Abstract
Abstract: :
Purpose: We present the first physiological recordings and visual responses to be identified with stained amacrine cells in the insect lamina, a primary visual neuropil analogous to the outer plexiform layer in the vertebrate retina. Lamina amacrines are proposed to participate in the circuitry for elementary motion detection (Douglass and Strausfeld 2003, Microscopy Res Tech 62:132–150), and may also mediate other early visual processing functions such as light/dark adaptation or lateral inhibition. Methods: Intracellular recordings from the lamina of the blow fly, Phaenicia sericata, were obtained during visual stimulation with flicker and motion on a CRT (200Hz horizontal refresh). Impaled cells were stained with fluorescent dye and studied with a confocal microscope. Golgi–stained sections and electron micrographs were obtained by standard methods. Results: Like insect photoreceptors, lamina amacrines exhibit nonspiking, "sign–conserving" sustained depolarizations in response to illumination. This behavior contrasts with the sign–inverting responses that are typical of other interneurons in the lamina as well as at deeper levels. Lamina amacrine recordings to date exhibit wider receptive fields than photoreceptors, suggesting that amacrine inputs originate primarily from a small number of neighboring visual sampling units (VSUs), with minor inputs from more distant VSUs. Contrast frequency tuning is similar to that of photoreceptors and lamina monopolar cells. Conclusions: With their known lateral connections among neighboring VSUs, lamina amacrines are well–suited to relay responses of neighboring VSUs to local intensity fluctuations. These observations are consistent with the proposal that lamina amacrines participate in a very peripheral, non–direction–selective motion detection circuit. We propose that this circuit, which lies at a level analogous to that of vertebrate horizontal cells, provides a common motion–selective input to deeper circuits that specialize in processing motion direction, orientation, speed or acceleration.
Keywords: amacrine cells • motion–2D • retina: distal (photoreceptors, horizontal cells, bipolar cells)