Steady-state visual evoked potentials and travelling waves.

OBJECTIVE: The amplitude and phase of the steady-state visual evoked potential (SSVEP) is sensitive to cognition and attention but the underlying mechanism is not well understood. This study examines stimulus evoked changes in the SSVEP phase topography and the putative role of travelling waves. METHODS: Eighteen subjects viewed a central-field checkerboard and full-field flicker stimulus temporally modulated at the peak alpha rhythm frequency. EEG was recorded from 10 midline scalp sites and the bipolar SSVEP obtained from differences between adjacent electrodes. RESULTS: The SSVEP phase comprised either progressive variations consistent with travelling waves or a phase reversal consistent with standing waves. The checkerboard pattern elicited travelling wave patterns in 14 subjects with estimated phase velocities ranging from 7 to 11 m/s after correcting for folded cortex. The flicker stimulus elicited phase reversals in 9 subjects, suggesting standing waves. Six subjects demonstrated a phase topography specific to the stimulus with travelling wave patterns associated with the checkerboard and standing wave patterns associated with the flicker. CONCLUSIONS: These differences suggest the emergence of travelling and standing waves under different spatial configurations of visual input to the cortex and that wave phenomena contribute to the spatiotemporal dynamics of the SSVEP.     

Subdurally recorded pattern and luminance EPs in the alert rhesus monkey

Two rhesus monkeys (Macaca mulatta) were trained to fixate a TV screen on which checkerboard and bar patterns could be presented. Stainless steel skull electrodes and electrode bundles under the dura, each containing 7 leads permitting 7 recording sites at 5 mm intervals, were placed over the temporal-occipital cortex and used to obtain visually evoked potentials (VEPs). The following conclusions were reached: VEPs recorded with skull electrodes resemble scalp recorded VEPs. Subdural electrodes may yield larger VEP amplitudes than skull or scalp electrodes. Subdural VEPs vary more in shape with recording site than scalp or skull recorded VEPs. Apart from differences in shape there are topographical differences between luminance and pattern EPs; the luminance EP is localized near the occiput and the pattern EP is most pronounced at the temporal site overlying the foveal projection area. For a given recording site it can be seen that: (a) the amplitude of the pattern reversal EP is smaller than that of the pattern onset EP; (b) the shape of the reversal EP resembles the pattern offset EP; and (c) the shape of the pattern onset EP is rather invariant to check size and bar width. On the basis of the pattern EP the striate and prestriate cortical areas can be distinguished as follows: (a) the prestriate cortex yields a positive-negative-positive (PNP) response complex to pattern onset and the striate cortex a PN response complex; and (b) the striate cortex responds to finer patterns than the prestriate cortex.     

Changes in pattern electroretinograms to equiluminant red-green and blue-yellow gratings in patients with early Parkinson's disease.

In Parkinson's disease (PD), the luminance pattern electroretinogram (PERG) is reported to be abnormal, indicating dysfunction of retinal ganglion cells (RGCs). To determine the vulnerability of different subpopulations of RGCs in PD patients, the authors recorded the PERG to stimuli of chromatic (red-green [R-G] and blue-yellow [B-Y]) and achromatic (yellow-black [Y-Bk]) contrast, known to emphasize the contribution of parvocellular, koniocellular, and magnocellular RGCs, respectively. Subjects were early PD patients (n = 12; mean age, 60.1 +/- 8.3 years; range, 46 to 74 years) not undergoing treatment with levodopa and age-sex-matched controls (n = 12). Pattern electroretinograms were recorded monocularly in response to equiluminant R-G, B-Y, and Y-Bk horizontal gratings of 0.3 c/deg and 90% contrast, reversed at 1Hz, and presented at a viewing distance of 24 cm (59.2 x 59 degree field). In PD patients, the PERG amplitude was significantly reduced (by 40 to 50% on average) for both chromatic and luminance stimuli. Pattern electroretinogram latency was significantly delayed (by about 15 ms) for B-Y stimuli only. Data indicate that, in addition to achromatic PERGs, chromatic PERGs are altered in PD before levodopa therapy. Overall, chromatic PERGs to B-Y equiluminant stimuli exhibited the largest changes. Data are consistent with previous findings in PD, showing that visual evoked potentials (VEP) to B-Y chromatic stimuli are more delayed than VEPs to R-G and achromatic stimuli. The results suggest that the koniocellular subpopulation of RGCs may be particularly vulnerable in early stages of Parkinson's disease.     

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.     

Sequential spatial maps of visual potentials evoked by checkerboard-pattern reversal: effect of the retinal field stimulated on response topography

Sequential color maps of visual potentials evoked by the reversal of various checkerboard patterns were recorded in 10 young adults using a 16 channel montage. It was found that each of the components of the N75-P100-N145 occipital complex had a specific spatial distribution on the scalp and was selectively influenced by the size, the spatial frequency, the luminance and possibly the wave length of the stimulus. Component N75 was found to be elicited by the more peripheral area of the TV stimulus (12 degrees X 16 degrees). Component P100 was associated with a frontal negativity of similar latency favoring the hypothesis of a dipolar occipital generator. With half-field stimulations the dipole orientation was modified, leading to a 'paradoxical' lateralization of P100 in most cases. However the reverse situation (P100 contralateral to the stimulated half-field) was observed in 4 and 3 subjects out of 10 with left and right half-field stimulations respectively. Thus VEP to full-field TV pattern reversal cannot be recommended to investigate hemianopic patients. Component N145 was of maximal amplitude when elicited by the reversal of small foveal patterns (2.18 degrees), especially red light emitting diodes.     

Alteration of visual evoked potentials and electroretinograms in Parkinson's disease

A group of 24 patients with Parkinson's disease (PD) with normal fundi and normal visual acuities was examined electrophysiologically. Checkerboard reversal VEPs and ERGs (P-ERGs) at various contrast levels as well as photopic and scotopic luminance ERGs were recorded and compared with an age-matched group of controls. Earlier reported latency increases of the VEPs of the patients were confirmed for patterns of high contrast only. Scotopic and photopic luminance ERGs of the patients showed normal latencies, but at all light intensities the amplitudes of the scotopic and photopic b wave, as well as the amplitudes of the photopic a waves, were significantly reduced, P-ERG amplitudes were reduced at 50% contrast. Identical results were obtained in patients under dopaminergic treatment (n = 17) and in patients who did not receive any treatment (n = 7). These results suggest that alterations occur already at the retinal level where dopamine receptors have been found. Thus the reported changes of the VEP are not caused by the visual cortex alone.     

The effect of adaptation to the stimulating pattern on the latency and wave form of visual evoked potentials

Visual evoked potentials (VEPs) elicited with a checkerboard stimulus after adaptation to an unpatterned grey field were compared to those obtained after adaptation to the stimulating pattern for an equal interval of time. When 3--4 sec elapsed between the adapting interval and recording of the VEP, the latency of the principal positive peak increased by 4 msec with pattern-reversal, but not with pattern-onset stimulation, while the amplitude was unchanged. However, when the pattern-onset VEP was recorded immediately after adaptation, the principal negative peak of the response was lost and the latency of its principal positive peak increased by 14 msec, causing it to mimic the conventional pattern-reversal VEP in both wave form and latency, and suggesting that adaptation may cause the wave form differences in the VEPs obtained by these two methods.     

The effect of background illumination on pattern onset visual evoked potentials

The early part (first 200 msec) of pattern onset VEPs elicited by a dartboard pattern was studied in conditions of varying level of background illumination. The effect of pattern adaptation and pattern blurr was also studied. The observed complex behaviour of the main negativity within this part of the VEP can be best described in terms of a composite of two independent negative peaks labelled N100 and N130. In high luminance conditions peak N100 was dominant and the presence of N130 was indicated only by a 'notch' on the rising slope of the negativity. As luminance decreased the situation was reversed and N130 became a dominant feature of the negative wave. This finding did not depend on the particular choice of reference site. For checkerboard stimulation the same features were present, but variability of the VEP wave form was greater than in the case of dartboard stimulation. Present results relate the well-known pattern specific properties of the negativity in onset VEPs to N100 only, whereas N130 is not pattern specific. Lower and upper half-field stimulation produced peaks of opposite polarity at 100 msec but no change was observed in polarity of N130. These findings support the suggestion that these two parts of the negativity in pattern onset VEPs may have different cortical sources.     

The spatial tuning of steady state pattern electroretinogram in multipie sclerosis

In normal subjects, the steady-state electroretinogram in response to contrast reversing gratings (PERG), is spatially band-pass tuned in amplitude, with a maximum at intermediate spatial frequencies and an attenuation at lower and higher ones. The amplitude attenuation at low spatial frequencies is believed to reflect centre-surround antagonistic interactions in the receptive fields of inner retinal neurons. The aim of this study was to evaluate the PERG spatial tuning in multiple sclerosis (MS) patients without a previous optic neuritis history. Steady- state PERGs in response to counterphase-modulated (8 Hz) sinusoidal gratings of variable spatial frequency (0.6, 1.0, 1.4, 2.2 and 4.8 c/deg), were recorded from 18 patients with definite or probable MS and no history of optic neuritis (ON-). Nine of them had no signs of subclinical optic nerve demyelination (asymptomatic) in either eye, while nine had symptoms or signs of optic pathways involvement (symptomatic) in one or both eyes. Results were compared with those obtained from 10 MS patients with a previous history of optic neuritis (ON+) in one or both eyes, as well as from 21 age-matched controls. The amplitudes and phases of the responses' 2nd harmonics were measured. Compared with the controls, asymptomatic ON- patients showed selective losses in mean PERG amplitudes at medium and high (1.0-4.8 c/deg) spatial frequencies. Symptomatic ON- patients and ON+ patients had reductions in mean PERG amplitudes, with respect to controls, involving the whole spatial frequency range, but with greater losses at medium-high (1.0-4.8 c/deg) than at lower spatial frequencies. In all patients' groups, the average PERG spatial tuning function differed significantly from that of the controls, assuming a low-pass instead of the normal band-pass shape. The PERG phase was delayed in ON+ but not in ON- patients, as compared to controls. However, the phase delay was independent of spatial frequency. In both ON- and ON+ patients, losses in PERG amplitude and spatial tuning tended to be associated with corresponding abnormalities in perimetric sensitivity, visual acuity, colour vision and transient visual evoked potential (VEP) latency. The results indicate that abnormalities of the spatial tuning of steady-state PERG can be found in MS patients without either optic neuritis or signs of subclinical optic nerve demyelination. These changes may reflect a retinal dysfunction, developing early in the course of MS, due to a loss of specific subpopulations of inner neurons, changes in lateral interactions of their receptive fields, or both.     

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.     

Coloured pattern displacement and VEP amplitude.

The results of this experiment confirm that evoked potential amplitude is linear with checkerboard pattern displacement. We have also found that this linear relationship holds for red, green and blue patterns. The hypothesis that there would be no significant differences between the amplitude of the evoked potential to colour/'black' stimulation and a white/'black' stimulus of equivalent luminance has been substituted. Differences were found between colours. The mean level for the blue pattern was lower than that for the red and the green patterns. We have further confirmed the relationship between luminance and the latency of the evoked potential. Differences in latency were found between all conditions in this study, suggesting that more accurate methods of measuring luminance are required in studies using colour pattern displacement if latency measurements are the main focus of the study.     

Visual evoked potential abnormalities in dyslexic children.

Developmental reading disability (dyslexia) has traditionally been attributed to impaired linguistic skills. Recent psychophysical data suggest that dyslexia may be related to a visual perceptual deficit. A few visual evoked potential (VEP) studies have addressed this hypothesis, but their results are far from consistent. We submitted 9 dyslexic subjects and 9 age- and sex-matched normal controls to checkerboard pattern reversal VEPs. The main experimental variables were: large (0.5 cycles per degree; cpd) and small (2 cpd) checks and two reversal frequencies (2.1 Hz and 8 Hz); mean luminance and contrast (60 cd/m2 and 50%, respectively) were kept constant in all four conditions. Transient VEP (2.1 Hz) parameters did not differ between controls and dyslexics at 2 cpd. At 0.5 cpd, N70 amplitude was significantly smaller and N70 latency significantly shorter in dyslexics. Amplitudes for the fundamental frequency (8 Hz), as well as for the second and third harmonics of the steady-state VEPs were smaller in dyslexics for both stimulus sizes. A discriminant analysis correctly classified each subject. Our data confirm the hypothesis of a perceptual deficit in dyslexic subjects. The abnormalities are related to spatial and temporal stimulus frequencies: they appear when large stimuli are presented, or when the stimulation frequency is high. These data support the hypothesis of selective magnocellular dysfunction in dyslexia.     

Visual evoked responses in multiple sclerosis: comparison of two methods for pattern reversal

Visual evoked responses have been elicited by reversal of a black and white checkerboard pattern and also by a display of light-emitting diodes (LED) generating a reversal of a pattern of illuminated red circular areas. In a control group the LED display produced responses of lower amplitude and shorter latency compared to responses to checkerboard stimulation. In patients with a definite or probable diagnosis of multiple sclerosis the stimulators were equally effective in evoking pathological responses; in patients with a possible but less certain diagnosis abnormal responses were found in a higher percentage when the LED stimulus was used. Differences in stimulus field size and in stimulus colour are discussed as possible reasons for this result.     

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.     

Evoked potentials in albinos: efficacy of pattern stimuli in detecting misrouted optic fibers

Misrouting of retinogeniculostriate projections associated with retinal hypopigmentation has been found to be a general phenomenon in mammals, including humans. Anomalous optic pathway projections of albinos may be detected by recording visually evoked potentials (VEPs). To determine the efficacy of various stimulus conditions for detecting misrouted optic fibers, we compared flash, modulated light, pattern reversal and pattern onset/offset (appearance/disappearance) stimuli. Pattern onset/offset were found superior in detecting anomalies of misrouting of retinogeniculostriate projections. As a group, humans with retinal hypopigmentation have poor evoked potentials to offset and pattern reversal stimuli. Nystagmus seems to be most symptomatic of poor pattern reversal responses. In general onset/offset pattern stimuli are more versatile for detection of abnormalities in the visual system than pattern reversal stimuli.     

Follow-up studies of visual potentials in multiple sclerosis evoked by checkerboard and foveal stimulation

Visual cortical potentials were evoked by reversal of a checkerboard pattern and a small quadrangular foveal stimulus. Examination of 68 patients with MS showed the highest detection rate for abnormality of VEPs with a combination of both methods. Follow-up studies revealed changes (improvement or impairment) of VEP amplitudes or latencies in nearly half of the patients. Most of the changes were correlated with a history of acute optic neuritis. For the detection of changes the foveal stimulus is preferable to the checkerboard pattern.     

The effect of standing contrast and pattern adaptation on the human visually evoked potential

Human visually evoked potentials (VEPs) were recorded with sinusoidal gratings of low (0.5 c/deg), medium (4 c/deg) and high (16 c/deg) spatial frequency (SF). Simultaneous recording from Oz-A1, O2-A1 and Oz-O2 allowed the separation of the early and late VEP waves for most subjects. The effect of standing contrast and pattern adaptation on these waves was studied. At low SF both early and late waves were not affected by standing contrast or adaptation with motionless grating, while adaptation with a drifting grating reduced them. At medium and high SFs both principal negative waves. N1 and N2, were reduced by standing contrast and pattern adaptation. The amplitude of N2 saturated at low contrast level (0.1) and was reduced after adaptation regardless of the SF of test and adapting stimuli, while the amplitude of N1 did not saturate up to the contrast level of 0.3 and exhibited SF-selective adaptation. The adapted onset-VEP was similar to the offset-VEP. The data suggest that: the early wave is generated by multiple narrowly tuned SF-selective structures that are medially positioned while the generators of the late wave are not SF-selective and occupy a wider area centered laterally; the effects of standing contrast and pattern adaptation on VEPs, as well as the relationship of onset and offset VEPs, reflect the time-course of neural activity evoked by long-lasting stimuli.     

Source geometry and dynamics of the visual evoked potential

A method is developed for the analysis of the steady-state visual evoked potential (VEP) recorded at an array of electrodes. The VEP is assumed to result from the sum of a number of independent mechanisms. Each mechanism proves to have a fixed intracerebral source (independent of stimulus paradigm) and dynamics which depend on the stimulus. The steps in the analysis are: Fourier analysis of the responses and retention of the first 4 even harmonics; factor analysis in the frequency domain; an invariant representation ('the invariant fingerprint') of the geometric information contained in the factor analysis; and interpretation of the invariant fingerprint in terms of a simple biophysical model. This analysis is applied to responses elicited by a contrast-reversing checkerboard. The dependence of the midline VEP wave form on reversal frequency, check size, and area of stimulation (upper, lower, and full-field) reflects a change in the dynamics of the generators, rather than a change in their geometry.     

Simultaneous recording of pattern electroretinogram (PERG) and visual evoked potential (VEP) in multiple sclerosis

We studied 18 multiple sclerosis (MS) patients affected by retrobulbar neuritis (RBN). The patients were subdivided into two groups. Group 1: 14 patients with RBN. Group 2: 4 patients with optic atrophy. An ophthalmological examination (visual acuity, fundus oculi, visual field) was carried out in all the patients. A simultaneous visual evoked potential (VEP) and pattern electroretinogram (PERG) recording at two spatial frequencies (45' and 15') was performed. All the data obtained in Group 1 were compared (Student T-Test) with those of a control group of normal subjects matched for age and sex. Group 1. VEP: a comparison of the data in MS patients affected by RBN with the control group revealed a statistically significant P100 latency delay with both spatial frequencies (P less than 0.001). PERG: no "b" wave latency change at 45' and 15' spatial frequencies were seen. A "b" wave amplitude reduction was observed; this reduction reached significant values at 45' (P less than 0.001). Group 2. In optic atrophies the PERG was absent in 4 eyes at 45' and in 5 eyes at 15'.     

Contrast and stereoscopic visual stimuli yield lateralized scalp potential fields associated with different neural generators.

The use of dynamic random-dot stereograms (RDS) allows to investigate evoked potential components generated exclusively by cortical structures. We analyzed the scalp distribution of stereoscopically evoked or contrast evoked potential field by recording electrical brain activity in 20 channels simultaneously from an electrode array covering the occipital scalp areas. Evoked brain activity was obtained from 13 healthy adults with dynamic RDS stimuli presented as a stereoscopic checkerboard pattern in the center, or in the right or left visual half-field. Such stereoscopically evoked scalp potential distributions were compared to those elicited by a conventional 2-dimensional checkerboard reversal stimulus of the same mean luminance and retinal extent. We found that the latencies of the major evoked components were similar for contrast and stereoscopic stimuli, while significant differences were observed when we compared the strength of the evoked potential fields or the topographical pattern elicited by lateralized stereoscopic and contrast stimuli. The functional relation of evoked electrical brain activity to the retinal stimulus location was significantly different for stereoscopic and contrast stimuli. We present evidence that stereoscopic perception relies on the activation of cortical structures in the human visual system that are different from those activated by comparable contrast stimuli, supporting the conclusions derived from our earlier electrophysiological experiments on stereoscopic vision. These data on the physiological correlates of processing of stereoscopic information in humans are in line with the results obtained with single neuron recordings from the cat and monkey visual cortex.