Photoreceptors are responsible for transforming light energy impinging on the retina into neural signals; consequently, information contained in the stimulus but not transmitted by the photoreceptors is lost to the organism. Therefore, knowledge of the response characteristics of the photoreceptors not only provides information about the receptors themselves, but is also a prerequisite to an understanding of more central processing in the visual system. Intraretinal microelectrode recording techniques that provide access to the receptor response, in particular the late receptor potential (LRP), enable us to study photoreceptor signal processing in primates. By using cynomolgous macaque monkeys, which have human-like visual systems, we can address questions pertinent to human visual perception. The identification of psychophysical phenomena at the photoreceptor-horizontal cell level will provide a means of studying retinal diseases in man with retinogram (ERG) as a diagnostic tool. Accordingly, the foveal local ERG will be used to investigate the role of the photoreceptors in various perceptual phenomena by using stimuli capable of enhancing the LRP. Recent advances in microelectrode technology will be applied in an attempt to achieve intracellular recording from primate cones; in conjunction with this technical development, we will study the temporal sensitivity of primate ganglion cells.