. >> nancy kanwisher: face decades. >> nancy kanwisher: face recognitio a problem, because all faces are basically the same. >> stahl: m.i.t. neuroscientist nancy kanwisher. >> kanwisher: there are these two roundish things here. there's this thing there. there's this thing there. they're all the same. and so discriminating one face from another is a very computationally difficult thing, because it's those subtle differences in the same basic structure that distinguish one thing from another. >> stahl: and it is exactly those subtle differences face blind people like jo livingston miss. >> livingston: i could describe anything that i can put into words-- eye color, general overall shape, whether your ears stick out. but those things would bring it down perhaps from the population of the world to a few million. >> stahl: so, she could say this person has dark eyes, high cheekbones, an oval face, which would allow jo to distinguish her from this person. but this face and this face? impossible. can see. but i cannot say the micro- measurements that are what tell a normal person that it'

neuroscientist nancy kanwisher. >> kanwisher: there are these two roundish things here.s this thing there. there's this thing there. they're all the same. and so discriminating one face from another is a very computationally difficult thing, because it's those subtle differences in the same basic structure that distinguish one thing from another. >> stahl: and it is exactly those subtle differences face blind people like jo livingston miss. >> livingston: i could describe anything that i can put into words-- eye color, general overall shape, whether your ears stick out. showed me, which looked very similar. >> duchaine: maybe you don't even see that there's any difference. >> stahl: i see something different in the lower lip. eyes are a little different. >> duchaine: but then, if i show them to you upright... so, here's the one that you saw on the left there. looked perfectly normal. and then... >> stahl: oh. >> duchaine: here's the one you saw on the right, you saw upside-down. >> stahl: oh, my goodness. the eyes and mouth in the photo on the right had been turned upsid

. >> nancy kanwisher: face recognition is a very difficult are basically the same. >> stahl: m.i.t. neuroscientist nancy kanwisher.ese two roundish things here. there's this thing there. there's this thing there. they're all the same. from another is a veryy difficult thing, because it's those subtle differences in the same basic structure that distinguish one thing from another.exactly those subtle differences face blind people like jo livingston miss. >> livingston: i could describeto words-- eye color, general overall shape, whether your ears stick out. would bring it down perhaps from the population of the world to a few million. >> stahl: so, she could say this cheekbones, an oval face, which would allow jo to distinguish her from this person. face? impossible. >> livingston: i can say what i can see. micro- measurements that are what tell a normal person that it's you anspecification. >> stahl: but how is it that the rest of us can perceive these two people as distinct the an important clue comes from what we can't distinguish-- as showed me, which looked very >> duchaine: mayb