A novel system for measuring visual potentials evoked by passive head-mounted display stimulators

Documenta Ophthalmologica - The objective of this work is to evaluate the performances of a novel integrated device, based on passive head-mounted display (HMD), for the pattern reversal visual...     

Objective assessment of senile changes in visual function by visually evoked cortical potentials

The P100 peak latency of pattern visually evoked cortical potentials (P-VECP) was found to increase and amplitude decrease in the elderly depending on stimulus conditions. Using either of these as a criterion, changes of visual function with aging were quantitatively assessed both in humans and animals. Contrast threshold was found to increase at higher spatial frequency ranges and the luminance threshold increased more than 0.8 log unit. Also, the contrast threshold increased due to a smaller pupillary area and there was progressive decrease of the temporal frequency curves with age for lower frequency ranges of less than 10 rev/sec. In addition, a sensitivity decrease for the upper visual field was detected plus blue-yellow defects and a decrease in the amplitude of accommodation. In order to exclude the effect of senile changes of the crystalline lens, the luminance threshold, the accommodation power and color sense were investigated in pseudo-phakic eyes with a posterior chamber lens. No significant differences were found between phakic and pseudo-phakic eyes. Accordingly, it was suggested that reduced transparency and yellowish changes of the crystalline lens do not essentially contribute to the loss of function in the elderly found in the present study, but the neuronal pathway was responsible. As a clinical model of senescence, cases of juvenile parkinsonism were investigated, during L-dopa treatment and after ceasing it. The deficiency of the neural-transmitter of the higher visual pathway was indicated in the elderly, also. Animal experiments on neuronal dysfunction in rats and mice suggested no aging effects in ERGs, whereas VECP peak latency for higher temporal frequencies increased with age. The assumption that the elderly changes occur at the neuronal level was supported by a loss of optic nerve fibers in mice with age. The numbers of optic nerve fibers measured were 48,115, 50,875 in the 3-month-old and 6-month-old groups, respectively, and decreased to 43,175 in the 30-month-old group. Though our results indicated the senescence of visual function at the neuronal level, it was not as much as shown by other sensory organs. It was therefore presumed that there might be a certain feed-back system from the brain to the retina.     

Visual evoked potentials to luminance and chromatic contrast in rhesus monkeys

The spatial and temporal tuning of the luminance and chromatic systems were investigated using visual evoked potentials (VEPs) recorded from rhesus monkeys. VEPs were recorded from eight bipolar electrodes which were chronically implanted in the foveal projection region of area 17 in two monkeys. They were elicited by luminance (yellow-black) and chromatic (red-green) since-wave gratings which varied in spatial frequency (1, 4, 8 and 12 c/deg) and temporal frequency (3, 6, 10 and 15 Hz). The findings showed that chromatic VEPs exhibit low-pass tuning in both the spatial and temporal domains, whereas luminance VEPs exhibit bandpass tuning in the spatial domain and low-pass tuning in the temporal domain. The spatial tuning findings are consistent with the psychophysical literature, while the low-pass temporal tuning shown by the luminance VEPs was attributed to various cancellations and distortions in the suprathreshold VEP at high flicker rates and to the effects of the anesthesia.     

Apparent accommodation in pseudophakic eyes as measured with visually evoked potentials.

We measured apparent accommodation with pattern reversal visually evoked cortical potentials in 27 pseudophakic eyes after the implantation of posterior chamber intraocular lenses. The patients ranged in age from 47-83 years. The amplitude of accommodation was not significantly different between pseudophakic eyes and normal phakic eyes. Apparent accommodation was inversely proportional to the pupillary diameter and was significantly correlated with corneal astigmatism. No significant relationship was noted between apparent accommodation and age of patients, corrected visual acuity, or refractive error.     

Binocular interaction reflected in visually evoked cortical potentials as studied with pseudorandom stimuli

PURPOSE: To study the interaction of visual signals from both eyes with visual evoked cortical potentials (VECPs) elicited by pseudorandom binary sequence (PRBS) stimuli. METHODS: A PRBS was used to drive two independent LED arrays to elicit VECPs. The right and left eyes were simultaneously stimulated by two different series of PRBS stimuli. The impulse response function of each eye was calculated from the raw data by cross-correlating the PRBS and the response. The effect of changing the luminosity of the LEDs parametrically on the responses obtained from the two eyes was evaluated. RESULTS: The impulse response, obtained from 10 volunteers with normal vision, had characteristics similar to responses to the conventional VECP, with a major positive peak at 110 ms (P110). When two eyes were bilaterally exposed to two PRBS stimuli of the same luminosity, the P110 amplitudes of both eyes were decreased by the same amount from that obtained by stimulating only one eye. When the luminosity in the two eyes was different, the VECP amplitude in the eye with the dimmer stimuli decreased, and the amplitude in the contralateral eye increased. CONCLUSIONS: The results clearly demonstrate that interocular luminance interaction can be detected electrophysiologically in normal subjects by using PRBS- driven stimuli in which the feasibility of recording response from the two eyes independently.     

The effect of electrode position on flash visual evoked potentials in the newborn

We examined how changes in electrode position affected the visual evoked potential in 74 high-risk newborns using a multiple electrode technique. The variation in the visual evoked potential across the occiput was documented. Visual evoked potentials that were visible at one occipital electrode position were absent at a different electrode position in 21 of 87 recordings (24%). Changes of > 20 msec between electrodes in latency of waves P1 or N2 occurred in 34% and 29% of recordings, respectively. The amplitude of response varied by a factor of 2.36 between mid occipital and lateral occipital electrodes. The results demonstrated that large changes in morphologic characteristics, latency, and amplitude in the visual evoked potential of the newborn result from small changes in the position of recording electrodes. These findings underscored the importance of electrode position and accurate electrode placement. Our observations also indicated that recordings from a single electrode are inadequate in providing an accurate representation of the neonatal visual evoked potential.     

Visual evoked potentials to flash as an indicator of brain dysfunction following concussive head injuries in children

Sixty-six randomly selected children with concussive head injuries of different degrees were subjected to visual evoked potential to flash examination (VECP). Evaluating only the differences in occipital potential between both hemispheres and/or both eyes, the VECP was pathological in 23 cases. Estimating the severity of the head injuries, according to the proposal of Lange-Cosack and Tepfer (grades I–VI), gave rise to a close correlation between the incidence of pathological VECP and increasing degree of trauma. VECP changes were usually transitory in nature with slight injuries (I and II) and constant in traumata of higher degrees (III–VI). As to the agreement of VECP and EEG findings, in two-thirds of the cases, the results of both methods coincided with respect to the indication of pathological or nonpathological. In one-third the methods gave different results.

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Zusammenfassung Bei 66 nicht vorselektierten Kindern mit Schädel-Hirn-Traumen (SHT) unterschiedlicher Schweregrade wurden die visuell evozierten Potentiale (VECP) nach Blitzreizen abgeleitet. Bewertet wurden lediglich Differenzen im corticalen Potential zwischen beiden Hemisphären und/oder zwischen beiden Augen. Dabei ergab sich in 23 Fällen ein pathologisches VECP. Eine Einteilung des SHT Schweregrades nach Lange-Cosack und Tepfer (I–VI) führt zu einer engen Korrelation zwischen der Häufigkeit pathologischer VECP und dem Schweregrad des Traumas. Bei leichten Verletzungen (I–II) waren die VECP-Veränderungen nur vorübergehend, bei schweren Traumen (III–VI) waren sie dauerhaft. VECP- und EEG-Befunde stimmten in 2/3 der Fälle bezüglich der Aussage pathologisch oder nicht pathologisch überein. In 1/3 der Fälle ergaben sich Nichtübereinstimmungen insoweit, als beide Methoden zu unterschiedlichen Bewertungen kamen.

     

Visual evoked potentials to a light flash in patients with optic nerve atrophy

Informativeness of different methods used for registration of optic generated potentials in response to light flash (FOP) has been studied in 27 healthy persons and 21 patients (32 eyes) with partial atrophy of the optic nerve. It is shown that the most informative one for diagnosing optic nerve atrophy and determining visual acuity in such patients is a method of investigation of the critical incidence of rythm reproduction of FOP light flickerings. Considering the complexity and difficulty of the mentioned method for its practical usage, a simplified method is proposed; a registration of FOP of a stable state with the incidence of stimulations of 33 Hz, that allows to detect cases of pathology with visual acuity not less than 0.2.     

Critical period plasticity for visual function: definition in monocularly deprived rats using visually evoked potentials

VEPs were used to study the effects of visual deprivation and to define the critical period in the hooded rat. It is shown that monocular deprivation in young animals reduces both the visual acuity and the contrast sensitivity of the deprived eye and that the depth of attenuation depends upon the age at onset of deprivation. This suggests that the critical period is almost over by 6 weeks postnatally. This simple recording technique might be useful in studying amblyopic children and in defining more clearly the critical period for human vision.     

Comparison of cathode ray tube and liquid crystal display stimulators for use in multifocal VEP

Purpose

To compare the modified signal-to-noise ratio (SNR*) of multifocal visual evoked potential (mfVEP) responses elicited by a cathode ray tube (CRT) and liquid crystal display (LCD) monitor in normal subjects.

Methods

An LCD monitor and CRT monitor were luminance and contrast matched. Luminance stability and the effect of viewing angle on luminance and contrast was measured for both screens. The SNR* of mfVEP responses from 15 normal subjects was compared between the stimulators using repeated measures analysis of variance.

Results

The CRT monitor took 10 min from switch on to reach the desired luminance compared to 60 min for the LCD monitor. LCD luminance was sensitive to variations in ambient temperature, fluctuating by 10 cd/m^?2 over approximately 20–27 °C, whereas CRT luminance was stable. Luminance variation from the centre to the edge of the CRT screen was 8 % when viewed perpendicularly and 28 % when viewed at an angle of 25°, compared to 24 and 46 %, respectively, for the LCD screen. Contrast was >94 % and varied by <3 % across both monitors for both viewing conditions. There was no significant difference in SNR* between responses elicited by the two stimulators (p = 0.76).

Conclusions

CRT and LCD stimulators elicited mfVEP responses with similar SNR* in normal subjects. This study highlighted practical issues with the use of LCD monitors as visual stimulators, particularly with regard to warm-up time, luminance stability and luminance uniformity.     

Senile changes of crystalline lens: effects on the delayed latency of pattern visually evoked potentials in phakic and pseudophakic eyes

We recorded visually evoked cortical potentials to pattern stimuli (PVECPs) in 22 patients who received a posterior chamber intraocular lens implant. The mean latency of the P100 obtained from patients was not significantly different from that of age-matched normal phakic subjects. Furthermore, we found that both groups showed a similar increased delay of the P100 latency with age. The results of this study indicated that the senile opacity of the crystalline lens does not contribute to changes of PVECPs with age. These alterations may be due to senile changes of the neural pathway.     

Asymmetric responses in cortical visually evoked potentials to motion are not derived from eye movements.

PURPOSE: Normal neonates and many adults after abnormal visual development have directional preferences for visual stimulus motions; i.e., they give better responses for optokinetic nystagmus (OKN) and visually evoked potentials (VEPs) in one direction than to those in the opposite direction. The authors tested whether the VEP responses were asymmetrical because of abnormal eye movements. METHODS: VEPs were recorded from the visual cortices of five macaque monkeys: one normal, one neonate, and three reared with alternating monocular occlusion (AMO). They were lightly anesthetized, followed by paralysis to prevent eye movements. They then had "jittered" vertical grating patterns presented in their visual fields. The steady state VEPs were analyzed with discrete Fourier transforms to obtain the amplitudes and phases of the asymmetries. RESULTS: The normal, control monkey had small, insignificant amplitudes of its asymmetrical Fourier component and random phases that were not 180 degrees out of phase across the left and right eyes. The neonatal monkey and the AMO monkeys all had large, significant asymmetries that were approximately 180 degrees out of phase between the left and right eyes. CONCLUSIONS: The neonate and abnormally reared monkeys continued to have asymmetrical responses even after their eyes were paralyzed. Therefore, eye movements cannot be the source of the asymmetrical amplitudes of the VEPs, and the visual cortex is at least one source responsible for asymmetries observed in neonates and adults reared under abnormal visual inputs.     

Simultaneous electroretinograms and visually evoked potentials from adult amblyopes in response to a pattern stimulus.

Electroretinograms (ERGs) and visually evoked potentials (VEPs) were recorded simultaneously from each eye of three adult amblyopes. A spatially alternating checkerboard pattern stimulus of constant mean luminance was used to eliminate the effect of stray light on the ERG. The VEP was affected in the amblyopic eye of all subjects. In two subjects the VEP amplitude was reduced; in the third subject the amplitude was not attenuated, but the waveform of the VEP was markedly altered. Photopic ERGs recorded from the normal and amblyopic eye of each subject with an unpatterned flashing light were equal in amplitude. However, ERGs elicited by a patterned stimulus were affected in the amblyopic eye of all three subjects; the after-potential showed larger reductions in amplitude than the b-wave. These results suggest there may be some retinal involvement in human amblyopia.     

Rod and cone system contributions to oscillatory potentials: an explanation for the conditioning flash effect

The oscillatory potentials (OPs) of the human electroretinogram (ERG) are smaller in response to the initial flash of a series than to subsequent flashes. To investigate a possible rod system contribution to this "conditioning flash effect," we have examined OPs in normals and rod monochromats. The OPs recorded from rod monochromats were similar to those recorded from normals under test conditions that selectively stimulate rods. However, under conditions that in normals stimulate both rods and cones and that result in maximal amplitude of the OPs, the rod monochromats exhibit markedly reduced OPs. This finding suggests that the initial (conditioning) flash operates by adapting the rod system contribution to the OPs, so that the OPs in response to subsequent flashes result primarily from the cone system. In agreement with this hypothesis, the conditioning flash effect did not occur when flashes were presented against a background which eliminated the rod system response nor during the cone plateau phase of dark adaptation.     

Chiasmal coefficient of flash and pattern visual evoked potentials for detection of chiasmal misrouting in albinism

The diagnosis of albinism can be confirmed by electrophysiological examination, when chiasmal misrouting can be demonstrated. The present study describes a quantitative analysis method for this purpose. A chiasmal coefficient (CC) was calculated by correlating the differential potential over left and right hemisphere, when stimulating left versus right eye. This CC will be negative in albinism and positive in normal individuals. VEPs were recorded in 20 control subjects, four children with congenital motor nystagmus and six children with albinism. In up to 25% of the controls the CC was negative, when using flash VEP. However, with pattern VEP all had a positive CC. All children with albinism had a negative CC. Three of the four patients with congenital motor nystagmus had a positive CC, and one child had a small negative value with flash stimulation. In conclusion, determination of CC is a valuable and objective analysis method for electrophysiological determination of chiasmal misrouting. The method is relatively simple and only needs two electrode tracings. One should be aware of false-positive results when using flash stimulation. Whenever possible pattern stimulation should be used.     

The gap effect revisited: slow changes in chromatic sensitivity as affected by luminance and chromatic borders

Chromatic discrimination was studied with two half-fields that were either precisely juxtaposed or were separated by a narrow gap. When present, the gap was in some conditions filled with light isoluminant to the test fields. When the fields were juxtaposed, chromatic sensitivity declined with viewing duration. For a discrimination based solely on S cone activity, separating steadily-viewed fields by either a luminance or a purely chromatic gap caused similar enhancements of sensitivity. Neither type of gap had much effect when the fields were flashed. The results may be interpreted as showing that either a luminance or chromatic contour can spatially delimit the two half-fields, thus preventing a slow spatial integration from reducing the discriminability of the two sides of the field.     

闪光视诱发电位检查对白内障患者术后视力的预测

目的:评价术前闪光视诱发电位(flash visual evoked potential,FVEP)检查对白内障术后视力的预测价值.方法:试验组白内障患者95例130眼于术前1～3 d常规行FVEP检查,术后1wk复查FVEP,1mo复查最佳矫正视力(best corrected visual acuity,BCVA).对照组38例62眼.分析FVEP-P2波的振幅及潜伏期,并注重波型完整性的分析.结果:术前FVEP正常者78眼,术后视力改善者75眼(96.2%),无改善者3眼(3.8%).术前FVEP异常者52眼,术后视力改善者40眼(76.9%),无改善者12眼(23.1%).FVEP对白内障患者术后视力预测的假阳性率28.5%,假阴性率3.9%,预测可靠性66.9%(BCVA=0.3).各级晶状体混浊者FVEP的潜伏期和振幅差异无显著性,白内障术前与术后FVEP的潜伏期和振幅差异无显著性.FVEP的振幅与术后BCVA相关性较好.结论:白内障术前FVEP正常者高度提示术后视力预后良好,但术前FVEP异常者术后仍有76.9%视力提高.因此,在临床工作中对于术前FVEP异常的白内障患者,不应轻易放弃手术治疗.     

Acuity predictions from visually evoked potential to checkerboard pattern reversal stimuli: the effect of reversal rate

Introduction : A pattern reversal stimulus (check size 5.5 minutes of arc) was used to elicit the visually evoked potential (VEP). Methods : The peak to trough amplitude of the VEP wave was measured and compared to subjective visual acuity (Landolt C). The recordings were made at three, six and 12 pattern reversals per second. Results : The correlation between VEP amplitude and visual acuity was found to decrease at higher temporal frequencies. The decrease is not confined to the use of a television system reversing checkerboard pattern stimulus as suggested by previous workers. Discussion : It may well be concerned with a change in processing of the visual information as could be expected at these higher frequencies.