You may have seen the following optical illusion
posted by Japanese psychologist Akiyoshi Kitaoka It’s an extinction illusion and there are
twelve black dots although most people aren’t able to see all of them at the same time and
today I’m going to talk about what it is that drives this illusion. [INTRO by Caro Waro & Cristina de Manuel] You may recall from my previous episode that
we have plenty of flaws in our eyes, and today I’m going to acquaint you with two new ones.
The first one is to do with the way our photoreceptors are distributed in our eyes. Our central vision
has the highest concentration of photoreceptors which are mostly found in the macula, a 5.5mm
region in the centre of the eye, and within that, the fovea, which is 1.5mm and has the
highest concentration of cones in our eye. As you go out towards peripheral vision, the
density of photoreceptors drastically plummets and there are way more rods than cones that
don’t process colour – so it is actually the brain’s job to fill in the blanks so as to
give us the illusion of seeing quite well. And in fact, our brain does such a great job
of that that people who have degenerative diseases where they lose their peripheral
vision, such as retinitis pigmentosa, often take a while to discover that they’re actually
going blind. However, our poorer peripheral vision isn’t
the only thing that’s at the heart of this illusion. If I take the black dots and move
them from the intersections up to the alleys, you will be able to see them, and this is
due to another factor called lateral inhibition. Lateral inhibition is an effect that’s strongest
in the photoreceptors that are in the periphery of our vision. Basically, when one of them
is stimulated by a photon, it inhibits all the neighbouring photoreceptors. This on the
one hand eases the processing load on the brain, but it also allows the brain to be
able to detect edges and contrast a lot better. So in the case of this optical illusion, when
you have a black dot in the centre of all of these intersections, it’s much more likely
to be sampled and picked up on the dark grey areas, which will then wash over the black
and the brain won’t display it for you. However, this is not the case on the alleys,
and that is why you’re able to see it because it actually stands out in comparison to the
rest of the background. That said, the way you perceive this optical
illusion right now is going to vary depending on the relative size the circles have on your
screen, and whereabouts in the visual field they are – and you’ll probably be able to
see them either perfectly, or you might not see them at all, or they may appear to
be scintillating. However, don’t get me wrong, lateral inhibition
and poorer peripheral vision actually does make perfect sense – even though it does come
with some flaws. The biggest one (positive thing) is that it actually allows our brain
to process the images that we see a lot quicker whilst giving us a mostly functional image,
and provided you’re not looking at optical illusions like this most of the time, you’re
never going to even notice and it’s never going to affect you (…unless you have RP) Anyway – this was actually going to be a snippet
science, then I got a little bit too carried away – but if you want to find out even more
about extinction illusions and Hermann’s grids, I’ve actually rambled way more on my blogpost
at drawcuriosity.com. I also want to say thank you very much to the person who tweeted this
visual illusion to me, because you inspired this episode! And as always – thank you so much for watching
me, and I’ll see you in the next one! Bye! [MUSIC: Thastor & Cryosleepkitten]
[Channel Art: Caro Waro & Cristina de Manuel] [Hosting, Scripting, Editing: Inés Dawson]