Neuronal generators of visual evoked potentials in humans: visual processing in the human cortex

PURPOSE: We wished to define the localization of cortical generators of visual (pattern) evoked potentials (VEP) and the temporal sequence of activation in the occipital region. METHODS: In 4 candidates for epilepsy surgery, a large array of subdural electrodes was placed over occipital areas. Checkerboard pattern reversal stimuli were generated and the epileptogenic focus was localized and functionally mapped. Magnetic resonance imaging did not show any occipital lesions in any of the 4 patients. RESULTS: The area first activated was the lingual gyrus in the mesial occipital lobe (negative potential peaks at approximately 70 ms), followed by an area superior to the calcarine fissure (negative peaks at approximately 80 ms). Later (starting at approximately 90 ms), there were positive potentials over the occipital pole and lingual gyrus, followed by potentials at the lateral occipital lobe. CONCLUSIONS: These data support the idea that VEP are generated in the mesial and lateral occipital cortex by different circumscribed neuronal generators with different latencies of activation. The scalp-recorded N1 and P1 potential peaks most likely derive from the progressive activation of neuronal masses in different regions of the occipital lobe.     

Assessment of visual field impairment by objective technique

An objective psychophysical technique for investigating visual fields by averaged scalp potentials evoked by pattern gratings of alternating contrast and by sinusoidally modulated flicerking light is applied to a child with a right homonomous hemianopsia. This technique illustrates a value, in obtaining clinical data on visual fields in situations were subjective perimetry is difficult to administer or where conservating diagnostic methods do not yield evidence of pathology.     

Glutathione reductase, selenium-dependent glutathione peroxidase, glutathione levels, and lipid peroxidation in freshwater bivalves, Unio tumidus, as biomarkers of aquatic contamination in field studies

Somatosensory evoked potential (SEP) recordings in patients suffering from cortical myoclonus (CM) are characterised by evidence of abnormally enhanced scalp components. Our aim was to verify whether enhanced activity in giant SEPs arises from the same generators as in healthy subjects. We used the brain electrical source analysis (BESA) to compare scalp SEP generators of healthy subjects to those calculated in 3 patients with CM of varying causes. Firstly, we built a 4-dipole model explaining scalp distribution of early SEPs in normal subjects and then applied it to traces recorded from CM patients. Our model, issued from the right median nerve grand average and applied also to recordings from single individuals, included a dipole at the base of the skull and three other perirolandic dipoles. The first of the latter dipoles was tangentially oriented and was active at the same latencies as the N20/P20 potentials and, with opposite polarity, the P24/ N24 responses; the second dipole explained the central P22 distribution and the third had a peak of activity corresponding to the N30 component. When we applied our 4-dipole model to CM recordings, the first perirolandic dipole had a third peak of activity in all patients at the same latency as a parietal negativity and a frontal positivity, both following giant P24/N24 components; on the other hand, in one patient the second perirolandic dipole showed a later activation corresponding to a high central negativity, following a giant P22 response. We suggest that only the initial giant SEPs correspond to physiological potentials evoked in healthy subjects. The occurrence of late giant SEPs could be explained by hyperpolarization, following the postsynaptic excitatory potentials responsible for the early giant components.     

Intra-operative localization of sensorimotor cortex by cortical somatosensory evoked potentials: from analysis of waveforms to dipole source modeling

Intra-operative localization of sensorimotor cortex is of increasing importance as neurosurgical techniques allow safe and accurate removal of lesions around the central sulcus. Although direct cortical recordings of somatosensory evoked potentials (SEPs) are known to be helpful for cortical localization, source localization models can provide more precise estimates than subjective visual analysis. In addition to intra-operative analysis of waveforms and amplitudes of SEPs to median nerve stimulation in 20 neurosurgical patients, we used a spatiotemporal dipole model to determine the location of the equivalent dipoles consistent with the cortical distribution of the SEPs. The early cortical SEPs were modeled by 2 equivalent dipoles located in the postcentral gyrus. The first dipole was primarily tangentially oriented and explained N20 and P20 peaks. The second dipole was primarily radially oriented and explained P25 activity. We found consistent localization of the first dipole in the postcentral gyrus, which was always located within 8 mm of the central sulcus, with an average distance of 3 mm. This finding provides an objective basis for using the SEP phase reversal method for cortical localization. We conclude that dipole source modeling of the cortical SEPs can be considered as an objective way of localizing the cortical hand sensory area.     

The early component of the visual evoked magnetic field

Several EEG studies have reported an early component of the visual evoked potential. However, it is controversial whether this component is cortical or subcortical. Our study has aimed to clarify this problem using MEG and EEG in nine normal volunteers. A total of 4000 stimuli were presented to the monocular visual hemifield through a light-proof stimulating goggle and the visual evoked magnetic field and visual evoked potential was measured above the occipital lobe. The early component was observed in three of the nine subjects. The latency ranged from 40 to 45 ms in MEG and from 39 to 47 ms in EEG. The result of dipole localization analysis showed that its origin was cortical, and specifically, the striate cortex.     

Luminance and spatial attention effects on early visual processing

Event-related potentials (ERPs) were recorded from healthy subjects in response to unilaterally flashed high and low luminance bar stimuli presented randomly to left and right field locations. Their task was to covertly and selectively attend to either the left or right stimulus locations (separate blocks) in order to detect infrequent shorter target bars of either luminance. Independent of attention, higher stimulus luminance resulted in higher ERP amplitudes for the posterior N95 (80-110 ms), occipital P1 (110-140 ms), and parietal N1 (130-180 ms). Brighter stimuli also resulted in shorter peak latency for the occipital N1 component (135-220 ms); this effect was not observed for the N1 components over parietal, central or frontal regions. Significant attention-related amplitude modulations were obtained for the occipital P1, occipital, parietal and central N1, the occipital and parietal P2, and the parietal N2 components; these components were larger to stimuli at the attended location. In contrast to the relatively short latencies of both spatial attention and luminance effects, the first interaction between luminance and spatial attention effects was observed for the P3 component to the target stimuli (350-750 ms). This suggests that interactions of spatial attention and stimulus luminance previously reported for reaction time measures may not reflect the earliest stages of sensory/perceptual processing. Differences in the way in which luminance and attention affected the occipital P1, occipital N1 and parietal N1 components suggest dissociations among these ERPs in the mechanisms of visual and attentional processing they reflect.(ABSTRACT TRUNCATED AT 250 WORDS)     

Fast visual evoked potential input into human area V5

Studies of the human visual cortex have demonstrated that an area for motion processing (V5) is located in the lateral occipito-temporal cortex. To study the timing of arrival of signals in V5 we recorded multi-channel visual evoked potentials (VEPs) to checkerboard stimuli. We then applied dipole source analysis which was computed on a grand average of 10 subjects, and on five individual subjects, respectively. We demonstrate an early VEP component with onset before 30 ms and with a peak around 45 ms, located in the vicinity of V5. This early component was independent of a second activity, which started around 50 ms and peaked around 70 ms, and was located within the striate cortex (V1). These results provide further evidence for a very fast input to V5 before activation of V1.     

Dipolar sources of the early scalp somatosensory evoked potentials to upper limb stimulation. Effect of increasing stimulus rates

Brain electrical source analysis (BESA) of the scalp electroencephalographic activity is well adapted to distinguish neighbouring cerebral generators precisely. Therefore, we performed dipolar source modelling in scalp medium nerve somatosensory evoked potentials (SEPs) recorded at 1.5-Hz stimulation rate, where all the early components should be identifiable. We built a four-dipole model, which was issued from the grand average, and applied it also to recordings from single individuals. Our model included a dipole at the base of the skull and three other perirolandic dipoles. The first of the latter dipoles was tangentially oriented and was active at the same latencies as the N20/P20 potential and, with opposite polarity, the P24/N24 response. The second perirolandic dipole showed an initial peak of activity slightly earlier than that of the N20/P20 dipolar source and, later, it was active at the same latency as the central P22 potential. Lastly, the third perirolandic dipole explaining the fronto-central N30 potential scalp distribution was constantly more posterior than the first one. In order to evaluate the effect of an increasing repetition frequency on the activity of SEP dipolar sources, we applied the model built from 1.5-Hz SEPs to traces recorded at 3-Hz and 10-Hz repetition rates. We found that the 10-Hz stimulus frequency reduced selectively the later of the two activity phases of the first perirolandic dipole. The decrement in strength of this dipolar source can be explained if we assume that: (a) the later activity of the first perirolandic dipole can represent the inhibitory phase of a "primary response"; (b) two different clusters of cells generate the opposite activities of the tangential perirolandic dipole. An additional finding in our model was that two different perirolandic dipoles contribute to the centro-parietal N20 potential generation.     

Magnetoencephalographic analysis of rolandic discharges in benign childhood epilepsy

We report the detailed analysis of the generator and propagation of rolandic discharges in benign childhood epilepsy with centrotemporal spikes by means of 37-channel magnetoencephalography with neuromagnetic three-dimensional dipole localization. Equivalent current dipoles of prominent negative sharp waves of rolandic discharges appeared as tangential dipoles in the rolandic region, positive poles being situated anteriorly. These equivalent current dipoles showed a relatively limited localization and regular directions compared with other components. Equivalent current dipoles of preceding small positive waves, positive waves following negative sharp waves, and negative slow waves appeared in the vicinity of negative sharp waves. Equivalent current dipoles of rolandic discharges were located around the generator of somatosensory evoked magnetic fields stimulated at the lower lip. These findings suggest that rolandic discharges are generated through basically a mechanism similar to that for the middle-latency components of somatosensory evoked responses.     

Impact of chromosome 14q loss on survival in primary head and neck squamous cell carcinoma

OBJECTIVE: To investigate somesthetic functions of the perisylvian cortex. METHODS: Somatosensory evoked magnetic fields (SEFs) and somatosensory evoked potentials (SEPs) of the perisylvian cortex were recorded directly from subdural electrodes in a patient with a left frontal brain tumour. RESULTS: The most prominent SEP components after electrical stimulation of the right and left hands and the right foot were double peaked negativity recorded just above the sylvian fissure (latency 80 to 150 ms), respectively (N1a and N1b). Generator sources for the magnetoencephalographic counterparts of those peaks (N1a(m) and N1b(m)) were both localised at the upper bank of the sylvian fissure, and those of N1a(m) were more anteromedially located than those of N1b(m). CONCLUSIONS: These findings suggest the existence of at least two separate somatosensory areas within the human perisylvian cortex.     

Positive correlation of plasma transforming growth factor-beta 1 levels with tumor vascularity in hepatocellular carcinoma

The initial somatosensory evoked magnetic fields following painful heat stimulation by CO2 laser beam applied to the upper and lower limb were investigated in normal subjects. The main deflections, 'Pain MA' and 'Pain ML' following the arm and leg stimulation, respectively, were identified in the bilateral second sensory cortices (SII). The onset latencies of Pain MA and Pain ML were approximately 150 and 200 ms, respectively. No consistent equivalent current dipole was found in other areas including the primary sensory cortex in each hemisphere. Therefore, we consider that neurons in the bilateral SII are initially activated following painful heat stimulation.     

Differential roles of p300 and PCAF acetyltransferases in muscle differentiation

To determine the possibility of discriminating multi-sources in the brain by 3D vector magnetic field measurement of a magnetoencephalogram (MEG), measurements were made of magnetic fields produced by two current dipoles implanted in a spherical head model. The 3D vector magnetic field measurements were made by using a 3D second-order gradiometer connected to three rf-SQUIDs, which can detect magnetic field components perpendicular to and tangential to the scalp. The MEG distribution measuring the magnetic field perpendicular to the scalp was not helpful in estimating the location and number of sources because of the lack of a dipole pattern. By referring to the MEG distribution measuring the magnetic field distribution tangential to the scalp, however, two current sources could be clearly discriminated in a spherical head model. It was found that this MEG distribution measuring tangential to the scalp could provide information on new constraint conditions for the calculation of inverse problems with multi-sources. These results were also confirmed by measurement of the mixed somatosensory evoked fields elicited by simultaneous electric stimulation to the median nerve and the thumb.     

Immunohistochemical characterization of hepatic lymphocytes in acute hepatitis A, B, and C

Mechanically evoked cerebral potentials (MECP) were studied in humans standing on a movable platform with three different stance widths. A sudden platform tilt of 4 degrees produced ankle dorsiflexion and resulted in scalp potentials of five distinct components, the earliest being a positive deflection at 35/60 ms. Their latencies have shown fairly consistent values among the three stance widths, while the amplitudes underwent some significant changes under the wide stance condition as compared with tightly close feet. These findings were interpreted as purporting evidence of altered somaesthetic afferent input from lower limbs during standing with widely apart support surface.     

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.     

NF-kappaB2 is a putative target gene of activated Notch-1 via RBP-Jkappa

Cutaneous stimulation of the face and hand with a CO2 laser in three awake patients evoked potentials (LEPs) recorded from the dominant left parasylvian cortex. These were recorded by means of a subdural grid of electrodes implanted for evaluation of epilepsy. Stimulation of the contralateral face resulted in waveforms consisting of a negative potential (N2, 162 +/- 5 ms; mean +/- SE) followed by a positive potential (P2, 340 +/- 18 ms). Both waves occurred at longer latency after hand than after facial stimulation. N2 and P2 potentials recorded from the grid correspond well in morphology to those recorded from the scalp in four additional patients tested with the same stimulation paradigm. The N2 waves recorded from the subdural grid occurred at significantly shorter latencies than did those recorded from the scalp (184 +/- 6 ms), but the P2 waves at the grid occurred at significantly longer latencies than did those recorded at the scalp (281 +/- 13 ms). The amplitudes of the potentials recorded from the grid were maximal over the parietal operculum both for contralateral stimulation of the face or hand and for ipsilateral stimulation of the face. Potentials also were recorded in this area after stimulation of the ipsilateral hand. The cortical distributions of these potentials suggest that their generators are located in the parietal operculum or in the insula, or in both, consistent with previous PET, magnetoencephalographic, and scalp LEP source analyses. These previous analyses provide indirect evidence of nociceptive input to parasylvian cortex because the interpretation of each analysis incorporates multiple assumptions. The present results are the first direct evidence of nociceptive input to the human parasylvian cortex.     

An efficient algorithm for computing multishell spherical volume conductor models in EEG dipole source localization

Computationally localizing electrical current sources of the electroencephalographic signal requires a volume conductor model which relates theoretical scalp potentials to the dipolar source located within the modeled brain. The commonly used multishell spherical model provides this source-potential relationship using a sum of infinite series whose computation is difficult. This paper provides a closed-form approximation to this sum based on an optimal fitting to the weights of the Legendre polynomials. The second-order (third-order) approximation algorithm, implemented by a provided C-routine, requires only 100 (140) floating point operations to compute a single scalp potential in response to an arbitrary current dipole located within a four-shell spherical volume conductor model. This cost of computation represents only 6.3% (8.9%) of that required by the direct method. The relative mean square error, measured by using 20,000 random dipoles distributed within the modeled brain, in only 0.29% (0.066%).     

Pathology of Minamata disease

Minamata disease, or methylmercury poisoning, was first discovered in 1956 around Minamata Bay, Kumamoto Prefecture, Japan. A similar epidemic occurred in 1965 along the Agano River, Niigata Prefecture, Japan. The neuropathology of Minamata disease has been well studied; this review focuses on human cases of Minamata disease in Kumamoto Prefecture. Nervous system lesions associated with Minamata disease have a characteristic distribution. In the cerebral cortex, the calcarine cortex was found to be involved in all cases of Minamata disease, particularly along the calcarine fissure. The destruction of nerve tissue was prominent in the anterior portions of the calcarine cortex. Occasionally, the centrifugal route from the visual and visual association areas (internal sagittal stratum) showed secondary degeneration in prolonged cases after acute onset. Postcentral, precentral, and temporal transverse cortices showed similar changes, though they were less severe. Intense lesions in the precentral cortex caused the development of secondary bilateral degeneration of the pyramidal tracts. In the cerebellum, the lesions occurred deeper in the hemisphere. The granule cell population was most affected. In the peripheral nerves, sensory nerves were more affected than motor nerves. Secondary degeneration of Goll's tracts was occasionally seen in prolonged or chronic cases.     

Vecuronium dose requirement and pupillary response in a patient with olivopontocerebellar atrophy (OPCA)

BACKGROUND: Anticholinergic drugs are known to impair the motor function of the oesophagus but their effects on the oesophageal afferent pathways are unknown. AIM: To determine the effects of a peripherally-acting (trospium chloride) and a centrally-acting (biperiden) anticholinergic drug on the motility and the evoked potentials of the oesophagus. METHODS: Nine healthy volunteers were randomized to receive 1.2 mg trospium chloride (TC), 5 mg biperiden (BIP) or saline i.v. Primary peristalsis was elicited by swallowing a 5 mL water bolus and secondary peristalsis by insufflation of 20 mL air, 10 times each. Oesophageal potentials were evoked by electrical stimulation in the distal and proximal oesophagus (30 stimulations at 0.4 Hz, two runs). RESULTS: Both anticholinergic drugs reduced by a similiar amount the contraction amplitudes (TC 17 mmHg, BIP 25 mmHg, saline 67 mmHg; P < 0.01) and the rate of secondary contractions (TC 60%, BIP 70%, saline 95%; P < 0.01). In contrast, only biperiden prolonged the latencies of the evoked potentials (N1 peak, distal oesophagus: BIP 191 ms, TC 102 ms, saline 101 ms; P < 0.01; P1 peak: BIP 322 ms, TC 161 ms, saline 144 ms; P < 0.01). CONCLUSIONS: Both anticholinergic drugs depress oesophageal motility, but only the centrally-acting anticholinergic drug biperiden modifies the oesophageal evoked potentials, suggesting a central cholinergic transmission of the oesophageal afferent pathways.     

Human visual evoked responses are related to heart rate.

Attempted to determine the relation between changes in heart rate (HR) and averaged evoked potentials. 16 male undergraduates with variable HRs received 50 flashes of light during fast heart beats, 50 during slow heart beats, and 50 during midrange HR level. Evoked responses were recorded from the scalp overlying the right and left occipital lobes. HR, respiration, eye movements, and cephalic blood flow were also recorded. Results indicate that spontaneous changes in HR were related to changes in visual evoked responses and that this relation differed for the cerebral hemispheres. Cephalic pulse amplitude was largest following slow heart beats and smallest following fast heart beats, which suggests that changes in HR are related to changes in cerebrovascular as well as electrocortical activity. Results are discussed in terms of formulations derived from behavioral studies regarding the relation between cardiovascular activity and attentional processes. (54 ref) (PsycINFO Database Record (c) 2010 APA, all rights reserved)     

Differential effect of two calcium channel blockers--nifedipine and diltiazem--on atherogenesis in hypercholesterolemic hamster

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.