MATLAB Programming/Psychtoolbox/Screen Commands/FrameRate
Frame rate, in Hz, as most recently measured for this screen (typically when you first opened it).
MacOS: FrameRate counts frames over some measured period and calculates the rate. Setting hz doesn't affect the screen, only the returned value. Setting hz to NaN or  forces SCREEN to remeasure it now, eg after you use a Control Panel to change the screen resolution. The cached value is flushed when you close the last window on this screen.
Windows: The returned value is the nominal frame rate, not the measured rate. The returned value is always current. FrameRate will accept an argument, for compatibility with the MacOS implementation, but it ignores the value passed.