A clinical method for obtaining pattern visual evoked responses

Effects of experimental scotomata on sequential pattern-onset, pattern-reversal and pattern-offset visual evoked potentials

The effect of experimental scotomata on visual evoked potentials to half-field stimulation using sequential checkerboard onset, reversal and offset was investigated in 10 normal subjects to assess the relative sensitivity of the three stimulus modes, and the contributions of pathways subserving macular and paramacular parts of the visual field. Four scotoma sizes (0-1.5 degrees , 0 degrees - 2 degrees, 0 degrees -3 degrees and 0 degrees -4.5 degrees ) were used to mask the central part of the stimulus field (0 degrees -12 degrees ). Five check sizes (6', 12', 20', 50' and 80') were presented for each scotoma size. Peak-to-peak amplitudes and peak latencies of components on the ipsilateral and contralateral sides of the scalp to the stimulated half-field were measured. Scotoma size was highly significant in influencing component amplitude (p < 0.0001) and latency (p < 0.03) of all ipsilateral and contralateral onset components and all ipsilateral reversal and offset components. Components that were attenuated to the greatest extent with the smallest 0 degrees -1.5 degrees scotoma were the contralateral onset P105 and ipsilateral reversal P100 and N145. Onset CIII, reversal N80, and offset N85 and P110 only showed a significant attenuation after the use of scotomata of 0 degrees -3 degrees and larger. Our results show that scotoma size is a significant factor in influencing all the major visual evoked potential components, with the exception of reversal and offset contralateral potentials (N105 and N115), probably reflecting their paramacular origins. Reversal, ipsilateral P100 and N145, and onset, contralateral P105, appear to be predominantly of macular origin and the most sensitive potentials for detecting effects of small central scotomata.

The effect of stimulus size on human cortical potentials evoked by chromatic patterns

The effect of stimulus size on the pattern onset-offset visual evoked potential elicited with stimuli of two different wavelengths is studied under intensive yellow adaptation: (1) The onset response obtained with a 460 nm pattern is of negative polarity (N1) and saturates in amplitude with a stimulus radius of 7 deg. The onset response obtained with a 550 nm pattern is of positive polarity and continues to increase up to the maximum size (32.2 deg). (2) The peak time of N1 (460 nm) decreases with increasing stimulus size, that of P1 (550 nm) remains constant. These results are discussed as reflecting either varying retinal and brain anatomy, or cone activity, color-opponent activity, or luminance contrast activity.

Horizontal scalp distribution of steady-state pattern visual evoked cortical potentials in response to quadrant field stimulation

Scalp profiles of steady-state pattern visual evoked cortical potentials (VECPs) along the horizontal line at Pz (for upper field stimuli) and Oz (for lower field stimuli) were studied in 12 normal subjects by simultaneous five channel recordings from electrode positions 3 cm and 6 cm laterally on either side of, and at, Oz or Pz. Each electrode was referred to Fz or to the right earlobe. The amplitude and phase of the averaged VECPs (n = 20) were calculated by means of Fast Fourier Transform. With smaller check size the amplitude data verified that the scalp profile of the lower visual field peaked at the hemisphere contralateral to the quadrant stimulated. With larger check size the peak shifted to the ipsilateral hemisphere. With upper field quadrant stimulation, position of the reference electrode at Fz was found to be unsuitable since the VECP amplitude at the level of Pz was greatly reduced. Phase and apparent latency also varied with the field stimulated as well as with the position of electrodes.

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.

Correlation of evoked potentials (SEP and VEP), EEG and CT in the diagnosis of brain tumors and cerebrovascular diseases

In studying evoked potentials, EEG and CT in 29 patients with cerebral lesions (25 brain tumors and 4 with cerebrovascular diseases), a pathological EEG was found in 24 cases, pathological CT in 26 cases, pathological VEP in 10 of the 25 cases, pathological SEP in 10 cases out of 19. - Patients with pituitary tumors (chiasmatic tumors) showed pathological VEP and normal SEP due to compression of the visual pathway, and this correlated with the clinical site of the lesion, EEG and CT. - One patient with thalamic tumor showed pathological SEP and VEP due to affection of the somatic relay nucleus and the lateral geniculate body, this correlated with EEG and CT. - Patients with supratentorial tumors showed pathological SEP (absent cortical response), when there was cortical sensory loss and delayed conduction between N14-N20 when the posteior columns was affected. - Cases with infratentorial tumors showed pathological SEP only when there was affection of the ascending sensory pathway or the visual pathway. These cases showed pathological EEG and CT. - Cases with cerebrovascular disease showed pathological SEP when the lesion is at the thalamic level or affecting the thalamo-cortical pathway, evidenced clinically by cortical sensory loss or posterior column affection.

The effect of experimental 'scotomata' on the ipsilateral and contralateral responses to pattern-reversal in one half-field

The averaged cortical responses to a reversing checkerboard pattern presented monocularly in either left or right visual half-fields have been recorded from the occipital scalp using a transverse chain of widely spaced electrodes referred to a common mid-frontal electrode. The half-field responses showed a consistent asymmetry, the dominant feature of which was a positive wave (P100) that was widespread on the ipsilateral scalp and maximally recorded from the midline and ipsilateral electrodes. This formed part of the triphasic negative-positive-negative complex, the other two components being an N75 and an N145. On the contralateral scalp it was generally possible to record a triphasic complex of opposite polarity, but this was usually of smaller amplitude and its components (P75, N105, P135) showed greater variation in latency and morphology than the ipsilateral components. With progressive occlusion of the pattern stimulus from the central regions of the visual half-field, the ipsilateral positive wave (P100) was increasingly attenuated, while components of the contralateral complex were relatively unaffected, or, in some cases, enhanced. By contrast, reducing the radius of the stimulated area had relatively little effect on the ipsilateral P100, while the contralaterally recorded response was attenuated. These differential effects on the half-field response components are discussed in relation to the anatomy of the central and paracentral cortical representation of the visual field. The implications for the interpretation of evoked potential recordings in patients with field defects are considered.

Visual evoked responses and visual symptoms in multiple sclerosis

Absolute latency, interocular difference in latency, and waveform of visual evoked responses (VER) to checkerboard reversal stimuli recorded from the midline of the skull were studied in 104 multiple sclerosis patients, 25 to 50 years of age, classified according to visual symptomatology. Group 1 had strong evidence of past or present optic neuritis. Patients with blurring of vision, diplopia, and undefined visual complaints were assigned to group 2, while group 3 contained patients with no visual symptoms but suspected diagnosis of multiple sclerosis on other grounds. The three parameters explored showed consistent association with the degree of visual involvement, as assessed by clinical impression, but their discriminatory power was diverse. Absolute latency was significantly longer in group 1 patients compared with groups 2 and 3, but it did not discriminate between the last two, whereas interocular difference in latency proved to be sensitive to differences between symptomatic (diplopia, blurring) and asymptomatic groups (2 and 3). Waveshapes were grouped into three categories based upon degree of distortion of the major positive peak, and their relative distribution among the three patient groups was found to be associated with symptomatology. We suggest that, in the production of symptoms such as diplopia, a temporal disparity of afferent impulses might be involved in much the same way that spatial incongruities between both eyes lead to impaired function. In this regard, interocular difference in latency rather than absolute latency would be a more accurate predictor of symptom development. The analysis of VER waveshape suggests, in addition, the importance of inhomogeneous involvement of the visual pathways in the production of symptoms during the evolution of multiple sclerosis.

Latencies of evoked potentials to flicker and to pattern speedily estimated by simultaneous method

This note describes a method for rapidly measuring the latency of a steady-state visual evoked potential (EP). Three sinewaves of different frequencies (F1, F2 and F3 c/sec) are summed, and the resulting waveform modulates either light intensity (for flicker stimulation) or spatial contrast (for pattern stimulation). Three evoked potentials of frequencies F1, F2 and F3 c/sec respectively are stimultaneously recorded by running three Fourier analysers in parallel. The outputs of the three analysers are presented in a single polar plot so as directly to display the three values of EP amplitude and phase. Practical methods for generating the pattern and flicker stimuli are described.

The pattern evoked responses (VER) in optic neuritis

Five patients presenting clinically accepted signs of optic neuritis served as subjects for a study on EP's (evoked potentials); the patients were tested during the acute phase, the chronic phase and at a stage when no impairment was clinically observed. At the acute phase of optic neuritis the VER was massively altered; in the chronic phase delayed latencies depending upon the impairment of visual acuity and visual fields could be recorded. At the recovery stage, while visual acuity and visual field showed no deficit, not only delayed latencies but also reduction in the amplitude at all stimulation frequencies were recorded.