The Ebbinghaus illusion.
A two centimetre circle surrounded by six four centimetre circles looks larger than a two centimetre circle surrounded by eight one centimetre circles. Known as the Ebbinghaus illusion, this bit of visual trickery is difficult for people to see through. Indeed, most struggle to see that the two central circles are actually the same size. An exception to this are the Himba. Found in the Kunene Region of Namibia, the Himba people are semi-nomadic pastoralists and, when presented with visual illusions by a team of psychologists in 2012, they proved remarkably resistant to this illusion. At the time, the team suggested that there was something about their intimate relationship with nature that was leading them to respond to illusions so differently from people living in the developed world*. Now new work is revealing that the relationship with nature that the Himba have is irrelevant here. Instead, their cultural resistance to this illusion appears to stem from having slower reaction times and being much less easily distracted.
The new work presented participants with a range of illusions and asked them what they saw at different points in time. For some illusions, like the Ebbinghaus, the power of the illusion weakened over time and with increased concentration by the participants. For others, like the double-decrement contrast illusion grey-scale contrast illusion, the power of the illusion strengthened the longer participants were presented with it and the more they concentrated. These findings alone are interesting in that they reveal some illusions as strengthening the longer they are studied and some weakening. Yet, where this work gets really interesting is when it is applied to the findings with the Himba.
The double-decrement contrast illusion.
The researchers in 2012 bribed the Himba people to participate with bags of sugar and flour. While a bag of sugar or flour might not seem all that exciting to you or me, these commodities were valuable to the Himba and the experience of being presented with visual illusions was an exciting novelty. This made more engaged in the task and led them to spend much more time studying the illusions than the British students in the control group. This, they argue, is why the Himba so consistently saw through the Ebbinghaus illusion while the British students could not. Lending support to this theory is Himba performance on a grey-scale contrast illusion that the new research shows gets stronger with time and attention. Unlike the British, who easily saw through this illusion, the Himba were almost always deceived by it.
All told, the findings reveal that the power of illusions varies with cultures and that a key force behind this variance is the novelty of the illusion itself and whether it is one that strengthens or weakens with increased attention.
This just published online in Psychological Science.
*They used a bunch of British students as a control group.