Dr. David Berson

Yeah, so this last pigment is a really peculiar one.

One can think about it as really the initial sensitive element in a system that's designed to tell your brain about how bright things are in your world.

And the thing that's really peculiar about this pigment is that it's in the wrong place in a sense.

When you think about the structure of the retina, you think about a layer cake, essentially.

You've got this thin membrane at the back of your eye, but it's actually a stack of thin layers.

And the outermost of those layers is where these photoreceptors you were talking about earlier are sitting.

That's where the film of your camera is, essentially.

That's where the photons do their magic with the photopigments and turn it into a neural signal.

It's the surface on which the light pattern is imaged by the optics of the eye.

And now you've got an array of sensors that's capturing that information and creating a bitmap, essentially.

but now it's in neural signals distributed across the surface of the retina.

But it turns out that this last photopigment is in the other end of the retina, the innermost part of the retina.

That's where the so-called ganglion cells are.

Those are the cells that talk to the brain, the ones that actually can communicate directly

what information comes to them from the photoreceptors.

And here you've got a case where actually some of the output neurons that we didn't think had any business being directly sensitive to light were actually making this photopigment.

absorbing light and converting that to neural signals and sending to the brain.

That's your circadian system.

It's keeping time and it's all built into our biology.

And this is actually one of the things that blind patients often complain about.