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Electrophysiological responses from 91 direction-selective (DS) units located in the basal optic root (nBOR) and the adjacent dorsomedial area have been recorded extracellularly in the frog ( Rana temporaria L.). Based on characteristics of the recorded responses, 23 of the units are considered to be retinal ganglion cells' (RGC; axon terminals). The majority of the remaining 68 units were considered to be DS neurons of the nBOR. The results of our study suggest that in the nBOR area there might be four subtypes of the tegmental and retinal DS units that respond selectively to stimuli moving in the dorso-ventral, ventro-dorsal, caudo-rostral and rostro-caudal directions. The receptive field (RF) sizes of the nBOR DS neurons were estimated to be about 30-, while those for the retinal units were significantly smaller - just 6-. In response to abrupt darkening within the units' RFs, both the nBOR DS neurons and the RGC DS units respond by a weak discharge. Our results indicate that the frog nBOR DS neurons integrate the inputs from the retinal OFF-type DS units over relatively large segments of the visual field. [ABSTRACT FROM AUTHOR]

In this paper, we found that spatial and temporal asymmetricity of excitatory connections are able to generate directional selectivity which can be enhanced by asymmetrical inhibitory connections by reconstructing a hexagonally-arranged three-layered simulation model of retina by NEURON simulator. Asymmetric excitatory inputs to ganglion cells with randomly arborizing dendrites were able to generate weaker directional selectivity to moving stimuli whose speed was less than 10 μm/msec. By just adding asymmetric inhibitory connections via inhibitory amacrine cells, directional selectivity became stronger to respond to moving stimuli at ten times faster speed (< 100 μm/msec). In conclusion, an excitatory mechanism appeared to generate directional selectivity while asymmetric inhibitory connections enhance directional selectivity in retina. [ABSTRACT FROM AUTHOR]

A reaction-diffusion model is presented to encapsulate calcium-induced calcium release (CICR) as a potential mechanism for somatofugal bias of dendritic calcium movement in starburst amacrine cells. Calcium dynamics involves a simple calcium extrusion (pump) and a buffering mechanism of calcium binding proteins homogeneously distributed over the plasma membrane of the endoplasmic reticulum within starburst amacrine cells. The system of reaction-diffusion equations in the excess buffer (or low calcium concentration) approximation are reformulated as a nonlinear Volterra integral equation which is solved analytically via a regular perturbation series expansion in response to calcium feedback from a continuously and uniformly distributed calcium sources. Calculation of luminal calcium diffusion in the absence of buffering enables a wave to travel at distances of 120 μm from the soma to distal tips of a starburst amacrine cell dendrite in 100 msec, yet in the presence of discretely distributed calcium-binding proteins it is unknown whether the propagating calcium wave-front in the somatofugal direction is further impeded by endogenous buffers. If so, this would indicate CICR to be an unlikely mechanism of retinal direction selectivity in starburst amacrine cells. [ABSTRACT FROM AUTHOR]