Aktivität von Neuronen im Tractus opticus und Corpus geniculatum laterale bei langdauernden Lichtreizen verschiedener Intensität

Summary

• 1. The activity of single fibres of the optic tract (TO) und of neurones of the lateral geniculate body (CGL) was recorded during long lasting (4–10 min) diffuse light stimuli of different intensity (18–3500 lux) and during dark periods of the same length. The experiments were done on cats without general anestesia (operation under ether, curare).

• 2. At light on, most neurones of the TO as well as of the CGL showed, after an initial peak (on-, on-off-neurones) or an initial depression (off-neurones) of activity an almost exponential transition towards steady-state activity. This transitional period lasted 80–120 sec. Further analysis revealed two exponential parts, a first one whose course was independent from light intensity and which lasted 2–3 sec, and a second one whose steepness was dependent from the light intensity. At higher intensities, in on-neurones a triphasio course of adaptation was found with a significant depression of activity following the initial peak and a slow increase towards the steady state rate. The steady state discharge rate during illumination was above the dark activity in on-neurones, below in most on-off and all off-neurones. Some neurones (mainly with an on-center) only showed a short change of activity at light on and off but no lasting change of their activity during light. Since the majority of neurones with light-intensity-dependent steady state-discharge thus shows a decrease of activity during illumination (off- and most on-off neurones), Arduini's observation (1962) of a decrease of mass activity in TO and CGL during illumination is understandable.

• 3. The time course of adaptation of discharge activity of off-neurones at light off corresponded to that of on-neurones at light on. On- and off-center neurones did not behave completely reciprocal throughout the whole course of adaptation.

• 4. If in a given neurone the steady state discharge rate showed a systematic relation to light intensity this relation could be expressed by logarithmic functions as well as by power functions. This is valid for on-center- (positive exponent) as well as for off-center-neurones (negative exponent). On-center-neurones in the geniculate, however, may show a non-linear relation with two functions of different steepness at low and high light intensities. The validity of power- as well as of logarithmic functions is due to the similarity of both functions found in these experiments.

• 5. Differences in the behaviour of TO and CGL-neurones are shown. These consist mainly of a faster adaptation and lower discharge rate of CGL neurones at all intensities so that the relation between light intensity and discharge activity in the CGL is reduced by a linear factor compared to that in the TO. Besides this, qualitative differences exist in the steady state discharge activity with a tendency towards grouped firing in the geniculate.