Hello! My task is creation of real-time system which can draw simple 
graphics figure on monitor (VGA) when the system register input pulse. 
The main difficulty is to draw this image only once (for measuring man's 
reaction time: time between image drawing and the button pressing). I 
think it'll possible to use FPGA to register input pulse and to generate 
VGA signal with required parameters. But I don't undestand can the ONLY 
ONE image frame be drawn at input pulse interrupting or not?
The main task is to minimize time delays and in this case I can't use 
the standart solutions.
For example, using Raspberry Pi or a similar device is accompanied by 
the necessary of screen refreshing and image drawing binding. This adds 
a random time delay from 0ms (if the screen refresh time coincides with 
time of input external pulse which starts drawing figures) to 
1/60Hz=16.7ms.
I need your advices or suggestions. Thanks!

Probably the simplest solution would be to synchronize drawing of the 
picture with vertical blanking. You'd switch to a framebuffer holding 
the graphics in the vsync period which garantees a full video frame and 
start time is determined at the same time - e.g. by starting a clock or 
timer. User then stops the timer or clock which contains his reaction 
time.
I do not think you will need a FPGA for this - even something simple 
like the AVGA library will be fast enough to cope with the timing.

http://www.avga.prometheus4.com/

Even then, given enough samples, you should be able to get an accurate 
representation of reaction times by subtracting 8.3ms.