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The obsolescence of CRT monitors requires replacing stimulators used for eliciting VEPs with new monitors. Currently, LCD monitors are the only suitable alternative, however other technologies, like OLED, may become a viable option [23]. So far, the ISCEV extended protocol for VEP methods of estimation of visual acuity recommends ensuring luminance artifacts caused by non-CRT stimulators [9], which can be achieved by reducing the stimulus contrast [23]. However, this may not be possible without falling below the minimum contrast values recommended for VEP [1, 23]. Since LCD stimulators have been shown to result in mostly a delay in the VEP responses [2,3,4, 23] but seem not to affect the size of the amplitudes [2], we expected no difference between the estimated visual acuity by using LCD or CRT monitors used as a stimulator for the sweep VEP.

The results of the first experiment show statistically significant effects of the monitor type on the time-to-peak after stimulus onset and the peak-to-trough amplitude (Table 1). The mean delay of the time-to-peak after stimulus onset between recordings obtained using the LCD and the CRT monitor was about 60 ms, which is quite high and possibly caused by the relatively old LCD monitor used. Accordingly, statistically significant effects on the time-to-peak after stimulus onset and the peak-to-trough amplitude were found for the monitor/contrast combination in the results of the second experiment (Table 4). Surprisingly, the mean delay of the time-to-peak after stimulus onset of the CRT monitors with high contrast was with up to 151 ms, longer (Table 5) than that of the LCD monitors (with low and high contrast), although one would expect modern monitors to have shorter or even no delays [24, 25]. Additionally, a statistically significant interaction between the spatial frequency and the monitor type was revealed in both experiments, causing an increased time delay for the intermediate spatial frequencies (1.4–10.3 cpd) with LCD stimulation (Fig. 2, top left) in the first experiment and an almost linear increase with the spatial frequencies in the second experiment (Fig. 2, bottom left). This may be explained by the semi-manual cursor placement, which is necessary because the amplitudes are less pronounced at frequencies below and above this frequency band. Another cause might be an input lag resulting from the time required by the monitor to prepare the image data to be displayed. This could be caused by, e.g., internal scaling for non-native resolutions, which may even be present when using the monitor’s native resolution. In the worst case, this leads to nonlinearities of the response timing of the LCD monitor when presenting patterns of low or high frequency [26, 27]. In doubt, the precise duration of the input lag should be measured using a photodiode attached to the display [28] and in case of being constant, the delay could then be subtracted from the respective time-to-peak values. Finally, the higher latencies may also be caused by the different software used for generating the stimuli: whereas in the first experiment, a custom-developed Java-based software was used, in the second experiment, the Python-based PsychoPy was employed. Nevertheless, these differences seem not to affect the estimated visual acuity. The mean peak-to-trough amplitude using the LCD monitor in the first experiment is reduced by about 0.9 µV with a confidence interval from − 1.6 to − 0.2 µV compared to the CRT stimulator, but increased by about 2.6 µV (confidence interval from 1.2 to 4.0 µV) when comparing the new LCD monitor with the CRT monitor (both with high contrast) in the second experiment (Table 5). However, these differences were, despite being statistically significant, within the expected standard deviation from about 0.5 to 7 µV of the P100 amplitude found in the literature [29,30,31] and therefore probably of no clinical relevance (Fig. 2, right). Interestingly, the results of Nagy et al. [2] suggest a similar reduction in the peak-to-trough amplitude when using an LC display for stimulation. In the first experiment, no statistically significant interaction between monitor type and spatial frequency on peak-to-trough amplitude was found but a tendency to smaller amplitudes at intermediate frequencies (Table 1), whereas in the second experiment, the effect of the interaction of stimulator and spatial frequency was statistically significant (Table 4). It has to be taken into account that the residuals of the models were heteroscedastic and therefore the statistical significance of the effects may be overestimated [32].

In the first experiment, the difference between the subjective visual acuity and that estimated by the second-order polynomial method, or by the modified Ricker function, was not statistically significant from a hypothetical assumed value of 0 logMAR (Table 2). Neither were the variances between CRT and LCD statistically different. Accordingly, the linear mixed-effects models revealed no statistically significant effects of neither the monitor type, the recording cycle, nor their interaction on the difference between subjective and estimated visual acuity for both estimation methods (Table 3).

In contrast in the second experiment, the differences between subjective visual acuity determined using FrACT and the visual acuities estimated using the modified Ricker function along with the conversion formula used in the first experiment were significantly different from the hypothesized difference of 0 logMAR for both, the new gaming LCD monitor and the old LCD monitor, at high and low contrast, but not for the CRT monitor. After using an individually adjusted conversion formula for each monitor/contrast combination, no statistically significant difference from the hypothesized difference of 0 logMAR was found (Table 7). However, one should keep in mind that using the results to calculate the conversion formula used to predict the results is circular reasoning. Nevertheless, it indicates, that using individual established conversion formulas calculated from a sufficiently large number of normative data will minimize the error between true visual acuity and estimated visual acuity.

Table 6 lists the signal-to-noise ratio calculated from the fitted Ricker model for the different combinations of monitors and contrasts. The highest SNR was found for the CRT monitor using high contrast. The LCDs showed lower SNR values. The on average higher amplitudes obtained using LCD monitors (Table 5) indicate that more noise is present when stimulating using LCDs. However, this effect could be caused by the different software used for the stimulus presentation and the lower number of sweeps recorded for averaging compared to the recordings using the CRT monitor. Nevertheless, none of the differences between the SNR values obtained from the different monitor types was statistically significant (Table 6), which corresponds to the findings of Fox et al. [28].

We want to point out the limitations of the current study: We included only healthy participants, so the possible effects of LCD stimulators on patients with reduced visual acuity remain unclear and should be further investigated, especially since we found a statistically significant, albeit not clinically relevant, effect of the monitor/contrast combination on peak-to-trough amplitude and time-to-peak after stimulus onset in the second experiment (Tables 4, 5). Further limitations are that the participants were not stratified by age and that the subjective visual acuity in the first experiment was determined using an eye chart projector, in contrast to the second experiment, where FrACT was used, limiting the accuracy of the estimated value. Finally, this study compared only three specific monitors; therefore, the results are not universally valid.

In conclusion, based on the results of this study, LCD monitors may substitute CRT monitors for presenting the stimuli for the sweep VEP to objectively estimate visual acuity. Newer LCD screens, especially with low response times in the range of 1–2 ms, therefore, allow for a reduction in luminance artifacts at required contrast levels [23], albeit the luminance artifact may not have a large effect on the recorded signals [28]. New technologies like OLED displays [23] may even be better suited, since one the one hand, the onset will be the same for the whole pattern, and on the other hand, LCDs and OLEDs provide a constant luminance level during stimulation, whereas CRTs need a constants pulses to keep the phosphor lit up, causing fast local luminance flashes all the time [28]. Therefore, in contrast to CRTs, LCD and OLED stimulators, e.g., may allow for recording true offset responses [33]. However, caution should be taken when leveraging modern displays for stimulation, since their in-built electronics perform all kinds of sophisticated image-enhancing procedures including color-correction, brightness boosting, contrast enhancement by real-time adjustments of the colors or the backlight, or eyestrain-reducing blue light filtering, with the aim to improve the users’ experience, or to increase the monitors lifetime. This applies in particular to consumer electronics like TVs. Gaming monitors, in addition, use special acceleration drivers, which shut down the backlight, insert black frames (Black Frame Insertion, BFI), or employ variable refresh rates (e.g., Nvidia G-SYNC or AMD FreeSync) to clean the retained image from the eye. Therefore, one should disable any image processing or enhancing functionality in the monitor settings, before using the monitor as stimulator for electrophysiological experiments. Finally, it is advisable to perform a calibration with healthy volunteers using best-corrected and artificially reduced visual acuity and to collect normative data for the employed setup, as always recommended by ISCEV [34], in order to establish an individual conversion formula between the sweep VEP outcome and the estimated visual acuity.

Vision science experiments have historically depended upon cathode-ray tube (CRT) monitors to present stimuli with high spatial and temporal acuity. However, due to competition from plasma screens and liquid crystal displays (LCDs), CRT production was reduced or ceased by most manufacturers throughout the mid-2000s, meaning that many vision scientists now largely depend upon old and increasingly unreliable CRT monitors. Although CRTs are far from perfect numerous studies have described their superior performance relative to LCDs (e.g., Elze et al., 2007; Elze and Tanner, 2011, 2012). In recent years, however, high-quality LCDs targeting gamers have become commercially available. Further, two specialized companies now produce LCDs that aim specifically to meet the needs of vision researchers (“ViewPixx,” VPixx Technologies Inc., Canada, and “Display++,” Cambridge Research Systems, UK). Here, we compare the spatial and temporal luminance characteristics of these high-end research LCD monitors with more readily available gaming LCDs and CRTs.

Although CRTs are often considered the gold-standard monitor for use in vision research, they still have a number of limiting features (García-Pérez and Peli, 2001). CRTs generate an image by focussing an electron beam onto a phosphor layer, which emits visible light when struck by an electron. Color monitors use three phosphor layers, which each emit light with a different wavelength. The electron beam is rapidly “raster” scanned in rows, from the top-left to the bottom-right of the monitor, meaning that the entire image cannot be updated simultaneously. The decay rate of the phosphor"s fluorescence, in combination with the rapid electron scanning, means that CRT monitors are unable to deliver continuous luminance patterns and the emitted light flickers at the frame rate used to generate images. This flicker is typically at 60–120 Hz, and while typically perceptually invisible, can strongly affect neural responses to visual stimuli (Wollman and Palmer, 1995; Krolak-Salmon et al., 2003; Williams et al., 2004). The relatively poor spatial independence in adjacent pixels in CRT monitors is also a major drawback, introducing artifacts to stimuli with high spatial frequency (Cowan, 1995; Bach et al., 1997; Pelli, 1997a; Krantz, 2000). Some older CRTs exhibit problems with focus of the electron beam and unreliable reproduction of low luminances.

LCDs use liquid crystals as voltage-controlled filters to control light emission. Light from a source at the back of the monitor (e.g., a light emitting diode or cold cathode fluorescent lamp) passes through three consecutive filtering layers: a polarizing filter; a layer of liquid crystals; and finally a second polarizing filter oriented orthogonal to the first. Light intensity is determined by the level of polarization change introduced by the liquid crystal layer: if no voltage is applied to the liquid crystals, they align such that the liquid crystal layer introduces a 90° change in polarization angle, and maximum light intensity will be achieved. As the voltage applied to the liquid crystals increases, they progressively change alignment, blocking more light. Unlike CRTs, each pixel in an LCD monitor is an independent filter element, allowing independent adjustment of the luminance of each pixel (Krantz, 2000; Wang and Nikolic, 2011). Nevertheless, LCD displays do exhibit two key temporal problems: first, temporal artifacts may arise depending on whether the light source is continuously on, flashed briefly once per frame, or subjected to pulse width modulation (PWM) to control brightness (Elze et al., 2007; Liang and Badano, 2007; Elze and Tanner, 2011, 2012). Second, significant temporal constraints occur due to the sluggish nature of switching in liquid crystals. This latency is undesirable in experiments with rapidly changing or fast moving stimuli, as the slow dynamics causes problems such as motion blur (Hong et al., 2005; Pan et al., 2005; Someya and Sugiura, 2007; Becker, 2008; Feng et al., 2008; Watson, 2010). Furthermore, the measured light intensity can dramatically change as the visual angle of the observer varies.

The demands of vision scientists using moving or reverse correlation stimuli, which are updated rapidly, are similar to the demands of many computer games. These games have driven the development of LCD monitors with high spatial resolution, high temporal refresh rates (100–120 Hz) and precise control over when and where light is emitted from the monitor on each frame refresh. Recent advancements in display technology have also facilitated the development of professional LCD monitors that are intended to meet the spatial and temporal requirements of vision science experiments. Given the constraints of CRT monitors, and the rapidly expanding market for LCD monitors, we examined whether any LCD monitors are suitable replacements for CRTs in the laboratory. In this study, we characterized and compared the spatial and temporal properties of a CRT monitor, two LCD monitors made specifically for vision sciences, two high quality gaming LCDs, and a consumer-grade LCD.

Our spatial tests demonstrate that most LCDs exhibit a dramatic decline in luminance toward their periphery, with effective luminance also dependent on viewing angle. The Display++ and VPixx are less strongly affected by this peripheral decline in luminance, and provide hardware based methods that partially compensate for luminance anisotropies in the vertical axis. All consumer-level and gaming LCDs showed difficulties in generating reliable temporal precision: (1) they were unable to reach the requested luminance within a single frame; and (2) they showed temporal dependence, meaning that the luminance of one frame affects the subsequent frame. The Display++ and VPixx were better in this regard, but care needs to be taken when calculating the actual duration of single-frame stimuli as the hardware mechanisms that control light generation and light transmission cannot be perfectly synchronized. We emphasize that depending on the type of experiments, extreme caution should be taken in selecting any LCD monitor to replace a CRT; however, we are confidently using the vision-science specific LCDs for electrophysiological and psychophysical studies.

First, we will provide a brief technical overview of functional principles as they relate to visual stimulus presentation. Detailed descriptions and parameter measurements are already available from the existing literature; however, our intention here is to equip readers with limited technical expertise with the necessary knowledge to set up computer experiments with LCD monitors. Thus, we keep our explanations relatively short and simplified.

LCD monitors work differently: Each pixel consists of liquid crystal threads that can be twisted or arranged in parallel by an electrical current applied to them. This leads to a polarization effect that either allows or prevents light passing through. A white light source located behind this crystal array uniformly and constantly illuminates the array. To display a black pixel, the crystal threads are twisted by 90° such that no light will pass through. A white pixel is achieved by aligning the crystals such that maximum light is allowed to pass through, until a different, non-white color needs to be displayed (see the lower panel of Fig. 1 for an LCD pixel’s brightness over time). This is a static process, not a pulsed one as in CRTs.

In theory, the difference in presentation methods, namely a strobing versus a static image, should be of no consequence if the light energy that falls onto the retina remains the same over the time period of one single frame. As the Talbot-Plateau law states2 is equally well detectable as a light flash presented for 60 ms at 40 cd/m2. This suggests that temporal integration can be easily described by energy summation”. Thus, in principle, LCD and CRT monitors should be able to yield comparable results.

However, due to the differences in technology, the visual signals produced by the two display types have different shapes (i.e., a different light energy-over-time-curve; see Fig. 1). Moreover, default luminance as well as visual-signal response times (in addition to other parameters, see below) differ between most CRT and LCD monitors

Table 1 reports the parameters we considered in setting up the CRT and LCD monitors. Certainly, most of them are commonly considered when setting up a computer experiment; nevertheless we deemed it important to mention them here explicitly, as their neglect might have unintended consequences. We used a 17” Fujitsu Siemens Scenicview P796-2 CRT color monitor previously used in several published studies including studies with masked presentation conditions

Our measurements revealed several interesting characteristics: First, luminance of the LCD monitor at default setting (i.e., maximum brightness) exceeded the CRT luminance at a ratio of 3.25:1. However, comparable average luminance can be (and was) achieved by downregulating the LCD monitor (the older CRT technology emits less energy even at maximum settings, see Table 2), without participants perceiving it as unnaturally dark. If one plans to upgrade from CRT to LCD monitors in an experimental laboratory, we therefore recommend measuring the CRT monitors’ brightness levels and matching them in the new LCD monitors’ user setup, if comparability with the old setup is needed. This will minimize hardware-dependent variability, thus contributing to better replicability. Please note that a brightness adaption is not a necessary precondition when employing LCD monitors; researchers should simply be aware that the brightness level can have an influence onto the resulting effects, especially in time-critical experiments with short and/or masked presentation. Thus, we recommend the adaptation for time-critical experiments in which researchers orient on existing empirical evidence gathered with CRT monitors. Furthermore, gray-to-gray response times varied slightly depending on the employed brightness levels2), so we suggest that researchers can rely on this more efficient method as an approximation.

For the empirical comparison of human performance with CRT and LCD monitors, we relied on these results and set the monitor settings accordingly (see Method section below).

Participants were administered a masked number priming task and a subsequent forced-choice prime discrimination task using both a CRT and an LCD monitor. In this well-established paradigm

Of central interest was the question whether both monitors would yield comparable masked priming effects. Monitors were set according to the parameters described in the previous section (see also Method section below). In order to obtain conclusive evidence, we decided for sequential hypothesis testing using Bayes factorshttps://osf.io/g842s/.

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You’ve probably seen terms like HD and Full HD on the boxes of monitors and TVs, but what does that mean? As you may have guessed, HD refers to “High Definition,” a quick way to refer to a high-quality video output. So if you see the term “Full HD” on a monitor box, that’s just a shorthand to denote its resolution, which would be 1920 by 1080, also called 1080p. The reason why it’s specified as “Full HD” is that there are also some TVs and monitors that output at 720p (high definition but not relatively as high as 1080p), which is 1280 by 720 pixels. 1080p is considered the current standard for monitors, and popular manufacturers, including Dell, Acer, Samsung, LG, BenQ and Viewsonic, offer a variety of 1080p monitors in their product lineups.

As you can imagine, the more pixels there are to display, the more critical it is that your monitor has a high refresh rate, especially when it comes to gaming. Typically, the standard has been a 120-hertz refresh rate in gaming monitors, but many features a 144-hertz refresh rate. The quicker a monitor can refresh the display, and the smoother the visual experience will be. This is because the refresh rate in the monitor works in tandem with a low response time (which specifies how quickly the monitor can send and receive new information) to make a seamless visual transition. Sometimes, if the response rate is not quick enough, some residual pixels can remain on the screen as the monitor is trying to refresh new ones. This is called ‘ghosting.’ Although it’s standard to have a four-millisecond response time on many gaming monitors, Samsung, LG, BenQ, Viewsonic, and more all offer 2k and 4k monitors with one-millisecond response times. It is also important to ensure refresh rates are identical if you plan to sync two monitors for your display.

Regarding the internal specs, response time and refresh rate are the main factors contributing to a smooth, immersive viewing experience. Still, the physical panel type of the monitor can also play into this. First, there’s the matter of how the monitor lights up: either with LCD or LED. The main difference lies in the material that is used to light the liquid crystals in the display. In LCD, it’s cold cathode fluorescent lamps (CCFLs), and in LEDs, it’s tiny light emitting and low-energy consuming diodes. This is the preferred type in most monitors because it consumes less power and produces less harsh light, so darker colors appear more vivid. Additionally, LED monitors can be much thinner than LCD ones.

Newer LCD monitors have improved with the implementation of IPS (In-Plane Switching) panels. For some, it’s a matter of preference, but where the IPS panels have shown their strength with accurate color reproduction, which is great for content creators who want to do photo editing or graphic design. The panel type you choose depends more on preference than anything else. Samsung is well known for championing the IPS panel in their monitors, and many people also enjoy using them for gaming.

Finally, another consideration is whether there are enough HDMI (High-Definition Multimedia Interface) ports. HDMI allows simultaneous digital video and audio transmission from one source to another. While HDMI ports are often standard, especially on gaming monitors, verifying that a monitor has enough HDMI compatibility for your setup before purchasing is essential.

Since monitors have to be lit in order for the viewer to see anything, the difference between the two types is in what is used to light up the crystals within the display. For LCD, that’s cold cathode fluorescent lamps (CCFL’s) and in LEDs, it’s tiny light emitting and low-energy consuming diodes. LED monitors tend to be thinner and more power-efficient, but improvements in the panel types have made LCDs more competitive.

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