[Activation-induced changes in visual functions]. / Aktivierungsinduzierte Veränderungen visueller Funktionen.

Visual functions were tested in 18 healthy male medical students on a control day and on the day of an oral examination. Stress induced impairment was observed in binocular tests measuring accommodation, convergence or their mutual relationship (near points of accommodation and convergence, Maddox-wing). Results of tests in which fusion operates as stabilizing factor (slightly dissociating test: Polatest) or of tests designed for far distance vision (synoptophore, Maddox-cross, Pola-cross) did not show significant impairment under examination stress. In slightly dissociating tests an activity-dependent improvement in stereoscopic vision became evident. Flicker fusion frequency values increased under stress. Colour vision analysis by means of the Nagel anomaloscope revealed that the absolute matching range remained unchanged, whereas the relative matching range broadened on the day of examination. The results indicate activity-dependence of binocular vision, with an improvement in central nervous functions on the one hand and an impairment of peripheral accommodative components on the other hand. Furthermore, it may be concluded that binocular coordination is achieved independently from accommodation dependent focusing of the image and is apparently processed separately and parallel to monocular image components.

The rabbit electroretinogram: effect of GABA and its antagonists.

Previous studies reported that the ERG b-wave is generated by changes of membrane potential of the Müller Cell, mediated by a proximal and a distal extracellular [K+] increase. From measurements with K+ sensitive electrodes there is some evidence that the proximal K+ source is generated by amacrine cell activity. It has been shown autoradiographically that in the rabbit retina GABA is located within a subgroup of amacrine cells, in interplexiform cells and horizontal cells. The effects of GABA, picrotoxin and bicuculline on the ERG of the isolated superfused retina were investigated. Picrotoxin between 10(-4) and 10(-3) M/l and bicuculline between 10(-5) and 10(-4) M/l enhanced the a- and b-wave amplitudes. The b-wave peaked at a later time and decayed at a slower rate than did the control ERG. In higher concentrations than 10(-5) M/l GABA had the opposite effect. In experiments with isolation of the PIII by means of 10(-2) M/l Aspartate or lowering of temperature to 22 degrees C, no effect of GABA on PIII could be shown. The present investigation supports participation of GABAergic cells in b-wave generation. As an underlying mechanism a proximal K+ source is assumed, which is modified by amacrine cell activity.

Possible role of amacrine cells in the generation of the mammalian ERG b-wave.

The ERG b-wave is believed to be generated by a change of membrane potential of Müller cells mediated by alteration in extracellular K+ activity. At least two K+ sources have been suggested. From studies with K+-sensitive electrodes there is some evidence that a proximal K+ source is generated by amacrine cell activity. It has been shown autoradiographically that in the rabbit retina gamma aminobutyric acid (GABA) is located in a subpopulation of amacrine cells. Therefore the effect of GABA on the b-wave amplitude of the isolated superfused rabbit retina was investigated by double-flash stimulation. Concentrations below 10(-5) MM GABA did not change the ERG; higher concentrations diminished the b-wave amplitude. With concentrations of 10(-3) MM GABA response to the second flash was clearly less reduced than that to the first. Furthermore the time course of the ERG was altered. Our investigation supports participation of amacrine cells in b-wave generation.

Spectral sensitivity of the European ground squirrel (Citellus citellus L.).

The spectral sensitivity of the European ground squirrel was investigated by electroretinography (ERG) and visual-evoked cortical potentials. Using chromatic adaptation, two spectral systems were found, one peaking between 520 and 530 nm, the other at 465 nm. The blue system was characterized by a slower course of the potential, a lower threshold and the lack of a d wave in the ERG. However, its threshold is higher than the rod threshold and it was shown to be more resistant to adapting light stimuli than rod systems are. It is concluded that the second spectral system is a blue cone system which in its rod-like properties is comparable to the blue cones found in cat, monkey and man.

A second order model of the optic generator potential and its relation to Stevens' power function.

The component PIII of the electroretinogram representing the optic generator potential was recorded after stimulation with short stimuli with different light intensity. It is shown that the impulse response function of a linear second order model with intensity-dependent coefficients can be well fit to the recordings. Two of its parameters, after logarithmic transformations, are linearly dependent on the luminance while the third parameter varies only within a small interval. It is therefore possible to describe the relation between PIII and luminance in a linearized second order model. Furthermore, both the type of the function relating the gain of the model to the luminance and its exponent are nearly identical with the psychophysic function relating luminance to subjective brightness. Further physiological implications are also considered.

[Aspartate transfer across the blood-retina barrier (author's transl)]. / Permeabilität der Blut-Retina-Schranke für Asparaginsäure.

Permeability of the blood-brain barrier is restricted with respect to amino acids involved in neurotransmission. This finding is well-documented in the case of gamma-amino-butyric acid (GABA) and glycine. Aspartic acid, which is also considered to be a transmitter, equally does not cross the blood-brain barrier in the rat with ease. This amino acid is also thought to be a transmitter in the retina. In order to examine the permeability of the blood-retina barrier with respect to aspartic acid, and investigation was undertaken of the effect of asparate on the light-induced sum potential of the retina in the isolated, perfused cat eyeball, a preparation which guarantees intact retinal circulation. The findings were compared with findings in the isolated retina where the substance was brought into direct contact with the retinal neurons. It was found that aspartate crossed the vascular barrier only to a limited extent and with delay. These results support the hypothesis that aspartic acid is involved in the retinal information processing.

[Experimental study of the problem of retinal ischemia (author's transl)]. / Experimentell-kritische Studie zum Problem der retinalen Ischämie

Survival and revival time of the retina in animal experiments are essentially dependent upon whether total ischemia occurs or not. Using the so-called barbiturate effect in the electroretinogram as a functional criterion, complete ischemia can be shown to take place in rabbit and cat at intraocular pressures of 100 mm Hg. At somewhat lower pressures, which already cause alterations of the ERG a remarked barbiturate effect can be observed, indicating the persistance of a certain amount of remaining blood flow. Possible mechanisms of the barbiturate effect in the retina are discussed.

Effect of azide on the ERG of the isolated mammalian retina.

Azide, which is known to affect the pigment epithelium strongly may be assumed to cause damage to the receptors, which are functionally connected to the pigment epithelium. To check this hypothesis the effect of azide on the ERG was investigated. An isolated retina preparation was used as in this preparation the P III component, which contains considerable receptor contribution, can be isolated. In 2 series of experiments the effects of azide on the P III and the complete ERG were investigated. Depending on the concentration azide was shown to abolish the b-wave, to cause delay and amplitude diminution of the P III and enhance a positive component in the off-effect. A number of plausible sites of origin of these azide effects on the ERG changes are discussed.