These dots are pimples that stick out.
These dots are dimples that are pushed in.
Here the dimples and pimples are mixed together.
Why do we see the first picture as pimples and the second as dimples? Why are some dots perceived as going in and others as going out? In our world, most of the light we see comes from above us - either as the sun above or indoor lighting overhead, so we perceive most pictures as having a light source that comes from above as well. That's why a dot with a light area around the top of the circle and a dark area at the bottom of the circle is read as a pimple.
If you can imagine it, move an imaginary light source from the top to the bottom of the picture, and the dots will shift ftom pimples to dimples.