Pattern component ratio in pattern-reversal VEP: normative data and clinical applications

It has been well established that there is a reliable implicit time to the VEP positive component (ca 100 msec) in response to pattern reversal. This has become a valuable test of the status of the visual system. Recent data suggest that activity around 200 msec is another useful measure of the cortical response to pattern alternation. We have carried out a normative study of the relative amplitudes of the two components. Among adults with normal visual acuity activity around 200 msec is directly related to the spatial frequency of the stimulus, peaking markedly in amplitude at a narrow range of spatial frequencies. This is in sharp contrast to the earlier component, which does not exhibit such sensitive size differentiation. Recently we have used the ratio between the late and early component amplitudes as a quantitative indicator of the efficiency of higher-level processing of pattern information. We present our normative data and illustrate the effects of decreased visual responses not related to known peripheral optical problems. The results of this work are also interpreted as showing basic characteristics of information processing in the visual system, in that higher spatial frequencies are processed by a continuum of rather sharply tuned cortical processes, with the higher spatial frequencies being processed later in time.     

Time-varying differences in evoked potentials elicited by high versus low spatial frequencies: a topographical and source analysis.

OBJECTIVE: To investigate time-varying differences in visual-evoked potentials (VEPs) and dipoles elicited by high versus low spatial frequencies. The main question was whether different spatial frequencies are processed in distinct cortical areas, especially after 100 ms. An additional question was whether and how a hemispheric balance in spatial frequency processing develops over time. METHODS: Stimuli were square-wave gratings, with spatial frequencies of 0.75, 1.5, and 6 c/d. VEPs and dipole models were analyzed at various latencies. RESULTS: For the time-window of 80-100 ms, spatial frequency-related differences in VEPs and dipoles in posterior regions as reported previously were replicated: lower spatial frequencies were associated with more positivity in the VEP and with more anterior and radial sources than high frequencies. However, after 100 ms differences in amplitude, but not in topography and dipoles, were found between the different spatial frequencies. Between 180-200 ms a right hemisphere dominance was found for all frequencies. CONCLUSIONS: After 100 ms, VEPs in response to different spatial frequencies seem to be generated in the same cortical areas. Also, no evidence for frequency-related hemispheric lateralization was found. SIGNIFICANCE: Insight is provided into the functional-anatomical basis of longer-latency frequency-related differences in processing.     

Intracortical generators of the flash VEP in monkeys

Flash visual evoked potentials (VEPs) in unanesthetized monkeys were recorded from the cortical surface and from closely spaced intracortical sites together with associated multiple unit activity (MUA). The VEP depth profiles were subjected to current source density (CSD) analysis to delineate the laminar pattern of transmembrane current flows manifested by extracellular source and sinks. The initial surface recorded components (P15 and P18) were generated subcortically within the thalamocortical radiations. The distribution of current sources and sinks associated with two subsequent surface negative components. N24 and N40. demonstrates their generation within laminae IVA and IVCb respectively, both parvocellular thalamorecipient layers. Oscillatory potentials resembling those seen in human VEPs are observed riding on N40; analysis of MUA in conjunction with sources and sinks coincident with these wavelets provides evidence that they derive from both thalamocortical and cortical activity. MUA in the 20-60 msec range shows phasic increases throughout lamina IV, which are maximum in amplitude within lamina IVA. This increased firing is concurrent with the sinks observed within the parvocellular thalamorecipient sublaminae IVCb and IVA. A subsequent component, P65, coincident with a decrease in MUA to below the spontaneous level co-located with a lamina IVCb current source, probably arises from intracortically generated inhibitory activity within IVCb. The next VEP component, a surface negative potential at 95 msec, is coincident with current sources and sinks in lamina III, and is consistent with stellate cell input to supragranular elements. VEP components after N95 are not associated with either MUA or CSD activity and are probably generated in extrastriate cortex. Human counterparts of the simian VEP are proposed.     

Binocularity in the little owl, Athene noctua. II. Properties of visually evoked potentials from the Wulst in response to monocular and binocular stimulation with sine wave gratings

Visually evoked potentials (VEPs) have been recorded from the Wulst surface of the little owl, Athene noctua, in response to counterphase-reversal of sinusoidal gratings with different contrast, spatial frequency and mean luminance, presented either monocularly or binocularly. Monocular full-field stimuli presented to either eye evoked VEPs of similar amplitude, waveform and latency. Under binocular viewing, VEPs approximately doubled in amplitude without waveform changes. VEPs with similar characteristics could be obtained in response to stimulation of the contralateral, but not ipsilateral, hemifield. These results suggest that a 50% recrossing occurs in thalamic efferents and that different ipsilateral and contralateral regions converge onto the same Wulst sites. The VEP amplitude progressively decreased with increase of the spatial frequency beyond 2 cycles/degree, and the high spatial frequency cut-off (VEP acuity) was under binocular viewing (8 cycles/degree) higher than under monocular (5 cycles/degree) viewing (200 cd/m2, 45% contrast). The VEP acuity increased with increase in the contrast and decreased with reduction of the mean luminance. The binocular gain in both VEP amplitude and VEP acuity was largest at the lowest luminance levels. Binocular VEP summation occurred in the medium-high contrast range. With decreased contrast, both monocular and binocular VEPs progressively decreased in amplitude and tended to the same contrast threshold. The VEP contrast threshold depended on the spatial frequency (0.6-1.8% in the range 0.12-2 cycles/degree). Binocular VEPs often showed facilitatory interaction (binocular/monocular amplitude ratio greater than 2), but the binocular VEP amplitude did not change either by changing the stimulus orientation (horizontal vs. vertical gratings) or by inducing different retinal disparities.(ABSTRACT TRUNCATED AT 250 WORDS)     

Visual evoked potentials in monkeys.

Visual evoked potentials (VEPs) were recorded from 2 cortical sites in stump-tailed macaques. VEPs recorded from striate cortex were basically consistent between animals (especially at low light intensity), remained remarkably stable over time, and compared favorably to VEPs reported by other investigators. We concluded that the VEP recorded from the striate cortex of day-active monkeys consists of 5 major peaks within the first 250 msec. The potentials recorded from post-central gyrus were simpler and more individualized and did not show intensity-related latency changes or increases in inter-subject variability. However, amplitudes of potentials recorded from both electrode placements increased with light intensity apparently reflecting the amplitude of individual potentials rather than the variability of these potentials from which the average VEPs were derived.     

Visual evoked potentials in preterm infants during the first hours of life

Flash evoked cortical potentials (VEPs) were studied in 33 preterm infants with gestational ages from 27 to 33 weeks. During the first 12 h after birth a visual response was evoked in all infants. Better estimates of the VEP latency and amplitude were obtained by using the values of 3 VEPs recorded at 30 sec intervals. The VEP latency decreased during the first hours of life, which was accounted for by an increase in core temperature; the latency decreased 6 msec/degrees C increase. Changes in amplitude were less influenced by changes in temperature. Both VEP latency and amplitude were inversely related to gestational age, but there was no association between head circumference and latency. The flash intensity could be reduced from 155 cd to 39 cd without any effect on VEP latency or amplitude. Similarly, a variation of background illumination below 200 lux did not cause VEP changes. The VEP was not affected by development of minor subependymal haemorrhages but it was severely attenuated during a short episode of hypoxia. It is suggested that when taking core temperature into account the VEP can be used to determine changes in the cerebral function in preterm infants.     

The pattern reversal VEP in short-gestation infants

Visual evoked potentials (VEPs) to pattern reversal stimulation have been obtained in 10 premature babies. The babies were born between 32 and 35 weeks gestational age and had pattern reversal VEPs recorded on at least 2 separate occasions. The responses were obtained from 33 weeks gestational age, and all babies had their initial recording at or before 37 weeks gestational age. The pattern reversal stimulation was produced on a small hand-held television monitor. The black and white checks subtended a visual angle of 2 degrees. Flash VEPs were also recorded on each occasion for comparison. The pattern reversal response consisted of a major positive (P1) component around 280 msec. The latency of the P1 component correlated negatively with gestational age. Although the flash VEP in the same babies was often dominated by the initial negative component no similar negative components were seen in the pattern reversal response.     

The processing of edge information in visual areas of the cortex as evidenced by evoked potentials

Using averaged visually evoked potentials (VEPs), recorded from bipolar cortical electrodes, as indicators of sensory information processing, the sensitivity to stimulus differences of parafoveal and foveal striate, foveal prestriate, and inferotemporal cortex was measured in 3 rhesus monkeys. The stimuli used were a blank field and a series of 5 checkerboard patterns in which check size was varied from 2°24′ to 9′ of retinal arc subtended per check. All stimuli had equal luminance (5.0 ft.-Lamberts) and duration (9 μsec).The results of this study indicate that: (1) VEPs obtained in all of the cortical regions were sensitive to stimulus differences simultaneously at several points along the temporal continuum following the stimulus; (2) the earliest signs of sensitivity to stimulus differences in the VEPs generally appeared initially in the parafoveal striate cortex at about 70 msec post stimulus and tended to be followed in succession by foveal striate, foveal prestriate, and inferotemporal cortex; (3) sensitivity was found for at least 190 msec and as long as 400 msec after stimulus presentation for all of the cortical regions examined and was generally maintained in foveal and parafoveal striate cortex for longer periods than in foveal prestriate and inferotemporal cortex; (4) the most significant signs of sensitivity were found in the parafoveal striate cortex and a simple transformation of edge information into VEP amplitude was shown to occur there; and (5) none of the cortical regions demonstrated long-term habituation of sensitivity to stimulus differences.These data give evidence for both serial and parallel processing of edge information between and within the different regions of the primary visual and visual association cortex with an apparent focus of edge information processing in parafoveal striate cortex.     

Visual evoked potential changes following renal transplantation

We have followed a group of 18 uremic patients through living-related donor renal transplantation (RTX) using pattern-reversal VEPs. Recordings were made prior to and 10 weeks after surgery at high, medium and low spatial frequencies. Prior to RTX, mean latency of the P100 component of the VEP was 107 msec. Individual values did not correlate with blood urea nitrogen or creatinine. Patients requiring hemodialysis did not differ from non-dialyzed patients. Ten weeks after RTX P100 latencies were significantly shortened while N75 latencies were unchanged. Several diabetic patients exhibited the appearance of previously unrecorded wave forms. P100 latency increased significantly with increasing spatial frequency before and after transplantation. Diabetic patients demonstrated a consistent increase in P100 amplitude while non-diabetic patients demonstrated a consistent decrease in P100 amplitude after RTX. The data indicate that renal transplantation has beneficial effects on the central nervous system of uremic patients not seen with chronic hemodialysis and that these effects may be quantitatively measured using the VEP. The data further suggest that electrophysiological effects of uremia and diabetes may be additive, but reversible after RTX. Alterations in the uremic and diabetic VEP may be related to retinal or more proximal central nervous system structures.     

The cumulative inhibitory effect of repetitively flashed stimuli on the recovery process of the human visual evoked potential to a test stimulus.

Cumulative inhibitory effects produced by visual response to flashed conditioning stimuli were measured by means of the amplitude reduction of the visual evoked potential (VEP) to a test stimulus. Four kinds of conditioning stimuli were used: a single conditioning stimulus and 3 repetitive conditioning stimuli with interstimulus intervals (ISIs) of 100, 200 and 400 msec, respectively. The VEP amplitude for the test stimulus was reduced most when the stimulus was paired with the single conditioning stimulus. It still remained small when the test stimulus was paired with the repetitive conditioning stimulus of 400 msec ISI, increased significantly when the ISI was shortened from 400 to 200 msec. Explanations of the VEP behaviors in terms of the stimulus relevance, task difficulty and the recovery rate of the cortical excitability were discussed.     

Equiluminant red-green and blue-yellow VEPs in multiple sclerosis.

The primate visual system is composed by two color-opponent pathways--red-green (R-G) and blue-yellow (B-Y)--subserved by the so-called parvo- and koniocellular streams respectively. The authors' aim was to compare the relative involvement of chromatic visual subsystems in multiple sclerosis (MS). In 30 MS patients with different forms of MS they recorded visual evoked potentials (VEPs) to onset (300 msec) and offset (700 msec) of equiluminant R-G and B-Y sinusoidal gratings of different contrast (90% and 25%). Equiluminance was established psychophysically by establishing the R-G and the B-Y color ratio at which chromatic gratings alternating at 15 and 10 Hz respectively had minimum visibility. The negative wave at stimulus onset with a peak latency of 120 to 160 msec was evaluated. Ordinary VEPs to luminance (LUM) contrast (black-white reversing checkerboards of 15' check size and 50% contrast) were also recorded for comparison. Latencies of R-G VEPs were abnormal in 53.3% and 58.3% of patients at 90% and 25% contrast respectively, whereas abnormal B-Y VEPs were 56.6% and 48.3%. Latencies of LUM VEPs were abnormal in 45% of patients. Interocular latency asymmetries were abnormal in 59.2% and 33.3% of patients for R-G, and 51.8% and 62.9% for B-Y. Latency asymmetries for LUM VEP were abnormal in 46.4% of patients. The higher rate of VEP abnormalities found with equiluminant chromatic stimuli compared with achromatic stimuli confirms the general vulnerability of color-opponent visual pathways in MS, even if the number of patients with abnormal findings was not significantly different when both test conditions were compared. VEPs to R-G and B-Y equiluminant stimuli appear to be involved approximately to the same extent.     

The effect of adaptation to the stimulating pattern on the latency and wave form of visual evoked potentials

Visual evoked potentials (VEPs) elicited with a checkerboard stimulus after adaptation to an unpatterned grey field were compared to those obtained after adaptation to the stimulating pattern for an equal interval of time. When 3--4 sec elapsed between the adapting interval and recording of the VEP, the latency of the principal positive peak increased by 4 msec with pattern-reversal, but not with pattern-onset stimulation, while the amplitude was unchanged. However, when the pattern-onset VEP was recorded immediately after adaptation, the principal negative peak of the response was lost and the latency of its principal positive peak increased by 14 msec, causing it to mimic the conventional pattern-reversal VEP in both wave form and latency, and suggesting that adaptation may cause the wave form differences in the VEPs obtained by these two methods.     

Effect of spatial frequency on simultaneous recorded steady-state pattern electroretinograms and visual evoked potentials.

Pattern electroretinograms (P-ERGs) and visual evoked potentials (VEPs) to 4 Hz alternating square-wave gratings were simultaneously recorded in 23 subjects. Responses were Fourier analyzed and amplitude and phase of the 2nd and 4th temporal harmonics were measured. The spatial frequency-amplitude function of the P-ERG 2nd harmonic component displayed either a bandpass tuning behavior, or a low-pass behavior. The peak amplitude for subjects with bandpass tuning was at 1.5 c/deg. The phase of the P-ERG 2nd harmonic decreased monotonically as spatial frequency increased. The VEP 2nd harmonic had a bimodal spatial frequency function with a peak at 3 c/deg and a second increase at spatial frequencies below 1 c/deg, regardless of the P-ERG characteristics. The phase of VEP 2nd and 4th harmonic had an inverted U-shaped function with peak at 3 c/deg and 1.5 c/deg respectively. Comparison of simultaneously recorded P-ERG and VEP spatial frequency functions demonstrated different tuning behavior for cortical and retinal responses. It is concluded that the proposed technique permits the separate analysis of retinal and cortical processing of visual information. The 2nd and 4th harmonic components of bEP behave independently of each other suggesting they may be generated by different subsystems.     

Early visual processing in neglect patients: A study with steady-state VEPs

Reliable steady-state visual evoked potentials (VEPs) were recorded in a group of 19 right brain-damaged patients with visuospatial hemineglect (Neglect), and two control groups: 15 left brain-damaged (LBD) patients and 12 right brain-damaged (RBD) patients without neglect. Moreover, VEPs were recorded in two rare cases of left brain damage and right visuospatial hemineglect. Stimuli were gratings phase-reversed at various temporal frequencies presented in the left and right visual field. In the Neglect group, VEPs to stimuli displayed in the left visual field (contralesional stimuli) had longer latencies. The delay was not present for the two control groups. As regards the VEP amplitudes, the Neglect group data showed a less distinctive pattern than in the case of latency. VEPs to stimuli contralateral to the lesion were smaller than those recorded for stimuli ipsilateral to the lesion in both Neglect and RBD groups. On the contrary, the VEP amplitudes for the two hemifields were comparable in the LBD group. In the case of left brain damage and neglect, VEPs to right visual field stimuli had longer latencies and lower amplitudes compared to the ipsilesional responses in both patients. Overall, the data support the view that, in most cases, early visual processing is not intact in the neglected hemifield.     

Visual evoked potentials to lateralized visual stimuli and the measurement of interhemispheric transmission time

Visual evoked potentials (VEPs) to lateralized light flashes were recorded from the parietal midline, and from homologous occipital and central sites, in a GO/NOGO reaction time task. The N160 component of the VEP was found to be larger over the hemisphere contralateral to the visual field of stimulus exposure at all pairs of lateral electrodes. At the occipital sites only, N160 latency was also shorter from the contralateral hemisphere, by an average of approximately 14msec. This was not so centrally, where a non-significant value of approximately 4 msec was obtained. These data are considered to be consistent with Milner and Lines' hypothesis that callosal transmission occurs at different rates in different functional regions of the corpus callosum.     

The effect of background illumination on pattern onset visual evoked potentials

The early part (first 200 msec) of pattern onset VEPs elicited by a dartboard pattern was studied in conditions of varying level of background illumination. The effect of pattern adaptation and pattern blurr was also studied. The observed complex behaviour of the main negativity within this part of the VEP can be best described in terms of a composite of two independent negative peaks labelled N100 and N130. In high luminance conditions peak N100 was dominant and the presence of N130 was indicated only by a 'notch' on the rising slope of the negativity. As luminance decreased the situation was reversed and N130 became a dominant feature of the negative wave. This finding did not depend on the particular choice of reference site. For checkerboard stimulation the same features were present, but variability of the VEP wave form was greater than in the case of dartboard stimulation. Present results relate the well-known pattern specific properties of the negativity in onset VEPs to N100 only, whereas N130 is not pattern specific. Lower and upper half-field stimulation produced peaks of opposite polarity at 100 msec but no change was observed in polarity of N130. These findings support the suggestion that these two parts of the negativity in pattern onset VEPs may have different cortical sources.     

The influence of pattern size on amplitude, latency and wave form of retinal and cortical potentials elicited by checkerboard pattern reversal and stimulus onset-offset.

Transient pattern electroretinograms (PERGs) and visual evoked potentials (VEPs) were recorded with checkerboard pattern reversal and equiluminance stimulus onset-offset, elicited by a high quality moving mirror stimulator. Different sized checkerboard patterns (0.35-4.2 c/deg) were used as stimulus patterns. The wave forms of the equiluminance stimulus onset responses were similar to ERGs evoked with luminance decrease and the stimulus offset PERGs were like ERGs elicited by luminance increase. The PERG c wave and the VEP showed spatial frequency tuning with pattern reversal and stimulus offset. Spatial frequency tuning was not detectable with PERG a and b waves. Pattern reversal and stimulus onset evoked PERGs had no major spectral components above 40 Hz; stimulus offset evoked PERGs contained components up to 55.3 Hz. Retino-cortical time--measured as a latency difference of the PERG b wave to VEP P100--was identical with pattern reversal and stimulus onset and about 12 msec longer with stimulus offset. Our results suggest that the 3 stimulation modes, reversal, onset and offset induce different types of processing at the retinal and cortical levels. PERG a and b waves to our high luminance/contrast stimuli contain no pattern specific information and the c waves are the sum of luminance and pattern specific responses.     

Transient visually evoked potentials to sinusoidal gratings in optic neuritis.

Transient visually evoked potentials (VEPs) to sinusoidal gratings over a range of spatial frequencies have been recorded in cases of optic neuritis. The use of the response to pattern onset in addition to the response to pattern reversal extended the range to higher spatial frequencies by up to two octaves. There was an increase in VEP delay and a greater degree of discrimination from a control group at higher spatial frequencies. This finding is discussed in the light of previous reports of luminance and checkerboard VEPs in demyelinating optic nerve disease. An attempt is made to relate amplitude changes in various VEP components to contrast sensitivity measurements in this group of patients.     

Effect of alcohol and task on hemispheric asymmetry of visually evoked potentials in man.

The study examined the effect of a dose of alcohol producing a mean blood alcohol content of 90 mg% on components of the scalp-recorded visually evoked potential (VEP) both with and without a visual discrimination task to control the level of attention, and the interaction of amplitudinal hemispheric asymmetry of the VEP with alcohol treatment and the discrimination task. Ingestion of ethyl alcohol producing a mean blood alcohol content of 90 mg% affected VEPs recorded from the central scalp by attenuating the overall amplitude of the later VEP components (60-200 msec) and by significantly reducing hemispheric asymmetry in the amplitude of these VEP components. Alcohol attenuates VEP components P90-N120 and N120-P180, and the task of counting flashes and attending to discriminate double flashes increased amplitude of VEP components N60-P90 and P90-N120 in control and placebo conditions. Several studies have reported that the VEP recorded from the right hemisphere of human beings is larger than the VEP recorded from the homologous location in the left hemisphere. Evoked potentials recorded under control and placebo conditions in this study also demonstrated a hemispheric asymmetry with right larger than left for component P90-N120. We also found a reliable alcohol by hemispheric asymmetry interaction. Alcohol selectively depressed the amplitude of the right hemisphere VEP (P90-N120) component to a significantly greater extent than the left hemisphere VEP was affected.     

Mapped distribution of pattern reversal VEPs to central field and lateral half-field stimuli of different spatial frequencies.

Visual evoked potentials (VEPs) to pattern reversal vertical bar stimuli of 3 different sizes (1, 2, 4 c/deg) were recorded from 19 scalp derivations in 50 controls. The stimuli were presented on a full-field (FF) screen of 24 degrees visual angle, and on left and right half-fields (HF) of 12 degrees radius. In 15 controls partial HF stimuli were presented on the central 3 and 6 degrees and as hemiannular stimuli of 12 degrees with occlusion of the central 3 and 6 degrees. An antero-posterior polarity reversal of the N1-P1-N2 sequence was observed for FF VEPs. A tangential polarity reversal was observed for HF VEPs. Also with central or hemiannular stimuli polarity reversals of all VEP components were observed within the scalp. Variants of VEP distribution, absence or prominence of some of the ipsi- or contralateral VEP components were observed in 8-40% of controls. The FF and HF VEP distribution, and the variant VEP asymmetries were partly dependent on the pattern spatial frequency.