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PubMed1997

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.

Brain Mapping↗
PubMed2002

Modulation of flash stimulation intensity and frequency: effects on visual evoked potentials and oscillatory potentials recorded in awake, freely moving mice.

Visual evoked potentials (VEP) responses to flash stimulation at nine intensities, from 0.611 to 945.6 cd/m(2)*s, and two frequencies (0.2 and 1 Hz) were recorded and oscillatory potentials (OPs) extracted after digital 50-Hz high pass filtering in unanaesthetized unrestrained mice. Both VEP and OPs morphology were replicable for all conditions and were similar to values reported in the literature. In particular OPs spectral analysis showed that the main frequency component remained stable at 66-77 Hz, for both stimulation frequencies, although it displayed an increase in amplitude, as a function of stimulus intensity. OPs amplitude at 1 Hz versus 0.2 Hz stimulus frequency was higher after taking into account the different noise contributions in the two conditions. Root mean square values calculated at selected time windows, revealed that, at 1 Hz, the main contribution to OPs occurs at the onset of the response (14-27 ms) while, at 0.2 Hz, the higher RMS was recorded later (42-56 ms). This difference accounts for the longer duration of the oscillatory event in the 0.2-Hz condition and suggests that oscillatory activity, modulated and carried along the visual pathway, is recorded at the cortical electrode after further elaboration at the cortical/subcortical level, depending on stimulus properties.

Animals↗
PubMed1992

Clinical application of motion-onset visual evoked potentials.

The results of motion-onset visual evoked potentials and pattern-reversal visual evoked potentials were compared in 5 adults with amblyopia, in 13 patients with unilateral retrobulbar neuritis and in 62 patients with multiple sclerosis. While the pattern-reversal visual evoked potentials had reduced amplitudes and prolonged latencies in all amblyopic eyes, the motion-onset visual evoked potentials were normal. Thus, motion-onset visual evoked potentials cannot be used for diagnosis of amblyopia. In patients with retrobulbar neuritis, both types of visual evoked potentials were delayed on stimulation of the affected eye. The latency increase was, however, greater for pattern-reversal visual evoked potentials than for motion-onset visual evoked potentials. Examination of the patients with multiple sclerosis showed that the additional use of motion-onset visual evoked potentials increased the sensitivity of the investigation. In some patients, only the motion-onset visual evoked potentials had pathologic latency increases, whereas the pattern-reversal visual evoked potentials stayed within normal limits.

Adult↗
PubMed1992

Comparison of preoperative 10-Hz visual evoked potentials to contrast sensitivity and visual acuity after cataract extraction.

Cataract patients whose surgical outcomes were in question were referred for testing by visual evoked potentials, elicited through closed eyelids by a luminance stimulus (flash) that appeared 10 times per second. Visual evoked potentials were rated as normal (predicted acuity of 20/50 or better) or abnormal (predicted acuity of 20/60 or worse). Postoperative Arden and Optronix contrast sensitivities and visual acuities were determined in 37 patients who had no intraoperative or early postoperative complications. Arden grating scores of less than 100 were rated as normal. The optimal and cutoff spatial frequency values were determined for the Optronix scores. Optimal and cutoff values of greater or equal to 1 c/deg and 12 c/deg, respectively, were rated as normal. Visual acuities were considered normal at 20/50 or better. Preoperative visual evoked potentials were quantitatively compared to the postoperative contrast sensitivities and visual acuities by 2 x 2 contingency tables. The accuracy of prediction was 79% for the visual acuities, 62% for the Optronix optimal values, 70% for the Optronix cutoff values and 62% for the Arden gratings.

Aged↗
PubMed2006

Deblurring visual evoked potentials using commercially available software.

Visual evoked potentials are useful clinical tools to study visual pathways of the brain. Although the temporal resolution is unsurpassed by other brain imaging technologies, the spatial resolution is diminished or blurred by the low conductance of the electrical signals through the skull. Methods have been proposed to improve the spatial resolution by downwardly projecting the electrical signals measured on the scalp to the surface of the cerebral cortex through the inverse solution of the equations governing static current flow. We describe the adaptation and combination of commercially available engineering software programs to solve this inverse problem and report the results of a sample run of the system. Before deblurring, the visual evoked potentials appeared to be diffusely localized over the posterior scalp. After deblurring, the visual evoked potentials were only found at the electrodes closest to the visual cortex, as would be predicted by our current knowledge of neuroanatomy.

Brain↗
PubMed1982

Short latency visual evoked potentials in man.

Contrary to auditory and somatosensory evoked potentials, surface recorded visual evoked potentials which arise in subcortical neural elements have rarely been described. Considerable disagreement exists between the reports in the literature on such visual potentials. In this study, flash stimuli were used to evoke the potentials which were recorded from the skin overlying the infra-orbital ridge, outer canthus, middle of the forehead, vertex, mastoid ipsilateral to the stimulated eye and inion, using a non-cephalic reference. The potentials were amplified in a band which was chosen to omit slow retinal and cortical potentials, and to enhance activity which might include compound neural activity. Potentials were recorded from 9 subjects (13 eyes), and for each one the effects of eye position and stimulus intensity were studied. The results indicate that the series of components recorded within the first 100 msec following photic stimulation were volume-conducted activity generated by a subset of the visual system which is activated by luminosity changes. The generators of the first 4 or 5 components seem to be situated within the retina, the subsequent components seem to be generated in the optic nerve or tracts, and the later components may be thalamo-cortical in origin. These potentials may complement pattern evoked potentials in a more accurate definition of sites of lesions along the visual pathway.

Adult↗
PubMed1999

An artefact desynchroniser for use with visual evoked potential stimulators.

When recording visual evoked potentials elicited by a CRT raster display, driven from a frame-synchronised stimulus generator, artefacts caused by magnetic fields originating from the CRT field coils may seriously contaminate the recordings. This interference cannot be removed by signal averaging as it is time locked to the frame rate of the CRT. The circuit described here effectively desynchronises the interference, enabling the signal averaging process to reduce the artefact without the bandwidth restriction and resultant distortion of the recording which would be caused by lowpass filtering.

Electronics, Medical↗
PubMed1987

Cortical generators of the CI component of the pattern-onset visual evoked potential.

Thirteen-channel visual evoked potentials (VEPs) to pattern-onset were recorded with stimuli restricted to individual octants of the peripheral field, to halves and to quadrants of the fovea. The voltage of the CI component was measured in each channel to define its topography for each stimulated sector. The potential fields so obtained were then analysed to find the orientation and location of a dipole that would produce a corresponding pattern of voltages at the scalp. The locations of the computed dipoles are consistent with the hypothesis that CI is generated in striate cortex. The computed locations and orientations are not compatible with alternative arrangements of sources in extrastriate cortex. A significant problem remains. If CI is indeed generated by the striate cortex then the orientation of the dipoles excited by stimulation of the peripheral field indicates that the cortex is surface negative. This leads to the prediction that foveal stimuli will elicit a CI which is negative at posterior electrodes. The experiments reported here confirm that CI is positive with such stimuli, and its source is calculated as a horizontal dipole with its positive pole oriented posterolaterally. Two possible explanations are considered for the reversed polarity of foveal CI: (a) that macropotentials associated with stimulation of the fovea are opposite in polarity from those associated with stimulation of the peripheral field; (b) that the foveal area of the retinotopic map extends into the lateral calcarine fissure with the effect that much of it faces in the reverse direction from the cortex at the pole.

Adolescent↗
PubMedDate not supplied

Stationary pattern adaptation and the early components in human visual evoked potentials.

Pattern-onset visual evoked potentials were elicited from humans by sinusoidal gratings of 0.5, 1, 2 and 4 cpd (cycles/degree) following adaptation to a blank field or one of the gratings. The wave forms recorded after blank field adaptation showed an early positive component, P0, which decreased in amplitude with spatial frequency, whereas the immediately succeeding negative component, N1, increased in amplitude with spatial frequency. P0 and N1 components of comparable size were recorded at 1 cpd. Stationary pattern adaptation to a grating of the same spatial frequency as the test grating significantly reduced N1 amplitude at 4, 2 and 1 cpd. The N1 component elicited at 4 cpd was attenuated in log-linear fashion as the spatial frequency of the adaptation grating increased. P0, on the other hand, was unaffected by stationary pattern adaptation at all combinations of test and adapting spatial frequencies, although P0 amplitude is known to be attenuated by adaptation to a drifting grating. Since N1, but not P0, was significantly attenuated following adaptation and testing at 1 cpd, it was concluded that the neurons generating these components are functionally distinct. The use of a common adaptation grating discounted the possibility that N1, but not P0, was affected due to a difference in the rates of retinal image modulation caused by eye movements made while viewing adaptation gratings of different spatial frequencies. The neurons generating N1 were adapted at a lower rate of retinal image modulation than that apparently required for adaptation of the neurons generating P0, which suggests a difference between these neurons in the rate of stimulus modulation necessary for activation.

Adaptation, Ocular↗
PubMed1983

Diurnal periodicity of lateral asymmetries of the visual evoked potential in healthy volunteers.

Visual evoked potential (VEP) was analyzed in 24 healthy male volunteers (age: 25-35 years) between 7.00 a.m. and 4.00 p.m. to evaluate possible diurnal variation in hemispheric differences of the response to a diffuse or 1 degree flash and checkerboard pattern-reversal (stimulation: binocular). VEP was recorded over O1-A1 and O2-A2, and 64 exposures were averaged during each session. After diffuse and 1 degree flash stimulation the amplitudes of early components (latencies less than 140 ms) were higher over the right hemisphere (O2-A2) than over the left (O1-A1) in the morning. These differences disappeared during the afternoon. Late components (latencies greater than 250 ms) exhibited higher amplitudes over the left than over the right hemisphere during the whole experiment. With checkerboard pattern-reversal stimulation such a time-dependent change in the amplitudes of VEP between both hemispheres was not measurable.

Adult↗
PubMed1984

Effects of age and alcohol abuse on pattern reversal visual evoked potentials.

The Pattern Reversal Visual Evoked Potential (PRVEP) was recorded in normal subjects and alcoholics. The recordings were made from the patients during withdrawal and repeated after three weeks of detoxification. It was found that the N76 latency was longer in the alcoholic patient in the withdrawal phase than in the normal subjects. The latency returned to normal range after detoxification in younger alcoholic patients but did not in the older alcoholics. The age-related increase in the N76 latency in the alcoholic patients was similar to that in normal subjects but more exaggerated. For alcoholics, the age-related change in the N76 latency reached significance, but was only a trend in normal subjects. The P100 latency demonstrated significant age-related change, but far less modification related to the alcoholism than the N76 latency. It is unclear at present whether the failure of the latency to return to normal in older patients after detoxification is related to longer periods of excessive drinking, or to a particular vulnerability of the older patients to continued use of alcohol.

Adolescent↗
PubMed1981

Visual evoked potentials in sarcoidosis.

The visual evoked potential to pattern reversal was recorded in 50 patients with sarcoidosis. Abnormalities of latency and amplitude were found in 15 patients (30%), including all 4 patients with clinically evident brain disease and 4 of 17 patients with overt ocular disease. Twenty-nine patients had no clinical evidence of ocular or neurologic disease, and 7 of them (24%) had abnormalities of the VEP, implying subclinical sarcoid lesions in structures at the base of the brain.

Adult↗
PubMed1989

Visual evoked potentials in diabetic patients.

Visual evoked potentials (VEPs) were assessed in 50 adult type I (insulin-dependent) and 19 type II (noninsulin-dependent) diabetes mellitus patients and in 54 controls. P100 wave latency was significantly longer in diabetic patients (P less than .001). Twenty-eight percent of diabetic patients had P100 wave latencies above the normal range. There was no correlation between P100 latency and type or duration of diabetes mellitus, quality of metabolic control, or presence of degenerative complications. The significance of VEP abnormalities in diabetes mellitus remains speculative.

Adult↗
PubMed1992

Acute alcoholic intoxication and naloxone. Effects on visual evoked potential.

Experimental assays analyzing visual evoked potential (VEP) changes during an acute alcoholic intoxication were carried out in two groups of cats: One with continuous ethanol (0.06 g/kg.min) i.v. perfusion. Another one with a naloxone (400 micrograms/kg) i.v. injection 10 min before ethylic perfusion. Naloxone potentiates alcohol effects on VEP parameters, and on the appearance of isoelectric postpotential and flat VEP.

Acute Disease↗
PubMedDate not supplied

[The picture of visual evoked potentials in Kimmerle anomaly].

Visual evoked potentials (VEP) were studied in 16 persons with radiological confirmed Kimmerle anomaly. Each eye was stimulated by the reversible checker-board pattern using the reversal and flash stimulation methods. Abnormal responses were noted in about 75% of the studied patients. Latency disturbances prevailed during reversal stimulation and abnormal configuration of responses predominated during flash stimulation.

Adult↗
PubMed1987

Properties of the evoked potential generators: current source-density analysis of visually evoked potentials in the cat cortex.

The depth profiles of visually evoked field potentials were recorded in areas 17 and 18 of the cat visual cortex. For comparison, potential profiles evoked by electrical stimulation of the primary afferents and of the nonspecific reticular system were also recorded. From these profiles the current source-density (CSD) distributions were calculated using the one-dimensional CSD method. CSD distributions evoked by the different types of stimuli differ in their amplitudes and time courses by approximately one and two orders of magnitude. Qualitatively, however, they are very similar. Thus, the CSDs can be interpreted as reflecting the same basic pattern of excitatory synaptic activations. This pattern consists of early activation components in the input layers, followed by excitatory synaptic activations in layer III, then in layer II, and in layer V. The basic pattern of cortical activation was found to be modulated by specific features of the visual stimuli. Modulations reflecting contour-versus contrast-contents as well as those reflecting characteristic features of moving patterns have been identified. Most of the CSD components of the cortical activation sequence were obtained from regions extending well beyond the cellular receptive fields in visual cortex. Thus, they reflect nonretinotopic activities. Parameters other than specific features of the visual stimuli have profound influence on cortical CSDs. Nonspecific parameters which have been considered are the general state of cortical excitability, the temporal interactions of successive activities (which are predominantly facilitatory), and the lateral interactions of simultaneous activations from different regions of the visual field (predominantly inhibitory).

Animals↗