Analysis of the Response Characteristics of Optic Tract and Geniculate Units and Their Mutual Relationship

Summary. 1. Responses to light stimuli of cat's optic tract and geniculate units are analyzed. Characteristics originating from the center and the surround of the receptive field are traced. These characteristics differ between ON- and OFF-center units. The origin of these differences is discussed. 2. Transfer characteristics of geniculate neurons have been determined by comparing the responses of optic tract and geniculate units. The main difference between these responses consists of a decreased responsiveness of the geniculate neurons. This is due to the fact that most recordings were made when the cat was asleep. During wakefulness the geniculate responses approximate most to the optic tract responses. Some other differences include an increase of inhibition in some periods of the LGN response. The firing pause of the ON-center geniculate neurons after the first excitation in particular is a characteristic feature of these neurons. 3. It is also possible to determine the input of geniculate neurons from the postsynaptic potentials visible in the quasi-intracellular recordings. Input-output relationships of these neurons confirm the differences mentioned above. These recordings show that the increase of inhibition is caused by intrageniculate hyperpolarizations. 4. It is generally accepted that the hyperpolarizations are mediated by the interneurons present in the LGN. Strong evidence exists that the input of these interneurons originates both from ON-center and OFF-center units as suggested by Burke and Sefton (1966a, b, c). Whether forward (Optic tract) or backward (LGN) fibres project to these interneurons is not completely clear. Arguments for the existence of both types of projections are given. The function of this intrageniculate inhibition is discussed.