The "Mona Lisa Effect"
This is the effect where when you look at the Mona Lisa, and then you change position, her eyes follow you around. Or at least that's how it seems. She seems to be looking at you, as much as you are are looking at her.
There has been much nonsense said about this, from the mystical to "theoretical explanations" including such things as da Vinci deliberately slightly misaligning her eyes in order to achieve the effect. I'm not saying that Mona Lisa'a eyes aren't slightly misaligned, whatever you take that to mean, but that's not the reason.
By way of illustration here are a couple of my paintings (below). Look at either of them on a big screen, or print them out big. Look at them from the front, and from each side, and you will find they are always looking at you. They will follow you around, paying careful attention to you. They are as aware of you, as you are them. Or so it seems.
It's true that in portrait painting there are certain things about a person's eyes that you have to pay particular attention to, because we are creatures who communicate through our eyes. But I didn't use a secret technique, or some special slight misaligning of the eyes, in order to achieve this particular so-called "Mona Lisa" effect. It's not that the effect doesn't exist. It does exist, but it exists in our mind. In our experience. And actually, in the way our brain works.
And really, that's the whole point of art. Not specifically the brain, but art is about what it creates in your experience. It's not about something outside of you. It's not about something separate from you. You don't experience anything except through your own mind and experience of being. Everything that you experience, and how you experience it, depends on that.
It is amazing, judging from all the noise about it on the Internet, that some people, especially scientists, seem to think we need science in order to show that this effect doesn't exist in the mona lisa. It exists, but not outside of you. You don't need science for it, at all, either to show you how it works, or what it takes for it not to work. If you stick something in front of the portrait, so that your attention is drawn to that too, you may well find that you then experience the subject to no longer be looking directly at you. It is all about the psychology of perception.
You often find science playing catch up with what is already perfectly well known by certain groups of people. Often, when you see some headline that begins "scientists have discovered that…" as if science is the only thing that brings truth or wisdom.
The truth about the "Mona Lisa effect" won't be news to many portrait artists, for example. Just as all the amazing things about psychological illusions in what we are looking at, the psychological relativity of color, the illusions of color, and form, and all the rest of it, that we occasionally see science apparently "revealing" as if it is for the first time, are generally things that artists with sufficient experience, everywhere, all over the world, already knew about. And probably have already been consciously using for years, in their work. Not just today, but down through the ages.