Optical illusions depend on your upbringing

The way you see the world literally depends on your upbringing.

The first hints that modernisation could change our vision came from the Victorian anthropologist WHR Rivers, who explored the islands of the Torres Strait, between Australia and Papua New Guinea at the turn of the 20th Century. As he met the locals, he offered them various sensory tests, including the following phenomenon, known as the Muller-Lyer illusion.

In reality, the lines are exactly the same, but if you ask people to estimate their size, most Westerners claim that the second line (with the 'feathers' pointing outwards) is around 20% longer than the top line. During his expedition to the Torres Strait, however, Rivers found that the locals were far more accurate – they just didn’t seem to be as susceptible to the illusion.

Westerners tend to see the central circle in the first picture as being smaller than the central circle in the second – when they are actually the same size. And just as Rivers had seen with the Muller Lyer illusion, Davidoff’s team found that the traditional Himba were far less susceptible than those of us living in modern societies.

The phenomenon seemed to reflect a basic bias towards “local processing” – they were more focused on the smaller details (the central circles) while ignoring the context (the surrounding ring) that warps your perception. More strikingly still, later experiments showed this enhanced focus also seemed to be reflected in their ability to hold their attention and ignore distraction: when they were asked to quickly search for shapes in a grid, for instance, they were less easily distracted by the movements of other objects on the screen. In fact, they appeared to be the most focused of any groups previously studied.

One explanation for their astonishing focus may come from the cattle rearing itself. Identifying each cow’s markings was apparently essential for their daily life – and this practice may perhaps train the eye with a focus and attention that was lacking in all modern societies. “I think that does come from their traditional lives – the powers to concentrate,” says Davidoff. But it could also be that modern life itself makes us more easily distracted by our surroundings. As Davidoff points out, urban environments are naturally more cluttered than the Kunene valley, with more objects vying for our attention. Just think about crossing the road, as your eyes dart from the traffic lights to the oncoming cars and the fellow pedestrians making their way towards you. Our attention needs to be more diffuse.
http://www.bbc.com/future/story/20170306-the-astonishing-focus-of-namibias-nomads