Effects of luminance on the pattern visual evoked potential in multiple sclerosis

VEP examination with new portable device

INTRODUCTION

We developed a new portable device called "VEPpeak" for the examination of visual evoked potentials (VEPs) to extend VEP examination beyond specialized electrophysiological laboratories and to simplify the use of this objective, noninvasive, and low-cost method for diagnostics of visual and central nervous system dysfunctions.

METHODS

VEPpeak consists of a plastic headset with a total weight of 390 g containing four EEG amplifiers, an A/D converter, a control unit, and a visual LED stimulator built in the front, vertically adjustable peak. The device is powered and controlled via USB connection from a standard PC/notebook using custom software for visual stimuli generation and for VEP recording and processing. Up to four electrodes can be placed at any scalp location or in combination with two dry electrodes incorporated into the headset. External visual stimulators, such as a tablet, can be used with synchronization. Feasibility and validation studies were conducted with 86 healthy subjects and 76 neuro-ophthalmological patients including 67 who were during the same session also tested with a conventional VEP system.

RESULTS

VEPpeak recordings to standard (pattern-reversal) and non-standard (motion-onset, red-green alternation) were robust and repeatable and obtained also in immobilized patients. Good comparability of results was achieved between VEPpeak and standard examination. Some systematic differences in peak latencies and amplitudes are consistent with differences in stimulus characteristics of the two compared systems.

DISCUSSION

VEPpeak provides an inexpensive system for clinical use requiring portability. In addition to ISCEV standard VEP protocols, free choice of stimuli and bio-signal recordings make the device universal for many electrophysiological purposes.

Childhood maltreatment impacts the early stage of facial emotion processing in young adults with negative schizotypy

Childhood maltreatment (CM) is a factor of risk for psychosis and is associated with alterations in facial emotion processing.Negative symptoms of schizophrenia-spectrum disorders are associated with deficits in facial emotion processing, but research findings on schizotypy are mixed. This study examined the early stage of facial emotion processing in young adults with high levels of negative schizotypy (NS) and explored the impact of childhood maltreatment. On the basis of the Social and Physical Anhedonia subscales of the Chapman Psychosis-Proneness Scales, a total of 74 high-NS and 52 low-NS individuals were recruited to complete the Childhood Trauma Questionnaire and the dot-probe task. The P100 and N170 components of event-related potentials were measured to assess the processing of four facial expressions of emotion. The high-NS group showed significantly reduced P100 amplitudes for all facial expressions. Angry and fearful expressions elicited larger N170 amplitudes than disgusted and happy expressions. Happy expressions elicited shorter N170 latencies than disgusted expressions. Compared to the high-NS group without CM, the high-NS group with CM had a longer latency of P100. Individuals with high NS, compared to individuals with low NS, have impaired fundamental visual processing, but intact processing of facial figurations. Childhood maltreatment may be a factor responsible for the pathological state of the visual pathway in high NS group.

Electrophysiological testing of visual function after mirror telescope implantation: a case report

PURPOSE

The implantation of an intraocular telescope increases life quality in patients with end-stage age-related macular degeneration (AMD). The present study monitored changes in electrophysiological markers of visual processing before and during seventeen months after a novel mirror telescope implantation in two patients (OV-male 90 years, MZ-female 70 years) with the final-stage form of AMD.

METHODS

Visual evoked potentials were recorded to high-contrast pattern-reversal (PR-VEP for check size 40' and 10'), low-contrast motion-onset stimuli (in visual periphery M-VEP M20°, and in central part M-VEP C8°), and event-related potentials (ERPs) in the oddball visual paradigm.

RESULTS

MZ's more systematic responses showed attenuation and prolongation of the M-VEP M20° and the PR-VEP 40' immediately after the telescope implantation with a slow amplitude recovery with unchanged prolonged latency. The implantation completely eradicated the M-VEP C8° without any restoration. The PR-VEP 10' were not readable. Only a part of OV's PR-VEP 40' and M-VEP M20' were of a repeatable and expected morphology. These OV's VEPs were consistent with MZ's findings. The ERPs did not show any effect of implantation in both patients. Post-implantation visual acuity and reaction time overcame the pre-implantation levels.

CONCLUSIONS

The mirror telescope preserved peripheral vision in contrast to classic telescopes; however, the telescope concurrently reduced the luminance of the magnified retinal image, which was likely responsible for the prolongation of the VEP latencies.

Attractiveness and affordance shape tools neural coding: insight from ERPs

The relation between attractiveness and motor affordance is a key topic in design and has not yet been investigated electrophysiologically. In this respect, action affordance and attractiveness represent two crucial dimensions in object processing (specifically for tools). In light of this evidence, Event Related Potentials (ERPs) enabled us to gain new insights into the time course of the interaction between these two dimensions during an explicit tool evaluation task. Behaviorally, tools that were judged as high affording and high attractive yielded faster response times than those judged as low affording and low attractive. The ERP results showed that early processes related to sensory gating and feature extraction (N100) were sensitive to both affordance and attractiveness; the P200 was dominated by affordance, indexing a facilitated access to motor action representation. The N300, P300 and the Late Positive Potential (LPP) showed enhanced responses for highly affording/attractive tools, reflecting the interconnection between attractiveness and affordance. Later responses were entirely affected by attractiveness, suggesting additional affective responses evoked by desirable tools. We are showing that things that are perceived as more functional and attractive have a privileged neural activation in the time course of tool evaluation, for the first time.

Early ERPs to faces: aging, luminance, and individual differences

Recently, Rousselet et al. reported a 1 ms/year delay in visual processing speed in a sample of healthy aged 62 subjects (Frontiers in Psychology 2010, 1:19). Here, we replicate this finding in an independent sample of 59 subjects and investigate the contribution of optical factors (pupil size and luminance) to the age-related slowdown and to individual differences in visual processing speed. We conducted two experiments. In experiment 1 we recorded EEG from subjects aged 18–79. Subjects viewed images of faces and phase scrambled noise textures under nine luminance conditions, ranging from 0.59 to 60.8 cd/m². We manipulated luminance using neutral density filters. In experiment 2, 10 young subjects (age < 35) viewed similar stimuli through pinholes ranging from 1 to 5 mm. In both experiments, subjects were tested twice. We found a 1 ms/year slowdown in visual processing that was independent of luminance. Aging effects became visible around 125 ms post-stimulus and did not affect the onsets of the face-texture ERP differences. Furthermore, luminance modulated the entire ERP time-course from 60 to 500 ms. Luminance effects peaked in the N170 time window and were independent of age. Importantly, senile miosis and individual differences in pupil size did not account for aging differences and inter-subject variability in processing speed. The pinhole manipulation also failed to match the ERPs of old subjects to those of young subjects. Overall, our results strongly suggest that early ERPs to faces (<200 ms) are delayed by aging and that these delays are of cortical, rather than optical origin. Our results also demonstrate that even late ERPs to faces are modulated by low-level factors.

Healthy aging delays scalp EEG sensitivity to noise in a face discrimination task

We used a single-trial ERP approach to quantify age-related changes in the time-course of noise sensitivity. A total of 62 healthy adults, aged between 19 and 98, performed a non-speeded discrimination task between two faces. Stimulus information was controlled by parametrically manipulating the phase spectrum of these faces. Behavioral 75% correct thresholds increased with age. This result may be explained by lower signal-to-noise ratios in older brains. ERP from each subject were entered into a single-trial general linear regression model to identify variations in neural activity statistically associated with changes in image structure. The fit of the model, indexed by R², was computed at multiple post-stimulus time points. The time-course of the R² function showed significantly delayed noise sensitivity in older observers. This age effect is reliable, as demonstrated by test–retest in 24 subjects, and started about 120 ms after stimulus onset. Our analyses suggest also a qualitative change from a young to an older pattern of brain activity at around 47 ± 4 years old.

Evoked potential (EP) alterations in experimental allergic encephalomyelitis (EAE): early delays and latency reductions without plaques

Experimental allergic encephalomyelitis (EAE) in its chronic relapsing (CR-EAE), chronic progressive (CP-EAE) and acute (A-EAE) forms was obtained in 24 juvenile strain 13 guinea pigs. Visual, brainstem acoustic and somatosensory evoked potentials (EPs) were recorded in these animals prior to the sensitizing injection and during the course of the disease. Delays in the EPs appeared 15 days post-sensitization (dps), preceding or simultaneously with clinical alterations: electron microscopy revealed myelin stripping and vacuolation in the animals sacrificed 25 dps. Decreases in EP latency were recorded 32 dps; when electron microscopy revealed myelin layers indicating remyelination, whereas light microscopy showed only inflammatory changes. When confluent plaques were revealed by light microscopy 120 dps, the EP wave shapes were distorted or absent. The discussion reviews the literature on early myelin and conduction changes during central demyelination.

Evaluation of vestibular and visual oculomotor function

The visual system interacts synergistically with the vestibular system. A normally functioning vestibulo-ocular reflex is necessary but not sufficient for optimum visual acuity during head motion. Studies of dynamic visual acuity, the acuity achieved during relative motion of visual targets or of the observer, indicate that motion of images on the retina markedly compromises vision. The vestibulo-ocular reflex normally provides a substantial measure of stabilization of the retina during head movements, but purely vestibular compensatory eye movements are not sufficiently precise for optimal vision under all circumstances. Other mechanisms, including visual tracking, motor preprogramming, prediction, and mental set, interact synergistically to optimize the gain (eye velocity divided by head velocity) of compensatory head movements. All of these mechanisms are limited in their capacity to produce effective visual-vestibular interaction at higher rotational frequencies and velocities. It is under these conditions that vestibular deficits give rise to symptoms of oscillopsia. Patients having vestibular lesions exploit mechanisms of visual-vestibular interaction to compensate by substitution for deficient vestibular function. Thus, for accurate topographic clinical diagnosis of vestibular lesions, testing conditions should isolate purely vestibular responses. This may be done by testing reflex eye movements during passively generated rotations in darkness, or perhaps by testing during other types of motion under conditions of extreme frequency and velocity sufficient to attenuate the effects of visual-vestibular interaction. This article reviews clinical tests of vestibular function in relation to synergistic interactions with vision.

Comparison of visual evoked potentials elicited by light-emitting diodes and TV monitor stimulation in patients with multiple sclerosis and potentially related conditions

Visual evoked potentials elicited by reversal of a checkerboard pattern constructed of square, red light-emitting diodes (LEDs) were compared with a conventional black and white pattern displayed on a TV monitor in control subjects and in 71 patients with established or suspected multiple sclerosis. Both stimuli elicited distinct responses in the control groups: the latencies were longer with LED stimulation while the amplitudes of the various components were differently altered. The frequency of abnormal responses among the patients was higher with LED stimulation than with TV stimulation, but the highest diagnostic yield was obtained when both methods were combined.

The pattern visual evoked potential. A multicenter study using standardized techniques

The peak latency of the pattern-reversal visual evoked potential is a sensitive measure of conduction delay in the optic nerve caused by demyelination. Despite its clinical utility, the pattern-reversal visual evoked potential has not previously been used in multicenter clinical trials, presumably because of difficulty in standardizing conditions between centers. To establish whether the pattern-reversal visual evoked potential could be adequately standardized for use as a measure in multicenter therapeutic trials for optic neuropathy or multiple sclerosis, stimulus and recording variables were equated at four centers and pattern-reversal visual evoked potentials were recorded from 64 normal subjects and 15 patients with resolved optic neuritis. Results showed equivalent latency and amplitude data from all centers, suggesting that stimulus and recording variables can be satisfactorily standardized for multicenter clinical trials. N70 and P100 peak latencies and N70-P100 interocular amplitude difference were sensitive measures of resolved optic neuritis.

Visual evoked potentials in multiple sclerosis: frequency response shows reduced alpha amplitude

Visual evoked potentials were measured in a group of 16 multiple sclerosis (MS) patients and in a control group of 20 subjects. With respect to vertex and occipital recordings, latencies of main peaks were prolonged and response amplitudes were reduced in the MS group. As a result of frequency domain analysis we found that the amplitude reduction was not uniform in all frequency ranges: alpha (7-12 (Hz) components of EPs were markedly reduced whereas theta (4-7 Hz) responses were not altered. It is remarkable that the frequency components were altered to a different degree--this may shed some light on the physiological roles of the frequency components: As MS is frequently associated with optic neuritis, our interpretation of this frequency-dependent pattern is based on regarding MS as a model of impaired sensory input to the brain: the fact that in this situation alpha responses are markedly reduced hints at a link between alpha responses and primary sensory processing. Theta responses turned out to be unaltered--i.e., less dependent on sensory inputs--and might thus reflect associative sensory processing. This conclusion for functional roles of EP frequency components has also been drawn from previous investigations of topographic differences of EP frequency components.

Auditory evoked potentials in multiple sclerosis: alpha responses are reduced in amplitude, but theta responses are not altered

Auditory evoked potentials (EPs) were measured in a group of 16 multiple sclerosis (MS) patients and in a control group of 20 subjects. In vertex recordings, response amplitudes were reduced in the MS group. Remarkably, EP frequency components computed from the averaged EPs showed different degrees of amplitude reduction in different frequency channels: alpha (7-12 Hz) components were reduced whereas theta (4-7 Hz) responses were not altered. Our interpretation takes into account results of our companion paper (Başar-Eroglu et al., this issue) on similar results in the visual modality and is based on considering MS as a disease with disturbed sensory input to the brain. The fact that in this disorder alpha responses are reduced while theta responses are not altered can be interpreted as follows: alpha components might be mainly dependent on sensory input and thus reflect primary sensory processing. Theta responses, being unaltered in MS, might mainly reflect associative processing. The results are in accordance with conclusions drawn from investigations of topographic differences of evoked electric and magnetic brain responses.

Colored focal visual evoked potentials by cathode ray tube versus scanning laser ophthalmoscope

We compared the focal visual evoked potentials obtained in 52 young subjects with normal vision, evoked by means of three alternating black/color checkerboards generated by a trichromic cathode ray tube (dominant wavelength, 514 nm; colorimetric purity, 0.45) and by means of a scanning laser ophthalmoscope (argon laser beam, 514 nm; colorimetric purity, approximately 1). These three checkerboards, with an area of 3.5 degrees x 3.5 degrees (stimulating the fovea), then with an area of 3.5 degrees x 3.5 degrees with a central exclusion of 1.5 degrees x 1.5 degrees (stimulating the perifoveola) and finally with an area of 1.5 degrees x 1.5 degrees (stimulating the foveola) were presented within a field (8 degrees x 8 degrees) of homogeneous luminance of 170 cd/m2 and 1500 cd/m2, respectively. Their check sizes were 30', with a reversal temporal frequency of 0.75 Hz. The transient focal visual evoked potentials recorded with these three stimuli generated by the two types of stimulators were clearly detected for at least 85% of subjects. Their characteristics (waveform, amplitude and culmination times of the different waves) were comparable, regardless of the stimulator used (cathode ray tube or scanning laser ophthalmoscope). These results suggest that, under these various conditions of luminance and colorimetric purity, the neurophysiologic circuits tested function in identical ways. The focal visual evoked potential signs, now clearly defined by means of stimuli generated by cathode ray tubes, therefore apparently can be applied to the focal visual evoked potential evoked by stimuli generated by the scanning laser ophthalmoscope.

Age-related changes in pattern visual evoked potentials: differential effects of luminance, contrast and check size

We recorded visual evoked potentials (VEPs) to checkerboard pattern-reversal stimulation in 109 normal subjects (51 males and 59 females; aged 19-84 years) in order to study the aging effect on the multiple channels of the visual system in humans. Transient VEPs to 3 check sizes (15', 30' and 50') were obtained by monocular stimulation. Two test conditions were employed: (1) a high luminance (180 cd/m2) and a low luminance (11 cd/m2) both with a fixed contrast (90%), and (2) a high contrast (85%) and a low contrast (10%) both at a fixed luminance (57 cd/m2). The major features of our results included: (1) the presence of a curvilinear relationship between P100 latency and age for all conditions, while the P100 amplitude did not show any such aging effect, (2) the age-latency function was similar between the two luminance conditions, while it was different between the two contrast conditions, and (3) the differential age effect on the P100 latency caused by changes in contrast depended on the check size. These results suggest that age-related changes in the human visual system are not uniform, but rather are different in the specific functional subdivisions. It is thus hypothesized that aging may differentially influence the separate channels of the human visual system.

Pattern visual evoked potential luminance and multiple sclerosis

Pattern visual evoked potentials (PVEPs) were recorded from 111 patients classified as having possible, probable or definite multiple sclerosis. Patients were stimulated with a checkerboard pattern using high and low luminances in order to test the hypothesis that an attenuated pattern luminance increases the detection rate of PVEP abnormalities. With increasing certainty of diagnosis, there was a concomitant increase in the incidence of PVEP abnormalities. However, there was no evidence that stimulating with a lower luminance pattern enhanced the sensitivity of the test. The same findings were also apparent when the patient data was analyzed according to the presence or absence of a history of optic neuritis or other visual symptoms. It is concluded that, within the luminance limits used in this study, the role of varied luminance in detecting demyelinating lesions in the optic nerves using the PVEP is minimal, although there was some limited evidence that a high level of luminance may be more appropriate than a low level.

Comparative effects of luminance and scatter on the pattern visual evoked potential and eye-hand reaction time

We investigated the effect of reduced luminance and increased scatter on the pattern visual evoked potential and eye-hand reaction time evoked to a check size of 0.5 degrees in 10 normal subjects. Data analysis indicated that a reduction in luminance as well as an increase in scatter caused a statistically significant increase in the peak time of the pattern visual evoked potential P100 wave. The reaction time, however, was not significantly affected by the initial 0.9-log unit attenuation of the stimulus luminance or the 0.3 scatter filter. Further attenuation of luminance or increase of scatter also yielded statistically significant increases. Our results suggest that the reaction time is less affected by a reduction in luminance or an increase in scatter of a 0.5 degrees stimulus than the pattern visual evoked potential is and therefore represents a more reliable test to assess visual function, especially in the presence of medial opacities, which are known to reduce luminance and produce scatter.

Clinical relevance of phase of steady-state VEPs to P100 latency of transient VEPs

Pattern visual evoked potentials (VEPs) to transient and steady-state stimulation were recorded in 10 normal subjects at 4 levels of luminance (180, 57, 22 and 11 cd/m2). VEPs were also recorded in 5 patients with optic neuropathy at a fixed luminance (180 cd/m2). The relationship between P100 latency of transient VEPs (T-VEPs) and the phase of steady-state VEPs (S-VEPs) was analyzed. As luminance decreased in normal subjects, P100 latency was prolonged and the phase lag increased. A significant linear relationship between the P100 latency and phase was found. Patients showed both the prolonged P100 latency and the delayed phase. The simple linear regression line of the phase-P100 latency function of normal subjects closely matched the patients' values. These results suggest that changes in the phase may be equivalent to changes in the P100 latency. S-VEPs, therefore, may be clinically useful in assessing visual function.

Brain electrical activity mapping of monocular P100 reveals another type of uncrossed asymmetry

Reference-independent analysis of monocular full-field P100 reveals another type of uncrossed asymmetry, not hitherto appreciated, and perhaps dependent on skull-impedance characteristics.

The utility of visual evoked potentials using hemifield stimulation and several check sizes in the evaluation of suspected multiple sclerosis

One hundred and eleven patients with suspected multiple sclerosis (64 possible, 47 probable) and 16 with a definite diagnosis of MS were evaluated with pattern-reversal visual evoked potentials (VEPs), employing monocular full-field checks subtending 7', 14' and 28' of visual angle, and right and left hemifield 28' checks. Thirty-seven patients (29%) had a completely normal study. Sixty-six patients (52%) had abnormal responses to full-field 28' checks, and in 13 (10%) of these, additional abnormalities were found in one or more of the other test conditions, which indicated the presence of an additional site of dysfunction in the visual pathway. Twenty-four patients (19%) with 'normal' full-field 28' responses had abnormalities in one or more of the other test conditions; these included prolonged latency to small size (7' and 14') full-field checks, abnormal responses in homonymous hemifields, and abnormal responses limited to a hemifield of one eye. Thus, the use of several check sizes and hemifield stimulation not only increases the sensitivity of VEPs in the evaluation of patients with suspected demyelinating disease, but enhances the capability of the VEP to demonstrate more than one area of visual system impairment.

Effects of scopolamine parenteral administration on the electroretinogram, visual evoked potentials, and quantitative electroencephalogram of healthy volunteers

Single intramuscular doses of scopolamine and matching placebo were administered to young, healthy, and emmetropic volunteers. Electroretinograms and visual evoked potentials (flash and pattern stimuli) were recorded prior to and 30, 90, and 120 min after administration. The effects of scopolamine at the central nervous system level were monitored also by quantitative electroencephalographic methods. Scopolamine reduced the peak-to-peak amplitude of the late components of the flash-evoked potential without affecting latencies. A decrease of the N75 latency and increment of N175 latency of pattern-evoked potentials were observed without any apparent modification of the amplitude values. These changes were not produced by administering topical cyclopentholate. Electroencephalographic effects were apparent with a longer delay after administration than were those on the visual evoked potentials. No significant modification was observed in the electroretinogram under these recording conditions.

Delayed visual evoked potentials are independent of pattern orientation in macular disease

Visual evoked potentials (VEPs) and contrast sensitivity (CS) were studied in patients affected by maculopathy. VEP delays and CS reduction were demonstrated in each affected eye. In distinction to patients affected by multiple sclerosis (MS), in maculopathy patients VEP latency is independent of the orientation of the grating stimulus. It is proposed that stimulating with more than one pattern orientation is useful in the differential diagnostic use of VEPs.

How might demyelinating disorders and glaucoma modify synaptic function?

Latencies of visually evoked potentials (VEPs) tend to be abnormally long in multiple sclerosis (MS). Similar VEP delays are seen in glaucoma. Such delays could result in part from reduced intensities of synaptic inputs at post-retinal synaptic relays, and defects of axoplasmic transport might be one cause for this. The effective rate of synaptic activation of a given postsynaptic neuron can be decreased either by reducing the arrival-rate of presynaptic action potentials (e.g., by complete or partial blockage of conduction in some presynaptic axons), or by reducing the quantity of neurotransmitter released per action potential (e.g., as a consequence of presynaptic neurotransmitter depletion). It is proposed that in both glaucoma and MS, delayed VEPs may result from either or both of these mechanisms. Firstly, loss and functional impairment of optic nerve axons occurs in each disorder. Secondly, in glaucoma the increased intraocular pressure tends to block the rapid anterograde axoplasmic transport (RAAT) which brings neurotransmitter supplies to the axon terminals. This could result in neurotransmitter depletion in the lateral geniculate relay, decreased synaptic effectiveness of remaining normally-conducting optic nerve axons, and thereby increased VEP latencies. RAAT is also blocked by demyelinated lesions that have been produced experimentally by injection of diphtheria toxin. If it is impaired by the demyelinated plaques of multiple sclerosis, then VEP slowing by a similar presynaptic depletion mechanism could ensue.

An instrument to monitor the luminance of pattern visual evoked potential stimuli

When recording pattern visual evoked potentials it is important that the overall luminance of the pattern and the relative luminance of the pattern elements (for example checks or bars) are maintained at a constant level. This paper describes a low-cost battery-powered instrument designed and constructed to measure the luminance of specific areas of the screen on which the pattern is projected.

A survey of the pattern reversal visual evoked response (PRVER) in 1428 consecutive patients referred to a clinical neurophysiology department

The records of 1428 neurological patients referred to a department of Clinical Neurophysiology for PRVER testing have been reviewed. Half field studies with multichannel recordings were carried out in the majority. 1186 of these patients had a provisional diagnosis of MS; 42% had abnormal PRVERs compared with 25% for those patients who did not have MS. Symmetrical latency increases from stimulation of either eye, interocular latency abnormalities and amplitude abnormalities were as frequent in the non-MS patients as the group with a provisional diagnosis of MS. A small proportion of patients had neurophysiological evidence of field defects; homonymous hemianopias occurred as frequently as "central" defects in the MS group. The PRVER abnormalities are considered in relation to the current models of conduction in demyelinated fibres.

Reproducibility of the visual evoked potential using a light-emitting diode stimulator

The intersubject and intrasubject reproducibility of the VEP was evaluated using two different methods of producing pattern reversal--a mirror/projector system and a light-emitting diode (LED) system. Intersubject reproducibility was determined in 100 normal subjects (50 males, 50 females). Ten subjects were studied on ten different occasions over 11 months to establish intrasubject reproducibility. The two methods gave comparable results. It is concluded that the mirror/projector system has no advantages over the LED system, which is cheaper, more robust and more convenient. Most of the intersubject variability was found to be due to subject variables and it is concluded that technical improvements are unlikely to reduce further the variability of the test. A variability in the latency of P100 of up to 7.7 ms was recorded in the serial studies on the ten subjects, indicating that in longitudinal studies on patients, changes in latency must exceed 9-10 ms to be significant. With both stimulus systems, there were significant sex-related differences in latency (P50 and P100 but not N70) and amplitude (N70-P100). The extent of the difference was such that the upper limit of normal latency for P100 (2.5-3 SD from the mean) was 4.2-4.7 ms longer for males than females-a value which exceeds 1 SD. Separate control values for males and females are advisable whichever method is used to produce pattern reversal.

Deliberate alteration of the visual evoked potential

Seven out of 12 normal subjects could deliberately produce abnormal pattern-reversal visual evoked potentials (VEPs) which simulated disorders of the anterior visual pathways without detection. In six the mechanism was near-point accommodation and in one eccentric fixation. If voluntary suppression of the VEPs is suspected various modifications to the recording technique may be of value. These include the use of a large check size and stimulus field and binocular stimulation.

Serial pattern shift visual evoked potentials in multiple sclerosis

Forty patients with MS initially tested in our laboratory were recalled for repeat PSVEP testing approximately two years later. Twelve normal controls were tested in a similar manner approximately two years apart. The PSVEP positive peak latency changed little in the 24 control eyes (mean 1.4 msec, range 0-6) over the study interval. Most MS patient eyes also showed little change in PSVEP latency over the two year study interval. Fifty-eight eyes changed 8 msec or less. Eighteen eyes showed a PSVEP latency increase of 10 msec or more. Six of these eighteen eyes were symptomatic (attack of clinical optic neuritis), twelve asymptomatic during the study interval. Symptomatic eyes tended to have greater latency increases during the study interval than asymptomatic eyes. Significant latency increases occurred with equal frequency in previously normal eyes (normal PSVEP on first test) and abnormal eyes (abnormal PSVEP on first test or previous clinical optic neuritis). Significant latency increases occurred with greater frequency in patients with a mixed or progressive course than in patients with a remitting-relapsing course, and in patients with greater disability rating (Kurtzke 3-7) than in patients with lower disability ratings (Kurtzke 0-2). Bilateral latency increases occurred during the study interval more frequently than expected by chance. Patient age and disease duration did not significantly influence the number of PSVEP latency increases seen during the study interval. Four eyes decreased in latency by 10 msec or more during the study interval. All these eyes had had an episode of acute optic neuritis which began in the 5 weeks immediately preceding the 1st PSVEP test.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The recovery cycle of the pattern visual evoked potential in normal subjects and patients with multiple sclerosis

The recovery cycle of amplitude and latency of the P100 of the chequerboard pattern reversal visual evoked response was studied in 16 normal subjects and 20 patients with multiple sclerosis. There was no clear-cut pattern of recovery with respect to amplitude but at interstimulus intervals of less than 40 msec the latency of the test response P100 tended to be significantly delayed, the magnitude of this delay being virtually constant at intervals of 20 msec or less. Only 8 of 40 eyes from the 20 multiple sclerosis patients showed a pattern of recovery of latency significantly different from the normal subjects. Possible mechanisms for these observations are discussed with particular reference to current concepts of the pathophysiology of multiple sclerosis.

Normal checkerboard pattern reversal evoked potentials in parkinsonism

The utility of checkerboard pattern reversal visual evoked potentials in the evaluation of Parkinson disease (Pd) was studied in 25 patients and controls without eye pathology. The results did not reveal differences in potential latency but the amplitude appeared slightly larger in PD than controls. Major intraocular differences were observed only in Pd patients with glaucoma, cataracts or retinal degeneration. The commonly employed high contrast and luminance, checkerboard pattern reversal technique did not appear useful in the evaluation of Pd. Selection of proper stimulus parameters may be critical in detecting VEP abnormality in Pd.

Orientation-specific visual evoked potential deficits in multiple sclerosis

Checkerboard pattern reversal visual evoked potentials (VEPs) have proved useful in the confirmation of optic nerve disease in patients with multiple sclerosis (MS). Recently evidence of orientation-specific loss in contrast sensitivity and the presence of orientation-specific visual evoked potential (VEP) deficits in MS patients has been obtained using sinusoidal gratings as stimuli. This study reports the presence of orientation-specific VEP delay in MS using the conventional checkerboard pattern presented in two orientations: normally oriented (check condition) or diagonally oriented (diamond condition). Peak latency values of the N70 and P100 components of the VEP were statistically analyzed using appropriate ANOVA and nonparametric statistics. As a group of MS patients showed significant VEP delays under check and diamond pattern conditions. However, individual subject analysis revealed that about 20% of the MS population show VEP delay to only one pattern orientation. It was shown that by including a diamond pattern condition the diagnostic yield of VEP delay in these clinically definite MS patients was increased 11% over that obtained with check stimulation alone.

The significance of luminance on visual evoked potentials in diagnosis of MS

In view of the fact that with psychophysical methods monocular luminance changes may increase the detection rate of pathological interocular-latency differences in MS patients, we studied the influence of stimulus luminance on the detection rate of MS using checkerboard visual evoked potentials. Decrements of stimulus luminance covering a range of three log units were unable to increase the detection rate of VEP. Regression coefficients of the luminance-latency functions did not differ in MS patients and normals. Contrary to the hypothesis tested, the diagnostic significance of VEP decreases with decreasing stimulus intensity.

Evoked potentials in the clinical neurosciences

The use of evoked potentials for the evaluation of disorders of the nervous system has become a most valuable aid to the neurosurgeon and neurologist, often providing information of critical value without recourse to invasive techniques. In order to employ these techniques, it is helpful to understand the principles of evoked potential electrogenesis and the methodology used for analysis of evoked potential clinical data. This article is aimed at providing the clinical neurosurgeon with this type of information and with a review of current clinical applications in this rapidly developing field.

A sequential technique for half-field pattern visual evoked potential testing

A stimulus technique is described where left and right half fields of a video checkerboard pattern reverse sequentially at a 245 msec interval. The pattern VEPs are recorded on a 512 msce timebase. This allows VEPs to left and right half-field reversal to be obtained on the same sweep, cutting total stimulus time required for testing in half. Normal values for absolute and relative latency and amplitude are given.