Identification of the hemisphere activated by hemifield visual stimulation using a single equivalent dipole model

Right visual field advantage in parafoveal processing: evidence from eye-fixation-related potentials

Readers acquire information outside the current eye fixation. Previous research indicates that having only the fixated word available slows reading, but when the next word is visible, reading is almost as fast as when the whole line is seen. Parafoveal-on-foveal effects are interpreted to reflect that the characteristics of a parafoveal word can influence fixation on a current word. Prior studies also show that words presented to the right visual field (RVF) are processed faster and more accurately than words in the left visual field (LVF). This asymmetry results either from an attentional bias, reading direction, or the cerebral asymmetry of language processing. We used eye-fixation-related potentials (EFRP), a technique that combines eye-tracking and electroencephalography, to investigate visual field differences in parafoveal-on-foveal effects. After a central fixation, a prime word appeared in the middle of the screen together with a parafoveal target that was presented either to the LVF or to the RVF. Both hemifield presentations included three semantic conditions: the words were either semantically associated, non-associated, or the target was a non-word. The participants began reading from the prime and then made a saccade towards the target, subsequently they judged the semantic association. Between 200 and 280ms from the fixation onset, an occipital P2 EFRP-component differentiated between parafoveal word and non-word stimuli when the parafoveal word appeared in the RVF. The results suggest that the extraction of parafoveal information is affected by attention, which is oriented as a function of reading direction.

Study of the human visual cortex: direct cortical evoked potentials and stimulation

The authors studied the visual cortex of 15 patients undergoing studies for medically intractable epilepsy. Although the subdural and strip electrode placement varied in each of these patients, there were enough electrodes over the visual cortex to complete studies involving evoked potentials and direct cortical stimulation. Visual evoked potentials were elicited using two check sizes (50 and 16 min) for pattern reversal studies, 50 min checks for on-off stimulation, 50 min checks for horizontal and vertical hemifields and simple flash for the VEP. These studies demonstrated that the pattern reversal and on-off stimuli caused very complex, multipotential waveforms in striate and vision associational cortex that do not resemble the response obtained at the scalp. Different volumes of visual cortex are activated by stimulation with 16 min checks, 50 min checks and simple flash. Flash activates the largest volume of visual cortex and it is likely that this finding is what makes this test of so little value clinically. Direct cortical stimulation shows that colored responses are generated primarily in the posterior striate cortex and inferior occipital lobe, while movement is primarily generated by the visual association cortex. No complex visual images were obtained by stimulation of either the striate cortex or visual association cortex. The brain mechanisms that lead to formed visual images remain to be identified.

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.

EEG source localization and global dimensional complexity in high- and low- hypnotizable subjects: a pilot study

Individuals differ in hypnotizability. Information on hypnotizability-related EEG characteristics is controversial and incomplete, particularly on intracerebral source localization and EEG dimensionality. 19-channel, eyes-closed resting EEGs from right-handed, healthy, 8 high- and 4 low-hynotizable subjects (age: 26.7 +/- 7.3 years) were analyzed. Hypnotizability was rated after the subjects' ability to attain a deep hypnotic stage (amnesia). FFT Dipole Approximation analysis in seven EEG frequency bands showed significant differences (p < 0.04) of source gravity center locations for theta (6.5-8 Hz, more posterior and more left for highs), beta-1 and beta-2 frequencies (12.5-18 and 18.5-21 Hz; both more posterior and more right for highs). Low Resolution Electromagnetic Tomography (LORETA) specified the cortical anteriorization of beta-1 and beta-2 in low hypnotizables. Power spectral analysis of Global Field Power time series (curves) showed no overall power differences in any band. Full-band Global Dimensional Complexity was higher in high-hypnotizable subjects (p < 0.02). Thus, before hypnosis, high and low hypnotizables were in different brain electric states, with more posterior brain activity gravity centers (excitatory right, routine or relaxation left) and higher dimensional complexity (higher arousal) in high than low hypnotizables.

Identification of the stimulated hemiretina in primary school children and adults based on left and right hemifield pattern reversal visual evoked potentials--a comparative study

OBJECTIVES

The analysis of left and right hemifield pattern-reversal visual evoked potentials (PVEPs) in children and the identification of the stimulated hemiretina testing different identification procedures previously applied to adults.

METHODS

Lateral hemifield PVEPs were recorded in 40 children (6-11 years) and 27 adults (25-40 years) from, at least, 19 standard electrodes. Two procedures were tested for the determination of the stimulated hemifield: firstly, the evaluation of the values of instantaneous frequency at the occipital electrodes at P100 latency (determined by the global field power), and secondly, the application of a generalised dynamic neural network (GDNN) using the PVEP time course at selected electrode positions as the external input.

RESULTS

P100 latency as well as P100 amplitude over the contralateral occiput in children were significantly greater than in adults. Contrary to the behaviour in adults, instantaneous frequency is not a robust identifier of left and right hemiretina stimulation in children. The best identification performances were achieved when using group trained GDNNs with the bipolar difference signals of electrodes P3/P4 or T5/T6 as the external input.

CONCLUSIONS

The PVEPs at electrodes P3/P4 and T5/T6 contain essential information for the determination of the stimulated hemifield. This should be further considered during the development of on-line procedures for automatic PVEP detection in future studies.

The visual word form area: spatial and temporal characterization of an initial stage of reading in normal subjects and posterior split-brain patients

A standard model of word reading postulates that visual information is initially processed by occipitotemporal areas contralateral to the stimulated hemifield, from whence it is subsequently transferred to the visual word form (VWF) system, a left inferior temporal region specifically devoted to the processing of letter strings. For stimuli displayed in the left visual field, this transfer proceeds from the right to the left hemisphere through the posterior portion of the corpus callosum. In order to characterize the spatial and temporal organization of these processes, reading tasks with split-field presentation were performed by five control subjects and by two patients suffering from left hemialexia following posterior callosal lesions. The subjects' responses were studied using behavioural measures and functional brain imaging techniques, providing both high spatial resolution (functional MRI, fMRI) and high temporal resolution (high-density event-related potentials, ERPs). Early visual processing was revealed as activations contralateral to stimulation, located by fMRI in the inferior occipitotemporal region and presumably coincident with area V4. A negative wave occurring 150-160 ms post-stimulus, also strictly contralateral to stimulation, was recorded over posterior electrodes. In contrast with these hemifield-dependent effects, the VWF system was revealed as a strictly left-hemispheric activation which, in control subjects, was identical for stimuli presented in the left or in the right hemifield and was located in the middle portion of the left fusiform gyrus. The electrical signature of the VWF system consisted of a unilateral sharp negativity, recorded 180-200 ms post-stimulus over left inferior temporal electrodes. In callosal patients, due to the inability of visual information to pass across the posterior part of the corpus callosum, the VWF system was activated only by stimuli presented in the right visual field. Similarly, a significant influence of the word/non-word status on ERPs recorded over the left hemisphere was discernible for either hemifield in controls, while it affected only right-hemifield stimuli in callosal patients. These findings provide direct support for the main components of the classical model of reading and help specify their timing and cerebral substrates.

Identification of hemifield single trial PVEP on the basis of generalized dynamic neural network classifiers

This paper is concerned with the application of generalized dynamic neural networks for the identification of hemifield pattern-reversal visual evoked potentials. The identification process is performed by different networks with time-varying weights using signals from different electrode positions as external inputs. Since dynamic neural networks are able to process time-varying signals, the identification of the stimulated hemiretinae is performed without feature extraction. The performance of the method presented is compared with a reference method based on the values of instantaneous frequency at the occipital electrode positions at P100 latency.

Vector analysis of visual evoked potentials elicited by pattern reversal and photic stimuli

OBJECTIVE

The 2D VEPs to pattern reversal (PR) and LED goggle were studied in order to obtain a stable parameter for the functional assessing of posterior visual pathways regardless of the stimulus type used.

DESIGN AND METHODS

Apex c latency, bc segment amplitude (V), and bc vector orientation angle (theta) in voltage space were computed from VEPs recorded in 50 normal human beings and two patients with left posterior brain lesions, in an orthogonal Fpz-Oz and T3-T4 montage and displayed as a two channel Lissajous' trajectory. The effects of stimulus type and stimulated eye were analyzed in the normal group by a two-way ANOVA.

RESULTS

The stimulated eye had no effect on any parameter. Apex c latency was slightly longer, and V was greater and more variable in the responses to goggle stimuli, but there was no significant difference in theta, oriented to mid-occipital scalp with very low variability for both stimulus types. The patients showed significant deviations of theta towards the affected hemisphere.

CONCLUSIONS

The bc vector orientation (theta) is a stable parameter for the evaluation of the posterior visual pathways using both pattern reversal and LED stimuli, specially the latter, useful in unconscious or uncooperative patients.

Affective attitudes to face images associated with intracerebral EEG source location before face viewing

We investigated whether different, personality-related affective attitudes are associated with different brain electric field (EEG) sources before any emotional challenge (stimulus exposure). A 27-channel EEG was recorded in 15 subjects during eyes-closed resting. After recording, subjects rated 32 images of human faces for affective appeal. The subjects in the first (i.e., most negative) and fourth (i.e., most positive) quartile of general affective attitude were further analyzed. The EEG data (mean=25+/-4. 8 s/subject) were subjected to frequency-domain model dipole source analysis (FFT-Dipole-Approximation), resulting in 3-dimensional intracerebral source locations and strengths for the delta-theta, alpha, and beta EEG frequency band, and for the full range (1.5-30 Hz) band. Subjects with negative attitude (compared to those with positive attitude) showed the following source locations: more inferior for all frequency bands, more anterior for the delta-theta band, more posterior and more right for the alpha, beta and 1.5-30 Hz bands. One year later, the subjects were asked to rate the face images again. The rating scores for the same face images were highly correlated for all subjects, and original and retest affective mean attitude was highly correlated across subjects. The present results show that subjects with different affective attitudes to face images had different active, cerebral, neural populations in a task-free condition prior to viewing the images. We conclude that the brain functional state which implements affective attitude towards face images as a personality feature exists without elicitors, as a continuously present, dynamic feature of brain functioning.

Identification of the stimulated hemiretinae using a reduced number of PVEP trials

Left and right hemifield pattern-reversal visual evoked potentials (PVEP) at P100 latency are characterised by typical field distributions of potential and the spectral parameter instantaneous frequency (IF). Both parameters can be utilised for the correct identification of the stimulated hemiretina in healthy volunteers (Hoffmann, K. et al. Electroenceph. clin. Neurophysiol., 100, 1996: 569-578). The aim of this study was the investigation of the robustness of instantaneous frequency for reduced numbers of averages. Hemifield PVEP of 15 volunteers (20 channel records) were analysed. The number of averages was reduced step-by-step (64-32-16-8-4-1). For each average, the time of P100 latency was determined by the Global field power maximum between 90 and 125 ms. The stimulated hemifield was identified using the potential or instantaneous frequency values at the occipital electrode positions O1 and O2: by maximal potential and instantaneous frequency on the stimulus-contralateral side. In summary, by reduced numbers of averages (as well as by single trials) the stimulated hemiretina was correctly identified more frequently on the basis of instantaneous frequency than of potential distribution. A number of 8 averages seems to be sufficient for the correct identification of the stimulus condition. Consequently, for the identification of left and right hemifield stimulations the recording time could be reduced immensely. Instantaneous frequency is suggested as an additional and robust parameter for the selective averaging of artefact-free trials during the recording of hemifield PVEP.

Faces and emotions: brain electric field sources during covert emotional processing

Covert brain activity related to task-free, spontaneous (i.e. unrequested), emotional evaluation of human face images was analysed in 27-channel averaged event-related potential (ERP) map series recorded from 18 healthy subjects while observing random sequences of face images without further instructions. After recording, subjects self-rated each face image on a scale from "liked" to "disliked". These ratings were used to dichotomize the face images into the affective evaluation categories of "liked" and "disliked" for each subject and the subjects into the affective attitudes of "philanthropists" and "misanthropists" (depending on their mean rating across images). Event-related map series were averaged for "liked" and "disliked" face images and for "philanthropists" and "misanthropists". The spatial configuration (landscape) of the electric field maps was assessed numerically by the electric gravity center, a conservative estimate of the mean location of all intracerebral, active, electric sources. Differences in electric gravity center location indicate activity of different neuronal populations. The electric gravity center locations of all event-related maps were averaged over the entire stimulus-on time (450 ms). The mean electric gravity center for disliked faces was located (significant across subjects) more to the right and somewhat more posterior than for liked faces. Similar differences were found between the mean electric gravity centers of misanthropists (more right and posterior) and philanthropists. Our neurophysiological findings are in line with neuropsychological findings, revealing visual emotional processing to depend on affective evaluation category and affective attitude, and extending the conclusions to a paradigm without directed task.

Functional magnetic resonance imaging during recognition of written words: Chinese characters for concrete objects versus abstract concepts

An attempt was made to apply functional magnetic resonance imaging (fMRI) to reveal cortical areas activated upon presentation of two groups of Chinese characters in six normal right-handed, male, Japanese subjects. Presentation of the characters representing 'abstract concepts' activated the bilateral occipital region without a significant difference between the bilateral occipital and temporal regions. Presentation of the characters representing 'concrete objects' resulted in significantly stronger activation in the left occipital and temporal regions. These results suggest that recognition of concrete characters involves a stronger initial process in the left occipital temporal cortices than recognition of abstract characters.

Functional magnetic resonance imaging during recognition of written words: Chinese characters for concrete objects versus abstract concepts

An attempt was made to apply functional magnetic resonance imaging (fMRI) to reveal cortical areas activated upon presentation of two groups of Chinese characters in six normal right-handed, male, Japanese subjects. Presentation of the characters representing 'abstract concepts' activated the bilateral occipital region without a significant difference between the bilateral occipital and temporal regions. Presentation of the characters representing 'concrete objects' resulted in significantly stronger activation in the left occipital and temporal regions. These results suggest that recognition of concrete characters involves a stronger initial process in the left occipital temporal cortices than recognition of abstract characters.

Visual evoked cortical magnetic fields to pattern reversal stimulation

We studied visual evoked magnetic fields to pattern reversal stimulation in six healthy subjects. Similar to the N75-P100-N145 components in visual evoked potentials, triphasic deflections, N75m-P100m-N145m, were clearly observed around the midoccipital position. A very small component, P50m, was occasionally observed preceding the N75m. Equivalent current dipoles (ECDs) of the main deflection, P100m, to quadrant-field stimulation were estimated near or around the calcarine fissure contralateral to the stimulation. The vertical ECD location of the P100m to the upper quadrant-field stimulation was estimated significantly lower (0.81 +/- 0.45 cm) than those to lower stimulation. These results were compatible with the retinotopic organization of the visual cortex (cruciform model) and suggested that the P100m originated in the striate cortex. The small P50m, although only a small number of ECDs could be estimated reliably, was located in the contralateral visual cortex. ECDs of the N75m were estimated mainly near or around the contralateral calcarine fissure. ECDs of the N145m were estimated also retinotopically, but with a greater vertical distance (2.90 +/- 1.09 cm) between upper and lower quadrant-field stimulation. MR-overlaid ECDs of the N145m suggested that these originated in the extrastriate cortex. No ECD was estimated when a probe was placed at the midfrontal position.

Multichannel EEG fields during and without visual input: frequency domain model source locations and dimensional complexities

27-Channel EEG potential map series were recorded from 12 normals with closed and open eyes. Intracerebral dipole model source locations in the frequency domain were computed. Eye opening (visual input) caused centralization (convergence and elevation) of the source locations of the seven frequency bands, indicative of generalized activity; especially, there was clear anteriorization of alpha-2 (10.5-12 Hz) and beta-2 (18.5-21 Hz) sources (alpha-2 also to the left). Complexity of the map series' trajectories in state space (assessed by Global Dimensional Complexity and Global OMEGA Complexity) increased significantly with eye opening, indicative of more independent, parallel, active processes. Contrary to PET and fMRI, these results suggest that brain activity is more distributed and independent during visual input than after eye closing (when it is more localized and more posterior).

Instantaneous frequency maps, dipole models and potential distributions of pattern reversal-evoked potential fields for correct recognition of stimulated hemiretinae

Lateral hemifield pattern-reversal visual evoked potential (PVEP) field data were evaluated using potential distributions, dipole modelling and distributions of Hilbert transformation-based instantaneous frequency in order to determine the stimulated hemisphere. Twenty channel records were collected from 35 normal volunteers in two laboratories using similar stimulus conditions (11-20.5 degrees target, 60-75 min checks, 2/s reversal, 500 ms analysis epoch). P100 latency was determined in each average by the global field power maximum between 90 and 120 ms. Using the data from O1 and O2 at P100 latency, the stimulated hemisphere was identified by maximal potential or minimal instantaneous frequency on the stimulus-contralateral side, or, using the 20-electrodes data at P100 by the ipsilateral lateralization of the dipole model. Correct classification of the stimulated 70 hemiretinae was achieved by potential distribution in 44 cases, by dipole modelling in 54 cases and by instantaneous frequencies in 68 cases. Errors in the classification by potential distribution and dipole location were twice as frequent for decisions based on expected locations over the left than over the right hemisphere. This finding might be caused by the relatively larger size of the left occipital lobe. We conclude that a single value of instantaneous frequency which implies a massive data reduction can serve as a robust parameter for the characterization of the input conditions of hemifield PVEP (i.e. the stimulated hemiretina). It is more successful than potential distribution or dipole modelling, probably because instantaneous frequency incorporates considerably more information than the other two measures. It is suggested to explore instantaneous frequency as a parameter to recognize small retinal area stimuli in perimetry studies.

Dipole-modeling of the visual evoked P300

We collected visual event-related potentials (ERPs) from 6 normal subjects using an "oddball" paradigm. Subjects were required to count the occurrences of matching shapes presented in the left and right visual field. Shapes matched on 20% of trials. ERPs were recorded from 20 or 43 electrodes distributed over the scalp. A multiple spatio-temporal equivalent dipole (ED) model was used to fit the early sensory and P300 component. A latency window to analyze the P300 was determined using the global field power statistic. The spatial topography of the P300 over this window was characterized by a midline positivity that decreased in amplitude with spatial distance from the peak. After sensory components were fit, the source of P300 could be accounted for by 1 or 2 EDs, which were usually located near medial temporal areas. This result is at odds with evidence from depth recordings during the oddball paradigm, showing that multiple regions of the brain are active during this interval.

The effects of luminance and chromatic background flicker on the human visual evoked potential

Previous studies report that background luminance flicker, which is asynchronous with signal averaging, reduces the amplitude and increases the latency of the pattern-onset visual evoked potential (VEP). This effect has been attributed to saturation of the magnocellular (m-) pathway by the flicker stimulus. In the current study, we evaluate this hypothesis and further characterize this effect. We found that flicker had similar effects on the pattern-onset and pattern-reversal VEP, suggesting that the reversal and onset responses have similar generators. Chromatic flicker decreased latency of the chromatic VEP whereas luminance flicker increased peak latency to luminance targets. This result indicates that luminance flicker saturates a rapidly conducting m-pathway whereas chromatic flicker saturates a more slowly conducting parvocellular (p-) pathway. Finally, evoked potentials to chromatic and luminance stimuli were recorded from 34 electrodes over the scalp in the presence of static and asynchronously modulated backgrounds. An equivalent dipole model was used to assess occipital, parietal, and temporal lobe components of the surface response topography. Results showed that chromatic flicker reduced activity to a greater extent in the ventral visual pathway whereas luminance flicker reduced activity to a greater extent in the dorsal visual pathway to parietal lobe. We conclude that the VEP to isoluminant color and luminance stimuli contains both m- and p-pathway components. Asynchronous flicker can be used to selectively reduce the contribution of these pathways to the surface recorded VEP. Our results provide evidence of parallel pathways in the human visual system, with a dorsal luminance channel projecting predominantly to the posterior parietal lobe and a ventral color channel projecting predominantly to inferior temporal lobe.

Visual evoked potentials generator model derived from different spatial frequency stimuli of visual field regions and magnetic resonance imaging coordinates of V1, V2, V3 areas in man

Visual evoked potentials (VEPs) to pattern reversal vertical bar stimuli were recorded from 24 scalp derivations (including zygomatic and inion) referenced to digitally linked earlobes in 50 controls. 1, 2 and 4 cpd patterns were presented as full field (FF) stimuli, on Upper Hemifields (UHF) and Lower Hemifields (LHF), upper and lower quadrants and with the occlusion of central and peripheral UHF and LHF. VEPs to octant stimuli were also recorded with 2 cpd patterns. N1, P1 and N2 components were recorded from posterior and inion derivations with FF stimuli, from posterior derivations with LHF stimuli, only from inion leads with UHF stimuli, from derivations ipsilateral to stimuli with quadrants and octants, and consistently from midline derivations only with lower quadrants. Polarity inverted sequences (iN1-iP1-iN2) were recorded from the other scalp derivations, with similar latency and spatial frequency sensitivity as N1-P1-N2. Single Equivalent Dipole (ED) calculations were performed on N1 and P1 recorded in the different stimulus conditions. Our findings contradict previous hypotheses on VEP generators and contradict the predictions of VEPs polarity and distribution based on the "cruciform model" of VEPs generators. In order to explain the distribution of VEPs to upper and lower half fields and to quadrant and octants, we propose a model based on the position of the medial and occipito-polar surface of visual cortex in man.

Brain electrical source analysis of laser evoked potentials in response to painful trigeminal nerve stimulation

Cerebral generators of long latency brain potentials in response to painful heat stimuli were identified from potential distributions in 31 EEG leads, using the brain electrical source analysis (BESA) programme in the multiple spatio-temporal dipole mode. Data were taken from a study with 10 young healthy male subjects who participated in 3 identical sessions, 1 week apart, with 4 blocks of 40 stimuli (randomized intensities above mean pain threshold). Brief infrared laser heat pulses were applied to the right temple; laser evoked brain potentials (LEPs) were averaged over 40 stimuli per block. BESA was applied to the grand mean maps averaged over the 10 subjects, 3 sessions and 4 stimulus blocks per session, as well as to the individual maps. In all cases 4 generators could consistently be identified by BESA, which were able to explain up to 98.8% of the total variance in scalp distributions at certain time intervals: dipole I with a maximum activity at 106.3 msec in the contralateral somatosensory trigeminal cortex, 19.0 mm beneath the surface; dipole II with a maximum activity at 112.1 msec at the corresponding ipsilateral area at a depth of 13.6 mm; dipole III with a maximum activity at 130.4 msec in the frontal cortex; dipole IV with 2 relative maximum activities at 150.6 and 220.5 msec, localized centrally under the vertex at a depth of 33.1 mm, which described both the late vertex negativity and the consecutive positivity. BESA applied to the individual LEP maps of each individual and session yielded again 4 major generators with sites, strengths and orientations comparable to those of the grand mean evaluations. The standard deviation (S.D.) of site coordinates within subjects was less than 3 mm for dipoles I, II and IV (5 mm for dipole III). The between-subject standard deviation was considerably larger (15 mm), which was attributed to individual differences in head geometry, size and anatomy. Dipoles I and II are assumed to be generators in secondary somatosensory areas of the trigeminal nerve system with bilateral representation, though significantly stronger in the contralateral site. Dipole III in the frontal cortex may be related to attention and arousal processes, as well as to motor cortical initiation for eye movements and muscle effects. The central dipole IV describing all late activity between 150 and 220 msec is probably a representative of perceptual activation and cognitive information processing; it was located in deep midline brain structure, e.g., the cingular gyrus.

Source model and scalp topography of pattern reversal visual evoked potentials to altitudinal stimuli suggest that infoldings of calcarine fissure are not part of VEP generators

Visual evoked potentials (VEPs) to pattern reversal vertical bar stimuli were recorded from 19 scalp, 2 zygomatic and 3 inion derivations referenced to digitally linked earlobes in 50 controls. 1, 2 and 4 cycles per degree (cpd) patterns were presented as full field (FF) stimuli, on upper and lower hemifields (UHF-LHF), upper and lower quadrants and with the occlusion of central and peripheral UHF and LHF. VEPs to octant stimuli were also recorded with 2 cpd patterns. N1, P1 and N2 components were recorded from posterior and inion derivations with FF stimuli, from posterior derivations with LHF stimuli, only from inion leads with UHF stimuli, from derivations ipsilateral to stimuli with quadrants and octants, and from midline derivations only with lower quadrants. Polarity inverted sequences (iP1-iN1-iP2) were recorded from the other scalp derivations, with similar latency and spatial frequency sensitivity as N1-P1-N2. The orientation of Equivalent Dipoles (ED) was orthogonal with surface coordinates of mesial and occipito-polar calcarine cortex, measured on Magnetic Resonance Imaging. A model of VEP generators is proposed, suggesting that the VEP sequence is elicited only in mesial and occipito-polar surfaces of calcarine cortex.

Locating VEP equivalent dipoles in magnetic resonance images

Pattern-reversal and diffuse flash visual evoked potentials (VEPs) were obtained from 4 normal adults. A spatiotemporal dipole model was used to determine the location of the hypothetical equivalent dipoles consistent with the scalp distribution of the VEPs. Equivalent dipoles representing ERG and VEP activity were placed within 3-D magnetic resonance images of the brain. Most of the localization error appeared to be due to inadequate sampling of the potential field in frontal and occipital areas by the 10-20 system of electrode placement. Locating electrophysiologic dipoles within magnetic resonance images of brain structure allows evaluation of dipole localization techniques.