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At least 289 records · Page 16Linked to original sources
PubMed1994

Effect of experimental scotoma size and shape on the binocular and monocular pattern visual evoked potential.

A small experimental, central scotoma significantly attenuates the human pattern visual evoked potential. The steady-state pattern visual evoked potential was recorded from seven visually normal adults who viewed a reversing checkerboard with 24' checks and a central scotoma that varied in size and shape. We found that square scotomas had to be at least 3 x 3 degrees to significantly (p < 0.05) attenuate the pattern visual evoked potential. Receptor density has been shown to be greater along the horizontal meridian than the vertical meridian. We hypothesized that this results in greater cortical representation of the horizontal meridian than the vertical meridian and, therefore, the pattern visual evoked potential might be significantly attenuated by a smaller rectangular scotoma oriented along the horizontal meridian than along the vertical meridian. One dimension of the rectangular scotoma was fixed at either 1 degree or 3 degrees, while the other dimension was varied from 1 degree to 8 degrees. The threshold scotoma size that significantly (p < 0.05) attenuated the pattern visual evoked potential was a horizontal scotoma subtending 1 x 4 degrees and a vertical scotoma subtending 5 x 1 degree (vertical x horizontal). Meridional differences in cortical representation were not apparent to the larger scotoma series in which the fixed dimension subtended 3 degrees (3 x 2 degrees and 2 x 3 degrees). Further analysis of the data revealed that the apparent meridional difference for the 1 degree scotoma series was a function of data variability. The determinant of the PVEP amplitude was scotoma area, not orientation. Monocular and binocular threshold scotoma sizes were the same, which could be due to the level of binocular summation demonstrated by our subjects.

Adult↗
PubMedDate not supplied

Monocular contribution to the peak time of the binocular pattern visual evoked potential.

The contribution of each monocular pathway to the timing of the binocular pattern visual evoked potential was assessed in situations where a significant interocular timing discrepancy was observed. Monocular and binocular pattern visual evoked potentials to 0.5 degree checks were recorded from normal subjects, normal subjects in whom one eye was blurred, patients with monocular amblyopia, and patients with resolved unilateral optic neuritis. Normal subjects showed facilitation, while suppression was evidenced in subjects with monocular blurring. In patients with amblyopia, the affected pathway had no effect on binocular pattern visual evoked potential latency, suggesting that the amblyopic eye was suppressed. In contrast, all patients with optic neuritis showed binocular averaging. Our results show that different forms of binocular interaction are evidenced in normal subjects, in amblyopia and in optic neuritis, and suggest that a comparative analysis of monocular and binocular pattern visual evoked potential peak times brings valuable information to the clinical evaluation that could be used to distinguish disease processes further.

Adolescent↗
PubMed1993

Pattern electroretinogram and visual evoked potential amplitudes are influenced by different stimulus field sizes and scotomata.

The pattern electroretinogram and the visual evoked potential were recorded simultaneously with various stimulus fields and artificial scotomata of increasing sizes. In contrast to an earlier study, a smaller check size (20') and two stimulus field sizes (20 degrees x 20 degrees and 10 degrees x 10 degrees) for the scotomata were used. With a concentric decreasing stimulus field, a reduction of both the pattern electroretinogram and visual evoked potential was found. Both showed a simultaneous reduction of amplitudes, but, compared with the amplitude in the full field, the reduction was more extensive for the pattern electroretinogram at each test field size. This implies a greater contribution to the pattern electroretinogram from more eccentric retinal parts. An artificial central scotoma of increasing size in the 20 degrees x 20 degrees field had less influence on the pattern electroretinogram than on the visual evoked potential. The percentage amplitude loss of the visual evoked potential was more pronounced. The visual evoked potential was eventually abolished by a scotoma size from 10 degrees x 10 degrees upward, while the pattern electroretinogram was still registrable. When scotomata of similar size were introduced in a smaller (10 degrees x 10 degrees) field, percentage pattern electroretinogram and visual evoked potential amplitude losses were less separated than in a larger (20 degrees x 20 degrees) test field.

Adult↗
PubMed1985

Measurement of contrast sensitivity function using pattern-reversal visual evoked responses.

In order to determine whether pattern-reversal visual evoked response (VER) can be used to measure contrast sensitivity function (CSF), we investigated the effect of change of contrast upon pattern-reversal VER. Contrast thresholds for VER were extrapolated in five spatial frequencies. The CSF curve obtained from the VER showed the inverted U-shape as the psychophysical CSF curve. However, the low frequency fall-off that is usually seen in psychophysical CSF was less evident in the electrophysiological CSF. The difference between the electrophysiological CSF and the psychophysical CSF increased along with the spatial frequency. Proper stimulus conditions would make this method of evaluation of CSF by pattern reversal VER useful in relatively young children in whom psychophysical tests cannot be performed.

Adult↗
PubMed1984

An automated system for visual studies.

An interactive computer-controlled system is described that is used for visual studies including Visual Evoked Potentials in humans and animals and Visual Receptive Field recordings in animals. Visual stimuli are generated by a display system and the brain activity is monitored by microelectrodes (for animal recordings) and scalp electrodes (for human recordings). The signals are amplified, digitized, and stored. The software uses a response feedback algorithm for mapping the receptive fields. Initially random patterns are presented on a TV monitor and the neural response is recorded. Depending on the response to the pattern and the light distribution in it, the algorithm calculates a new pattern, always trying to maximize the response. As the process goes on, the stimuli patterns become near optimal and thus the receptive field of the neuron is mapped automatically, as a result that for many years has been formed by trial and error. The same system is used for analysis of the recorded results and recordings of the Visual Evoked Potentials in animals and humans. For the human evoked potentials different patterns are generated on the display monitor with a variety of choices, ranging from the simplest (checkerboard and gratings) to the most complicated ones (faces and scenes).

Animals↗
PubMedDate not supplied

Simultaneous pattern electroretinogram and visual evoked potential recordings in dyslexic children.

To help clarify the conflicting evidence of neurophysiologic abnormalities in children with reading problems (dyslexia), we examined pattern electroretinograms and visual evoked potentials to stimulation with checks of 24', 49' and 180', each at 5%, 42% and 100% contrast, in a group of dyslexic children and a group of normal (i.e., normally reading) children. Neurophysiologic difference between the groups was restricted to the visual evoked potential, which showed a significant prolongation of the P100 wave in dyslexic children at the highest contrast (100%) and the smallest checks (24'). There was no significant difference between normal and dyslexic children in the P50 and N95 pattern electroretinogram waves. These results support the assumption of a visual deficit in dyslexic children. However, they are not consistent with the evidence of an isolated deficit of the magnocellular function, which, theoretically, would cause more prominent visual evoked potential changes to lower contrast and the largest check stimuli.

Adolescent↗
PubMedDate not supplied

Pattern-onset visual evoked potentials: more useful than reversal for patients with nystagmus.

PURPOSE: The visual evoked potential is often used to assess visual function in neurologically impaired patients, a group in whom nystagmus is a common feature. Pattern-reversal stimuli are commonly used to produce visual evoked potentials in clinical practice. Previous reports have shown that this stimulus is not optimal when subjects have nystagmus. The present study aimed to compare the efficacy of pattern-onset and reversal stimuli when used to measure visual evoked potentials from subjects with idiopathic nystagmus. METHODS: In five adults with congenital nystagmus and 10 visually normal adults, VEPs were recorded and reproduced for checkerboard stimuli of two sizes (120' and 60'). Each size was presented as both pattern-onset and reversal check. RESULTS: Visually normal adults demonstrated similar visual evoked potential amplitudes and quality in response to pattern-reversal and pattern-onset. However, in the presence of nystagmus, visual evoked potentials recorded to pattern-reversal stimuli were significantly smaller and of poorer quality than those obtained to pattern-onset stimuli (analysis of variance p<0.05; Kendall's tau, p<0.05). CONCLUSIONS: Pattern-onset stimuli produce larger and clearer visual evoked potentials in patients with nystagmus compared with those produced to pattern-reversal stimuli.

Adult↗
PubMed1991

Saccade onset and offset lambda waves: relation to pattern movement visually evoked potentials.

The lambda (lambda) wave is an occipital EEG potential which occurs when saccadic eye movements are made against an illuminated contrast background. There is some disagreement concerning the presence of sub-components to the lambda-wave, and its relationship to visually evoked potentials. In the present study, lambda-waves were recorded with saccades of different durations (30-110 ms) and compared to VEPs associated with pattern movements of similar durations and velocity. It was found that the lambda-wave consisted of a saccade onset component with positive sub-components at 59 and 100 ms after saccade onset, and a saccade offset component with a positive potential at 74 ms after saccade offset. With small saccades of 30 ms duration or less, these components superimposed to form a single lambda-wave. In the case of pattern movement VEPs, a movement onset component of latency 110 ms following movement onset, and a movement offset component at 89 ms after movement offset, were identified. The similar behaviour of the lambda-wave and VEP under these conditions supports the view that the lambda-wave is a visually evoked potential resulting from movement of the visual field across the retina during a saccadic eye movement.

Adult↗
PubMed1993

Suppression of visual responses of neurons in inferior temporal cortex of the awake macaque by addition of a second stimulus.

The responses of neurons, in inferior temporal cortex of the awake macaque, to single stimuli and pairs of stimuli were examined. The responses of most neurons were weaker to pairs of stimuli than to the best single stimulus of that pair presented alone. This 'suppression by a second stimulus' did not appear to be stimulus-selective and the suppression was greater when the second stimulus appeared in receptive field locations that exhibited weaker responses. This phenomenon suggests competitive interactions between IT neurons that may be involved in visual attention or learning or both.

Animals↗
PubMed1982

The early wave of the visual evoked potential to sinusoidal gratings: responses to quadrant stimulation as a function of spatial frequency.

VEPs were elicited by sinusoidal grating patterns of differing spatial frequencies presented in each of the 4 quadrants of the visual field. Analysis of the early wave of the VEP indicated a pattern of polarity inversions when wave forms were compared across the horizontal meridian of the visual field. With the exception of low spatial frequencies, clear polarity inversions also occurred across the vertical meridian of the visual field in the case of laterally placed electrodes. Comparison of the clearly identifiable early peak with records for the same subject using central field stimulation showed the complexity of the latter records and the inherent difficulty of isolating and interpreting specific component waves of the VEP without adequate topographical data from quadrant stimulation.

Brain Mapping↗
PubMed1982

Pattern-reversal visual evoked potentials and retinal eccentricity.

The effect of stimulation of discrete areas of the retina on visual evoked potentials (VEP) was studied in 16 normal volunteers. The stimulus consisted of a constant luminance 2 degrees 18' field containing checks of 34'30' reversing at a frequency of 500 msec. The amplitude of the VEP was highest at the fixation point and inside the 2 degrees isopter. Rapid amplitude decrement was noted with stimuli located within the 2-4 degrees isopters. No identifiable response was obtained outside the 4 degrees or 6 degrees isopter with a 2 degrees 18' stimulus. VEP could, however, be elicited by increasing the size of the stimulus. The smallest size of a field required to evoke a detectable response also varied in relations to retinal eccentricity. Stimulation at 0 degree, 8 degrees and 14 degrees horizontal eccentricities with fields of a size estimated to activate an equivalent amount visual cortex produced VEPs of similar amplitude.

Electroencephalography↗
PubMed1983

Transient visually evoked potentials to the pattern reversal and onset of sinusoidal gratings.

Transient visually evoked potentials (VEPs) recorded in response to the contrast reversal and onset of spatially sinusoidal gratings have been investigated. Previous reports of an increase in the latency of the response at higher spatial frequencies have been confirmed but only for spatial frequencies higher than 2 c/deg. At lower spatial frequencies it is suggested that two positive components interact resulting in a departure from a monotonic relationship between latency and spatial frequency. Below 1 c/deg pattern reversal and pattern onset modes of stimulation produced VEPs of similar amplitude and wave form. Above 1 c/deg the amplitude of the pattern onset response peaked at a higher spatial frequency than the response to pattern reversal, and the response was dominated by a negative (N1) rather than a positive component (P1). This distinction has been corroborated by investigating the effect of field size variation. The peak amplitudes shifted to a lower spatial frequency with increase in field size but at all field sizes the N1 component of the pattern onset response peaked at a higher spatial frequency than the other components measured. It is attempted to relate these findings to previous studies of both grating and checkerboard VEPs and to psychophysical studies of contrast sensitivity.

Electroencephalography↗
PubMed1983

The recovery cycle of the pattern visual evoked potential in normal subjects and patients with multiple sclerosis.

The recovery cycle of amplitude and latency of the P100 of the chequerboard pattern reversal visual evoked response was studied in 16 normal subjects and 20 patients with multiple sclerosis. There was no clear-cut pattern of recovery with respect to amplitude but at interstimulus intervals of less than 40 msec the latency of the test response P100 tended to be significantly delayed, the magnitude of this delay being virtually constant at intervals of 20 msec or less. Only 8 of 40 eyes from the 20 multiple sclerosis patients showed a pattern of recovery of latency significantly different from the normal subjects. Possible mechanisms for these observations are discussed with particular reference to current concepts of the pathophysiology of multiple sclerosis.

Electroencephalography↗
PubMed1984

Scalp potential fields evoked by grating stimuli: effects of spatial frequency and orientation.

Visually evoked potentials were recorded from an array of 16 electrodes over the occipital cortex in 12 subjects. Grating stimuli of different spatial frequencies were presented on a circular test field, and potential fields evoked by vertical, horizontal and oblique stimuli were compared. Component latencies were determined by computing a reference-independent measure of field power. Scalp field maps were constructed at component latencies and compared over all conditions. Significant influences of both spatial frequency and orientation on component latencies were found. Component latencies were shortest for 2.3 c/deg gratings, and for vertically oriented stimuli. Latencies increased for horizontal, and further for oblique, gratings. The effect of spatial frequency or orientation on the location of the major components on the scalp was less pronounced. On the other hand, there were significant differences both in component latency and scalp distribution when upper and lower hemiretinal stimuli were compared. Upper hemiretinal stimulation yielded shorter latencies and a more anteriorly located potential peak on the scalp.

Adult↗
PubMed1984

Hemispheric asymmetry of visual evoked potentials in patients with well-defined occipital lesions.

Visual evoked potentials were examined in 3 patients who had well defined unilateral occipital lobectomy. In an attempt to resolve discrepancies in the interpretation of hemispheric asymmetries in evoked potentials, recordings were obtained from a wide array of electrodes with multiple montages during full-field and hemi-field stimulation with checkerboard pattern reversal and during full-field flash stimulation. The discrepancies appear to be due primarily to differences in electrode montages used in recording. The use of a midfrontal reference electrode reliably produces a paradoxical EP lateralization: abnormal EPs are recorded over the intact occipital lobe and normal EPs are recorded over the ablated occipital lobe. These findings are fully consistent with the interpretation of Barrett et al. (1976) that the generator of the P100 has an equivalent dipole located in the calcarine cortex on the posteromedial surface of the contralateral hemisphere. Our findings support the use of the Queen Square montage (Halliday et al. 1977). The use of electrodes at O1 and O2 referred to the ipsilateral earlobe yields inconsistent lateralization and should be discouraged. The lateralization of EP abnormalities did not differ for flash and pattern reversal, although the asymmetries were sometimes more prominent with pattern reversal.

Brain↗
PubMed1985

Neurophysiological signs of brain damage due to glue sniffing.

Recordings of the brain electrical responses to pattern reversal (checkerboard) visual stimuli showed significantly increased latencies in a group of 12 young people who had been sniffing glue over a prolonged period. Only small improvements were observed in two people who had repeat recordings after 6 months' abstinence. These results show that chronic glue sniffing can lead to long lasting brain damage.

Adhesives↗