of the luminance change was different and its influence on the retinal and optic nerve responses was unknown. Investigating the influence of the different properties on the human visual system will be interesting but here we have just investigated the possibility of substituting CRT monitor with OLED monitor as a visual stimulator for p-VEP. We investigated a single LCD and a single OLED monitor but the input lag and response time are unique in LCD and OLED screens. Therefore, a better LCD screen or a better OLED monitor as a visual stimulator may be found with further investigations.
In conclusion, the OLED screen can be a better substitute for the CRT screen and also LCD screens as a stimulator for eliciting p-VEPs. However, it will be important to collect normative data because how the different luminance changes will affect human perception of briefly presented stimuli is unknown.
H. Funada is an employee of Tomey Corp., Japan. Tomey Corp. had no role in study design, data collection and analysis, decision to publish, or preparation of the paper. All commercial identities mentioned in the paper were purchased by Teikyo University School of Medicine without receiving any financial gain. All authors have no additional competing interest that potentially influences the study, including financial relation. No author has a financial or proprietary interest in any material or method mentioned.
Support of this study was provided by Researches on Sensory and Communicative Disorders from the Ministry of Health, Labor, and Welfare, Japan, and from the Ministry of Education, Culture, Sports, Science and Technology, Japan.