This script displays flickering gratings in order to measure the SSVEP response to gratings of different contrasts. You can set the number of contrasts to use, the number of trials, and the spatial and temporal frequency of the gratings at the top of the script.
The rational behind this is that there should be a "saturation" of the contrast response at high contrasts, which is dependent on GABA.
You should also be able to use the matlab version of this code, but I recommend using the python one (more accessible code, more up-to-date). Also, python is open source.