Both of these claims are kinda misguided. The brain is able to detect very short flashes of light (say, 1 thousandth of a second), and other major changes in light perception. Especially an increase in light will be registered near instantly. However, since it doesn’t have a set frame rate, more minor changes in the light perception (say, 100 fps) are not going to be registered. And the brain does try to actively correct discontinuities, that’s why even 12 fps animation feels like movement, although a bit choppy.