VEP assessment of visual function

Pattern and sweep visual evoked potential in the objective determination of visual acuity

Purpose: To investigate the effects of pattern visual evoked potential (pVEP) and sweep visual evoked potential (sVEP) on the accurate visual acuity (VA) measurement in adults.  Methods: Medical files of 282 eyes of 141 patients who underwent VA measurement in our electrophysiology laboratory and did not undergo simulation were retrospectively analyzed. The VA was measured using the Snellen chart. Only those with a VA of higher than 1/10 on the Snellen chart were included in the study. The VA was assessed and reported by the pVEP (VA-pVEP) and sVEP test (VA-sVEP). The correlation analysis was performed using the Pearson correlation analysis. Results: Of 141 patients, 92 were males and 49 were females with a mean age of 37.7±18.4 years. There was a strong positive correlation between the VA measured by the Snellen chart and the VA measured by pVEP (VA-pVEP) (r=0.858, p<0.001). There was a weak positive correlation (r=0.267, p<0.001) between the VA measured by the Snellen chart and the VA measured by the sVEP (VA-sVEP). A weak positive correlation was found for the VA-pVEP and VA-sVEP (r=0.313, p<0.001). Conclusions: For the measurement of the degree of the VA, it seems reasonable to use pVEP initially, while sVEP should be used in cases with short attention span and those who are noncooperative and in infants. 

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.

Assessment of visual disability using visual evoked potentials

Background

The purpose of this study is to validate the use of visual evoked potential (VEP) to objectively quantify visual acuity in normal and amblyopic patients, and determine if it is possible to predict visual acuity in disability assessment to register visual pathway lesions.

Methods

A retrospective chart review was conducted of patients diagnosed with normal vision, unilateral amblyopia, optic neuritis, and visual disability who visited the university medical center for registration from March 2007 to October 2009. The study included 20 normal subjects (20 right eyes: 10 females, 10 males, ages 9–42 years), 18 unilateral amblyopic patients (18 amblyopic eyes, ages 19–36 years), 19 optic neuritis patients (19 eyes: ages 9–71 years), and 10 patients with visual disability having visual pathway lesions. Amplitude and latencies were analyzed and correlations with visual acuity (logMAR) were derived from 20 normal and 18 amblyopic subjects. Correlation of VEP amplitude and visual acuity (logMAR) of 19 optic neuritis patients confirmed relationships between visual acuity and amplitude. We calculated the objective visual acuity (logMAR) of 16 eyes from 10 patients to diagnose the presence or absence of visual disability using relations derived from 20 normal and 18 amblyopic eyes.

Results

Linear regression analyses between amplitude of pattern visual evoked potentials and visual acuity (logMAR) of 38 eyes from normal (right eyes) and amblyopic (amblyopic eyes) subjects were significant [y = −0.072x + 1.22, x: VEP amplitude, y: visual acuity (logMAR)]. There were no significant differences between visual acuity prediction values, which substituted amplitude values of 19 eyes with optic neuritis into function. We calculated the objective visual acuity of 16 eyes of 10 patients to diagnose the presence or absence of visual disability using relations of y = −0.072x + 1.22 (−0.072). This resulted in a prediction reference of visual acuity associated with malingering vs. real disability in a range >5.77 μV. The results could be useful, especially in cases of no obvious pale disc with trauma.

Conclusions

Visual acuity quantification using absolute value of amplitude in pattern visual evoked potentials was useful in confirming subjective visual acuity for cutoff values >5.77 μV in disability evaluation to discriminate the malingering from real disability.

The technique, validity and clinical use of the sweep VEP

Vision scientists have concentrated on studying two visual functions when it comes to assessing the sensory visual development in human: visual acuity and contrast sensitivity. The methods used to measure these visual functions can be either behavioral or electrophysiological. A relatively new technique for measuring the visual acuity and contrast sensitivity electrophysiologically is the sweep visual evoked potential (sVEP). This paper is a review of the literature on the sVEP technique: stimulus parameters, threshold determination, validity and reliability of sVEP are discussed. Different studies using the sVEP to study the development of visual acuity, contrast sensitivity, and vernier acuity are presented. Studies have demonstrated that the sVEP is a potentially important tool for assessing visual acuity and contrast sensitivity in non-verbal individuals with disorders affecting their visual system.

Pattern visual evoked potentials in malingering

OBJECTIVES

We previously developed a new method for estimating objective visual acuity by means of pattern visual evoked potentials (PVEP). In this study, this method was applied to the diagnosis of malingering.

MATERIALS AND METHODS

Six patients ranging in age from 40 to 54 years (mean 47 years) with suspected malingering were evaluated by means of the visual evoked potential test, optokinetic nystagmus (OKN) inhibition test, and the visual field test. In the PVEP study, the stimulus consisted of black and white checkerboards (39', 26', 15', and 9') with a visual angle of 8 degrees, contrast level of 15%, and a frequency of 0.7 Hz. One hundred PVEP responses were averaged per session.

RESULTS

Routine ophthalmic examinations were normal in all patients. Five patients had a tubularly constricted visual field, and the remaining patient had a normal visual field. The objective visual acuities of the six patients estimated from PVEP were better than their subjective visual acuities estimated with Landolt rings.

CONCLUSIONS

Among a variety of psychophysical and electrophysiologic ancillary tests, we consider our PVEP method a useful method for objectively determining visual acuity in a patient with signs of ocular malingering.

The development of monocular and binocular VEP acuity

The development of monocular and binocular grating acuity was measured in 87 infants, 2-52 weeks of age, using the sweep VEP technique. Average monocular and binocular acuity growth functions were nearly identical, with a small (less than 0.2 octaves) binocular acuity superiority occurring only under 6 months. Interocular acuity differences were small (averaging less than 1/4 octave, unsigned, with a 95% confidence interval of less than +/- 0.6 octaves) and were not significant at any age. These characteristics make the sweep VEP technique a potentially sensitive tool for the detection of monocular visual losses in the early stages of amblyopia.

Signal to noise ratio considerations in the analysis of sweep visual-evoked potentials

More investigators are recommending rapid sweep displays to estimate visual-evoked potential (VEP) sensory thresholds. Theoretically, phase-sensitive analysis offers a higher signal to noise ratio (SNR) than phase-insensitive techniques and, therefore, a more reliable and equally valid threshold estimate. Phase-sensitive analysis assumes that the VEP phase does not change over the period of one sweep. This study tests the assertion that the VEP phase is sufficiently stable for valid and reliable phase-sensitive detection. Mathematical analysis shows that phase-sensitive detection yields a lower SNR than phase-insensitive analysis if the phase error is < 45 degrees . We recorded the VEP to contrast reversing sinusoidal gratings of sweeping spatial frequency (12.5-0.2 epd) from 26 subjects. In most, phase varied > 180 degrees Over one sweep. Moreover, these large phase shifts could not be diminished by modifying contrast reversal rate, direction of spatial frequency sweep, or sweep time. We conclude that when using spatial frequency sweeps, phase-insensitive detection is superior to phase-sensitive. The filter's bandwidth and the effect of SNR on sensory threshold estimations also are discussed.

Electrophysiological assessment of the human depth-perception threshold

The cortical response to stereoscopic stimuli was measured as a function of disparity using visual evoked potentials (VEP). The stereoscopic stimulus consisted of a dynamic random-dot pattern that portrayed a three-dimensional horizontal grating. Disparity of the grating was variable between 0 and 18 arc min, step size being 4.22 arc s. Evoked responses were recorded using a "random-sequence sweep technique." The VEP amplitude increased approximately linearly with the logarithm of disparity and allowed an estimation of the stereo threshold by linear extrapolation. The evoked potential estimates of the stereo threshold of 16 subjects were compared to psychophysically obtained thresholds. Several threshold-estimation techniques are discussed. Evoked potential and psychophysical threshold estimates had the lowest discrepancy when the VEP amplitude was linearly extrapolated to the logarithmic disparity axis. The difference between the evoked potential estimate and the psychophysical threshold was less than 20% in 56% of all cases; 16 of 18 cases (89%) agreed within a factor of 2.

Spatial frequency sweep VEP: visual acuity during the first year of life

The grating acuity of 197 infants from 1 week to 53 weeks of age was measured using the visual evoked potential (VEP) in response to counterphase grating stimulation. The gratings were presented as a 10 sec spatial frequency sweep which spanned the acuity limit. The amplitude and phase of the second harmonic response were extracted by discrete Fourier analysis. The VEP amplitude versus spatial frequency function showed narrow spatial frequency tuning with amplitude peaks at one or more spatial frequencies. The phase of the response at medium to high spatial frequencies was generally constant at a spatial frequency peak, followed by a progressive phase lag with increasing spatial frequency. Grating acuity was estimated by linear extrapolation to zero microvolts of the highest spatial frequency peak in the VEP amplitude versus spatial frequency function. This visual acuity estimate increased from a mean of 4.5 c/deg during the first month to about 20 c/deg at 8-13 months of age. The VEP acuities at 1 month are a factor of three to five higher than previously reported for pattern reversal or pattern appearance stimuli. By 8 months VEP grating resolution was not reliably different from adult levels in the same apparatus.

Spatiotemporal conditions which elicit or abolish the oblique effect in man: direct measurement with swept evoked potential

Reversing sine wave gratings were electronically swept in spatial frequency and contrast. The acuity limits and contrast thresholds of 4 observers were inferred from evoked potential stimulus-response functions elicited by these stimuli and retrieved with a quadrature lock-in amplifier. The evoked potential functions, linearized in the case of contrast by increasing contrast logarithmically with time, were extrapolated to the point of zero response. This point provides an electrophysiologically defined threshold value for acuity and for contrast. An oblique effect (superior sensitivity for HV-oriented gratings) could reliably be demonstrated in both acuity and contrast threshold performance. This oblique effect could readily be abolished under low spatial/high temporal frequency conditions. The findings are discussed in terms of shifting relative strengths of X and Y contributions to the steady-state evoked potential.