OSCILLATORY POTENTIALS IN THE VISUAL SYSTEM OF CATS AND MONKEYS

Spontaneous membrane fluctuation in catfish type-N cells

Type-N (sustained) amacrine cells in catfish retina produce spontaneous membrane fluctuation of about 35 Hz. Fluctuations were seen either in dark or with a steady illumination. As all the distal cells and type-C (transient) amacrine cell did not produce any spontaneous fluctuation, type-N cells were the source of the oscillatory potentials.

Nasopharyngeal recordings separate retinal from optic nerve potentials

Electrical potentials in response to single flashes and to pattern-reversal stimuli presented in Maxwellian view were recorded from several intranasal locations by withdrawing a nasopharyngeal electrode from the epipharynx through the nose in steps of 1 cm. From the anterior parts of the nose a waveform could be obtained resembling the corneal electroretinogram. In the epipharynx an ERG was recorded which was inverted in polarity. This reversal in polarity was explained by assuming an electrical dipole of the retina oriented parallel to the electrode path. When the electrode was below and behind the posterior pole of the eye ball a high-frequency activity of increased amplitude was observed the origin of which could not be explained by the retinal dipole. It is suggested that these increased oscillations originate from the optic nerve. Their behavior is explained by a hypothetical electrical dipole of the optic nerve oriented vertical to the electrode path.

High-frequency components of human visual evoked potentials

This study was aimed to further our investigations on the high frequency components of human visual evoked potentials (VEP). By using stimulations in the form of optic step functions, we measured (Cz-ear lobe) VEPs with 512 real data points in a total recording time of 20.48 msec in normal adults (N = 12). The peaks in the transient evoked potentials occurred with approximate latencies of 0.5, 1.0, 1.5, 2.5 and 5.0 msec. Transforming the individual time responses to the frequency domain by the Fourier Transform, maxima were obtained in 200, 400, 700, 1200 and 2200 Hz positions in the amplitude frequency characteristics. The findings were compared with those of a similar study on the auditory system. It is stated that the response activities of 200 Hz and 2200 Hz are unique to the visual system.

Rhythmicity in rabbit retinal ganglion cell responses

An interesting pattern of rhythmic spike bursts was observed in the light responses of rabbit retinal ganglion cells when using a fast time scale. This rhythmicity was found in all ganglion cell types tested in all parts of the retina. The presence and extent of rhythmicity was related to the intensity, size and shape of the light stimulus. The best stimulus extended into the receptive field surround yet still evoked a strong response from the center. We suggest that rhythmicity results from time delays in the inhibitory interactions between central and surround pathways to the ganglion cells.

Origin of wavelets in the visual evoked potential

Low amplitude high frequency wavelets have been demonstrated to be ubiquitous in the visual system of animals and are observed in the ERG of man. Wavelets have also been observed superimposed upon large slow waves obtained from electrodes on occipital scalp. Presently, the rather stereotypic wavelet repetition rate permitted the use of active analog filters tuned to 100 Hz, with a 60-200 Hz bandpass which produced no measurable distortion at 100 Hz. With this method we recorded a series of wavelets in 15 of 16 subjects that usually began from 35 to 40 msec following the onset of a 10(4) troland, 25 degrees visual angle 100 msec flash. We then sought to determine the origin of these wavelets. Wavelets recorded between occiput (Oz) and vertex (Cz) had photopic spectral sensitivity that differed from that of ERG wavelets simultaneously recorded. Moreover, in a topographical study, wavelets recorded between Cz and various lateral positions reached a maximum at Oz; and when adjacent bipolar electrode pairs were used, wavelet polarities inverted as pairs were moved across the occipital region. Thus wavelets recorded between 35 and 70 msec were likely generated from occipital cortex rather than retinal, sub cortical, or diffuse cortical sites. In addition, the topographical distribution of slow wave ('P100') and wavelets differed. Wavelet latencies had a different relation to retinal illuminance than P100 latencies, suggesting that P100 and wavelets have different neurogeneses. Wavelets recorded between Oz and Cz thus reflect the earliest cortical visual processing recorded in man.

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.

Two retinal processes displayed in the cat electroretinogram

Local stimulation of dark adapted cat's retina with spots of light of various intensities elicits a complex b-wave in which four deflections can be consistently identified. An analysis of these wavelets recorded with the low frequency cut off at 80 Hz is made in an attempt to infer their origin. The experimental conditions under which these wavelets were studied were standardized by stimulation with a spot of light at different intensities, frequencies and positions in the visual field. The changes in amplitude and latency by those different conditions of stimulation and by the intravenous (i.v.) injection of additional doses of nembutal, suggests that these peaks are triggered by two retinal excitatory processes related to the scotopic system. It is also pointed out that components 1-2 are mainly determined by the activation of retinal cells which correspond to the area centralis and that components 3-4 represent the activity of retinal cells from the lateral regions (nasal and temporal). The possible clinical use of our results is discussed.

Oscillatory potentials. History, techniques and potential use in the evaluation of disturbances of retinal circulation

The oscillatory potentials seem to reflect severe disturbances in the retinal (and perhaps choroidal) circulation. In some cases of diabetic retinopathy with severe microangiopathy, the oscillatory potentials may be selectively reduced or extinguished while the amplitude of the a- and b-waves of the ERG remains normal. A correlation appears to exist between severely reduced oscillatory potentials and a circulatory deficiency in the retina. This selective reduction of the oscillatory potentials during advancing retinopathy is considered to be indirect evidence that they are generated independently from the mechanism producing the primary components (the a- and b-waves). The usefulness of the oscillatory potentials in the prognosis of retinal disease, particularly in diabetic retinopathy, is reviewed. The historical background, the techniques and instrumentation necessary to produce and record them, the experimental data available on the site of their origin, the clinical significance to date and the experimental efforts in our laboratory are summarized.

Analysis of the retina via suprafusion electroretinography

Electroretinographic (ERG) transient responses elicited in monkeys by abrupt changes in the periodicity of a rapidly intermittent (suprafusion) luminance stimulus were studied experimentally, and analyzed and interpreted through a theory of dynamic retinal responses. The suprafusion ERG transients are confirmed to behave in accord with theoretical expectation, as elemental responses (retinal Green's functions). By aid of the theory the ERG wave-forms can be reduced to two significant elements. One element, accounting for approximately two-thirds of the total ERG variance, is strictly linear, and correlates well with simultaneously evoked cortical (VEP) transients which were previously related to suprafusion perception in humans. The other element, comprising approximately one-third the ERG transient, is a rectification, with properties indicating that it may arise from a specific layer of retinal neurons (amacrine cells); on this assumption the theory demonstrates that high-frequency nonlinear ERG flicker can isolate activities proximal and distal to the rectifying (amacrine) layer. Thus, the hypothesis of an amacrine origin for the rectifying element entails the possibility that suprafusion ERG studies could accomplish in vivo "dissection" of the human retina.

Visual evoked potential in man: early oscillatory potentials

Short latency visual evoked oscillatory potentials to bright light stimulation were recorded from the scalp of 15 normal human adult subjects. The onset latencies of these potentials recorded over anterior frontal and posterior scalp regions were 9--17 msec and 13--24 msec, respectively. The frequency of the oscillations was about 100 c/sec. These potentials were widespread in their distribution over the scalp but were most prominent at midline and parasagittal recording locations. Like similar potentials recorded in animals, it seems that these potentials arise in both subcortical and cortical visual structures. The mechanism underlying the generation of these potentials and their possible functional significance are discussed.

Studies of visual function and its decay in mice with hereditary retinal degeneration

Functional implications of mouse hereditary retinal degeneration have been studied at the level of the superior colliculus and visual cortex in the C57BL/6J-le rd strain. On autoradiography at a light-microscopic level, following eye injection with radioactive compounds, central visual structures appeared normal. A slight reduction in ipsilateral retinal projection was probably related to reduced retinal pigmentation associated with the light ear (le) mutation. In recordings from visual cortex and tectum in rd mice older than five months the cells discharged with highly rhythmic maintained activity. This ongoing activity depended on retinal input, since temporary asphyxia of the eye stopped it immediately. The frequency of the rhythm was influenced by the anesthesia. In these older mice no visual receptive fields could be mapped, but in a few tectal recordings it was possible to suppress the maintained activity by diffuse, very intense illumination. As in normal mice, no auditory or somatosensory responses were observed in the visual cortex or upper tectal layers. In recordings from tectum before the age of three weeks retinotopic topography and receptive fields were normal. By day 24 no receptive fields could be recorded from parts of the tectum representing the central 90--100 degrees of the visual field, whereas within a peripheral ring responses were still roughly normal under photopic conditions. Over the following four months these peripheral responses faded away slowly. Incremental thresholds, especially in the scotopic range, were elevated, rising slowly to unmeasurable values. Similarly during dark adaptation the thresholds fell to values several log units above those reached in normal mice; these values of dark adapted thresholds in rd mice rose with age. This is consistent with morphological changes known to occur in the retina as a consequence, of the rd mutation the rods degenerating before the cones.

Procedures for routine clinical electroretinography (ERG) in dogs

Following pupillary dilatation and immobilization of the dog with a cataleptic drug (l-Polamivet, Hoechst) the electroretinogram (ERG) was performed with the technique of Ganzfeld stimulation. The head of the dog was kept within a sphere of 60 cm diameter, the white inner surface of which could be indirectly illuminated with a stroboscope producing light flashes of 10 musec duration. The ERG was recorded oscillographically by means of modified contact lenses. Dogs were tested for rod and cone function with luminance curves including white, blue and red stimuli and with trains of repetitive photic stimuli. Tests were performed under dark and light adaptation. The ERG of the dark-adapted dog, an indicator for the electrical activity of the rod system, was similar with that of man with respect to configuration and other characteristics. The electrical activity of the cone system was different from that of man by having a 10-fold lower sensitivity and a reduced capability for discrimination of red versus blue or white stimuli.

Tonic retinal influences in primates

A type of unit discharge, termed "luxotonic," has been found in the striate cortex of unanesthetized squirrel monkeys and macaques.6, 21 The firing frequency of these units shows relatively little adaptation, continues indefinitely (hours), and reflects the level of diffuse illumination of the eye. The more numerous "photergic" units discharge more rapidly in the light, whereas "scotergic" units fire fastest in the dark (or at luminance levels below threshold for cones). Luxotonic activity is abolished by anesthesia and has not been described for striate cortex of other species. Primates also display a profound alteration in the EEG of striate cortex following elimination of all retinal input.32 Since this change is far more drastic than that produced by blindness in other species, it is natural to inquire whether it is related to the loss of the normally prominent luxotonic activity. When the blind monkey sleeps, the bizarre EEG is replaced by patterns wholly normal in appearance,32 indicating that some nonvisual system has extensive access to striate cortex in this state.

Flash evoked potentials from rat superior colliculus

In view of reports that the superior colliculus evoked potential from rats is uniquely sensitive to toxic gases, the present study characterized normal flash evoked potentials from unanesthetized rats. The waveform was complex, with at least 5 positive and 5 negative peaks. The waveform originated in the SGS layer, and some components were stable over time if conditions of light intensity, stimulus frequency and dark adaptation were held constant. The greater complexity of the waveforms reported here compared to those described by others can be attributed to both an intense flash stimulus and unanesthetized preparation.

Electrophysiology of neural units in goldfish optic tectum

Axons of retinal ganglion cells showed responses not previously emphasized: (a) many tonic units discharged oscillations, 2--12 spikes per burst, interburst intervals 20--300 msec; (b) phasic units showed concentric or flanking ON and OFF fields, response frequency depended on balance of retinal excitation and inhibition; (c) directional sensitivity was maximal for retinal stimuli moving in naso-temporal direction; (d) in anterior tectum deep afferent layer (DAL) provides for deep electrical sink, fibers of DAL have small fields, mostly in front of fish; (e) color-opponent units are prevalent in the superficial terminal layers, color is spatially and temporally represented. Tectal cell responses were distinguished by large visual fields, spontaneity, multiple spikes and long latencies to optic nerve stimulation, failure to follow above 60 per sec, plasticity of response. Tectal neurons of three classes included (a) cells of one type in upper layers were inhibited in ongoing activity by visual input, receptive fields exceeded 100 degrees, were often oblong, responses did not habituate; (b) cells of second type were excited by visual stimuli, became unresponsive (habituated) or responsive only to stimuli in different position or direction (newness cells); lability precluded field mapping and dishabituation was produced by change in background, extraneous stimulation, and spontaneous firing; (c) pyriform cells in periventricular layer were abundant, difficult to isolate electrically, discharged spontaneously in bursts at intervals of several seconds and responded to visual input by interruption of firing. Some tectal cells responded to non-visual stimuli as well.

Oscillations in rod and horizontal cell membrane potential: evidence for feed-back to rods in the vertebrate retina

1. Rods and horizontal cells were studied with intracellular recordings in the retina of the toad, Bufo marinus; 161 cells were from the eyecup preparation and thirty were from the isolated perfused retina. 2. Of these cells, 39% exhibited either transient or sustained oscillations of membrane potential. Light flashes either evoked transient oscillations or temporarily abolished sustained oscillations. The amplitudes of the oscillations could be as large as 27 mV. The frequency of the oscillations at 25 degrees C was between 1-5 and 3-5 Hz and was strongly dependent on temperature and background illumination. 3. The rod oscillation amplitude and the peak of the horizontal cell light response increased similarly with increasing test flash diameters. They continued to grow for diameters much larger than those which increased the peak of the rod light response. 4. Perfusion of the isolated retina with 2 mM aspartate had only a small effect on the rod light response but it completely eliminated the horizontal cell light response as well as the oscillations recorded in both rods and horizontal cells. 5. It is believed that the oscillations result from a reverberating interaction between rods and neurones post-synaptic to rods. Thus, rods can be both post- as well as presynaptic retinal elements.

Correlation analysis of units recorded in the cat dorsal lateral geniculate nucleus

Spike activity was simultaneously recorded from pairs of units consisting of both optic tract fibers and relay cells in the cat dLGN. The unit pairs were classified with regard to the types of receptive fields involved, the relative location of receptive fields, and their occular drive. Statistical dependencies between the discharge patterns of the simultaneously observed units were assessed by computing their auto- and crosscorrelogram during a period of maintained discharge. Two distinctly different types of statistical dependence were observed. Unit pairs having like and overlapping receptive field centers exhibited statistical dependencies manifested in their crosscorrelograms by a peak near the origin. Conversely, unit pairs having opposite and overlapping receptive field centers were characterized by a prominent valley in their crosscorrelograms near the origin. These statistical dependencies are inferred to represent functional interaction most probably occurring in the retina and characterized by a common element exerting an influence on both units.

The function of the retina in the perfused eye

The data show that the enucleated eye of the cat can be maintained in apparently physiologically functioning condition by appropriate arterial perfusion. Under appropriate conditions, photically evoked electrical mass responses can be recorded from various parts of the isolated, perfused eye for 8 to 10 hours. ERGs as well as responses from axonal bundles of the optic nerve exhibit shapes, amplitudes and time courses comparable to their counterparts in vivo. Homeostasis of the perfusion ensures the stability of these light-evoked electrical responses. Transient changes in biophysical parameters of the perfusate rapidly induce marked, although reversible, changes in the amplitudes of b-waves of the ERGs. Increases or decreases in the flow rate of the perfusate induce parallel increases or decreases in the amplitudes of the b-waves as well as of the optic nerve responses. Similar alterations in the oxygen concentration of the perfusate induce similar and proportional changes in the amplitudes of the b-waves. It is concluded, that low flow rates of hemoglobin-free perfusate induce hypoxia; consequently, acceleration of the flow can compensate for hypoxia in a certain range. Previous studies on the effects of and recovery after transient hypoxia in mammalian retina are in concordance with the present data. Progressive decrease of temperature induces gradual and reversible reductions in the amplitudes of the b-waves and increases their latencies and peak-times. It is suggested, that initial hypothermia, which occurs during the period of cannulation, reduces the deliterious effects of the coincident unavoidable hypoxia on retinal neuronal elements. Since light-evoked electrical responses can be maintained for many hours in these preparations and since movements of cardiovascular and respiratory origin, invariably present to varying extent in the in vivo experiments, are eliminated, this preparation is suitable for intracellular recordings from neuronal elements of the retina. Potentials were recorded from cells in various layers of the retina of the cat; intracellular recordings from horizontal cells (S-potentials) are described in detail. Spectral analysis of S-potentials allowed to distinguish between three types according to their inputs: a mixed, rod-cone type, which was most frequently encountered, a pure cone- and a pure rod-type. Light- and electronmicroscopic investigation of the retina after perfusion revealed that (1) the extent of cellular damage depends on the flow rate of the perfusate; (2) little cellular damage is observed if medium flow rates, which maintain physiologic responsiveness of the isolated eye to light, were applied for two hours; (3) high flow rates applied for two hours, or medium flow rates applied for 7 hours appear to induce cystic changes in the pigment epithelium, but only minor changes in the cells of the inner nuclear layer.

A late component of flash-evoked potentials in the cat's optic chiasma and superior colliculus: its appearance due to background illumination

Flash-evoked potentials (FEPs) in the cat's optic chiasma and superior colliculus were recorded under the following two conditions: complete darkness and background illumination. Special attention was paid to a specific late component of FEP'S and comparison in the behavior of the late component was made between the two conditions. It was found that a late component of FEPs in the optic chiasma appeared in the presence of background illumination while it was not observed under the condition of complete darkness. A corresponding late component of FEPs was detected in the superior colliculus. The late component might be supposed to be driven from a class of W-cells in the cat's retina.

Single-cell responses associated with rhythmic slow-wave potentials in rat superior colliculi

The present study was designed to investigate the features of rhythmic slow-wave potentials in the superior colliculi of rats and to study the relationship between these potentials and the activity of single colliculi neurons. In contrast to studies in other portions of the visual system, rhythmic slow-wave potentials in the colliculi fell within a single limited range of frequencies from 9 to 26 cycl/sec. Single-cell recordings revealed that numerous collicular neurons were also responding rhythmically, with discharge frequencies generally coinciding with those established for collicular slow-wave potentials. In some animals slow-wave and single-cell potentials were recorded simultaneously through adjacent electrodes, and in all instances the frequency of discharge of single neurons coincided with the frequency of oscillation of the slow-wave potentials. These data suggest that rhythmic postsynaptic potentials in individual collicular neurons may be the mechanism by which rhythmic slow-wave potentials are generated.

Antidromic responses of orthodromically identified ganglion cells in monkey retina

1. Conduction velocities of two types of on-centre monkey ganglion cells, called phasic and tonic, have been measured by stimulating their axons in the optic tract while recording from their cell bodies in the retina.2. The average conduction velocity of twenty-two phasic and twenty-seven tonic cells is 3.8+/-S.D. 0.6 and 1.8+/-S.D. 0.4 m/sec respectively. Since the latter, but not the former, show opponent-colour responses, retinal signals carrying information about colour appear to be travelling in smaller axons than those not handling such information.3. Stimulation of the optic tract elicits several graded intraretinal potentials, which are negative in the optic nerve fibre layer and positive in the inner plexiform layer. One of these potentials, which is largest near the fovea, occurs simultaneously with the antidromic impulses of tonic ganglion cells and is considered to result from extracellular current generated by these cells.4. Stimulation of the optic tract suppresses the orthodromic responses of ganglion cells, more for phasic than for tonic ones. This suppression is only observed after a cell is antidromically driven and is considered most likely due to a transient hyperpolarization of the cell's membrane potential following an impulse.