Early Clinical and Subclinical Visual Evoked Potential and Humphrey's Visual Field Defects in Cryptococcal Meningitis

Cryptococcal induced visual loss is a devastating complication in survivors of cryptococcal meningitis (CM). Early detection is paramount in prevention and treatment. Subclinical optic nerve dysfunction in CM has not hitherto been investigated by electrophysiological means. We undertook a prospective study on 90 HIV sero-positive patients with culture confirmed CM. Seventy-four patients underwent visual evoked potential (VEP) testing and 47 patients underwent Humphrey''s visual field (HVF) testing. Decreased best corrected visual acuity (BCVA) was detected in 46.5% of patients. VEP was abnormal in 51/74 (68.9%) right eyes and 50/74 (67.6%) left eyes. VEP P100 latency was the main abnormality with mean latency values of 118.9 (±16.5) ms and 119.8 (±15.7) ms for the right and left eyes respectively, mildly prolonged when compared to our laboratory references of 104 (±10) ms (p<0.001). Subclinical VEP abnormality was detected in 56.5% of normal eyes and constituted mostly latency abnormality. VEP amplitude was also significantly reduced in this cohort but minimally so in the visually unimpaired. HVF was abnormal in 36/47 (76.6%) right eyes and 32/45 (71.1%) left eyes. The predominant field defect was peripheral constriction with an enlarged blind spot suggesting the greater impact by raised intracranial pressure over that of optic neuritis. Whether this was due to papilloedema or a compartment syndrome is open to further investigation. Subclinical HVF abnormalities were minimal and therefore a poor screening test for early optic nerve dysfunction. However, early optic nerve dysfunction can be detected by testing of VEP P100 latency, which may precede the onset of visual loss in CM.     

Clinical Utility and Limitations of Intraoperative Monitoring of Visual Evoked Potentials

Objectives

During surgeries that put the visual pathway at risk of injury, continuous monitoring of the visual function is desirable. However, the intraoperative monitoring of the visual evoked potential (VEP) is not yet widely used. We evaluate here the clinical utility of intraoperative VEP monitoring.

Methods

We analyzed retrospectively 46 consecutive surgeries in 2011-2013. High luminance stimulating devices delivered flash stimuli on the closed eyelid during intravenous anesthesia. We monitored VEP features N75 and P100 and took patients'' preoperative and postoperative visual function from patient charts. Postoperative ophthalmologic workup was performed in 25 (54%) patients and preoperatively in 28 (61%) patients.

Results

VEP recordings were feasible in 62 of 85 eyes (73%) in 46 patients. All 23 eyes without VEP had impaired vision. During surgery, VEPs remained stable throughout surgery in 50 eyes. In 44 of these, visual function did not deteriorate and three patients (6 eyes) developed hemianopia. VEP decreased transiently in 10 eyes and visual function of all was preserved. VEPs were lost permanently in 2 eyes in two patients without new postoperative visual impairment.

Conclusions

Satisfactory intraoperative VEP monitoring was feasible in all patients except in those with severe visual impairment. Preservation of VEPs predicted preserved visual function. During resection of lesions in the visual cortex, VEP monitoring could not detect new major visual field defects due to injury in the posterior visual pathway. Intraoperative VEPs were sensitive enough to detect vascular damage during aneurysm clipping and mechanical manipulation of the anterior visual pathway in an early reversible stage. Intraoperative VEP monitoring influenced surgical decisions in selected patients and proved to be a useful supplement to the toolbox of intraoperative neurophysiological monitoring.     

Modifications of Visual Evoked Potentials in Patients with Glaucoma

We compared the visual evoked EEG potentials (VEP) elicited by presentation of a reversal chess pattern in patients with glaucoma and in the control group. Amplitudes, peak latencies of the main VEP components (N75, P100, and N145), interpeak intervals, and interpeak magnitudes were measured, and a spectral analysis of the averaged VEP was performed. In patients suffering from glaucoma, the latencies of the N75 and P100 components were greater, while the interpeak intervals P100-N145 and N75-N145 were shorter, than those in the control group. Glaucoma-related changes in the VEP spectral characteristics, in particular a drop in the spectral power of oscillations corresponding to the alpha rhythm, were observed.     

N70 and P100 can be independently affected in multiple sclerosis

We have studied the relationship between N70 and P100 of the pattern visual evoked potential in 98 patients with multiple sclerosis and in 59 controls. In patients with multiple sclerosis, P100 was either absent or had prolonged latency in 121 eyes (61.7%), while N70 was absent or prolonged in 97 eyes (49.5%). The total number of eyes with either N70 and/or P100 abnormalities was 137 (69.9%). Eighty eyes (40.8%) had abnormal latency of both P100 and N70, while 41 eyes showed P100 delays without corresponding N70 changes. Seventeen eyes had abnormal N70, but normal P100 latency. N70 and P100 appear to be more often absent in the definite rather than in the possible multiple sclerosis group. These data show that N70 and P100 can be independently affected in patients with MS.     

Visual evoked potential in guinea pigs

The present study was designed to establish visual evoked potential (VEP) as one of clinical tests for veterinary medicine. Experiments were carried out on eight adult male guinea pigs weighed 350 to 750 g. We investigated influences of click sound, luminous intensity and habituation on VEP patterns. The VEP of the guinea pig was composed of primary (P 10, N 20, P 30, N 40) and secondary (P 55, N 75, P 100, N 140) components, followed by a rhythmic after-discharge. Click sound with flash produced some unclear peaks in VEP, while click sound without flash elicited clear six peaks. These different components of the response to stimulation suggested that the acoustically evoked potential induced some peaks in VEP. With the intensity used in the present study, changes in luminous intensity resulted in unrecognizable difference among the VEPs. Early components of VEP were not clearly influenced by the habituation to stimulation. As the stimulation was repeated, rhythmic after-discharge seemed to be suppressed in the half of experiments.     

Single-trial ERP evidence for the three-stage scheme of facial expression processing

Using a rapid serial visual presentation paradigm, we previously showed that the average amplitudes of six event-related potential (ERP) components were affected by different categories of emotional faces. In the current study, we investigated the six discriminating components on a single-trial level to clarify whether the amplitude difference between experimental conditions results from a difference in the real variability of single-trial amplitudes or from latency jitter across trials. It is found that there were consistent amplitude differences in the single-trial P1, N170, VPP, N3, and P3 components, demonstrating that a substantial proportion of the average amplitude differences can be explained by the pure variability in amplitudes on a single-trial basis between experimental conditions. These single-trial results verified the three-stage scheme of facial expression processing beyond multitrial ERP averaging, and showed the three processing stages of "fear popup", "emotional/unemotional discrimination", and "complete separation" based on the single-trial ERP dynamics.     

Factors that limit the use of flash visual evoked potentials for surgical monitoring

A study was conducted comparing the incidence with which the N2/P2/N3 was obtained after flash VEP in 3 groups: anterior visual pathway lesions, non-tumor craniotomies and non-cranial surgery. These groups allowed evaluation of the effects of anesthesia, visual pathway lesions and craniotomy on the stability of the flash VEP. It was found that the latency was not significantly affected in the 3 groups, whereas the incidence of obtainable peaks and the amplitudes were adversely affected by anesthesia, cranial surgical manipulation and especially by the presence of a visual pathway lesion. These adverse effects were so marked that the application of flash VEP for intraoperative monitoring seems of little use.     

Pattern-reversal visual evoked potentials in photosensitive epilepsy

VEPs to checkerboard pattern-reversal were recorded from 18 epileptic patients who had EEG photoparoxysmal responses to stroboscopic light. Patients were grouped according to whether seizures were precipitated by environmental light stimuli, or television viewing. Longitudinal studies were conducted on 8 patients treated with valproic acid. We concluded the following: (1) Latency of the major positive peak (P2) of the pattern-reversal VEP was shorter among photosensitive patients than among normal controls. This was especially true of television-sensitive patients. (2) Valproic acid, when effective in controlling seizures, lengthened the P2 latency and decreased VEP amplitude. Studies of drug effects on VEPs may help to elucidate neurochemical mechanisms of the visual cortex. (3) Because of overlap of values with normals, VEP measurements are not at present very sensitive in the diagnosis of photosensitivity. However, longitudinal studies in individuals parallel clinical changes and may be useful as objective measures of improvement.     

Pre-stimulus spectral EEG patterns and the visual evoked response

The relationship between the latencies and amplitudes of the N1 and P2 components of the visual evoked potential (VEP) and the psychophysiological state of the brain immediately preceding the time of the stimulus has been investigated in 7 male subjects. Power spectral measures in the delta, theta, alpha and beta bands of the 1 sec pre-stimulus EEG were used to assess the brain state, and low intensity flashes, delivered randomly between 2 and 6 whole seconds, were used as the stimuli. Trials were ranked separately according to the relative amounts of pre-stimulus power in each EEG band and were partitioned into groups by an equal pre-stimulus spectral power criterion. Averaged EPs were computed from these groups and multiple regression analysis was used to relate pre-stimulus spectral power values to EP features. Five of the 7 subjects displayed consistent increases in N1-P2 amplitude as a function of increasing pre-stimulus relative alpha power. The between-subjects effect of pre-stimulus EEG on N1 latency was small, but was moderate for P2 latency (both significant). Both N1 and P2 latency were found to decrease with increasing amounts of pre-stimulus relative delta and theta power.     

The temporal frequency response function of pattern ERG and VEP: changes in optic neuritis

Steady-state pattern electroretinograms (PERGs) and visual evoked potentials (VEPs) in response to sinusoidal gratings (2 c/deg), sinusoidally counterphased at closely spaced temporal frequencies (TFs) between 4 and 28 Hz, were recorded from 11 patients with unilateral optic neuritis (ON; 11 affected eyes and 10 healthy fellow eyes) and 7 age-matched normal subjects (7 eyes). Amplitude and phase of responses' second harmonics were measured. Responses' apparent latencies were estimated from the rate at which phase lagged with TF. When compared to control values, mean PERG and VEP amplitudes of ON eyes were reduced (by about 0.4 log units) at both low (5–10 Hz) and high (16–20 Hz) TFs. Mean PERG amplitudes of fellow eyes were selectively reduced at low TFs (by about 0.3 log units). Mean PERG apparent latencies of both ON and fellow eyes were delayed (by 15 and 9 ms, respectively). Mean VEP apparent latency of ON eyes was delayed at both low and high TFs (by 24 and 30 ms, respectively), while that of fellow eyes was selectively delayed at high TFs (by 28 ms). The results in ON eyes indicate non-selective abnormalities of PERG and VEP generators responding at both low and high TFs. VEP TF losses may be in part accounted for by corresponding PERG losses. In the fellow eyes of patients, more selective PERG and VEP TF abnormalities may suggest differential impairment of retinal and postretinal subsystems responding better to low and high TFs (i.e. parvo-and magnocellular streams).     

Scalp-recorded oscillatory potentials evoked by transient pattern-reversal visual stimulation in man

Replicable oscillatory potentials, time-locked to pattern stimuli (9.0° central; counterphase reversal at 2.13 Hz) were dissociated from conventional, broad-band VEPs recorded in healthy volunteers at occipital scalp locations by high-pass digital filtering at 17.0–20.0 Hz. Nine consecutive wavelets were identified with a 56.4 ± 8.4 msec mean latency of the first replicable wavelet and mean peak-to-peak amplitude varying between 0.9 and 2.0 μV. The first 2 wavelets had significantly shorter latencies than wave N70 of unfiltered VEP, whereas the last 2 wavelets had longer latencies than N145. Latency and amplitude values varied as a function of contrast and spatial frequency of the stimulus, with shorter latencies and larger amplitudes at 60–90% contrast level and tuning of amplitude at 5.0 c/deg. All wavelets were correlated with wave P100 of unfiltered VEP, while a correlation with N70 of VEP was observed only for those wavelets with latencies in the range of wave P100. Two patients with documented brain lesions involving the visual system are described as examples of oscillatory responses occurring irrespective of filter bandpass and instead of the expected conventional VEP when the generation of these is interfered with by brain pathology. A substantial cortical contribution to the origin of the oscillatory response is conceivable. It is suggested that the oscillatory response to pattern-reversal stimulation reflects events in the visual system that are parallel to, and partly independent of, the conventional VEP, with potential application in research or for clinical purposes.     

Effects of aging on event-related brain potentials (ERPs) in a visual detection task

Event-related brain potentials (ERPs) were recorded from 74 subjects (45 men) between 18 and 82 years of age in a simple visual detection task. On each trial the subject reported the location of a triangular flash of light presented briefly 20° laterally to the left or right visual field or to both fields simultaneously. ERPs to targets exhibited a similar morphology including P1, N1, P2, N2, and P3 components across all age groups. The principal effects of advancing age were (1) a marked reduction in amplitude of the posterior P1 component (75–150 latency) together with an amplitude increase of an anterior positivity at the same latency; (2) an increase in amplitude of the P3 component that was most prominent over frontal scalp areas; and (3) a linear increase in P3 peak latency. These results extend the findings of age-related changes in P3 peak latency and distribution to a non-oddball task in the visual modality and raise the possibility that short-latency ERPs may index changes in visual attention in the elderly.     

Multichannel visual evoked potentials in migraine

Multichannel recordings of visual evoked potentials (VEPs) have proved to be useful in the evaluation of visual field defects. We studied the topographic distribution of transient VEPs in 15 migraine patients (8 with visual aura and 7 without) and 15 age-matched controls during the migraine-free interval. All the subjects included in the study had normal visual fields. VEPs were recorded from 9 electrodes placed on the posterior scalp. Stimuli were full-field and hemifield reversing square wave grating patterns of medium spatial frequency (4 c/deg). The groups did not show significant differences in latencies and amplitudes of the major components (N70, P100) recorded from the midline. However, migraine patients with visual hemianopic aura showed definite asymmetries in the VEP amplitude distribution. Significantly reduced, absent or polarity-invered VEP responses were recorded ipsilateral to the side of the prodromic visual symptoms. Direct comparison of affected and unaffected hemispheres by partial field stimulation confirmed these findings. According to the VEP cortical generator theory, these abnormalities suggest a functional anomaly consistent with the clinical syndrome and detectable also in the migraine-free interval. None of the migraine patients without aura or the controls showed VEP amplitude asymmetries. We conclude that multichannel VEP recordings may discriminate between different subtypes of migraine and contribute important physiopathological information to the study of this disease.     

Clinical application of the P3 component of event-related potentials. II. Dementia,depression and schizophrenia

Patients with dementia, schizophrenia and depression were tested with analogous auditory and visual event-related potential (ERP) paradigms designed to elicit a large P3. The patient groups were compared to age normative predictions derived from a large control sample for a number of ERP and behavioral variables.The results were similar for the auditory and visual paradigms. P3 latency was prolonged two or more S.D.s beyond that predicted by age for less than one-half of the demented patients. This latency prolongation was significant for the group as a whole but would result in too many false negatives if used diagnostically for individuals. Furthermore, increased P3 latency was not specific, as the schizophrenic patients also had later P3s. The amplitude of P3 was reduced in the demented patients, but it was also smaller in other patient groups. The only variable which distinguished the demented patients from both controls and from the other patients was the single trial P3 latency/RT correlation. The demented patients, as a group, had significantly lower P3 latency/RT correlations, but this effect also was not sensitive enough to be diagnostic for individuals. The data from these two paradigms suggest that the P3 amplitude and latency abnormalities observed reflect a common, rather than a diagnostically specific deficit.This study is in contrast to some others which report much more sensitivity and specificity in the use of P3 latency in the diagnosis of dementia. Differences in task demands, patient samples and ERP analysis techniques might explain some of the discrepency.     

Recognition of joy, anger, and fear by face expression in humans

Behavioral and neurophysiological characteristics of a visual recognition of emotions of joy, anger, and fear were studied in 9 young healthy men and 10 women. It was shown that these emotions were identified by subjects with different rate and accuracy; significant gender differences in recognition of anger and fear were found. Recording of visual evoked potentials (VEP) from the occipital (O1/2), medial temporal (T3/4), inferior temporal (T5/6), and frontal (F3/4) areas revealed differences (related with the type of emotion) in the latencies of P150, N180, P250, and N350 waves and in the amplitude of VEP components with the latencies longer than 250 ms. These differences were maximally expressed in T3/4 derivation. The subjects could be divided in two groups. The first group was characterized by increased VEP latencies and higher amplitudes of VEP components later than 250 ms in response to anger (in comparison with other types of emotions). These phenomena were observed in all the derivations but were most pronounced in T3/4. In the second group, only late P250 and N350 components had shorter latencies during recognition of fear. VEP amplitude variations related with the type of emotions were insignificant and were recorded in the occipital and frontal areas. The two groups of subjects also differed in psychoemotional personality characteristics. It is suggested that primary recognition of facial expression takes place in the temporal cortical areas. A possible correlation of electrophysiological indices of emotion recognition with personality traits is discussed.     

兔先天性青光眼网络膜血管改变

目的　研究青光眼对视网膜脉络膜血液循环的影响。方法　选24月龄、体重3.5～4kg的先天性青光眼大耳白兔5只(7只眼),选10只同龄大耳白兔作为对照组。另选10只2月龄、体重2kg大耳白兔前房内灌注生理盐水制成急性高眼压模型。对三组兔进行眼底照像、闪光视诱发电位(FVEP)检查,观察视网膜脉络膜血管形态和FVEP的变化。对人工急性高眼压组还进行了闪光视网膜电流图(FERG)检查。结果　先天性青光眼组与同龄对照组相比视网膜脉络膜末梢血管网明显减少;人工急性高眼压组眼压升高后首先使视网膜脉络膜末梢血管网灌流不足,随着眼压的继续升高脉络膜大血管变细,末梢血管网灌流不足加重,眼压极度升高时脉络膜大血管血流中断。同龄正常对照组的FVEP的主波P100潜伏期是(83±9)ms,先天性青光眼组则为(112±14)ms,差异有非常显著意义(P＜0.01);人工急性高眼压组高眼压前为(69±5)ms,眼压60～80mm　Hg时延长为(81±7)ms,眼压在100～130mmHg时FVEP波形低平,近似直线;眼压恢复正常后2hFVEP的P100潜伏期为(82±8)ms。人工急性高眼压前后FERG变化显著。结论　青光眼可以影响视网膜脉络膜血液循环;可使FVEP、FERG发生变化。     

Gender Differences in Spontaneous and Evoked Activities of the Human Brain

The purpose of this study was to analyze sex-related differences in spontaneous and evoked brain activity. The study included 11 women and 10 men aged between 18 and 27 years; the subjects were adapted to the conditions of electrophysiological experiment. Differences in spontaneous activity were estimated by EEG spectral characteristics in frequency ranges corresponding to θ, α, β₁, β₂, γ₁, and γ₂ rhythms at rest with the eyes open and closed. The differences in evoked activity were estimated by the parameters of evoked potentials (EP) recorded upon the presentation of different types of stimuli (images and words) and categories (“fruits” and “vessels”). Behavioral differences were estimated by the efficiency of stimuli recognition. No gender differences were observed in the efficiency of stimuli recognition (number of errors, time of reaction). In the EEG of male subjects at rest, fast frequencies (β and γ) were less marked than in the female subjects, which may indicate a higher level of activation of CNS structures in women. Regardless of stimuli type and category, the amplitude of EP components in men is lower than that in women, which may be caused by lower level of CNS activation. However, the difference in the latency of EP components depended on the type of stimuli used. During the recognition of visual images, the latency of earlier EP components (N1, P2) in men was longer mostly on back and/or right electrodes. The latencies of the later EP components (P3, N3) in male subjects were longer on the back electrodes and shorter on the front electrodes than those in female ones. During the recognition of words, the latency of P1, N1, P2 and N2 components in men were shorter on temporal and temporoparietal electrodes of the left hemisphere. This may indicate that visual image in men are perceived slower but comprehended faster than in women. On the other hand, verbal stimuli in men were perceived faster but comprehended slower than in women.     

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.     

Clinical and neuroimaging correlates of abnormal short-latency Somatosensory Evoked Potentials in elderly vascular dementia patients: A psychophysiological exploratory study

BACKGROUND: Short Latency Somatosensory Evoked Potentials (SEPs) may serve to the testing of the somatosensory tract function, which is vulnerable and affected in vascular encephalopathy. The aim of the current study was to search for clinical and neuroimaging correlates of abnormal SEPs in vascular dementia (VD) patients. MATERIALS AND METHODS: The study included 14 VD patients, aged 72.93 PlusMinus; 4.73 years, and 10 controls aged 71.20 PlusMinus; 4.44 years. All subjects underwent a detailed clinical examination, blood and biochemical testing, brain MRI and were assessed with the MMSE. SEPs were recorded after stimulation from upper and lower limbs. The statistical Analysis included 1 and 2-way MANCOVAs and Factor analysis RESULTS: The N13 latency was significantly prolonged, the N19 amplitude was lower, the P27 amplitude was lower and the N11-P27 conduction time was prolonged in severely demented patients in comparison to controls. The N19 latency was prolonged in severely demented patients in comparison to both mildly demented and controls. The same was true for the N13-N19 conduction time, and for the P27 latency. Patients with subcortical lesions had all their latencies prolonged and lower P27 amplitude. DISCUSSION: The results of the current study suggest that there are significant differences between patients suffering from VD and healthy controls in SEPs, but these are detectable only when dementia is severe or there are lesions located in the subcortical regions. The results of the current study locate the abnormal SEPs in the white matter, and are in accord with the literature.     

Olfactory event-related potentials in young and elderly adults: evaluation of tracking task versus eyes open/closed recording.

The purpose of the present study was to evaluate olfactory event-related potentials (OERPs) elicited by amyl acetate from subjects performing a visuomotor tracking task compared with the no-task conditions of eyes open and eyes closed. Task condition did not produce any reliable effects for any amplitude measure. Task type weakly influenced only P2 latency. Elder adults evinced smaller P2 and N1/P2 amplitudes and longer N1 and P2 latencies than young adults. The results suggest that tracking task performance is not necessary to obtain robust OERPs from normal subjects of a wide age range.