We can never know what it is like to be a bat, philosopher Thomas Nagel famously agued. But what about monkeys? Do they see the world in the same way we do?
To find out, neuroscientists in India used an 鈥渙ddball鈥� test to compare visual performances between people and macaques. They found that, although the different species scored similarly on many aspects of perception, there were two intriguing differences: the monkeys were less phased by images being flipped upside down and they seemed far more focused on fine details.
There have been previous attempts to compare human and monkey vision, but the tests often require extensive training, says at the Indian Institute of Science in Bengaluru. 鈥淭he worry becomes: did their perception change because of all that training?鈥� Different aspects of vision have also been studied using different tests, so it is hard to compare the results.
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Now, Arun and his colleagues have chosen just one test, the oddball search task, which can be adapted to different visual features. Subjects are shown a set of pictures on a screen, identical apart from one, and must identify the odd one out by touching it.
When the oddball appears more similar to the other images, 鈥渋t takes us longer to find the odd one out鈥�, says Arun, providing an objective measure of perception. The researchers taught the test to three monkeys, using juice as a reward, and 24 people.
In several tasks, the people and monkeys performed similarly. For instance, they both categorised inanimate objects (such as a mushroom or leaf) as more similar to each other than to animate items (such as a bird or cat).
They also both obeyed Weber鈥檚 law, which states that the smallest noticeable change in a stimulus depends on its baseline magnitude. In this case, the researchers showed that the brighter a spot was, the bigger any change in brightness had to be before either monkeys or humans could perceive it.
But there were also intriguing differences. The researchers tested a phenomenon in which people find it harder to differentiate mirror images flipped across a vertical axis compared with a horizontal one. An example of this is when young children confuse the letters 鈥渂鈥� and 鈥渄鈥�. This 鈥渕irror confusion鈥� is thought to arise because we often see the things around us, such as trees or chairs, from different angles around a vertical axis, so we perceive these images as more interchangeable. We more rarely come across them upside down, so images flipped across a horizontal axis seem more different and therefore easier to tell apart.
This is exactly what happened among the human participants in the oddball test: they took longer to identify outlier images if they were flipped around a vertical axis compared with a horizontal one. But the monkeys didn鈥檛 experience mirror confusion, and consistently showed similar performances whether images were flipped vertically or horizontally. Arun suggests this might be because these agile creatures are more used to seeing familiar objects from a range of angles. 鈥淭hey are swinging across stuff, and sometimes they鈥檒l hang upside down.鈥�
The monkeys were also highly attuned to small features, yet seemed oblivious to larger shapes. The researchers tested this using large shapes 鈥� a diamond, circle or X 鈥� made up of smaller shapes. People find it easier to tell when the overall shape changes, compared with the smaller units. 鈥淵ou can see the forest before you see the trees,鈥� says Arun. But the monkeys only seemed interested in the details, and were no faster on the task when the overall shape changed as well. It isn鈥檛 clear why this is, says Arun.
at KU Leuven in Belgium says the differences identified in the study are 鈥渟triking鈥�, although he cautions the results could be affected by differences in attention as well as perception.
As well as providing a glimpse into the monkey visual world, the study could have broader implications for research, says Arun, as these animals are often used as models for human vision. Although they are similar in many respects, he says, 鈥渋t may not be OK to assume they see everything the way we do鈥�.
Arun hopes the oddball approach could be used to chart vision in other systems, too. The test is so simple, even birds and bees could be trained to do it, he says. His team is now using it to investigate vision in children of different ages, including those with ADHD and autism, to understand how they might perceive the world differently than adults.
bioRxiv
