Reduced contrast sensitivity, pattern electroretinogram ratio, and diminished a-wave amplitude in patients with major depressive disorder.

The electroretinogram (ERG), a non-invasive electrophysiological tool used in ophthalmology, is increasingly applied to investigate neural correlates of depression. The present study aimed to reconsider previous findings in major depressive disorder (MDD) reporting (1) a diminished contrast sensitivity and (2) a reduced patten ERG (PERG) amplitude ratio, and additionally, to assess (3) the photopic negative response (PhNR) from the flash ERG (fERG), with the RETeval® device, a more practical option for clinical routine use. We examined 30 patients with a MDD and 42 healthy controls (HC), assessing individual contrast sensitivity thresholds with an optotype-based contrast test. Moreover, we compared the PERG ratio, an established method for early glaucoma detection, between both groups. The handheld ERG device was used to measure amplitudes and peak times of the fERG components including a-wave, b-wave and PhNR in both MDD patients and HCs. MDD patients exhibited diminished contrast sensitivity together with a reduced PERG ratio, compared to HC. With the handheld ERG device, we found reduced a-wave amplitudes in MDD, whereas no significant differences were observed in the fERG b-wave or PhNR between patients and controls. The reduced contrast sensitivity and PERG ratio in MDD patients supports the hypothesis that depression is associated with altered visual processing. The findings underscore the PERG's potential as a possible objective marker for depression. The reduced a-wave amplitude recorded with the RETeval® system in MDD patients might open new avenues for using handheld ERG devices as simplified approaches for advancing depression research compared to the PERG.

The FreiBurger: a new optotype for P300-based acuity estimation.

PURPOSE: Accurate objective assessment of visual acuity is crucial, particularly in cases of suspected malingering, or when the patient's inability to cooperate makes standard psychophysical acuity tests unreliable. The P300 component of the event-related potentials offers a potential solution and even allows for the use of standard optotypes like the Landolt C. However, low-vision patients with large eccentric visual field defects often struggle to locate the Landolt C gap quickly enough for a P300 to be reliably produced. METHODS: Addressing this challenge, we introduce a novel optotype (the "FreiBurger") with a critical detail that extends through the optotype's center. Two experiments, with 16 and 12 participants, respectively, were conducted. In the first, psychophysical acuity estimates were obtained with both the FreiBurger and the Landolt C. In the second, we tested the performance of the FreiBurger, relative to the Landolt C, in eliciting a P300 with undegraded vision, simulated low vision, and in a simulated combination of low vision and visual field constriction. RESULTS: Comparable psychophysical acuity values (average difference 0.03 logMAR) were obtained for both optotypes. In the P300 recordings, both optotypes produced similar P300 responses under conditions of undegraded vision and low vision. However, with the combination of low vision and constricted visual field, the P300 could only be reliably obtained with the FreiBurger, while the amplitude was drastically reduced with the Landolt C (9.1 µV vs. 2.2 µV; p < 0.0005). CONCLUSION: The new optotype extends the applicability of P300-based acuity estimation to the frequently encountered combination of low vision and constricted visual field, where Landolt C optotypes fail. Although impairments were simulated in the present study, we assume that the advantages of the new optotype will also manifest in patients with such impairments. We furthermore expect the advantages to apply to time-sensitive psychophysical examinations as well.

Effect of eccentric fixation on the steady-state pattern electroretinogram.

PURPOSE: The steady-state pattern electroretinogram (ssPERG) is used to assess retinal ganglion cell function in a variety of research contexts and diagnostic applications. In certain groups of patients or study participants, stable central fixation of the stimulus is not guaranteed. The present study aimed at assessing the effects of misfixation on the ssPERG response to checkerboard reversal stimuli. METHODS: Using two check sizes (0.8° and 15°), we compared ssPERG responses for several amounts of fixation deviation, ranging from 0° to 19° horizontally and from 0° to 14° diagonally. The stimulus area extended to 15° eccentricity, stimulus reversal rate was 15/s. RESULTS: Up to around 7° eccentricity, there was no sizable effect of fixation deviation under most conditions. Effects were somewhat larger for nasal than for temporal deviation, in particular for small checks. Diagonal deviation was associated with a response to luminance onset/offset at 7.5 Hz (subharmonic of the reversal rate), most prominently when the interior of a large check was fixated. CONCLUSION: Generally, moderate inaccuracies of fixation do not have a sizable effect on ssPERG amplitude. However, with large checks, the luminance response has to be considered.

Temporal frequency dependence of the polarity inversion between upper and lower visual field in the pattern-onset steady-state visual evoked potential.

PURPOSE: According to the cruciform model, the upper and lower halves of the visual field representation in the primary visual cortex are located mainly on the opposite sides of the calcarine sulcus. Such a shape would have consequences for the surface-recorded visual evoked potential (VEP), as V1 responses to stimulation of the upper and lower hemifield manifest with opposite polarity (i.e., polarity inversion). However, the steady-state VEP results from a complex superposition of response components from different cortical sources, which can obscure the inversion of polarity. The present study assesses the issue for different stimulation frequencies which result in different patterns of superposition in the steady-state response. METHODS: Sequences of brief pattern-onset stimuli were presented at different stimulation rates ranging from 2 Hz (transient VEP) to 13 Hz (steady-state VEP). The upper and lower hemifields were tested separately and simultaneously. The data were assessed both in the time domain and in the frequency domain. RESULTS: Comparing the responses to the stimulation of upper and lower hemifield, polarity inversion was present within a limited time interval following individual stimulus onsets. With increasing frequency, this resulted in an approximate inversion of the full steady-state response and consequently in a phase shift of approximately 180° in the time-domain response. Polarity inversion was more prominent at electrode Pz, also for transient responses. Our data also demonstrated that the sum of the hemifield responses is a good approximation of the full-field response. CONCLUSION: While the basic phenomenon of polarity inversion occurs irrespective of the stimulus frequency, its relative impact on the steady-state response as a whole is the largest for high stimulation rates. We propose that this is because longer-lasting response components from other visual areas are not well represented in the steady-state VEP at higher frequencies.

Removing mains interference from the mfERG by applying a post-processing digital notch filter: for the good or the bad?

PURPOSE: Ideally, the multifocal electroretinogram (mfERG) is recorded without noticeable intrusion of mains interference. However, sometimes contamination is difficult to avoid. A post-processing digital notch filter can help to recover the retinal response even in severe cases of mains interference. While a digital filter can be designed to have little to no impact on peak times, filtering out mains interference also removes the retinal signal content of the same frequency, which may result in a change of amplitude. The present study addressed this issue in the standard first order kernel mfERG. METHODS: In 24 recordings from routine exams with no perceivable mains interference, the effects of 50-Hz and 60-Hz non-causal digital notch filters on amplitude and peak time were assessed. Furthermore, the effect of filtering on contaminated traces was demonstrated and simulated mains interference was used to provide an example of nonlinear superposition of retinal signal and mains interference. RESULTS: mfERG amplitudes were reduced by 0%-15% (median 6%) with the 50-Hz filter and remained virtually unaffected with the 60-Hz filter. Simulations illustrate that spurious high-frequency components can occur in the filtered signal if a strongly contaminated signal is clipped due to a limited input range of the analog-to-digital converter. CONCLUSION: The application of a 50-Hz digital notch filter to mfERG traces causes a mild amplitude reduction which will not normally affect the clinical interpretation of the data. The situation is even more favorable with a 60-Hz digital notch filter. Caution is necessary if the assumption of linear additivity of retinal signal and mains interference is violated.

Seizure triggered by flicker electroretinogram in a patient with no history of epilepsy.

PURPOSE: It is well known that repetitive flash stimulation may trigger seizures in susceptible individuals. Nevertheless, reports of such incidents occurring during recording of a flash electroretinogram (ERG) are extremely rare. Here, we describe the case of a photic-induced seizure triggered during an ERG recording in the absence of a history of epilepsy or other paroxysmal events. METHODS: A 14-year-old male patient presented with reduced visual acuity and impaired mesopic vision. Ophthalmological exams confirmed the patient's complaints but were inconclusive as to the underlying pathophysiology. An ERG recording was performed, during which the 30-Hz flicker stimulus triggered a seizure. RESULTS: The ERG was essentially normal, with the exception of a 7-Hz rhythm superimposed onto the flicker ERG response that was recorded when the seizure developed. CONCLUSIONS: The present case highlights the possibility that the 30-Hz ERG flash stimulus triggers a seizure in patients with no previous paroxysmal events. Literature evidence suggests that the likelihood of such an incident could be reduced by stimulating monocularly.

VEP-based acuity estimation: unaffected by translucency of contralateral occlusion.

PURPOSE: Visual evoked potential (VEP) recordings for objective visual acuity estimates are typically obtained monocularly with the contralateral eye occluded. Psychophysical studies suggest that the translucency of the occluder has only a minimal effect on the outcome of an acuity test. However, there is literature evidence for the VEP being susceptible to the type of occlusion. The present study assessed whether this has an impact on VEP-based estimates of visual acuity. METHODS: We obtained VEP-based acuity estimates with opaque, non-translucent occlusion of the contralateral eye, and with translucent occlusion that lets most of the light pass while abolishing the perception of any stimulus structure. The tested eye was measured with normal and artificially degraded vision, resulting in a total of 4 experimental conditions. Two different algorithms, a stepwise heuristic and a machine learning approach, were used to derive acuity from the VEP tuning curve. RESULTS: With normal vision, translucent occlusion resulted in slight, yet statistically significant better acuity estimates when analyzed with the heuristic algorithm (p = 0.014). The effect was small (mean ΔlogMAR = 0.06), not present in some participants, and without practical relevance. It was absent with the machine learning approach. With degraded vision, the difference was tiny and not statistically significant. CONCLUSION: The type of occlusion for the contralateral eye does not substantially affect the outcome of VEP-based acuity estimation.

Minor effect of inaccurate fixation on VEP-based acuity estimates.

PURPOSE: VEP-based estimation of visual acuity may be used in cases of suspected malingering to objectify subjective complaints. In such an application, a lack of cooperation needs to be expected. The same may apply to young children with suspected functional impairments. In the present study, we assessed how inaccurate fixation affects the acuity estimates obtained with a VEP technique. METHODS: VEP-based acuity estimates were obtained by stimulating with a series of different check sizes using a 'stepwise sweep' protocol. Sixteen participants were tested with normal and degraded vision under five different fixation conditions (central fixation and eccentric fixation at top, bottom, right, and left edge of the stimulus area). RESULTS: The majority of individual acuity estimates with eccentric fixation differed by less than 0.1 logMAR from central fixation, and almost all estimates differed by less than 0.3 logMAR. Median estimates with eccentric fixation differed only slightly (up to 0.08 logMAR) and, except for top fixation with normal vision, non-significantly. However, data quality was lower with eccentric fixation, which increased the probability that no acuity estimate could be derived from the recording. CONCLUSION: VEP-based acuity estimates are relatively insensitive to eccentric fixation. Unnoticed deviations from central fixation in routine applications will probably be smaller than in the present study and will have even less impact on the outcome.

VEP estimation of visual acuity: a systematic review.

PURPOSE: Visual evoked potentials (VEPs) can be used to measure visual resolution via a spatial frequency (SF) limit as an objective estimate of visual acuity. The aim of this systematic review is to collate descriptions of the VEP SF limit in humans, healthy and disordered, and to assess how accurately and precisely VEP SF limits reflect visual acuity. METHODS: The protocol methodology followed the PRISMA statement. Multiple databases were searched using "VEP" and "acuity" and associated terms, plus hand search: titles, abstracts or full text were reviewed for eligibility. Data extracted included VEP SF limits, stimulus protocols, VEP recording and analysis techniques and correspondence with behavioural acuity for normally sighted healthy adults, typically developing infants and children, healthy adults with artificially degraded vision and patients with ophthalmic or neurological conditions. RESULTS: A total of 155 studies are included. Commonly used stimulus, recording and analysis techniques are summarised. Average healthy adult VEP SF limits vary from 15 to 40 cpd, depend on stimulus, recording and analysis techniques and are often, but not always, poorer than behavioural acuity measured either psychophysically with an identical stimulus or with a clinical acuity test. The difference between VEP SF limit and behavioural acuity is variable and strongly dependent on the VEP stimulus and choice of acuity test. VEP SF limits mature rapidly, from 1.5 to 9 cpd by the end of the first month of life to 12-20 cpd by 8-12 months, with slower improvement to 20-40 cpd by 3-5 years. VEP SF limits are much better than behavioural thresholds in the youngest, typically developing infants. This difference lessens with age and reaches equivalence between 1 and 2 years; from around 3-5 years, behavioural acuity is better than the VEP SF limit, as for adults. Healthy, artificially blurred adults had slightly better behavioural acuity than VEP SF limits across a wide range of acuities, while adults with heterogeneous ophthalmic or neurological pathologies causing reduced acuity showed a much wider and less consistent relationship. For refractive error, ocular media opacity or pathology primarily affecting the retina, VEP SF limits and behavioural acuity had a fairly consistent relationship across a wide range of acuity. This relationship was much less consistent or close for primarily macular, optic nerve or neurological conditions such as amblyopia. VEP SF limits were almost always normal in patients with non-organic visual acuity loss. CONCLUSIONS: The VEP SF limit has great utility as an objective acuity estimator, especially in pre-verbal children or patients of any age with motor or learning impairments which prevent reliable measurement of behavioural acuity. Its diagnostic power depends heavily on adequate, age-stratified, reference data, age-stratified empirical calibration with behavioural acuity, and interpretation in the light of other electrophysiological and clinical findings. Future developments could encompass faster, more objective and robust techniques such as real-time, adaptive control. REGISTRATION: International prospective register of systematic reviews PROSPERO ( https://www.crd.york.ac.uk/PROSPERO/ ), registration number CRD42018085666.

ISCEV extended protocol for VEP methods of estimation of visual acuity.

The International Society for Clinical Electrophysiology of Vision (ISCEV) standard for visual evoked potentials (VEPs) describes a minimum procedure for clinical VEP testing and encourages more extensive testing. This ISCEV extended protocol is an extension to the VEP standard. It describes procedures for recording multiple VEPs to a range of sizes of pattern stimuli to establish the VEP spatial frequency limit (threshold) and for relating this limit to visual acuity.

Use of diffusing filters for artificially reducing visual acuity when testing equipment and procedures.

PURPOSE: When evaluating ophthalmological devices and procedures, for instance those for visual electrophysiology, it is often desirable to perform tests with reduced acuity. Doing this with individuals with actual visual impairments has a number of disadvantages, such as considerable recruitment efforts, especially when a specific acuity range is targeted, and little control about the actual perceptual characteristics of the impairment, which are normally not fully known. Lenses with positive diopters or blurring filters that are placed in front of the eyes of visually normal observers promise a simple solution to the problem. However, defocus results in considerable spurious resolution, and previous studies suggest that the frequently used Bangerter occluders are not optimal for the purpose. The present study therefore reviews a number of other options and tests a selection of filters with respect to their effect on acuity and contrast sensitivity with the aim of identifying filters that primarily degrade acuity while mostly sparing contrast sensitivity. METHODS: First, we screened several filters for potential usefulness. The Freiburg Acuity and Contrast Test was then used to measure visual acuity and contrast sensitivity with a subset of three filters (Luminit LSD 0.5° and 1°, and LEE 420) and, for comparison, with a Bangerter occluder with a nominal acuity grade of 0.1. A qualitative comparison of the filters' effect on the checkerboard-reversal VEP was also performed. RESULTS: With both Luminit filters, variability in acuity across participants was relatively small, and at least with the 0.5° version, contrast sensitivity was relativity little affected. The LEE filter and the Bangerter occluder resulted in more variability and, compared to the effect on acuity, a relatively strong reduction in contrast sensitivity. Comparing the Luminit 0.5° and 1° filters, the reduction of acuity was not proportional to physical stimulus degradation. The effect on VEP responses was consistent with the psychophysical data. CONCLUSIONS: The Luminit filters, which have a Gaussian light diffusion profile, appear to be a good choice for artificial reduction of acuity.

ERG shrinks by 10% when reducing dark adaptation time to 10 min, but only for weak flashes.

PURPOSE: To compare dark-adapted (DA) ERG between 10, 15 and 20 min of dark adaptation (DA). METHODS: In a counterbalanced random block design, 40 healthy adult subjects were dark-adapted for 10, 15 or 20 min before we recorded ERGs to nine flash strengths from 0.001 to 10.0 cd s/m2 (dilated pupils) with a DTL-like electrode. Before and between sessions, the room was lit. Apart from choosing a wider range of stimulus strengths, and adding shorter DA times, the recordings fully complied with the ISCEV ERG Standard, namely using corneal electrodes, mydriasis and a standard DA sequence. RESULTS: The a-wave amplitude was not affected by any adaptation condition. For the b-wave amplitude, effects of reduced DA time are stronger for weaker flashes: Reducing DA from 20 to 10 min had no measurable effect on the DA 3 ERG, but reduced the DA 0.01 b-wave significantly (p < 0.0001) to 87 ± 2% (mean ± SEM). The DA 0.001 b-wave (not part of the ISCEV ERG Standard) was more affected (down to 72 ± 4%). There was a small, but significant, increase, only for weak flashes, in a- and b-wave peak times for 20 compared to 10-min dark adaptation time. CONCLUSION: Reducing dark adaptation time from 20 to 10 min in normal participants has no effect on the ISCEV DA 3 and DA 10 ERG. The reduction in DA 0.01 ERGs to 87 ± 2% agrees with Hamilton and Graham (Doc Ophthalmol 133:11-19, 2016. https://doi.org/10.1007/s10633-016-9554-x ) who found 90 ± 2% and with Asakawa et al. (Doc Ophthalmol 139:33-44, 2019. https://doi.org/10.1007/s10633-019-09693-8 ) who found 83%. Pending verification in pathophysiological states, the current results suggest that one might be able to correct for the 10% amplitude loss when gaining 10 min through shortened DA.

The "speed" of acuity in scotopic vs. photopic vision.

PURPOSE: The effect of duration of optotype presentation on visual acuity measures has been extensively studied under photopic conditions. However, systematic data on duration dependence of acuity values under mesopic and scotopic conditions is scarce, despite being highly relevant for many visual tasks including night driving, and for clinical diagnostic applications. The present study aims to address this void. METHODS: We measured Landolt C acuity under photopic (90 cd/m2), mesopic (0.7 cd/m2), and scotopic (0.009 cd/m2) conditions for several optotype presentation durations ranging from 0.1 to 10 s using the Freiburg Acuity and Contrast Test. Two age groups were tested (young, 18-29 years, and older, 61-74 years). RESULTS: As expected, under all luminance conditions, better acuity values were found for longer presentation durations. Photopic acuity in young participants decreased by about 0.25 log units from 0.1 to 10 s; mesopic vision mimicked the photopic visual behavior. Scotopic acuities depended more strongly on presentation duration (difference > 0.78 log units) than photopic values. There was no consistent pattern of correlation between luminance conditions across participants. We found a qualitative similarity between younger and older participants, despite higher variability among the latter and differences in absolute acuity: Photopic acuity difference (0.1 vs. 10 s) for the older participants was 0.19 log units, and scotopic difference was > 0.62 log units. CONCLUSION: Scotopic acuity is more susceptible to changes in stimulus duration than photopic vision, with considerable interindividual variability. The latter may reflect differences in aging and sub-clinical pathophysiological processes and might have consequences for visual performance during nocturnal activities such as driving at night. Acuity testing with briefly presented scotopic stimuli might increase the usefulness of acuity assessment for tracking of the health state of the visual system.

Can VEP-based acuity estimates in one eye be improved by applying knowledge from the other eye?

PURPOSE: It is desirable to make VEP-based acuity estimates match standard subjective acuity numerically, as the latter is familiar to ophthalmologists and optometrists. This is achieved by applying an empirical conversion factor, and previous studies found the resulting values to be within ±1 octave of subjective acuity. This leaves room for improvement. In the present study, we tested for the case of a monocular acuity deficit whether the known difference between subjective and objective acuity in the trusted fellow eye can be used to get a more precise objective estimate in the eye of which the acuity is to be estimated. In other words, we tested whether it would make sense to determine a patient-specific conversion factor. METHODS: In 19 subjects, we obtained monocular objective and subjective acuity estimates with both eyes. Normal vision and artificially degraded vision were tested. Subjective acuity was taken as the veridical value. We computed the differences between objective and subjective acuity and reasoned that if these were correlated between eyes and acuity levels, the valid information from the trusted healthy eye could be used to improve the precision of the acuity estimate in the other, potentially impaired, eye. RESULTS: The difference between objective and subjective acuity values was neither correlated significantly between eyes, nor was it correlated significantly between acuity levels. CONCLUSIONS: Knowledge about the discrepancy between objective and subjective acuity values in one eye does not help improving the accuracy of acuity estimates in the other eye. The lack of a significant correlation between eyes even at the same acuity level suggests that a major part of the discrepancies between subjective acuity and VEP-based acuity is not the result of factors that would equally apply to both eyes, such as cortical morphology.

Acuity VEP: improved with machine learning.

PURPOSE: Acuity-VEP approaches basically all use the information obtained across a number of check sizes (or spatial frequencies) to derive a measure of acuity. Amplitude is always used, sometimes combined with phase or a noise measure. In our approach, we employ steady-state brief-onset low-contrast checkerboard stimulation and obtain amplitude and significance for six different check sizes, yielding 12 numbers. The rule-based "heuristic algorithm" (Bach et al. in Br J Ophthalmol 92:396-403, 2008. https://doi.org/10.1136/bjo.2007.130245 ) is successful in over 95% with a limit of agreement (LoA) of ± 0.3LogMAR between behavioral and objective acuity for 109 cases. We here aimed to test whether machine learning techniques with this relatively small dataset could achieve a similar LoA. METHODS: Given recent advances in machine learning (ML), we applied a wide class of ML algorithms to this dataset. This was done within the "caret" framework of R using altogether 89 methods, of which rule-based and multiple regression approaches performed best. For cross-validation, using a jackknife (leave-one-out) approach, we predicted each case based on an ML model having been trained on all remaining 108 cases. RESULTS: The ML approach predicted visual acuity well across many different types of ML algorithms. Using amplitude values only (discarding the p values) improved the outcome. Nearly half of the tested ML algorithms achieved an LoA better than the heuristic algorithm; several "Random Forest"- or "multiple regression"-type algorithms achieved an LoA of below ± 0.3. In the cases where the heuristic approach failed, acuity was predicted successfully. We then applied the ML model trained with the Bach et al. [1] dataset to a new dataset from 2018 (78 cases) and found both for the heuristic algorithm and for the ML approach an LoA of ± 0.259, a nearly one-line improvement. CONCLUSIONS: The ML approach appears to be a useful alternative to rule-based analysis of acuity-VEP data. The achieved accuracy is comparable or better (in no case the ML-based acuity differed more than ± 0.29 LogMAR from behavioral acuity), and testability is higher, nearly 100%. Possible pitfalls are examined.

Little effect of 0.01% atropine eye drops as used in myopia prevention on the pattern electroretinogram.

PURPOSE: Daily administration of 0.01% atropine eye drops is a promising approach for myopia control. The mechanism of action is believed to involve the dopaminergic system of the retina, triggering an increased release of dopamine. Previous studies in psychiatric condition such as major depression suggest that pattern electroretinogram (PERG) amplitudes are modulated by changes in retinal dopamine. It is thus plausible that atropine eye drops could have an effect on PERG amplitudes. The present study was designed to test this, assessing the difference in amplitude between contrast levels and the ratio of amplitudes between check sizes as primary endpoints. METHODS: We included 14 participants with no more than ± 2 diopters of ametropia and visual acuity of at least 1.0. One eye was chosen randomly in each participant for atropine application (14 days, one drop of 0.01% atropine solution once daily before bedtime). We recorded two sets of steady-state PERG recordings: one with different contrasts (25% and 98%) and one with different check sizes (0.8° and 17°). Near-point distance, near visual acuity, and pupil diameter were measured additionally. RESULTS: The recordings to different contrasts did not show atropine-related changes of PERG amplitude. A small increase by 6% of the amplitude difference between contrast levels with atropine application was not significant (p = 0.08). Raw amplitudes in the check size condition increased with atropine by 17% (p < 0.01) and 10% (p < 0.03) for small and large checks, respectively, without a significant concomitant effect on the amplitude ratio. Pupil size was significantly affected (median increase 0.5 mm, p < 0.002). However, neither of the experimental conditions was associated with a significant correlation between pupil size and PERG effects. CONCLUSION: The effects on PERG primary endpoints after the 14-day period of atropine administration were small, especially compared to effect sizes in major depression, and statistically insignificant. Effects on raw amplitude were inconsistent. The present results suggest that retinal processing as reflected by PERG does not sizably change following a treatment regimen with atropine that is typical for myopia control.

P300-based acuity estimation in imitated amblyopia.

PURPOSE: A frequent approach to estimating visual acuity objectively is the recording of visual evoked potentials (VEPs) to patterns of different coarseness. This, however, overestimates acuity in patients with fragmented and distorted vision such as in amblyopia. This is likely due to VEP-based techniques using checkerboard or grating stimuli. We hypothesized that no overestimation would occur when the event-related potential P300 in response to optotype stimuli is used for acuity estimation. METHODS: In 29 visually normal participants, we recorded P300 responses to Landolt C optotypes of different sizes. Vision was artificially degraded by placing a patterned polymethyl methacrylate pane in front of the monitor, which resulted in fragmentation and distortion of the stimulus. As control, a frosted pane was used. Both panes were adjusted to yield the same reduction of acuity in a standard subjective acuity test. A difference of less than 0.1 log MAR was defined as criterion to judge the outcomes of the objective tests as equivalent for both types of artificial visual impairment. RESULTS: The average difference of P300-based objective acuity estimates between types of visual degradation was significantly smaller than 0.1 log MAR, indicating that the performance of the objective acuity test was equivalent for both types of visual degradation. CONCLUSION: Our data suggest that P300-based objective acuity testing with optotype stimuli is more akin to standard psychophysical acuity testing and thus a suitable approach in cases of visual impairment where VEP-based methods fail to yield reliable results.

[Visual Acuity, Contrast Sensitivity, Colour Vision: Thoughts on Psychophysical Exams in Neuro-ophthalmology]. / Sehschärfe, Kontrastempfindlichkeit, Farbensehen: Gedanken zu psychophysischen Untersuchungen in der Neuroophthalmologie.

Despite many technical advances, psychophysical testing of perceptual performance continues to play an important role in ophthalmic diagnostics, including neuro-ophthalmology. The present article considers methodological and clinical aspects of examining visual acuity, contrast sensitivity, and colour vision. In contrast to acuity, contrast sensitivity is at present rarely tested, despite often being a more sensitive marker of disease. In general, standardised procedures have the advantage of a high degree of comparability. However, tests that deviate from the standard might be better adapted to detect the characteristic impairments associated with a specific visual disorder, which justifies their application in certain cases. This also applies to colour vision testing. Preliminary evidence suggests that the measurement of colour saturation thresholds might be a more efficient and more sensitive alternative to conventional colour vision tests in cases of acquired colour vision deficiencies.

[Reaching Beyond the Retina with Clinical Electrophysiology]. / Mit klinischer Elektrophysiologie hinter die Netzhaut.

Electrophysiological recordings from the retina and cortex are pivotal to reach beyond the retina for ophthalmological and neuro-ophthalmological diagnostic testing. Pattern electroretinograms (PERG) can be used to examine retinal ganglia cells and visual evoked potentials (VEP) help to investigate overall visual pathways. Thus, they support objective functional tests of visual pathways, as well as differential diagnosis. Conventional electrophysiology is of limited value in detecting local defects in the visual field. This gap is filled by applications of multifocal electrophysiology. This permits spatially resolved testing with multifocal PERG (mfPERG) and multifocal VEP (mfVEP), and eventually objective visual field testing with mfVEP. It is important for this spectrum of methods to consider possible confounds when performing the measurements and when interpreting the results. This is explained in the present article on the basis of a series of typical examples.