Information Processing by the First Optic Ganglion in Dipterans

Information processing properties of the dipteran first optic ganglion were studied by observing and analyzing the discharge behavior of two units in the intermediate chiasma and the slow potential behavior of two units in the first optic ganglion. Both types of chiasma units (on-off, on-maintained) were centripetal and corres­ponded to second order units in the first optic ganglion. The on-off unit was characterized by a transient discharge following the onset and cessation of a light pulse presented anywhere in the receptive field which had an elliptical configuration with average major and minor half directional sensitivity angles of 5.5 and 4.3 degrees, respectively. The receptive field of the on-maintained unit was composed of three roughly circular regions arranged adjacently along a line, and stimulation of the center region elicited a sustained dis charge while stimulation of either adjacent region elicited an off discharge. The average half directional sensitivity angle of the center region was 2.5 degrees which compared well with the acceptance angle of the photo receptors. The orientation of the major axes of the on-off and on-maintained unit receptive fields was always that of the medio-lateral axis of the compound eye. The on and off regions of the on-maintained unit receptive field were antagonistic, for stimulation of either off region inhibited or suppressed a discharge resulting from region stimulation. Furthermore, the off response was inhibited by region stimulation if the cessation of on region stimulation preceded, by not more than 200 msec., the cessation of off region stimulation. The discharge patterns of both units were independent of the stimulus spectral wavelength, and all units studied, of both type, possessed the same spectral sensitivity which was characterized by two peaks of approximately equal maximum sensitivity centered at 350 mµ and 485 mµ. It was concluded that these units belonged to the system served by photoreceptors 1-6 and that a photopigment with two absorption peaks was responsible for the observed spectral sensitivities. Neither unit displayed any sensitivity to the plane of polarized light. Positive slow potentials were recorded from the first optic ganglion with fine micropipettes, and they were believed to originate from the photo receptor axons. A hyperpolarizing slow potential, most likely of intracellular origin, was also recorded, but its prop­erties were not studied in detail. Based on their information processing properties, the on-off and on-maintained units were identified with the two type I monopolar neurons of each cartridge. A model was developed which adequately explained the behavior of both types of units, and its structure was compared with the known anatomical structure of the first optic ganglion.