Computed tomography and pattern reversal visual evoked potentials in chronic schizophrenic patients

Eighteen chronic schizophrenic subjects treated with a uniform dosage (4-6 mg/day p.o.) of haloperidol were submitted to computed tomography (CT) and to pattern reversal visual evoked potentials (VEPs). Compared to age-matched controls, schizophrenic patients showed lateral and third ventricular enlargement, greatly delayed VEP latencies and reduced amplitudes. These abnormalities were not related to diagnostic subgroups. Schizophrenic patients with a positive family history for major psychiatric disorders showed normal CT scan measures and greatly abnormal VEP measures, whereas patients with a negative family history showed CT scan signs of atrophy and less pronounced VEP abnormalities.     

Visual evoked potentials in phenylketonuria: association with brain MRI, dietary state, and IQ.

At separate institutions, pattern reversal visual evoked potentials (VEPs) were recorded in children and older patients with phenylketonuria and compared with MRI of the brain. In nine patients aged less than 14 years, who were still on a diet low in phenylalanine, VEPs were clearly abnormal in only one and the abnormalities seen on MRI were mild. In 27 patients aged 14-31 years VEPs were abnormal in more than 80%, with significant reduction of amplitude and prolongation of latency despite the general absence of visual symptoms and abnormalities on routine neuro-ophthalmological examination. Among the older patients there was no significant correlation between VEP measures and plasma phenylalanine or tyrosine concentrations; neither was the incidence of VEP abnormalities dependent on whether or not the patients were still on a low phenylalanine diet. Some VEP amplitude measures were inversely correlated with the MRI lesion score, perhaps reflecting the severity of white matter abnormalities in the parieto-occipital region. In the older patients the amplitude of VEPs to stimulation of the central 8 degrees of the visual field was significantly correlated with IQ. The study confirms the high incidence of subclinical visual pathway involvement in older children and adults with phenylketonuria, and suggests the possibility of a link between the abnormal appearance of subcortical white matter on MRI and a physiological index of function of the CNS. As there was no evidence of general intellectual decline, it is suggested that the correlation between central field VEP amplitude and IQ may reflect abnormal development during infancy. Abnormalities on MRI, on the other hand, seem to be more closely related to current dietary state and phenylalanine concentration.     

A study on visual evoked responses in childhood epilepsy with occipital paroxysms.

As some apparently idiopatic epilepsies may occasionally pose diagnostic difficulties in regard to their precise status of etiology, evoked potentials, particularly visual evoked potential (VEP), may contribute to the diagnosis of childhood epilepsy with occipital paroxysms (CEOP) as a subsidiary method of evaluation. This study includes 19 children (10 boys 52.6%; 9 girls 47.4%) ranging in age from 5 to 17 years (mean SD = 9.68 3.28) suffering from CEOP and a control group of 30 normal children, matched for chronological age and sex. Peak amplitudes and latencies of the P100 component for pattern-shift VEP (PVEP) and of major positivity for flash VEP (FVEP) are measured, respectively. The results from this study demonstrate that amplitude and latency values in patients with CEOP differs insignificantly when compared with controls. Although, non-significantly, mean values of amplitudes for both PVEP and FVEP were higher in the patients than in the normal children, whereas latencies in FVEP were somewhat longer. There may be some tendency for the amplitudes to increase and the latencies to be delayed in VEPs in patients with CEOP, when an overall interpretation of our and similar studies are considered. In certain cases of diagnostic difficulty, VEP values may provide further information for the clinician, regarding either a symptomatic or an idiopathic nature of the underlying disorder.     

Abnormal spatial frequency processing in high-functioning children with pervasive developmental disorder (PDD).

OBJECTIVE: Basic abnormalities in visual information processing could be associated with the local visual bias often found in subjects with PDD. Therefore, the present study investigated the existence of deficits in spatial frequency processing at an early sensory level in children with PDD. METHODS: Visual evoked potentials (VEPs) and VEP dipole sources elicited by high and low spatial frequency gratings were analyzed in high-functioning children with PDD and matched controls. RESULTS: Around 80 ms (N80-latency) children with PDD did not show the same robust differences between high and low spatial frequencies in VEP amplitude and VEP brain sources as controls, because of atypical processing of high frequencies. Analyses at the P1-latency (130 ms) revealed that, although similar inferior-medial brain sources were activated for the processing of both spatial frequencies in the PDD and control group, source strength in response to both frequencies was weaker in the PDD compared to control group. Moreover, additional superior-lateral brain sources were activated during the processing of both frequencies in the PDD group. CONCLUSIONS: Decreased specialized processing of high and low spatial frequencies might be a robust characteristic of PDD. Early in processing abnormalities in high spatial frequency processing seem to occur in PDD. At a later phase in processing there seems to be both atypical high and low spatial frequency processing. Considering that the processing of specific spatial frequencies plays an important role in the processing of global and local aspects of hierarchical stimuli and faces and of emotions, present data suggest that peculiarities in PDD subjects with respect to these stimuli might be related to an abnormality in more fundamental visual processes. SIGNIFICANCE: A basic abnormality in visual frequency processing is established in children with PDD.     

Brain stem auditory and visual evoked potentials in multiple sclerosis

The diagnostic value of the checkerboard pattern-reversal visual evoked potential (VEP) and the random, low rate stimulated brain stem auditory evoked potential (BAEP) was compared in 99 patients with established or suspected multiple sclerosis (MS). In normal subjects examined by both techniques no abnormal recordings were found. In 49 patients with definite MS an incidence of abnormality was found in 100% of VEP and in 84% of BAEP recordings. In 50 patients with probable or possible MS an abnormal VEP was found in 70% and an abnormal BAEP in 50%. When the two examinations were combined, the diagnostic yield increased to 100 and 80%, respectively. 22 patients had only spinal symptoms; in these the VEP gave 73%, the BAEP 55% and the combination 82% abnormalities. The combination of the two techniques was found useful for demonstrating demyelinating lesions in the central nervous system, the diagnostic value being greatest when these lesions were clinically silent.     

Specificity of visual evoked potential alterations in Alzheimer's disease]

We have studied visual evoked potentials (VEP) in four groups: healthy young and aged subjects and aged patients suffering from depression or Alzheimer's disease (AD). In the flash modality, peak IV delay in AD patients as compared to aged controls corresponded to lengthened III-IV interlatency (IL). When AD and depressed patients were compared, peak IV delay in the former did not reach significance. Normal aging resulted in delayed peaks II and III, without IL modification. In the pattern reversal modality, no difference was observed between AD patients and aged, healthy or depressed controls. Aging delayed peaks P50 to P180. The abnormality exhibited by AD patients was specific to this group. However, the lack of significant difference between AD and depressed groups argues against the ability of VEP to differentiate those conditions.     

Vector analysis of visual evoked potentials in migraineurs with visual aura.

OBJECTIVE: To assess the capability of vector analysis of visual evoked potentials (VEPs) for revealing alterations in posterior visual pathways in migraineurs with visual aura. METHODS: VEPs to pattern reversal (PR) and LED goggle stimulation were obtained in 23 patients suffering from migraine with visual aura, in an orthogonal Fpz-Oz and T3-T4 montage and displayed as a two-channel Lissajous' trajectory. VEP latency and amplitude at Fpz-Oz, bc segment amplitude (V) and bc vector orientation angle (theta) were compared with a previously collected normative database for individual assessment, and group comparisons with the previously collected normal sample were made. Electrophysiological measures were also correlated with time from onset of disease and from the last crisis, and with the side of symptoms. RESULTS: No individual alterations in VEP latency or amplitude were observed. However, 36.4% of patients showed alterations in vector orientation to PR and 78% to LED goggles. Group differences with respect to normal subjects were found not only in vector orientation but also in midline VEP and V, only for PR stimulation. A significant relationship was found between the laterality of vector deviation and the laterality of symptoms. CONCLUSIONS: Vector analysis of VEP revealed alterations possibly corresponding to asymmetrical visual cortex activation in migraineurs with visual aura, mainly to diffuse light stimulation. SIGNIFICANCE: An electrophysiological parameter of individual value for revealing asymmetric activation of visual cortex in migraineurs is proposed.     

Visual evoked potentials in infants and children.

Visual evoked potential (VEP) studies are of great value in a wide variety of pediatric patients, including those with disorders of the sensory visual pathway and those at risk for visual pathway damage. VEPs are simple, non-invasive, and are particularly appropriate for infants and young children who cannot communicate visual symptoms or cooperate for standard vision assessment. VEPs in pediatric patients have the following main purposes: (1) detecting lesions causing dysfunction of the sensory visual pathways (the VEP is a sensitive indicator of subclinical lesions and can be used to differentiate visual impairment from visual inattention in young infants); (2) confirming functional loss when disorders of the visual system are present; (3) quantifying visual impairment in patients with known visual disorders, accomplished either empirically by noting the severity of the VEP abnormality to flash and pattern stimuli or by visual acuity estimation studies (early quantification of vision loss allows referral to early intervention programs, which can ameliorate the long-term consequences of the disability); (4) monitoring patients who are at risk for visual complications either from diseases (such as hydrocephalus or neurofibromatosis) or as a complication of therapeutic intervention (e.g., neurosurgery, chemotherapy) to help detect and avoid long-term sequelae of such therapies on the developing nervous system; (5) establishing prognosis for visual and systemic recovery based on flash VEPs for specific pediatric disorders including perinatal asphyxia in full-term neonates, acute-onset cortical blindness, and, to a fair extent, in comatose children; and (6) in some cases, contributing to the differential diagnosis. Abnormalities of flash and/or pattern VEPs are generally nonspecific to the type of exact location of the lesion, except in distinguishing prefrom postchiasmal lesions. However, in certain conditions, such as the hereditary ataxias of childhood, VEP abnormalities may help in the diagnosis. Similarly, deterioration in VEPs may help differentiate progressive from static encephalopathies. VEPs have become an indispensable tool in pediatric ophthalmology and neurology. They will probably play an increasingly important role in the future, primarily due to the difficulty in assessing visual system function in young or ill children and the VEP's sensitivity to subclinical damage in this aspect of the central nervous system.     

Electrophysiological assessment of visual function in patients with non-arteritic ischaemic optic neuropathy

Background and purpose:  Our study aims to evaluate retinal function and neural conduction in post-retinal visual pathways of patients with non-arteritic ischaemic optic neuropathy (NION).
Methods:  Twenty patients (mean age: 63.7 ± 5.96 year) with NION and 20 age-similar control subjects were enrolled. Simultaneous recording of pattern electroretinograms (PERGs) and visual evoked potentials (VEPs), and Log of minimum angle resolution (MAR) visual acuity (VA) were assessed in NION patients and controls.
Results:  Significantly ( anova , P  < 0.01) abnormal PERG and VEP responses, delayed retinocortical time (RCT, difference between VEP P100 and PERG P50 implicit times), and reduced VA were found in NION patients with respect to control subjects. The delay in RCT was not significantly (Pearson's test, P  > 0.01) correlated with the PERG impairment. The reduction in VA was significantly (Pearson's test, P  < 0.01) correlated to the increase in VEP P100 implicit time and RCT, whereas no correlations ( P  > 0.01) were found with PERG abnormalities.
Conclusions:  Non-arteritic ischaemic optic neuropathy patients with a reduction in VA may present two different, unrelated impairments: a dysfunction of the inner retinal layer (abnormal PERG) and abnormal post-retinal neural conduction (abnormal VEP and RCT). The reduction in VA seems to be related to the post-retinal impairment and seems to be independent from the retinal dysfunction.     

The value of brain stem auditory, visual and somatosensory evoked potentials and blink reflexes in the diagnosis of multiple sclerosis

Cervical and cortical somatosensory evoked potentials (SEP) following electrical stimulation of the median nerve and blink reflexes (BR) following electrical stimulation of the supraorbital nerve were recorded in 30 normal subjects aged 20–49 years. Subjects aged 40–49 had longer SEP latencies than subjects aged 20–39 years.
A total of 29 slightly affected patients with multiple sclerosis (MS) aged 26–49 years, including four patients without clinical signs (suspected MS) and 19 patients with signs indicating only one lesion (possible MS) were examined by low-rate random-stimulated brain stem auditory (BAEP), checkerboard pattern-reversal visual evoked potentials (VEP), SEP and BR. Abnormal recordings by at least one of the examinations were found in all but three patients, and by all four tests in five patients.
In patients with definite or probable MS, demonstration of clinically recognized or subclinical lesions was of minor diagnostic value, in contrast to the importance such findings had in patients with suspected or possible MS. Silent lesions were shown by at least one of the tests in the four suspected and in 13 of the possible MS patients, so these 17 patients could be transferred to a more certain diagnostic category. This reclassification was most often due to the BAEP recording.
In patients with spinal signs, the combination of BAEP and VEP recording was sufficiently efficient. In patients with optic neuritis a combination of BAEP and SEP was preferred. No abnormal recordings were found in 15 normal subjects examined by all four tests.     

The effect of age, sex and interstimulus interval on augmenting and reducing of occipital VEPs

Three intensities of patterned and unpatterned flashes were used to elicit occipital visually evoked potentials (VEPs) from 4 groups of 15 subjects: boys and girls aged 7-9 years and boys and 15 girls aged 13-15 years. Interstimulus intervals (ISIs) of 1, 2 and 4 sec were used. Amplitude/intensity (A/I) slope was computed for each ISI and flash type condition and an estimate was made of the ability to differentiate patterned from unpatterned stimuli. The latter was inferred from coefficients of correlation which reflected the degree of similarity between VEP wave forms elicited by the two types of flashes. Responses of younger children attenuated to increasing stimulus intensity (negative A/I slope), while VEP amplitude of older subjects augmented to brighter flashes. VEP amplitude reduction may be the response of an immature brain to overstimulation. Amplitude reducing by 7-9-year-old children does not appear to be related to insufficient time between flashes for visual system recovery, since ISI did not differentially affect A/I slope of the younger and older subjects. Longer ISIs did, however, tend to produce more positive A/I slopes for all subjects, but only for patterned stimuli, suggesting that pattern detectors do not fully recover with ISIs of 1 and 2 sec. The visual system of young children was relatively less efficient than that of older children at differentiating patterned from unpatterned stimuli, indicating a differential sensitivity of pattern detectors as a function of age. Gender was not a factor in either magnitude of A/I slope or similarity of VEP wave forms.     

The role of magnetic stimulation in the diagnosis of multiple sclerosis.

Magnetic stimulation was used to measure motor conduction time (MCT) between head and neck, and head and lumbar region, as well as amplitude of the motor evoked potential (MEP) in normal subjects and patients with multiple sclerosis (MS). Patients with definite MS had significantly longer MCTs and smaller amplitude MEPs than normal subjects when recording from arm and leg muscles. In a comparison with visual evoked potential (VEP) recordings, head to neck MCTs were abnormal less often than VEPs, and VEPs detected more silent lesions. Recording from leg as well as arm muscles significantly increased the yield of abnormal MCT measurements. The detection of silent lesions in the patients with definite MS was improved, but there was no improvement in the non-definite cases. Amplitude measurements provided very little extra diagnostic information over MCT measurements alone and did not improve the detection of silent lesions. Interside MCT differences yielded extra abnormalities when recording from the arms but not the legs. Interside MCT abnormalities increased the detection of silent lesions in both the definite and non-definite categories. It was concluded that the majority of useful diagnostic information in patients with MS should be obtainable from bilateral MCT (head to neck) measurements, together with estimation of interside MCT differences. However, VEP recording is a better diagnostic test for MS than MEP recording as more silent lesions are detected. This may be because MCT abnormalities tend to reflect the degree of pyramidal disability.     

Topographical analysis of the onset VEP in the detection of paracentral visual field defects.

Transient visual evoked potentials (VEP) were recorded simultaneously from 16 electrodes evenly placed over posterior scalp locations covering the occipital, posterior parietal and temporal areas. Interhemispheric amplitude difference of the N70 deflection was established across 6 homologous lateral electrode pairs in 15 normal controls and 32 patients with chiasmatic or retrochiasmatic cerebral lesions. Twenty-three of these had known homonymous or bitemporal field defects while 9 had normal fields on routine perimetry. Significant interhemispheric asymmetry of any single electrode pair occurred in 55% of the 32 patients with known pathology, while the cumulative yield of all electrode pairs was over 80 percent. The diagnostic yield of individual electrode pairs was significantly different: the electrode pair placed over the temporo-parietal junction detected the highest number of abnormalities. The cumulative abnormality taken over all lateral electrode pairs could be described with a curve well fitted with a probability summation function. It is inferred that the contribution of several independent generator sources is reflected in the N70 of the pattern onset VEP. The results suggest that multichannel recording of the interhemispheric amplitude distribution of the N70 of the onset VEP is useful for the evaluation of paracentral visual field defects.     

Visual sensory processing deficits in Schizophrenia and their relationship to disease state

CONTEXT: Visual Evoked Potential (VEP) abnormalities have been a fairly consistent finding in patients with schizophrenia, and it has been suggested that electrophysiological markers of early sensory processing may be useful as trait markers for the illness, and for development as potential diagnostic measures. OBJECTIVE: Clear amplitude reductions in the occipital P1 component of the VEP (~100 ms), have been repeatedly demonstrated in patients with schizophrenia. Here, we investigated whether the extent of this deficit was related to age, clinical symptoms, medication status and length of illness, in a large cohort of ethnically homogenous patients. DESIGN, SETTING AND PARTICIPANTS: VEP responses to simple isolated-check stimuli were examined in 52 DSM-IV diagnosed patients with schizophrenia, and compared with responses from 26 healthy age-matched control subjects. Using high-density electrical scalp recordings, we assessed the integrity of the visual P1 component across the two groups. This study was conducted at St.Vincent's Psychiatric Hospital in Fairview, Dublin, Ireland. RESULTS: Substantially reduced P1 amplitude was demonstrated in the patient group compared to controls. The deficit was not linked to age, length of illness or medication status. A small positive correlation, accounting for about 11% of the variance, was found between P1 amplitude and clinical symptoms scales (BPRS and SANS). In addition, we found that a slightly later (~110 ms) fronto-central component was relatively increased in the patient group, and was inversely correlated with the occipital P1 amplitude in the patients, but not in the healthy control subjects. CONCLUSIONS: Our findings clearly demonstrate a deficit in early visual processing in patients with schizophrenia (with a large effect size; Cohen's d = 0.7) that is unrelated to chronicity. The results are consistent with recent findings showing that the P1 deficit is endophenotypic of the disorder and related to genetic risk factors rather than the disease process itself.     

Neurophysiological evaluation of visual function in iRBD: potential role in stratifying RBD conversion risk

Study objectivesTo evaluate neurophysiological alterations of visual function in idiopathic REM sleep Behavior Disorder (iRBD) both as markers and predictors of neurodegenerative disorders.MethodsIn a longitudinal follow-up study of 46 consecutive iRBD patients (follow-up duration 8.4 ± 3.4 years), the baseline parameters in luminance-contrast pattern (VEPp), red-green color (VEPc) and motion-onset (VEPm) Visual Evoked Potentials in iRBD were compared to early (ePD) and advanced (aPD) Parkinson's Disease subjects. Parameters of latency and amplitude of iRBD converters to neurodegenerative disease were compared with those of the non-converters.ResultsThe VEP P100 mean latency values for both eyes and for both stimulation checks (30′ and 15’) were significantly longer in all the three groups of patients as compared to controls; moreover latencies were longer in aPD than in the iRBD group who did not differ from the ePD group. The same held true when we analyzed the number of abnormal subjects belonging to each diagnostic group with a higher number of abnormal subjects in the aPD group compared to both the ePD and in iRBD groups. Chromatic and motion potentials were not different from controls and did not differ in the 3 diagnostic groups. The iRBD subjects who converted to a neurodegenerative disorder showed longer P100 latencies and a higher occurrence of VEPp abnormalities than those who did not convert. Again chromatic and motion VEPs were not different depending on conversion.ConclusionsIn iRBD patients the detection of an abnormal VEPp should be considered as a red flag for possible synnucleinopathy, eventually contributing in stratifying the risk of phenoconversion.     

Visual dysfunction in treated schizophrenia suggested by visual evoked potentials from pattern-reversal stimulation

Summary Steady-state visual evoked potentials (steadystate VEPs) from pattern-reversal stimulations were compared in treated schizophrenic patients and normal subjects matched for sex and age. The VEP amplitudes were more variable in the patients than in the controls. Furthermore, the VEP amplitudes of the patients mostly showed little or no change when the check size was varied, in contrast to the controls who showed a marked check size effect. These results suggest that schizophrenics receiving drugs have dysfunction of the visual system, especially an inability to respond adequately to changes of visual information.     

Visual involvement in Friedreich's ataxia: PERG and VEP study

Neuro-ophthalmological assessment, including red-free light retinography, in conjunction with simultaneous visual evoked potential (VEP) and pattern electroretinogram (PERG) recordings were performed in 10 Friedreich's ataxia patients: 9 patients showed marked VEP abnormalities. Moderate PERG amplitude reduction, with normal latencies, was related to mild and scattered fiber loss revealed by red-free light retinography. The initial part of the visual pathways does not seem to be the main site of electrophysiological abnormalities as demonstrated by the greater extent and relative independence of VEP versus PERG alterations. Primary axonal degeneration of the optic nerve and tracts cannot account for all features of VEP abnormalities, thus implying some dysfunction in succeeding visual structures as well.     

Visual evoked potentials in spinocerebellar degenerations

Pattern reversal visual evoked potentials were studied in 21 patients with spinocerebellar ataxias among whom 6 had Friedreich's ataxia, 10 had hereditary spastic ataxia and 5 had spinocerebellar degeneration with slow eye movements (olivopontocerebellar degeneration). The VHP abnormalities found in 4 cases of Friedreich's ataxia and one with spinocerebellar degeneration with slow eye movements, consisted of, bilaterally absent VEP in 3 patients and bilaterally abnormal responses with asymmetry in two. All the patients with spastic ataxia had normal VEP latencies. The N 70 – P 100 amplitudes, in patients with hereditary ataxias were significantly reduced compared to controls (P < 0.001). The VEP abnormalities correlated best with neuroopthalmic findings, but had no relation to age, sex, inheritance or duration of illness. The VEP findings are probably suggestive of progressive nerve fibre loss in the visual pathways with associated slowing of conduction. The higher incidence of visual pathway involvement in Friedreich's ataxia compared to other hereditary ataxias as reported in recent studies is confirmed.     

A fast visual evoked potential method for functional assessment and follow-up of childhood optic gliomas.

OBJECTIVE: To evaluate a fast technique of visual evoked potentials (VEPs) recording, in response to steady-state luminance stimuli (SS-LVEPs), for functional assessment and follow-up of childhood optic gliomas (OGs). METHODS: Eighteen OG patients (age range: 3.5-18 years), with different degrees of optic pathway damage severity, were examined. Sixteen age-matched normal subjects served as controls. Ten of the 18 OG patients were re-tested 1-3 months after the first examination. SS-LVEPs were elicited by a sinusoidally-modulated flickering (8 Hz) uniform field, generated by a light emitting diode (LED)-array and presented monocularly in a mini-ganzfeld. Amplitude and phase of the Fourier-analyzed response fundamental (1F) and second harmonic (2F) were measured. The full VEP protocol had a median duration of 6 min (range: 4-12). RESULTS: When compared to normal control values, median 1F and 2F SS-LVEP amplitudes of OG patients were reduced (P<0.01), with a borderline increase in 2F phase lag (P<0.05). In 11 OG patients with asymmetric optic pathway damage in between-eye comparisons, median 1F amplitude losses were greater (P<0.01) in fellow eyes with more severe damage. No significant interocular difference was observed in control subjects. Median test-retest changes of 1F and 2F component were <20% and 30 degrees for amplitude and phase, respectively. In individual OG patients, 1F and 2F amplitudes were positively correlated (P<0.01) with visual acuity. 1F amplitude losses were correlated (P=0.01) with the severity of optic disc atrophy. Considering both 1F and 2F abnormalities, diagnostic sensitivity of SS-LVEP in detecting OG-induced optic pathways damage was 83.3%. CONCLUSIONS: The present findings support the use of this technique, as an alternative to pattern VEPs, for functional assessment and follow-up of OG in uncooperative children.     

Visual evoked potential in patients with cerebral asthenopia

Cerebral asthenopia is often overlooked as a symptom in diffuse brain lesion. An objective correlate of this symptom has so far never been demonstrated. Averaged visual evoked potential (VEP) in 10 patients with asthenopia was compared with 20 normal subjects. Both eyes and each eye alone were stimulated using bipolar recording in the midline and over each of the occipital lobes. There was no difference of latency of the VEPs in the two groups, but the amplitude of the most prominent component was significantly reduced in the patients. There was also a difference in the two groups regarding habituation and lateralisation. No amplitude difference could be found in the somatosensory evoked potential. The amplitude difference in VEP, as an objective correlate of asthenopia, is probably modal specific and suggests involvement of the visual cortex. VEP is unsuitable as a diagnostic tool due to the great overlap between amplitudes in asthenopic patients and control subjects.