Biomedical subjects
Publications and source records attributed to J R Brunette.
The results of a previous series of experiments in rabbits suggested that under scotopic conditions miosis elevates the electroretinogram (ERG) threshold by casting a peripheral ring scotoma. In the present work similar experiments failed to show any change in the ERG under photopic conditions.
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The response time is an extremely important element of the clinical electroretinogram (ERG) with respect to the recording's clinical interpretation. It is related to the intensity of stimulation. A series of simple experiments was devised in which light penetration to the retina was impaired. We expected simple and straightforward results, with all situations resulting in a decreased effective intensity of stimulation. It seems, however, that different types of decreased ocular light penetration may have different effects on ERG sensitivity and response time. When light penetration to the retina is decreased by filters the peak time is normal at the threshold (50 microV) and the sensitivity is decreased. Miosis produces quite a different picture:both the peak time at the threshold and the sensitivity are decreased. The results with miosis are similar to those obtained with models of photoreceptor destruction. Vitreous hemorrhages seem to act, according to the amount and diffusion of blood, either as filters or by producing a focal loss of photoreceptor function.
Electroretinographic responses can be modified by pathologic processes in two respects: response time and amplitude (or criterion threshold). Electrophysiologic syndromes can thus be defined. It is of clinical importance to recognize the effects of retinal metallosis on the electroretinographic response in cases of metallic intraocular foreign bodies: the sensitivity decreases while the response time remains normal for the intensity of stimulation. Iron appears to be more toxic than copper, but the secondary fibrosis induced by iron protects the retina more efficiently than that induced by copper.
The time response of the Electroretinogram (ERG) is an essential part of the examination. Peak time of response increases with dark adaptation and decreases with the intensity of stimulation. When working with constant amplitude during dark adaptation sensitivity increases but the intensity of stimulation decreases. It is essential to clarify the relationship of these two factors for valid interpretation of results. The present work suggests that the state of adaptation or retinal sensitivity affects amplitude, while peak time of the response is modified by the absolute value of the stimulating source whatever the state of adaptation. This observation has been done on rod responses and is valid as long as pigments remain unbleached and able to respond.
We demonstrate the main electrophysiological events during dark adaptation. We correlate simultaneously the separate rod and cone responses, the ERC sensitivity curves, their timing and the biphasic dark adaptation curve.
We exposed adult Rhesus monkeys who were fully dark adapted to paired flashes of white light which increased in intensity by increments of 0.1 log. units. As intensity increased, the amplitudes of both the cone and rod systems increased while the implicit times decreased so that the responses overlapped. High intensities produce a complex waveform which prevents separation of the rod and cone components or timing of the peaks. On the other hand, low intensities separate the early and late receptor potentials (L.R.P.) of cones and rods well and make it easy to time normal and pathological variations.
When Royal College of Surgeons (RCS) rats are bred with Wistar albinos the offspring are deeply pigmented and have normally functioning retinas. From this second generation, various breeding possibilities exist. In the course of successive cross-breedings one can obtain albinos, tan striped (RCS type) or pigmented animals, any one of which may be normal, carrier or affected. The skin pigmentation trait and that of retinal degeneration are independently inherited and follow autosomal recessive Mendelian laws. However normals for ERG studies vary with skin pigmentation. It is suggested that only one group should be chosen and preferably the pigmented variety.
We studied tapeto-retinal degeneration in the offspring of Royal College of Surgeons (RCS) and Wistar albino rats. Normal ERG values vary according to skin pigmentation: thresholds and implicit times are different. We believe the best experimental material is to be found in the affected pigmented offspring. Evolution of the degeneration over more than 9 months has been observed before extinction and the expressivity of the disease varies from one individual to another. This animal model closely resembles the ERG findings observed clinically in certain pigmentary degenerations of the retina.
The corneal electrical responses of the retina to light OFF are described in this report. The results reveal that high intensities of stimulation evoke an OFF response containing a positive wave. Since this wave was resistant to Sodium aspartate it is concluded that it arises from the receptor level. Negative wave are of double origin a) a negative potential evoked with low light intensities which is resistant to Sodium aspartate b) a second negative wave which is sensitive to the latter drug and thus arises from the internal layers of the retina. It is concluded that, because of the absence of a prominent b wave, the OFF responses permit a more sensitive evaluation of retinal electrophysiology.
In rabbit, the depression of the visual cortex by KCl showed an enhancement of the collicular evoked potentials. This enhancement revealed that the visual cortex had exerted a tonic type influence upon the superior colliculus.
The field potentials recorded simultaneously at various depth of the rabbit's visual cortex and superior colliculus were analysed following light ON and light OFF. The collicular ON and OFF potentials exhibited three slow components superimposed by fast rhythmic oscillations. Only the first slow component reversed its polarity with penetration from surface negative to positive in depth. The cortical ON and OFF responses similarly contained three slow waves which all reversed their polarity with electrode penetration: from surface positive to negative in deeper layers. The most striking difference between ON and OFF cortical responses is the absence of fast rhythmic oscillations in the cortical ON response.
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