Full-field Stimulation Research Articles

Surface distribution of steady-state cortical potentials evoked by visual half-field stimulation.

The surface distribution of steady-state cortical potentials (VECP) evoked by visual half-field stimulation was analyzed by using the fast Fourier transform (FFT). For all ten normal subjects, checkerboard stimulation of the lower visual half-field evoked the largest response at electrode location Oz, with stimulation for the upper visual half-field at Pz. Recording at Pz, a phase difference of approximately 180 degrees was observed for cortical potentials evoked by lower and upper half-field stimulation, the response being bigger for upper field than for full-field stimulation. Comparing the VECP elicited by stimulation of the right and the left half-field, six of ten subjects displayed the highest amplitude at the contralateral cortex (anatomical type), three subjects at the midline (central type), and one subject at the ipsilateral cortex (paradoxical lateralization type). At Oz there was little phase difference in the VECP with stimulation of the right and left visual half-field, and the amplitude was about half that for full-field stimulation. However, with the electrode placed at the temporal region of the skull there was a big phase difference of VECP signals; consequently, the sum of the amplitudes did not amount to the amplitude for full-field stimulation. The dipole theory of scalp distribution of VECP signals was found to be also applicable to conditions of steady-state stimulation, including polarity reversal for upper and lower visual half-field stimulation.

Clinical applications of visual evoked potentials for detection of chiasmal and postchiasmal lesions (author's transl)

Visual evoked potentials (VEP) were recorded in 18 patients with pathologic processes confirmed by computerized tomography in the chiasmal (n = 9) and parietooccipital region (n = 9). Reactions from the right and left hemisphere could be recorded separately in spite of using a simple one-channel apparatus and electrodes only at Oz and Cz. In 17 cases changes of the VEP provided information concerning the localization and extension of the lesion. In chiasmal processes we found a prolongation of monocular latencies, and a delayed or extinguished reaction to half-field stimulation from temporal retinal areas. However, the VEP was often pathologic for half-field stimulation of the nasal hemiretina. Pathologic VEPs were not always accompanied by visual field defects. In contrary to patients with chiasmal processes no pathologic reaction could be found to full-field stimulation in parieto-occipital lesions. Only when the affected hemisphere was stimulated selectively were diminution of amplitudes, prolongation of latencies, or extinguished responses observed. The VEP changes were uniform despite the cause of the lesion (tumor, ischemia). In chiasmal and parieto-occipital processes the VEP supplements computerized tomography by detecting deficits in function. This method appears suitable for monitoring the course of disease before and after neurosurgery.

Vector Analysis of Pattern VEP

A special new method for vector analysis of the visually evoked potential by pattern-reversal stimulation was devised. The initial major vector co nponent acquired by full-field stimulation showed a vertical and large vector stretching from anterior to posterior. The initial major vector component acquired by half-field stimulation was in the ipsilateral and anterior-posterior direction.

Objective perimetry by evoked potential recording: Limitations

Several authors have attempted to use evoked potential recording as a means of objective perimetry. This note describes two limitations. These are: 1. (1) The topological distribution of both flicker and pattern EPs over the scalp depends on which retinal site is stimulated. Thus, in normal subjects, EPs can erroneously suggest that one part of the visual field is defective at electrode A while erroneously suggesting at electrode B that a different part of the visual field is defective. 2. (2) Plots of EP amplitude vs. flicker frequency are different for different retinal quadrants. Thus, in normal subjects, flicker EPs can erroneously suggest a field defect in quadrant A at one flicker frequency while erroneously suggesting a field defect in quadrant B at a different flicker frequency. Further complications are that flicker EPs for individual quadrants can differ markedly between subjects and between left and right eyes of individual subjects (though these differences seem less for fullfield stimulation). These two points limit EP perimetry in the sense that quite misleading conclusions can be drawn from either transient or flicker EPs from a single electrode or flicker EPs at a single flicker frequency. It is necessary at least to compare records from several electrode sites and at several flicker frequencies; in itself a laborious procedure unless a computer is used. Even then, the development of an appropriate computer programme would not be trivial work, because the appropriate criteria for abnormality remain to be empirically established by recording from an adequately large population of control subjects.

Cortical Potentials Evoked by a TV Pattern Reversal Stimulus Varying Check Sizes and Position of the Stimulus Field

Using pattern reversal stimuli, produced by a TV system, visually evoked cortical potentials, obtained by half field and quadrant stimuli, are compared with those of full field stimulation. The lower

A paradox in the lateralisation of the visual evoked response.

THE average evoked response in man to stimulation of the visual field with a reversing checkerboard pattern and recorded from the scalp over the occipital region has a waveform which is consistent both within and between subjects. The most characteristic feature for full-field stimulation is a major positive component at about 100 ms which is recorded maximally 5 cm above the inion in the midline and is distributed fairly symmetrically over both sides of the head (Fig. 1). The pathways from the retina to the visual cortex undergo partial decussation in the chiasma, so that information presented to the left half of the visual field passes to the right hemisphere, whereas the left hemisphere receives signals from the right half of the visual field. Consequently, it might be predicted that stimulation of one half field will produce an evoked response which is maximal over the contralateral hemisphere. The data presented here show that, contrary to this prediction, the maximal response occurs at the scalp electrodes situated over the hemisphere ipsilateral to the field stimulated.

Ocular motor abnormalities in hereditary cerebellar ataxia.

Twelve members of a family with hereditary cerebellar ataxia of late onset were examined and, in 5, quantitative recording of eye movements were obtained. The initial and most severe symptom in all patients was ataxia of gait, followed by dysarthria and later by dysmetria of the limbs. Clinical examination did not reveal involvement of structures other than the cerebellum. Ocular motor examination showed: (1) inability to hold eccentric gaze resulting in gaze-paretic nystagmus; (2) downward beating nystagmus, accentuated on lateral gaze; (3) defective smooth pursuit, with relative preservation of optokinetic nystagmus induced by full-field stimulation; (4) rebound nystagmus; (5) enhanced gain (eye velocity/head velocity) of the vestibulo-ocular reflex during rotation in darkness; (6) decreased ability to suppress the vestibulo-ocular reflex during fixation of an object rotating with the patient; (7) saccadic dysmetria, especially downward overshoot; and (8) square wave-jerks. Although each of these signs can probably occur with lesions elsewhere in the brain, in combination they are highly suggestive of cerebellar involvement. With the reservation that we do not yet have pathological confirmation of the location of our patients' lesions, our results support the suggestion that the cerebellum specifically: (1) helps maintain eccentric gaze; (2) produces smooth pursuit eye movements; and (3) modulates the amplitude of saccadic eye movements. Many of the characteristics of the altered vestibulo-ocular responses and rebound nystagmus could be explained by the underlying anomaly in the smooth pursuit system.

Rod responses in retinitis pigmentosa, dominantly inherited.

Psychophysical and electrophysiological methods are used to evaluate retinal function in a family with dominantly inherited retinitis pigmentosa. Techniques of full-field (ganzfeld) stimulation, scotopically balanced lights, and a ganzfeld adapting field are introduced to analyze the electroretinogram (ERG). Rod ERG responses, reduced in amplitude and delayed in latency, are detectable in the early stages of the disease. Cone ERG responses are normal. Dark adaptation testing shows normal cone but abnormal rod function with a slight delay in the rate of rod recovery. Scotopic luminosity curves and ERG responses to scotopically matched lights suggest that the rhodopsin present has a normal absorption spectrum. A reduction in the amount or effectiveness of rhodopsin in each rod throughout the entire retina can explain the findings. The literature on light-induced retinal degenerations is reviewed, and the effects of light exclusion on this disease are considered.