Scotopic and mesopic light adaptation in the cat´s retina

The effects of light adaptation on retinal on-center ganglion cell firing were measured in the scotopic-mesopic range (10−5–1 cd/m2). Using diffuse adapting stimuli, the maintained discharge rate of all on-center units increases with increasing adapting luminance up to 10−3–10−2 cd/m2. Above this luminance it levels off or decreases. The change of the maintained discharge rate with increasing adapting luminances was related to changes in the receptive field organization of the unit and to changes in retinal sensitivity. The sensitivity of the retina at the different adapting luminances was measured by determining the luminance ΔI of a test spot which elicited a constant criterion ganglion cell response. ΔI increased with increasing adapting luminance not proportional toI A (Weber's law) but proportional toI A n (n varying between 0.45 and 0.75). Correspondingly, by changing the adaptation level, the intensity: response curve of a ganglion cell for small center stimuli was displaced on the intensity-axis by less than the adapting luminance step. The intensity response-curves at the different adaptation levels were parallel and S-shaped in the semilogarithmic coordinates with a straight course over 1.5 log units. The intensity-response relation for small center stimuli was best fitted by a log-function at all adaptation levels. It is concluded that the ganglion cell itself is not involved in the adaptation mechanism. Adaptation and spatial summation of signals occur at different levels of the retina.Die Auswirkungen der Helladaptation auf das Entladungsverhalten von on-Zentrum Neuronen der Katzenretina wurden im skotopischen und mesopischen Leuchtdichtebereich untersucht (10−5–1 cd/m2). Bei Verwendung diffuserLichtreize steigt die Daueraktivität aller on-Zentrum Neurone mit steigender Adaptationleuchtdichte bis zu Leuchtdichten zwischen 10−3 und 10−2 cd/m2 an. Bei weiterer Erhöhung der Leuchtdichte bleibt die Daueraktivität konstant oder nimmt wieder ab. Der Verlauf der Daueraktivität eines Neurons bei steigender Adaptationsleuchtdichte steht in Zusammenhang mit Veränderungen der receptiven Feldstruktur und der Empfindlichkeit der Retina. Die Empfindlichkeit der Retina wurde bei verschiedenen AdaptationsleuchtdichtenI A durch die Leuchtdichte ΔI eines Testreizes gemessen, der zu einer konstanten Schwellenreaktion führte. Bei ansteigendemI A nimmt ΔI proportionalI A n zu, wobein zwischen 0,45 und 0,75 liegt. Entsprechend wurde die Reiz-Reaktionskennlinie eines Neurons bei Veränderung des Adaptationszustandes um weniger als die logarithmische Differenz der Adaptationsleuchtdichten auf der Intenistätsachse verschoben. Die Reiz-Reaktionskennlinien eines Neurons bei verschiedenen Adaptationszuständen sind bei halblogarithmischer Auftragung parallel zueinander verschoben. Sie sind S-förmig mit einem geraden Anteil über 1,5 log Einheiten. Bei allen Adaptationszuständen wurden sie am besten durch eine Logarithmus-Funktion beschrieben. Es wird gefolgert, daß Adaptation und räumliche Summation Funktionen verschiedener Schichten der Retina sind und die Ganglienzellen nicht aktiv am Adaptationsmechanismus beteiligt sind

Postsynaptic potentials in the cat's visual cortex following electrical stimulation of afferent pathways

Postsynaptic potentials (PSP) of neurones in the visual cortex after electrical stimulation of optic tract and radiation fibres were analysed. Special attention is drawn to the disappearance of excitatory reactions, if cells are partially depolarized to a membrane potential of 30–40 mV, as it is the case in many visual cortex cells after impalement. This might lead to misinterpretation of the records obtained. In those cells in which excitatory responses were still visible, stimulation of the optic tract lead to sub- and suprathreshold EPSP's at an average latency of 2.8±0.3 msec. This mean latency could be divided into two sub-groups (2.4 and 3.8 msec). After radiation stimulation the latency for excitation was 1.2±0.3 msec; antidromic responses due to stimulation of corticofugal fibres were sometimes observed. The excitatory responses were followed by a gradable IPSP which sometimes was the first recordable event after a stimulus. Because of the above mentioned limitations, the number of cells with true primary inhibition could not be evaluated. Latencies of IPSP's after optic tract and radiation stimulation were, in the average 1 msec longer than those of EPSP's and both latency groups were clearly separated. After optic tract stimulation two different groups of IPSP-latencies could be distinguished (3.4 and 4.6 msec), which again were 1 msec longer than the two groups of EPSP-latencies. The findings suggest a recurrent collateral inhibitory feedback mechanism within the visual cortex. No indication for special inhibitory interneurones or for direct inhibitory geniculocortical fibres was found. The distinction of two latency groups after optic tract stimulation can be explained by two fibre groups with different conduction velocity in the optic radiation. Interaction of PSP's induced by light and electrically was demonstrated