From "Значение ретино-эпифизарной системы для психофармакологического эффекта" by Karen Ovanesov, who is co-author of one of those Russian articles on Pubmed to which you refer:
(...) the visual perception is non-stationary in time and changes during the day. The use of stimulants of mental processes, such as caffeine can not only enhance the functional activity of the retina, but also eliminate its daily fluctuations.
(...) At the higher dose (0.4), the drug caused an increase of sensitivity to the retina (13%), and significantly (by 20%) shortened the latency of the motor response to visual stimuli. This shift was more pronounced in the peripheral parts of the retina. A similar pattern is set, and in the study of color vision, and a substance markedly improves the perception of red and green colors, as compared with the blue and white stimuli and control data definitions.
(...) the use of caffeine leads to a smoothing of the daily rhythm of the sensitivity of the retina and improve light perception. This effect is more pronounced when using stimulant in the morning.
There are more russian articles about this in bibliography section.