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Visual evoked potentials were studied in 23 patients with Parkinson's disease at varying stages, and in 23 normal subjects of the same age. The latency of the P wave is increased in the Parkinson group in a significant manner and becomes more marked as the disease progresses. The difference in latency between the potentials evoked by stimuli applied to both eyes is also significantly increased in Parkinson's disease. The interpretation of these results remains obscure, but "peripheral" retinal and "central" mechanisms are probably involved. From a practical point of view, these results demonstrate that increases in latency of visual evoked potentials are not always due to localized demyelinization. Study of visual evoked potentials could be of values when investigating patients with Parkinson's disease.
With the multifocal technique, visual evoked potentials (VEPs) can be recorded simultaneously from many regions of the visual field. For the multifocal VEP (mfVEP), the patient views a display that typically contains 60 sectors, each with a checkerboard pattern. The display covers about the same retinal area as the 24-2 Humphrey visual field (HVF). However, due to the scaling of the sectors of the mfVEP display, the fields are sampled differently by the mfVEP and HVF. To assess local defects in the visual field, the mfVEP responses must be compared with normal controls. These comparisons require relatively sophisticated analyses and software. Whereas the mfVEP can be recorded relatively easily with the same equipment used to record multifocal electroretinograms (mfERGs), the software needed to perform the analysis is not yet widely available. The mfVEP is valuable for ruling out non-organic visual loss, diagnosing and following patients with optic neuritis/multiple sclerosis, evaluating patients with unreliable or questionable HVFs, and following disease progression. When combined with the mfERG, diseases of the outer retina (before the retinal ganglion cells) can be distinguished from diseases of the ganglion cells and/or optic nerve. The difficulties encountered in recording and analyzing mfVEP responses are greater than those involved in full-field VEP testing. Thus, in its current form, the mfVEP is best recorded and interpreted by ophthalmologists and electrophysiologists experienced with the technique. However, this technique is developing rapidly; advances in commercial hardware and software are expected in the near future.
We investigated whether the visual evoked potential (VEP) reflects cortical processing associated with preattentive texture segregation. On a visual display unit we presented stimuli with various arrangements of oriented line segments that either led to the appearance of a "preattentive" checkerboard or did not. Two presentation modes were used (pattern onset at 1 Hz and rapid pattern change at 4.3 Hz), while luminance (57 cd/m2) and contrast (92%) of the line segments remained constant. VEPs were recorded in 7 human subjects. The VEP was analyzed as a linear combination of putative components, which are evoked by either local pattern, quasi-local orientation contrast or global preattentive structure. In the transient VEP, we found a negativity over the posterior pole at a latency between 161 and 225 msec (FWHM) in the linear combination designed to extract segregation-specific components. Peak amplitude reached 3.1 +/- 0.8 microV (mean +/- SEM) at 199 msec. This negative peak appeared only for textures containing orientation contrast. Steady-state analysis of the rapid presentation also revealed a significant component (P = 0.002) associated with texture segregation. These potentials either represent processing of orientation contrast or global processing of texture segregation. The results suggest that specific surface potentials, differing from cognitive potentials, can be derived which are associated with preattentive processing.
PURPOSE: The visual evoked potential is often used to assess visual function in neurologically impaired patients, a group in whom nystagmus is a common feature. Pattern-reversal stimuli are commonly used to produce visual evoked potentials in clinical practice. Previous reports have shown that this stimulus is not optimal when subjects have nystagmus. The present study aimed to compare the efficacy of pattern-onset and reversal stimuli when used to measure visual evoked potentials from subjects with idiopathic nystagmus. METHODS: In five adults with congenital nystagmus and 10 visually normal adults, VEPs were recorded and reproduced for checkerboard stimuli of two sizes (120' and 60'). Each size was presented as both pattern-onset and reversal check. RESULTS: Visually normal adults demonstrated similar visual evoked potential amplitudes and quality in response to pattern-reversal and pattern-onset. However, in the presence of nystagmus, visual evoked potentials recorded to pattern-reversal stimuli were significantly smaller and of poorer quality than those obtained to pattern-onset stimuli (analysis of variance p<0.05; Kendall's tau, p<0.05). CONCLUSIONS: Pattern-onset stimuli produce larger and clearer visual evoked potentials in patients with nystagmus compared with those produced to pattern-reversal stimuli.
Visual evoked potential (VEP) studies are of great value in a wide variety of pediatric patients, including those with disorders of the sensory visual pathway and those at risk for visual pathway damage. VEPs are simple, non-invasive, and are particularly appropriate for infants and young children who cannot communicate visual symptoms or cooperate for standard vision assessment. VEPs in pediatric patients have the following main purposes: (1) detecting lesions causing dysfunction of the sensory visual pathways (the VEP is a sensitive indicator of subclinical lesions and can be used to differentiate visual impairment from visual inattention in young infants); (2) confirming functional loss when disorders of the visual system are present; (3) quantifying visual impairment in patients with known visual disorders, accomplished either empirically by noting the severity of the VEP abnormality to flash and pattern stimuli or by visual acuity estimation studies (early quantification of vision loss allows referral to early intervention programs, which can ameliorate the long-term consequences of the disability); (4) monitoring patients who are at risk for visual complications either from diseases (such as hydrocephalus or neurofibromatosis) or as a complication of therapeutic intervention (e.g., neurosurgery, chemotherapy) to help detect and avoid long-term sequelae of such therapies on the developing nervous system; (5) establishing prognosis for visual and systemic recovery based on flash VEPs for specific pediatric disorders including perinatal asphyxia in full-term neonates, acute-onset cortical blindness, and, to a fair extent, in comatose children; and (6) in some cases, contributing to the differential diagnosis. Abnormalities of flash and/or pattern VEPs are generally nonspecific to the type of exact location of the lesion, except in distinguishing prefrom postchiasmal lesions. However, in certain conditions, such as the hereditary ataxias of childhood, VEP abnormalities may help in the diagnosis. Similarly, deterioration in VEPs may help differentiate progressive from static encephalopathies. VEPs have become an indispensable tool in pediatric ophthalmology and neurology. They will probably play an increasingly important role in the future, primarily due to the difficulty in assessing visual system function in young or ill children and the VEP's sensitivity to subclinical damage in this aspect of the central nervous system.
A small experimental, central scotoma significantly attenuates the human pattern visual evoked potential. The steady-state pattern visual evoked potential was recorded from seven visually normal adults who viewed a reversing checkerboard with 24' checks and a central scotoma that varied in size and shape. We found that square scotomas had to be at least 3 x 3 degrees to significantly (p < 0.05) attenuate the pattern visual evoked potential. Receptor density has been shown to be greater along the horizontal meridian than the vertical meridian. We hypothesized that this results in greater cortical representation of the horizontal meridian than the vertical meridian and, therefore, the pattern visual evoked potential might be significantly attenuated by a smaller rectangular scotoma oriented along the horizontal meridian than along the vertical meridian. One dimension of the rectangular scotoma was fixed at either 1 degree or 3 degrees, while the other dimension was varied from 1 degree to 8 degrees. The threshold scotoma size that significantly (p < 0.05) attenuated the pattern visual evoked potential was a horizontal scotoma subtending 1 x 4 degrees and a vertical scotoma subtending 5 x 1 degree (vertical x horizontal). Meridional differences in cortical representation were not apparent to the larger scotoma series in which the fixed dimension subtended 3 degrees (3 x 2 degrees and 2 x 3 degrees). Further analysis of the data revealed that the apparent meridional difference for the 1 degree scotoma series was a function of data variability. The determinant of the PVEP amplitude was scotoma area, not orientation. Monocular and binocular threshold scotoma sizes were the same, which could be due to the level of binocular summation demonstrated by our subjects.
PURPOSE: To investigate how central retinal function contributes to pattern visual evoked cortical potentials (PVECPs), we recorded transient and steady-state PVECPs in patients with macular disease and compared the results with those of patients with optic neuritis. The results were also evaluated for correlation with visual acuity and kinetic perimetric measurements. METHODS: PVECPs to 3 rev/s stimulation (transient) and 12 rev/s stimulation (steady-state) for check sizes 15' and 30' were obtained from all patients and age-matched healthy controls. The peak latency and amplitude of the P100 component of the transient VECP and the amplitudes of the steady-state VECP were measured and evaluated. RESULTS: Patients with macular disease produced significant delay of P100 latency and amplitude attenuation of the steady-state VECP, as compared with healthy subjects. P100 latency delay was less in patients with macular disease than in those with optic neuritis, while the attenuation rate of the steady-state VECP amplitude was similar between groups. Visual acuity was correlated with P100 latency and steady-state VECP amplitude reduction. CONCLUSIONS: Although the delay of latency was less in patients with macular disease than in those with optic neuritis, the electrophysiologic alterations that occur in macular disease demonstrate a potential for diagnostic value at the level of the retina.
Visual evoked cortical potentials elicited by long-lasting stimuli of red light were examined in eight subjects under different conditions of vigilance. The DC components of the evoked potentials were compared with the delta and theta activity (0-7.5 Hz) of the EEG by means of frequency power spectra. The results are: 1) A proportional relationship exists between the amplitude of the negative DC component of the evoked potential and the theta and delta activity in the EEG. 2) Under conditions of low vigilance the negative DC potential does not return immediately to the baseline after the off-effect. Instead it may reach a second peak. 3) Under conditions of high vigilance, the DC potential may reverse its polarity. Its return to the baseline following the off-effect is immediate. 4) No dependence of the cortical DC potential on the intensity of the light stimulus could be demonstrated. 5) Since the negative potential outlasts the visual stimulus and could be recorded at the occiput and the vertex, it may be regarded as a nonspecific arousal reaction. Furthermore, since the positive DC potential can only be recorded at the occiput, it is regarded as a specific stimulus-correlated response.
The flash visual evoked potential (F-VEP), elicited by a 100 ms diffuse light flash presented at 2 Hz, was examined in the cat primary visual cortex (Area 17). Intracortical F-VEP depth profiles were recorded to characterize waveform changes with electrode depth. A positive surface component, with a latency of 200 ms, was the dominant waveform feature within the cortex, reversing in polarity and increasing in magnitude as the cortex was penetrated. Other prominent components with latencies of 30, 50, 100, and 125 ms were also observed. Changes in the waveform with stimulus duration and illumination were examined and revealed the sensitivity of prominent components to stimulus parameters.
Flash visual-evoked potentials were studied in 20 infants with iron-deficiency anemia to determine the effect of iron deficiency on visual function by using visual-evoked potentials in this type of anemia. After iron therapy for 12 weeks, visual-evoked potentials were retested in these otherwise healthy infants. All infants showed an excellent hematological response to iron therapy. Post-treatment visual-evoked potential N2 latencies (negative deflections) decreased significantly compared to the pre-treatment values (p < 0.05). These results suggest that iron-deficiency anemia causes subclinical visual impairment, and visual-evoked potentials may be a useful non-invasive means of detecting subtle effects of nutritional deficiencies and monitoring the nutritional status of infants.
Pattern reversal visual stimuli are used to evoke potentials (VEPs) for assessment of visual acuity and for localizing defects along the visual pathways. Our goal was to assess the importance of attention and defocusing to the recordings of pattern VEP. Forty-one volunteers with normal (6/6) corrected visual acuity participated in this study. Twenty-one were asked to defocus intentionally the visual stimulus (located 200 cm away) by fixating at a target 25 or 50 cm from the eye. Twenty other subjects performed auditory tasks to distract their attention from the visual stimulus. Pattern VEPs were elicited by different check sizes. The amplitude and time-to-peak of the P100 wave were measured. Intentional defocusing caused amplitude reduction and prolongation of the time-to-peak in young subjects (20-34 years old). With the smallest checks used (7.5') we could not record a reliable response from 43% of the young subjects (6 out of 14). In older patients (35-61 years old), intentional defocusing induced negligible effects on pattern VEPs regardless of check size. There were no effects of auditory distraction upon the pattern VEPs. Our data suggest that intentional defocusing can produce false positive results (reduced VEP with prolonged time-to-peak) only when small checks are used in young subjects. Divided attention has negligible effect on the recordings of pattern VEPs. With proper controls, the pattern VEP test can be used for objective assessment of visual function.
Visual evoked potentials to flash (FVEP) were recorded in 23 symmetrically growth retarded newborns of between 32 and 39 weeks gestational age and 41 normally grown controls. At 9 months post term FVEP recordings were repeated in 14 of the growth retarded and 26 of the control infants. The development of two long latency negative components of the wave form of the neonatal FVEP was delayed in the growth retarded infants. The amplitude of a long latency negative peak in the 9 month post term FVEP was reduced in the growth retarded infants. We suggest that intrauterine growth retardation may affect the development of secondary activity in the visual cortex.
PURPOSE: Because there is a broad spectrum of visual dysfunction in eyes with optic nerve hypoplasia, we determined whether any clinical observations, including optic disk measurements, could predict visual outcome in eyes with hypoplastic optic nerves. METHODS: We prospectively studied the eyes of 19 children (eight consecutive and 11 randomly selected), ages 7 to 92 months, in whom a diagnosis of optic nerve hypoplasia was made by an ophthalmologist. Clinical assessment included visual acuity, pupillary responses, and nystagmus. The ratio of the horizontal disk diameter to the disk-macula distance was calculated from fundus photographs. The amplitude and latency of the flash visual-evoked potential as well as the threshold response to pattern reversal checks were recorded. RESULTS: Visual acuity was associated with pupil reactivity, nystagmus, the ratio of the horizontal disk diameter to the disk-macula distance, pattern visual-evoked potential threshold, and flash visual-evoked potential amplitude (P < .001 for all parameters). All eyes with a ratio of the horizontal disk diameter to the disk-macula distance greater than .30 had good visual acuity, and all eyes with a ratio of the horizontal disk diameter to the disk-macula distance less than .30 had poor visual acuity. No eye with a ratio of the horizontal disk diameter to the disk-macula distance less than or equal to .15 had visual acuity better than light perception. CONCLUSIONS: Visual-evoked potential and photogrammetric measurement of relative disk size can be used to approximate visual acuity in eyes with optic nerve hypoplasia. This finding could lead to a diagnostic criterion or a severity classification for optic nerve hypoplasia.
Visual evoked potentials to pattern-reversal stimulation have been recorded in 45 patients with Friedreich's ataxia. In six patients, no response could be obtained because of optic atrophy; altogether, pathological results were found in 25 patients (55%). Normal latencies were recorded in 20 patients (45%). No statistically significant connection could be drawn between the result of the VEP testing and visual acuity, age, or onset and duration of sickness, but there was a strikingly familiar accumulation of results. There is a good conformity between our results and the incidence of optic atrophy in later stages of the disease as shown by pathological-anatomical and epidemiological studies. We have demonstrated that optic nerve lesions in Friedreich's ataxia are not as rare as is frequently assumed and that the VEP is a valuable tool for detecting them. On the other hand, unfortunately, the VEP is of no help in the differential diagnosis of Friedreich's ataxia versus multiple sclerosis.
To determine the normal pattern of the short latency visual evoked potentials (SVEP) in clinical diagnosis, in a previous report, the normal responses were classified into the following three types: uniform type, brainstem dominant type and occipital dominant type, according to the strength of the potentials. In the present report the latencies of N40 and P50, which constitute the SVEP, were investigated. In addition, the burst origin of these potentials was investigated using the dipole tracing method for P50 obtained through dynamic topography. Optic nerve potentials of N40 appeared first in the anterior region (from 37.1msec to 40.4msec following the stimulus) after the fourth retinal oscillatory potentials and soon extended to the posterior region (after about 2msec). This potential appeared in approximately about 65%, because of the low amplitude. P50 appeared 45.8-53.6msec after the stimulus about 100% and burst 2-4 times in the same place. Using the dipole tracing method, the origin of P50 was estimated to the near the deep center of the brain, probably around the brainstem. The same results were obtained using dynamic topography.
Visually evoked potentials (VEP) were measured in multiple sclerosis patients with five sizes of reversing check stimuli. The VEPs were obtained using random binary sequence triggered check reversals. The random binary sequence was cross-correlated with scalp potential responses to obtain the estimates of the linear response of the system. For each evaluation a series of five VEPs, one for each of five check sizes, was done. A correlation coefficient was calculated to evaluate check size versus amplitude of the major negative wave. A tabulation was made to compare Snellen chart visual acuity with these correlation coefficients. For the eyes with a Snellen acuity of 20/20, 63% of the VEP amplitudes increased as the check size was decreased with correlation coefficients of -0.6 to -1. For the eyes with a Snellen acuity of 20/70 or less the amplitude decreased with the check size decrease, showing a correlation coefficient of +0.6 to +1 in 45% of the evaluations. The check size giving the largest amplitude was also tabulated with respect to the visual acuity, but did not appear to have as strong a relationship to visual acuity.
PURPOSE: To determine whether the temporal frequency characteristics of the visual system as determined by visually evoked potentials (VEP) can be used for a preoperative estimation of the visual function in eyes with cataracts. METHODS: Light stimuli driven by a pseudorandom binary sequence (PRBS) of 40950 ms duration were presented and EEG recordings were made from 13 control and 20 patients with cataracts preoperatively and 1 week after cataract operation. The first kernel of the PRBS-VEP was obtained as the first-order cross-correlation function between PRBS and PRBS-VEP. The Fourier transform of this function was used as the temporal frequency characteristic (TFC). RESULTS: The mean +/- standard deviation of the latency and amplitude of the VEP in normal controls were 110.8+/-4.3 ms and 2.01+/-0.67 microV, respectively. A high correlation (r>0.7) between the pre-and postoperative VEP waveform was obtained in 13 eyes (65%), and 14 eyes (70%) in the VEP-TFC curves. The sensitivity of the examination was 73%, 27%, and 91% for the latency, amplitude and TFC of the VEP, respectively. The specificity of the examination was 67%, 100%, and 89% for the same measures. Eleven of 12 eyes with abnormal TFC preoperatively showed retinal or optic nerve lesions postoperatively. False-negative results were seen in cases with delayed corneal edema. CONCLUSION: Postoperative visual function of patients with cataracts can be predicted by preoperative measurement of the TFC obtained by PRBS-VEP.