An evaluation of organic light emitting diode monitors for medical applications: great timing, but luminance artifacts

Fullfield and extrafoveal visual evoked potentials in healthy eyes: reference data for a curved OLED display

PURPOSE

Visual evoked potentials (VEP) present an important diagnostic tool in various ophthalmologic and neurologic diseases. Quantitative response data varied among patients but are also dependent on the recording and stimulating equipment. We established VEP reference values for our setting which was recently modified by using a curved OLED display as visual stimulator. Distinction is made between fullfield (FF) and extrafoveal (EF) conduction, and the effect of sex, age and lens status was determined.

METHODS

This prospective cross-sectional study included 162 healthy eyes of 162 test persons older than 10 years. A fullfield pattern-reversal visual evoked potential (FF-PR-VEP) with two stimulus sizes (ss) (20.4' and 1.4°) as well as an extrafoveal pattern onset-offset VEP (EF-P-ON/OFF-VEP) (ss 1.4° and 2.8°) was derived in accordance with the International Society for Clinical Electrophysiology of Vision guidelines. Amplitudes and latencies were recorded, and the mean values as well as standard deviations were calculated. Age- and sex-dependent influences and the difference between phakic and pseudophakic eyes were examined. A subanalysis of EF-P-ON/OFF-VEP and fullfield pattern onset-offset VEP (FF-P-ON/OFF-VEP) was performed. A 55-inch curved OLED display (LG55EC930V, LG Electronics Inc., Seoul, South Korea) was used as visual stimulator.

RESULTS

Mean P100 latency of the FF-PR-VEP was 103.81 ± 7.77 ms (ss 20.4') and 102.58 ± 7.26 ms (ss 1.4°), and mean C2 latency of the EF-P-ON/OFF-VEP was 102.95 ± 11.84 ms (ss 1.4°) and 113.58 ± 9.87 ms (ss 2.8°). For all stimulation settings (FF-PR-VEP, EF-P-ON/OFF-VEP), a significant effect of age with longer latencies and smaller amplitudes in older subjects and higher amplitudes in women was observed. We saw no significant difference in latency or amplitude between phakic and pseudophakic eyes and between EF-P-ON/OFF-VEP and FF-P-ON/OFF-VEP.

CONCLUSIONS

A curved OLED visual stimulator is well suited to obtain VEP response curves with a reasonable interindividual variability. We found significant effects of age and gender in our responses but no effect of the lens status. EF-P-ON/OFF-VEP tends to show smaller amplitudes.

Accuracy and precision of stimulus timing and reaction times with Unreal Engine and SteamVR

The increasing interest in Virtual Reality (VR) as a tool for neuroscientific research contrasts with the current lack of established toolboxes and standards. In several recent studies, game engines like Unity or Unreal Engine were used. It remains to be tested whether these software packages provide sufficiently precise and accurate stimulus timing and time measurements that allow inferring ongoing mental and neural processes. We here investigated the precision and accuracy of the timing mechanisms of Unreal Engine 4 and SteamVR in combination with the HTC Vive VR system. In a first experiment, objective external measures revealed that stimulus durations were highly accurate. In contrast, in a second experiment, the assessment of the precision of built-in timing procedures revealed highly variable reaction time measurements and inaccurate determination of stimulus onsets. Hence, we developed a new software-based method that allows precise and accurate reaction time measurements with Unreal Engine and SteamVR. Instead of using the standard timing procedures implemented within Unreal Engine, time acquisition was outsourced to a background application. Timing benchmarks revealed that the newly developed method allows reaction time measurements with a precision and accuracy in the millisecond range. Overall, the present results indicate that the HTC Vive together with Unreal Engine and SteamVR can achieve high levels of precision and accuracy both concerning stimulus duration and critical time measurements. The latter can be achieved using a newly developed routine that allows not only accurate reaction time measures but also provides precise timing parameters that can be used in combination with time-sensitive functional measures such as electroencephalography (EEG) or transcranial magnetic stimulation (TMS).

Clinical evaluation of a medical high dynamic range display

PURPOSE

Recent new medical displays do have higher contrast and higher luminance but do not have a High Dynamic Range (HDR). HDR implies a minimum luminance value close to zero. A medical HDR display prototype based on two Liquid Crystal layers has been developed. The goal of this study is to evaluate the potential clinical benefit of such display in comparison with a low dynamic range (LDR) display.

METHODS

The study evaluated the clinical performance of the displays in a search and detection task. Eight radiologists read chest x-ray images some of which contained simulated lung nodules. The study used a JAFROC (Jacknife Free Receiver Operating Characteristic) approach for analyzing FROC data. The calculated figure of merit (FoM) is the probability that a lesion is rated higher than all rated nonlesions on all images. Time per case and accuracy for locating the center of the nodules were also compared. The nodules were simulated using Samei's model. 214 CR and DR images [half were "healthy images" (chest nodule-free) and half "diseased images"] were used resulting in a total number of nodules equal to 199 with 25 images with 1 nodule, 51 images with 2 nodules, and 24 images with 3 nodules. A dedicated software interface was designed for visualizing the images for each session. For the JAFROC1 statistical analysis, the study is done per nodule category: all nodules, difficult nodules, and very difficult nodules.

RESULTS

For all nodules, the averaged FoMHDR is slightly higher than FoMLDR with 0.09% of difference. For the difficult nodules, the averaged FoMHDR is slightly higher than FoMLDR with 1.38% of difference. The averaged FoMHDR is slightly higher than FoMLDR with 0.71% of difference. For the true positive fraction (TPF), both displays (the HDR and the LDR ones) have similar TPF for all nodules, but looking at difficult and very difficult nodules, there are more TP for the HDR display. The true positive fraction has been also computed in function of the local average luminance around the nodules. For the lowest luminance range, there is more than 30% in favor of the HDR display. For the highest luminance range, there is less than 6% in favor of the LDR display.

CONCLUSIONS

This study shows the potential benefit of using a HDR display in radiology.

On the number of perceivable blur levels in naturalistic images

Blur is a useful cue for depth. Natural images contain objects at a range of depths whose depth can be signaled by their perceived blur. Here, to evaluate the usefulness of blur as a depth cue, we estimate the number blur levels that observers can perceive simultaneously. To estimate this value, observers discriminated and classified dead leaves patterns that contained a controlled distribution of blur levels but are more complex or naturalistic than stimuli typically used in blur research. We used a 2-IFC discrimination task, in which observers reported the interval that contained more blur levels and a classification task, in which observers reported the number of perceived blur levels. In both tasks, observers could not discriminate or classify more than four levels of blur in the stimulus reliably. In isolation from other cues, blur may provide only a coarse cue to depth and add limited depth information when present in natural scenes with complex distributions of blur and multiple depth cues.

Humans perceive flicker artifacts at 500 Hz

Humans perceive a stable average intensity image without flicker artifacts when a television or monitor updates at a sufficiently fast rate. This rate, known as the critical flicker fusion rate, has been studied for both spatially uniform lights, and spatio-temporal displays. These studies have included both stabilized and unstablized retinal images, and report the maximum observable rate as 50–90 Hz. A separate line of research has reported that fast eye movements known as saccades allow simple modulated LEDs to be observed at very high rates. Here we show that humans perceive visual flicker artifacts at rates over 500 Hz when a display includes high frequency spatial edges. This rate is many times higher than previously reported. As a result, modern display designs which use complex spatio-temporal coding need to update much faster than conventional TVs, which traditionally presented a simple sequence of natural images.

Pattern visual evoked potentials elicited by organic electroluminescence screen

Purpose. To determine whether organic electroluminescence (OLED) screens can be used as visual stimulators to elicit pattern-reversal visual evoked potentials (p-VEPs). Method. Checkerboard patterns were generated on a conventional cathode-ray tube (S710, Compaq Computer Co., USA) screen and on an OLED (17 inches, 320 × 230 mm, PVM-1741, Sony, Tokyo, Japan) screen. The time course of the luminance changes of each monitor was measured with a photodiode. The p-VEPs elicited by these two screens were recorded from 15 eyes of 9 healthy volunteers (22.0 ± 0.8 years). Results. The OLED screen had a constant time delay from the onset of the trigger signal to the start of the luminescence change. The delay during the reversal phase from black to white for the pattern was 1.0 msec on the cathode-ray tube (CRT) screen and 0.5 msec on the OLED screen. No significant differences in the amplitudes of P100 and the implicit times of N75 and P100 were observed in the p-VEPs elicited by the CRT and the OLED screens. Conclusion. The OLED screen can be used as a visual stimulator to elicit p-VEPs; however the time delay and the specific properties in the luminance change must be taken into account.