Visual Evoked Cortical Potentials Research Articles

Long-term maturation of visual pathways.

Previous research in adults has demonstrated the utility of the visual evoked potential (VEP) to measure the integrity of the chromatic and achromatic visual pathways. The VEP has also been shown to be a valuable indicator of maturation of these pathways in infants up to 1 year of age. The present manuscript reports changes in the visual pathways from 2 years to adulthood as measured by the spatio-chromatic VEP. The responses to achromatic reversal stimuli designed to preferentially activate the low spatial-frequency achromatic (luminance) pathways appear adult-like by 1 year of age. The responses to low spatial-frequency isoluminant onset stimuli designed to preferentially activate the chromatic pathway do not appear as they do in the adult until after 12-13 years of age. The shapes of the chromatic VEP waveforms shift from a positive-negative complex to a negative-positive complex. These changes can be modeled by a decrease in the latency of a large negative component between the ages of 1 year and adulthood. The results suggest that for low spatial-frequency stimuli, there are long-term changes in the development of the chromatic pathways that are not observed in the low spatial-frequency achromatic pathways. The changes in the chromatic VEP waveforms with age may be a physiological correlate of reported behavioral changes.

The efficacy of methylprednisolone in aconite-induced myelo-optic neuropathy in the rabbit

Studies in animals indicate that aconitine or aconite has toxic effects on the visual system of a rabbit model. The toxic effects include myelo-optic neuropathy, as proven in the visual evoked cortical potentials (VECP) and histopathological studies. We investigated the effect of intravenous high-dose methylprednisolone (MP) on myelo-optic neuropathy caused by aconite. The group treated with MP (30 mg/kg, twice a day, for 3 days followed by 15 mg/kg for 3 days) in addition to aconite (1.5 ml/kg, equivalent to 0.7 mg/kg of aconitine) was compared with an aconite only-injected group and a normal control group. In the MP-treated group, increased recovery of onset latency, peak latency, and amplitude in VECP was recorded at two weeks (p<0.05) when comparied with the aconite only-injected group. In comparison with the normal control group, the MP-treated group showed a significant delay in onset latency at one and two months (p<0.05). The MP-treated group also showed a significant difference in peak latency at all observation periods when compared with the aconite only-injected group. However, the amplitude in both the MP-treated group and the aconite only-injected group increased at two months and did not show a significant difference when compared with the normal control group. Histopathological findings of the myelin sheath in the MP-treated group generally showed less severe damage than in the aconite only-injected group. The true benefits of high-dose MP were clear within two weeks. The authors conclude that treatment with intravenous high-dose MP immediately after aconite injection may have some beneficial effects on the aconite-induced myelo-optic neuropathy, although such treatment does not show a definite recovery.

The effect of spatial frequency on chromatic and achromatic steady-state visual evoked potentials

Objective: Little is known about the physiological properties of the major components of steady-state visual evoked potentials (VEPs). Based on the hypothesis that isoluminant color and high contrast pattern differentially activate the parvo- and magnocellular pathways, we studied difference in spatial frequency function between chromatic and achromatic VEPs to sinusoidal gratings.Methods: Steady-state VEPs to isoluminant chromatic (red-green) and high contrast (90%) achromatic (black-white) sinusoidal gratings with nine spatial frequencies (0.5 to 8.0 cycles/° (cpd)) at 4 Hz (8 reversals/s) were recorded in 13 normal subjects. VEPs were Fourier analyzed to obtain phase and amplitude of the second (2F) and fourth (4F) harmonic responses.Results: The 2F amplitude of chromatic VEPs decreased above 4.0 cpd in a low-pass function while that of achromatic VEPs showed a band-pass function with a peak at 4.0 cpd. The 4F amplitude of chromatic VEPs was not affected significantly by spatial frequency whereas that of achromatic VEPs exhibited a high-pass function. The phases of 2F and 4F showed a non-monotonic function of spatial frequency in both chromatic and achromatic stimuli with peaks at middle spatial frequencies.Conclusion: Chromatic and achromatic visual stimuli differently affected 2F and 4F components, which thus suggests that 2F and 4F components are generated from different neuronal subgroups largely in the parvocellular pathway.

Electrophysiological and histological measures of retinal injury in chronic ocular hypertensive monkeys.

This study was performed to determine whether components of the standard ERG (electroretinogram), multifocal ERG, and flash VECP (visually-evoked cortical potential) response might provide a sensitive measure of retinal ganglion cell injury in a monkey model for chronic ocular hypertension. Argon laser treatment of the aqueous outflow tissue was used to induce chronic elevation of intraocular pressure (IOP) in the right eye of 18 young adult cynomolgous monkeys. At 15 months post- IOP elevation, standard methods were used to record ERG and VECP responses. Multifocal ERG responses were also recorded at this time. Loss of retinal ganglion cells due to ocular hypertensive injury was determined by histological analysis of all retinas. Ocular hypertensive retinal injury was associated with a loss of retinal ganglion cells. There was no histological or electrophysiological evidence for injury to any other retinal cell type. Correlation of electrophysiological response amplitudes with histological measures of retinal ganglion cell loss/survival yielded results which suggest that activity in retinal ganglion cells makes a substantial contribution to components of the 30 Hz flicker ERG, the flash VECP, and both first and second order multifocal ERG responses. Of the electrophysiological measures used in this study, multifocal ERG response amplitude had the greatest sensitivity to retinal ganglion cell loss. Components of the multifocal ERG provide a sensitive measure of ganglion cell injury in a monkey model of chronic ocular hypertension. These same measures may have utility in the clinical diagnosis and management of glaucoma.

Macular pigment and the colour-specificity of visual evoked potentials

A modified Moreland anomaloscope was used to examine two subjects, one with dense macular pigment, the other with relatively light pigmentation. Chromatic visual evoked potentials (VEPs) were elicited from these two subjects using coarse, isoluminant gratings of different sizes. Colour-specificity was verified by comparing chromatic onset VEPs (reflecting sustained activity) with chromatic reversal VEPs (reflecting mainly transient activity) and also by Fourier analysis (colour-specific responses are dominated by the fundamental, transient activity by the second harmonic). Achromatic (transient-type) intrusions, produced by large blue-green gratings could be related to the extent of subject-specific macular pigmentation.

Visual acuity, contrast sensitivity and retinal magnification in a marsupial, the tammar wallaby (Macropus eugenii).

The visual acuity of the tammar wallaby was estimated using a behavioural discrimination task. The wallabies were trained to discriminate a high-contrast (86%) square-wave grating from a grey field of equal luminance (1000-6000 cd m-2). Visual-evoked cortical potentials were used to measure the complete contrast sensitivity function. The stimulus was a sinusoidal phase reversal of a sinusoidally modulated grating of various spatial frequencies and contrasts with a mean luminance of 40 cd m-2). The behavioural acuity was estimated to be about 4.8 cycles/deg. The contrast sensitivity peaked at about 0.15 cycles/deg and declined towards both lower and higher spatial frequencies. The cut-off frequency of the contrast sensitivity function is slightly lower than the behaviourally measured acuity at about 2.7 cycles/deg. The retinal magnification factor was estimated anatomically from laser lesions to be about 0.16 mm/deg. Based on the known ganglion cell density and the retinal magnification factor, an anatomical upper limit to visual acuity of about 6 cycles/deg can be calculated. The differences in estimates of visual acuity between the behavioural and anatomical methods on the one side and physiology on the other side are discussed.

Temporal analysis of the chromatic flash VEP--separate colour and luminance contrast components.

Temporal analysis of the chromatic flash visual evoked potential (VEP) was studied in human subjects with normal and anomalous colour vision using a deterministic pseudo-random binary stimulus (VERIS). Five experiments were carried out on four normal subjects investigating heterochromatic red-green exchange and single colour/achromatic (either red/grey or green/grey) exchange over a wide range of luminance ratios for the two stimuli, the effects of lowered mean luminance on the chromatic VEP and the effects of colour desaturation at constant mean luminance and constant luminance contrast. Finally, the performance of three dichromats, a protanope and two deuteranopes, on heterochromatic exchange VEP and on colour desaturation were investigated. In contrast to the chromatic electroretinogram, which shows great symmetry with respect to luminance ratio on opposite sides of the isoluminant point, the chromatic VEP demonstrated a distinct asymmetry when the colours exchanged included red. On the red side of isoluminance (red more luminant than green), a wave with longer latency and altered waveform became dominant. The effects of green stimulation were indistinguishable from those of achromatic stimulation at the same luminance contrast over the whole range of chromatic contrast and for all levels of desaturation studied. Desaturation of red with constant luminance contrast (desaturated red/grey stimulation) resulted in a systematic alteration in the evoked waveform. Subtraction of the achromatic first- and second-order responses from responses recorded in the red desaturation series resulted in remarkably uniform waveforms, with peak amplitudes growing linearly with saturation. The absence of interaction between achromatic and coloured components for all (including the most intense colour) stimulus parameters used suggests that the generators of these components are separate. Recordings from the dichromats showed that the contrast response minimum shifted from the point of photopic isoluminance to the point of zero cone contrast (at the silent substitution point) for the remaining cone type. The waveforms recorded with a series of luminance ratios were much simpler than those recorded from trichromats and symmetrical with respect to their isoluminant points. Despite the indication of the presence of L cones of apparently normal spectral sensitivity in the deuteranopes (on the basis of flicker photometry), there was no evidence for a red-sensitive component in the desaturation or heterochromatic stimulation series. The results are discussed in terms of the possibility of separate generation of chromatic and achromatic contributions to the VEP.

Cortical and thalamic visual evoked potentials during sleep-wake states and spike-wave discharges in the rat

Flash visual evoked potentials (VEP) were simultaneously recorded from the primary visual cortex and the dorsal lateral geniculate nucleus in freely-moving WAG/Rij rats, to investigate whether the thalamic VEP shows the same state-dependent alterations as the cortical VEP. VEPs obtained during active and passive wakefulness (AW and PW), slow-wave sleep (SWS), REM sleep and during the occurrence of spike-wave discharges (SWD), a specific trait of the genetically epileptic WAG/Rij rat, were compared. The general architecture of the thalamic VEP resembles the cortical VEP, although its polarity is reversed. This facilitated the interpretation of components in terms of underlying neuronal events. The primary excitation peak is differently modulated in cortex and thalamus. Whereas the thalamic component (P30) is not affected by brain-state, the cortical component (N1) shows a strong increase in latency during SWS and SWD. In contrast, the modulation of later components is highly similar for cortex and thalamus. VEPs obtained during AW and REM resemble each other. During SWS and SWD there is a considerable, and during PW a moderate, enlargement of primarily inhibitory components. After-discharges are enhanced during SWS, SWD and REM. No evidence is found for a major impairment of sensory transmission during SWD.

Temporal response characteristics of the spatiochromatic visual evoked potential: nonlinearities and departures from psychophysics.

Although the visual evoked potential (VEP) for isoluminant stimuli has been characterized in terms of spatiochromatic parameters, temporal tuning along various chromatic directions has received less systematic attention. Additionally, there has been little categorical comparison of psychophysical appearance with VEP responses obtained for temporal variation of these patterns. At appropriate contrasts the VEP's for color axes (LM, S) show a robust and contrast-sensitive temporal tuning peak at 4 Hz. Contrast response functions at 4 Hz for the LM color axis are markedly nonmonotonic. However, there is a clear monotonicity with contrast for VEP latencies along these color axes. The anomalous behavior does not appear to be due to interactions between chromatic signals, to luminance artifact, or to rod intrusion. These anomalies in the temporal characteristics of the chromatic VEP may reflect interactions between chromatic responses and inherent cortical responsivity not linked to psychophysical behavior.

Regenerating ganglion cell axons in the adult rat establish retinofugal topography and restore visual function

The mechanisms of neuronal network response to axotomy are poorly understood. In one of the favoured models used to study the fate of injured neurons in the adult rat visual system, appreciable numbers of retinal neurons survive optic nerve injury under conditions of microglia-targeted neuroprotection. Rescued neurons can regenerate their axons and become target-dependently stabilised after reconnection with their natural visual centres by means of a peripheral nerve graft, which, in addition to guidance, actively supports axonal growth. The mechanisms that control regenerative axonal growth and resynaptogenesis include coordinated cell-cell interactions between growing neurites and target cells in order to establish a meaningful reconnectivity. Here the function of the regenerating visual circuitry was first studied by monitoring the ability of animals to discriminate spatial patterns, and second by recording visual evoked cortical potentials (VEPs) in the same animals. These functions were correlated with neuroanatomical studies of the retinotopic organisation of regenerating axons. To achieve these goals, adult rats were behaviourally trained in a Y-maze to discriminate between vertical and horizontal stripes. Both optic nerves were transected, and the regenerating axons of one optic nerve were guided into the area of optic tract with a peripheral nerve graft according to the protocols of neuroprotection and simultaneous grafting, in order to enable large numbers of axons to reinnervate the major visual targets in the midbrain and thalamus. Postoperative testing of the animals showed a marked improvement of visual perception and behaviour. The VEPs of the same animals were measurable indicating a restoration of the visual circuitry including the ascending corticopedal connections. Neuroanatomical assessment of the fibre topography within the graft and the area of termination revealed a rough topographic organisation that may account for restoration of the function. These results suggest that interrupted central pathways can be functionally reconnected by providing a neuroprotective environment in combination with peripheral nerve grafts to bypass lesions.

The study of visual evoked potentials in patients with thyroid-associated ophthalmopathy identifies asymptomatic optic nerve involvement.

In the present study we have recorded visual evoked cortical potentials (VECP) in 88 patients affected by autoimmune thyroid disease and thyroid-associated ophthalmopathy (TAO) without clinical signs of optic neuropathy. At the time of ophthalmological examination, 37 of these patients were hyperthyroid, 41 were euthyroid, and 8 were hypothyroid; 2 were not assessed. Twenty-nine normal subjects served as controls. We performed pattern reversal visual stimulation and recorded the amplitude and latency of the cortical electric response at 100 ms (P100 wave). There were no differences in the mean P100 amplitude of TAO patients and normal subjects. The mean P100 latency in patients was 105.6 +/- 0.5 ms, significantly higher than that in normal subjects (102.0 +/- 0.5 ms; P < 0.00003). Latency in euthyroid patients did not differ from that in either hypo- or hyperthyroid patients. The VECP test was positive (latency, > or = 110.0 ms) in 21 (23.8%) TAO patients. In patients with proptosis greater than 21 mm, latency was 106.7 +/- 0.7 ms, significantly higher than that in patients with normal Hertel measurements (104.3 +/- 0.6 ms; P < 0.01). Latency was not increased in patients with acute inflammatory signs compared to those with inactive eye disease and in patients with altered extrinsic motility. In patients with an abnormal visual field study, the mean latency was 110.3 +/- 1.5 ms, significantly higher than that in patients with a normal visual field (104.7 +/- 0.4; by t test, P < 0.000003). In conclusion, we observed a prolongation of the latency of the evoked cortical response in patients with TAO without subjective visual complaints and without optic nerve compression. We believe that the study of VECP in TAO is complementary to the study of the visual field in identifying early optic nerve dysfunction in the absence of decreased visual acuity.

Chromatic and achromatic visual evoked potentials in Parkinson's disease

Chromatic and achromatic visual evoked potentials (VEP) were evaluated in 39 patients with idiopathic Parkinson's disease (PD) (age 64.0 ± 8.6 years) and 43 healthy controls (age 62.8 ± 8.7 years). The following pattern-reversal checkerboard stimuli were performed: (1) achromatic with luminance contrast 86% (achr.hk.); (2) achromatic with luminance contrast 20% (achr.lk.); (3) chromatic isoluminant blue-yellow (by.); (4) chromatic isoluminant red-green (rg.). The mean latencies N70, P100, and N135 of chromatic and achromatic VEP were significantly delayed in patients with PD as compared to controls. The highest rate (41.0%) of pathological findings could be demonstrated by achromatic stimulation (luminance contrast 86%). Isolated abnormalities of chromatic VEP (in combination with normal achromatic VEP) were found in 5 (12.8%) patients. The delay of VEP-latencies was significantly correlated with the severity of motor symptoms in PD patients. We conclude that VEP are valuable tools to demonstrate a dysfunction of the visual system in PD. Although chromatic VEP are less sensitive than achromatic VEP, the combination of both will increase the diagnostic yield. Therefore, there seems to exist a variety of individual characters of visual impairment in PD.

Chromatic and achromatic visual evoked potentials in Parkinson's disease

Chromatic and achromatic visual evoked potentials (VEP) were evaluated in 39 patients with idiopathic Parkinson's disease (PD) (age 64.0 ± 8.6 years) and 43 healthy controls (age 62.8 ± 8.7 years). The following pattern-reversal checkerboard stimuli were performed: (1) achromatic with luminance contrast 86% (achr.hk.); (2) achromatic with luminance contrast 20% (achr.lk.); (3) chromatic isoluminant blue-yellow (by.); (4) chromatic isoluminant red-green (rg.). The mean latencies N70, P100, and N135 of chromatic and achromatic VEP were significantly delayed in patients with PD as compared to controls. The highest rate (41.0%) of pathological findings could be demonstrated by achromatic stimulation (luminance contrast 86%). Isolated abnormalities of chromatic VEP (in combination with normal achromatic VEP) were found in 5 (12.8%) patients. The delay of VEP-latencies was significantly correlated with the severity of motor symptoms in PD patients. We conclude that VEP are valuable tools to demonstrate a dysfunction of the visual system in PD. Although chromatic VEP are less sensitive than achromatic VEP, the combination of both will increase the diagnostic yield. Therefore, there seems to exist a variety of individual characters of visual impairment in PD.

The development of the chromatic visual evoked potential

Evidence exists to suggest that visual function of very young infants is not cortically controlled and may in fact be mediated by a more primitive subcortical system. Visual evoked potentials (VEP) are used widely as a clinical and research tool and in this study the development of the chromatic VEP was investigated. Chromatic vision is a cortical function, hence the presence of a chromatic VEP may suggest the presence of a functioning cortical pathway.Using pattern reversal stimuli of red and green checks, the presence and morphology of the transient chromatic VEP were assessed. The checkerboards reversed in chromaticity and control of the relative brightness (luminance) of the checks allowed the production of a range of stimuli of different red/green luminance ratios. A black/white stimulus was included in all trails for comparison. VEPs to these stimuli were recorded from 36 infants aged between 1 and 14 weeks post‐term. Fourteen of these infants were then assessed on two further occasions to monitor the longitudinal development of the chromatic VEP. All infants tested demonstrated clear VEPs to achromatic stimuli and to those red/green stimuli that contained luminance information. As the luminance information within the stimuli decreased, the morphology of the wave changed with the major component (P1) becoming diminished and delayed. No VEP could be recorded from any infant less than 7 weeks post‐term at time of testing when using an isoluminant stimulus. In the longitudinal study, all of the infants, who initially demonstrated no response to isoluminant stimuli, did so on reaching their eighth post‐term week.These results would suggest that those channels responsible for the processing of chromatic information do not function until the eighth post‐term week. During the period prior to this, visual processing takes place via a colour‐insensitive pathway, probably subcortical.

Objective detection of hemifield and quadrantic field defects by visual evoked cortical potentials.

An objective method for detecting hemifield and quadrantic visual field defects has been developed using steady state visual evoked cortical potentials (VECPs), an adaptive noise canceller (ANC), and Hotelling's t2 statistic. The purpose of this study was to determine the sensitivity and specificity of the technique. Nine subjects (mean age 44 years) were investigated with field loss due to a variety of causes including both anterior and posterior visual pathway lesions. Dynamic perimetry was performed by means of a Goldmann or Tübingen perimeter. VECP recordings were made from each visual field quadrant (23 degrees X 23 degrees) by means of a steady state reversing checkerboard (7.7 rev/s). The central 5 degrees of the visual field and the vertical and horizontal meridians were masked during these measurements. Recordings were made from three electrode sites, positioned over the visual cortex, relative to a mid frontal electrode. Each recording lasted 2 minutes, during which time fixation was monitored. The data from each recording were divided into 4 second segments, and the amplitude and phase of the VECP signal measured using the ANC. Hotelling's t2 statistic was applied to determine the probability of signal detection. Receiver operating characteristic curves were used to find the optimum signal detection threshold for identification of the visual field defects. The results of the study confirmed patterns of subjective visual field loss. The technique had a sensitivity and a specificity of 81% and 85%, respectively, for detecting 'non-seeing' areas in the inferior visual field, and 82% and 89%, respectively, for detecting 'non-seeing' areas in the superior visual field. These results demonstrate that the technique is of potential clinical value to ophthalmologists and neurologists when subjective perimetry is not possible.

Intraocular motility, electrophysiological tests and visual fields in drug addicts

The effects of drugs on intraocular motility, retina function and central pathways in a drug addicts population have been studied. The study group were made up of 15 drug addicts. Fifteen healthy subjects who had never used drugs acted as a control group. Refractive state, accommodation, tonometry, assessment of pupillary reflexes, campimetry, visual evoked cortical potential (VECP) and ERG were performed in all cases. All the subjects studied used heroine, 12 used cannabis and 12 used cocaine. Refraction was normal in all cases. Accommodation amplitude in normal subjects ranged from 8.5 to 11.5 D, and from 3.5 to 9.5 D in the drug addicts group. Tonometry indicated a tendency to lower intraocular pressure values. Direct and consensual pupillary reflexes were normal in eight subjects, but were sluggish in the rest. Campimetry showed relative and absolute scotoma in five subjects. VECP were found to be normal in two of the cases, moderately pathological in six and pathological in the remaining seven cases. As for the ERG, it showed non-specific altered traces in 11 cases. Accommodation amplitude was lower than normal in 20 eyes. The decrease of VECP with altered traces but with an increase of latency, seem to indicate the presence of an alteration in the optical pathway or in the cortical centres. The findings in ERG point out a decrease of amplitudes and an increase of latency in more than half the population studied.

Intraocular motility, electrophysiological tests and visual fields in drug addicts

SummaryThe effects of drugs on intraocular motility, retina function and central pathways in a drug addicts population have been studied. The study group were made up of 15 drug addicts. Fifteen healthy subjects who had never used drugs acted as a control group. Refractive state, accommodation, tonometry, assessment of pupillary reflexes, campimetry, visual evoked cortical potential (VECP) and ERG were performed in all cases. All the subjects studied used heroine, 12 used cannabis and 12 used cocaine. Refraction was normal in all cases. Accommodation amplitude in normal subjects ranged from 8.5 to 11.5 D, and from 3.5 to 9.5 D in the drug addicts group. Tonometry indicated a tendency to lower intraocular pressure values. Direct and consensual pupillary reflexes were normal in eight subjects, but were sluggish in the rest. Campimetry showed relative and absolute scotoma in five subjects. VECP were found to be normal in two of the cases, moderately pathological in six and pathological in the remaining seven cases. As for the ERG, it showed non‐specific altered traces in 11 cases. Accommodation amplitude was lower than normal in 20 eyes. The decrease of VECP with altered traces but with an increase of latency, seem to indicate the presence of an alteration in the optical pathway or in the cortical centres. The findings in ERG point out a decrease of amplitudes and an increase of latency in more than half the population studied.

The VEP thresholds for full-field stimuli in dark-adapted infants.

Dark-adapted adults' electroretinographic b-wave thresholds are approximately 2 log units below a-wave thresholds and approximately 3 log units above the perceptual threshold, and their perceptual and the visually evoked cortical potential (VEP) thresholds are similar. Dark-adapted infants' scotopic a- and b-wave thresholds for full-field stimuli are both about 0.5 log units above those of adults, but their scotopic VEP thresholds for such stimuli have not been studied. We obtained scotopic VEP thresholds for brief, full-field stimuli from dark-adapted, infants and adults to consider the relationships of the cortical responses to the responses of more distal structures, namely the rod photoreceptors (scotopic a-wave) and ON-bipolars (scotopic b-wave). The median VEP threshold of infants is 0.5 log unit above that of adults. Thus, the relationships of a- and b-wave and VEP thresholds in infants are similar to those in adults. These results are consistent with rod cell immaturities being the primary determinant of the difference between infants' and adults' thresholds.

Simultane Ableitung von VECP und Muster-ERG bei künstlicher Erhöhung des Augeninnendrucks

The visual evoked cortical potentials (VECP) are changed by the artificial increase in intraocular pressure (IOP). The exact location of the damage is still unclear. One possibility is that the increased ocular pressure reduces the blood flow in the optic nerve head. The function of the optic nerve fiber function would be decreased by this effect. Another possibility is that the generation of the signals in the retinal ganglion cells could be influenced by the artificial pressure enhancement. We developed a method of simultaneous recording pattern reversal electroretinograms (PERG) and VECP during artificially raised IOP. The PERG was recorded by cutaneous electrodes (Grass, USA; diameter 5 mm), which were positioned close to the lid margins. The pattern reversal rate was 7.9 reversals/s. In a pilot study, we examined 10 healthy volunteers with artificially increased ocular pressure. In 9 cases, amplitudes showed a more stable behavior in the VECP than in the PERG: the mathematical behavior of the amplitude/pressure curves was more stable in the VECP of 6 volunteers and once in the PERG. Signs of very sufficient autoregulation could be found in the VECP of 7 volunteers and once in the PERG. The critical pressure, at which a further increase in IOP causes a continuous amplitude decrease to the noise level, was 9 times more stable in VECP (median 53 mmHg) than in PERG (median 48 mmHg). The results lead to the conclusion that in healthy persons the artificial pressure rise influences the ganglion cells at an earlier timepoint than the signal transmission in the axons.

The Behavior of Visual Evoked Cortical Potentials during and after an Artificial Increase in Intraocular pressure

Previously, it has been shown that in healthy persons the function of the eye assessed by visual evoked cortical potential (VECP) amplitudes changes in a characteristic way during stepwise artificial elevation of intraocular pressure (IOP). It has been demonstrated that a stepwise increase in IOP leads to a decrease in the amplitude followed by an increase or a stabilization of the function during a further rise of IOP. At pressure values of 62 mmHg, the amplitude falls to the noise level. In order to examine whether this behaviour can also be observed if the procedure is reversed, we initially increased the IOP to values, at which the VECP amplitudes could not be distinguished from noise (standstill pressure). During a subsequent stepwise decrease in the artificially increased IOP, we investigated 5 persons. The function did not recover before the IOP had reached values below 46 mmHg. Thus, at the same IOP value, the VECP can assume different amplitude values depending on wether the pressure rises or falls. In 24 further persons, the artificial IOP increase was decreased directly to the starting values. The function recovered immediately. Factors influencing the eye shape (e.g. astigmatism), as ascribed at artificially increased IOP, cannot explain the different behaviour of VECP amplitudes in the experiments done here, because mechanical changes are the same in IOP increase and its reverse. But the behaviour observed here is not contradictory and can be explained by the known characteristics of microcirculation.(ABSTRACT TRUNCATED AT 250 WORDS)