How to change the brightness of a stimulus?

Hi, I am using Psychtoolbox with MATLAB R2020a on a MacOS. I am currently coding a task that involves a change in brightness of 3 simultaneously presented ‘spots’ on the screen whereby I can specify the brightness of the stimuli based on input from a Gaussian distribution. That is, a number is drawn from a Guassian distribution and a particular stimulus brightness is presented based on that. However, I am yet to find any functions/ code that can do this. Any help would be greatly appreciated

Thanks in advance.