Visual evoked potentials in the vicinity of the optic tract during stereotactic pallidotomy

We recorded visual evoked responses in eight patients with Parkinson's disease, using a depth electrode either at or below the stereotactic target in the ventral part of the globus pallidus internus (GPi), which is located immediately dorsal to the optic tract. Simultaneously, scalp visual evoked potentials (VEPs) were also recorded from a mid-occipital electrode with a mid-frontal reference electrode. A black-and-white checkerboard pattern was phase reversed at 1 Hz; check size was 50 min of arc. Pallidal VEPs to full field stimulation showed an initial positive deflection, with a latency of about 50 ms (P50), followed by a negativity with a mean latency of 80 ms (N80). The mean onset latency of P50 was about 30 ms. P50 and N80 were limited to the ventralmost of the GPi and the ansa lenticularis. Left half field stimulation evoked responses in the right ansa lenticularis region while right half field stimulation did not, and vice versa. These potentials thus seemed to originate posterior to the optic chiasm. The scalp VEPs showed typical triphasic wave forms consisting of N75, P100 and N145. The location of the recording electrode in the ansa lenticularis region did not modify the scalp VEP. These results suggest that P50 and N80 are near-field potentials reflecting the compound action potentials from the optic tract. Therefore, N75 of the scalp VEPs may represent an initial response of the striate cortex but not of the lateral geniculate nucleus.     

Visual event-related potentials evoked by using a virtual reality display

This study aimed at investigating whether a virtual reality display (VRD) is an appropriate tool for evoking visual event-related potentials (VEPs). VEPs evoked by VRD stimuli were highly similar in form to VEPs evoked by using a computer monitor, both having two dominant peaks, labeled P100 and N200. Monitor and VRD N200 latencies and amplitudes were highly correlated. However, peak latencies were longer and the peaks were broader when stimuli were presented on the VRD. Besides, VRD P100 amplitude was smaller, and an N75 peak could be seen usually only on monitor VEPs.     

The effect of binocular stimulation on each component of transient and steady-state VEPs

We recorded the monocular and binocular VEPs to the alternation of sinusoidal gratings in order to evaluate the binocular interaction in each component of transient and steady-state VEPs in 13 normal subjects. Three spatial frequencies (1.3, 2.6 and 5.3 c/deg) with a 90% contrast were used as visual stimuli. The latencies and amplitudes of N70 and P100 of the transient VEPs were measured. The steady-state VEPs were Fourier analyzed, and both the phase and amplitude of the second (2F) and fourth (4F) harmonic responses were obtained. Binocular interaction was influenced by spatial frequency such that a binocular summation or even an inhibition occurred. For the transient VEPs, a binocular summation was more pronounced in the amplitude of N70 than in that of P100 at all spatial frequencies. There were no significant effects of binocular stimulation on latencies of N70 or P100. However, the latencies of N70 and P100 showed different spatial frequency characteristics. For the steady-state VEPs, the amplitude of 2F revealed a binocular summation that was more pronounced at 5.3 c/deg, whereas the 4F amplitude showed binocular inhibition at 2.6 and 5.3 c/deg. The 2F phase showed binocular inhibition at all spatial frequencies, whereas no such inhibition was observed in the 4F phase. These results suggest that individual components of transient and steady-state VEPs are physiologically distinct and may therefore be generated from different neuronal populations in striate cortex.     

Interocular interaction assessed by VEPs to pattern-onset, -reversal,and -offset in normally sighted and amblyopic subjects

The extent of interocular interaction reflected in sequentially averaged VEPs to checkerboard onset, reversal and offset stimulation was investigated to assess the relative efficacy of the three modes of pattern stimulation. Thirty-one controls and 18 amblyopic children were studied. Components on the side of the scalp ipsilateral and contralateral to the stimulated left half-field were measured for checksizes 12′, 20′, 50′ and 80′. Binocular:monocular amplitude ratios for normals were compared with `binocular:good eye' amplitude ratios for amblyopes. The reversal P100 ratio was found to differ significantly between normals and amblyopes for 12′, 20′ and 50′ checks. Ipsilateral (CII) and contralateral (P105) onset components also differed significantly but for the smallest 12′ checks only. In controls, onset components (P105 and CIII) and, reversal components (N80 and P100) showed significantly shorter binocular as compared with monocular latencies. These latency differences were not found in amblyopes. Our results show that interocular interaction in normals is best shown by potentials which predominantly reflect macular pathway activation, and are most conspicuous for reversal N80 and P100 components. Similarly, these components demonstrated the clearest differences when comparing binocular interaction effects between controls and amblyopic subjects.     

Comparison of the pattern reversal visual evoked potential mediated by separate cone systems

With the purpose of recording responses mediated by the 3 cone systems visual evoked potentials (VEPs) were elicited by the reversal of monochromatic checkerboards superimposed upon strong monochromatic backgrounds (yellow, purple and blue-green).The sensitivity to light of various wave lengths were measured as the reciprocal of the intensity necessary to elicit a VEP amplitude of 3 μV. The spectral sensitivity curves based on this VEP amplitude criterion in the presence of blue-green, purple and yellow adaptation showed peak sensitivities in the red, the green and the blue part of the spectrum, respectively. This indicates that the responses reflect separate modulation of the 3 different cone mechanisms.The potentials obtained with yellow adaptation differed from those obtained with purple and blue-green adaptation. The amplitude versus log intensity function was flatter and the latency of the major positive peak was increased by 20–25 msec.Repeated examinations of 4 subjects suggest that the method yields reliable latency measurements of responses mediated by separate cone mechanisms.     

Spatial dissociation of early and late colour evoked components

Evoked potentials to the primary colours red, green, yellow and blue were recorded and compared with those evoked by white. The unpatterned 10° × 13° stimuli were generated with the aid of a colour monitor. Activity was depicted with 5 electrodes, the central electrode 5 cm above the inion and two on each side 5 and 10 cm apart from the central electrode.With equally bright colour stimuli a previously described early negative colour-dominated component N87 was localized in all subjects at the central occipital electrode while the following positivity P100 was clearly more lateralized to the peripheral electrodes. With half-field stimulation N87 showed a similar — paradoxical — lateralization to the ipsilateral electrodes as has been demonstrated for pattern reversal.The existence and localization of N87 could be confirmed also for blue colours, its amplitude independent of the blue luminance, its latency decreasing for definite additional brightness increments and decrements. The N87 to blue was of the same latency as the N87 components to other colours.N87 is mainly generated foveally and parafoveally, its amplitude saturates with stimuli larger than 6–8° in diameter.     

The phases of visual cortex and hippocampal evoked potentials in rabbit reflect the orienting reaction in case of visual stimuli intensity changes

The oddball paradigm was applied in experiments with waking rabbits using rare (deviant) and frequent (standard) stimuli, which were similar in color but different in intensity, deviant ones being of lower intensity. In addition, the VEPs to the single deviants were averaged. Such single deviants (without standard stimuli) were given at the beginning and at the end of the stimulation. The positivity of cortical and hippocampal visual evoked potentials to deviant stimuli increased in comparison to responses to standards and single deviants. The VEP-peaks P1 and P2 in the visual cortex and the VEP P1, N1 and P2 peaks in hippocampus increased. The most prominent significant changes were demonstrated for cortical VEP P2 (P130) peak. It is suggested that the increase of positivity to oddball deviants is due to the orienting reflex arising in response to rare stimuli. The increase of P2-peak can be connected with the transfer of information signaling changes of light intensity. It was demonstrated that the most clear and contrast differences in the VEPs to deviants and standards took place for the case of minimal distinction in their intensities. This fact may reflect the orienting reactionto threshold stimuli.     

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.     

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.     

Amplitude asymmetry of hemifield pattern reversal VEPs in healthy subjects

The amplitudes of transient and steady-state visual evoked potentials (VEPs) were measured during hemifield stimulation of the left eye in 10 healthy adults. Pattern reversal of a checkerboard was produced at 4 stimulation frequencies: 1, 5, 10 and 15 Hz. The amplitudes of pattern VEPs were evaluated using the paired t test to determine significant differences between right and left hemifields. The transient VEP amplitudes from midoccipital, midparietal, ipsilateral occipital and contralateral occipital electrodes were significantly greater with right hemifield stimulation. The steady-state VEP amplitudes from the midoccipital electrode during 15 Hz stimulation were significantly greater with right hemifield stimulation. Our neurophysiological data may be compatible with neuroanatomical asymmetries of the occipital lobes in humans.     

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

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

视觉图形诱发的瞳孔反应

以图形变化刺激进行瞳孔反应研究,实验表明：（１）空间平均亮度守恒的光栅或棋盘格图形翻转能激发起瞳孔反应,为瞬态收缩波形,与ｐｕｐｉｌｌａｒｙ ｅｓｃａｐｅ相似;（２）光栅或棋盘格的空间频率的变化也能引起瞳孔反应,且反应幅度随空间频率差别增大而变大;（３）从均匀亮背景变化到棋盘格图形或者从棋盘格图形变化到黑暗背景,虽然不存在任何局部亮度增强,皆能引起瞳孔反应。实验结果明确证明了人的瞳孔反应系统除接收     

Three-dimensional human pattern visual evoked potentials. II. Multiple sclerosis patients

Pattern visual evoked potentials were obtained from 46 patients with definite relapsing/remitting multiple sclerosis, using both a conventional 5-channel occipital array and a 3-D recording technique consisting of three bipolar derivations approximating the three dimensions of space. These three orthogonal wave forms were displayed as a 3-D Lissajous trajectory for each subject. Two of the 15 patients with completely normal conventional pattern VEPs had abnormalities of the orientation of the B-C curvilinear segment of the 3-D pattern VEPs. Delays in the first major occipital positive component (P100) were evident using both techniques; the correlation between P100 latency and the latency of the corresponding trajectory apex was r = 0.99 (P < 0.01). Post-chiasmal MRI abnormalities were associated with 3-D VEP orientation abnormalities. Three-dimensional pattern VEPs are moderately more sensitive than conventional pattern VEPs at detecting dysfunction posterior to the optic chiasm in demyelinating disease and do not require the use of eccentric fixation to do so.     

Amplitudes of visually evoked potentials to patterned stimuli: Age and sex comparisons

Electrophysiological age and sex differences in visual pattern responsivity were investigated. Pattern reversal evoked potentials (PREPs) and visual evoked potentials (VEPs) to patterned and unpatterned flashes were recorded from 20 normal subjects in each of 4 groups: young females and males aged 25–35 years and older females and males aged 55–70 years. PREP waves N70-P100 and P100-N150 from the older women were significantly larger than those from subjects in the other groups; mean amplitudes for the young females, young males and older males were not different. A similar effect, unusually large potentials for the older women, was obtained for VEPs, but only for VEPs elicited by patterned flashes and recorded from occipital scalp, i.e., an area overlying visual cortex which is sensitive to lines and edges. Our findings suggest that the visual system of older females is unusually responsive to patterned stimuli.     

Visual evoked potentials in response to dichoptic presentation of sinusoidal grating and noise background

Dichoptic stimulation was used in comparison of visual evoked potentials (VEPs) with those obtained with monocular stimulation (recordings made from the occipital area). 16 subjects viewed sinusoidal gratings with the right eye while a visual noise was added via a mirror for the left eye. In presence of the noise, amplitude of the early VEP components' N1, P1b, and the late component P2 decreased, P1a is not changed in presence of the noise, and the late negative wave N2 increased for all spatial frequencies. The effect of noise on the amplitude of VEPs obtained for monocular and dichoptic stimulation was similar. The data suggest that external noise is filtered by the V1 cortical neurons--matched filters for the gratings.     

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.     

Brain potentials associated with pattern displacement and saccadic eye movements in humans and rhesus monkeys

Visual evoked potentials (VEPs) to the onset of motion of visual patterns and brain responses associated with saccadic eye movements (SRPs) were compared in human subjects and in rhesus monkeys. Three different velocities of pattern motion were employed. In humans, brain responses were recorded from six scalp areas. In monkeys, transcortical recordings were obtained from chronically implanted electrodes in the occipital, temporo-parietal, and frontal areas. In humans there was a clear difference in VEPs to the pattern motion between the anterior (Fz, Cz) and posterior (Pz, Oz) scalp regions. The earliest component was a positive peak at 85 ms at Oz followed by a negativity around 110 ms. In the fronto-central leads the VEP was characterized by a negativity at 145 ms and a subsequent broad positive component around 250 ms. SRP responses differed in the early components from the VEPs to pattern motion but a good correspondence was found in the morphology of the late components of the two types of brain potentials. Furthermore, flashed-on VEPs and SRPs elicited a late positivity of more pronounced amplitude than VEPs to pattern displacement. In monkeys similar findings were found: an early negative component of the pattern-displacement VEP could not be observed in the SRP responses over the visual cortex while the late portion of the SRP waveform was greater than the late positivity of the VEP to motion-onset.     

Neuromagnetic index of hemispheric asymmetry prognosticating the outcome of sudden hearing loss

The longitudinal relationship between central plastic changes and clinical presentations of peripheral hearing impairment remains unknown. Previously, we reported a unique plastic pattern of "healthy-side dominance" in acute unilateral idiopathic sudden sensorineural hearing loss (ISSNHL). This study aimed to explore whether such hemispheric asymmetry bears any prognostic relevance to ISSNHL along the disease course. Using magnetoencephalography (MEG), inter-hemispheric differences in peak dipole amplitude and latency of N100m to monaural tones were evaluated in 21 controls and 21 ISSNHL patients at two stages: initial and fixed stage (1 month later). Dynamics/Prognostication of hemispheric asymmetry were assessed by the interplay between hearing level/hearing gain and ipsilateral/contralateral ratio (I/C) of N100m latency and amplitude. Healthy-side dominance of N100m amplitude was observed in ISSNHL initially. The pattern changed with disease process. There is a strong correlation between the hearing level at the fixed stage and initial I/C(amplitude) on affected-ear stimulation in ISSNHL. The optimal cut-off value with the best prognostication effect for the hearing improvement at the fixed stage was an initial I/C(latency) on affected-ear stimulation of 1.34 (between subgroups of complete and partial recovery) and an initial I/C(latency) on healthy-ear stimulation of 0.76 (between subgroups of partial and no recovery), respectively. This study suggested that a dynamic process of central auditory plasticity can be induced by peripheral lesions. The hemispheric asymmetry at the initial stage bears an excellent prognostic potential for the treatment outcomes and hearing level at the fixed stage in ISSNHL. Our study demonstrated that such brain signature of central auditory plasticity in terms of both N100m latency and amplitude at defined time can serve as a prognostication predictor for ISSNHL. Further studies are needed to explore the long-term temporal scenario of auditory hemispheric asymmetry and to get better psychoacoustic correlates of pathological hemispheric asymmetry in ISSNHL.     

Visual evoked potentials in a patient with prosopagnosia

Visual evoked potentials (VEPs) were recorded from a 53-year-old man with prosopagnosia during presentation of slides of known and unknown faces and under two control conditions. ANOVA comparisons with a normal male group showed no differences in P100 amplitude, P300 amplitude or P300 latency. There were no significant evoked potential differences between the patient and controls specifically related to the face conditions.There was, however, a significant delay in the latency of P100 from both hemispheres during all types of stimuli. This prolonged latency was asymmetrical, showing a right sided emphasis with the control conditions: pattern reversal and slides of geometric designs. This finding, of a dissociation in the interhemispheric delay, provides physiological evidence of stimulus-specific organisation at an early, sensory level.The fact that the P100 component showed a marked delay, yet P300 fell within normal limits for amplitude and latency, suggests that this patient's problem lies at a perceptual level.     

Binary matrices and checkerboard distributions of birds in the Bismarck Archipelago

Aim We examine a presence–absence matrix of the avifauna of the Bismarck Archipelago, for which the concept of competitively driven community assembly rules was formulated, to determine whether data support widespread competitive determination of geographical distributions. Location Bismarck Archipelago. Methods We obtained occurrences of 154 land and freshwater bird species on 31 islands. We calculated the observed number of checkerboards for all species pairs, for congeneric species pairs and for pairs of species within guilds, and employed randomization techniques to detect unusual co‐occurrence patterns. Results Compared with random expectations, there are excesses of checkerboard pairs within both genera and defined guilds, but a detailed examination shows that competition is a cogent possible explanation in few instances. For many checkerboard pairs, species are not widely interspersed but are regionally allopatric, which probably reflects historical biogeography and dispersal limitation. Most congeneric and intraguild checkerboards include a species classified as a supertramp; when supertramps are omitted, there are 11 congeneric checkerboards and four intraguild but heterogeneric checkerboards. Main conclusions In isolation, presence–absence matrices provide limited insight into the role of competition in structuring bird communities of the Bismarcks. A major problem is disentangling historical geography and colonization history of the archipelago from the present‐day ecology of the species. Examination of observed checkerboards from a geographically explicit perspective and with knowledge of colonization routes suggests that many checkerboards are likely to result, at least in part, from historical biogeography and supertramps. Although species may be forced into supertramp status by competition, other factors (e.g. habitat preference) may be causal, and biogeographical distributions alone cannot distinguish between causes.