When you are asked to write the numbers one through ten in order, how do you put them in order? Horizontally? vertically? From left to right? From top to bottom? Will you place them randomly?
It has often been assumed and taught in schools in Western countries that the ‘correct’ order of numbers is from right to left (10, 9, 8, 7…).
The arrangement of numbers along the horizontal dimension is known as the “mental number line” and describes an important way we represent numbers and quantities in space.
Studies show that humans prefer to place large numbers on the right and small numbers on the left.
People are generally faster and more accurate when comparing numbers when larger ones are on the right and smaller ones on the left, and people with brain damage that disrupts their spatial processing have similar number processing problems.
But until now, there has been little research into whether the horizontal dimension is the most important one we associate with numbers. In new research published in PLOS ONE, we found that humans actually process numbers faster when displayed vertically, with the smallest number at the bottom and the largest number at the bottom.
Not just humans.
Our association between number and space is influenced by language and culture, but these associations are not unique to humans.
A test of three-day-old chicks shows that they find smaller numbers biased to the left and larger numbers biased right. Pigeons and blue jays have a left-to-right or right-to-left mental number line, depending on the individual.
These findings suggest that the connection between space and numbers may be hardwired into the brains of humans and other animals.
However, while many studies have examined left-to-right and right-to-left horizontal mental number lines, few have explored whether our dominant mental number line is horizontal.
How do we test these spatial numerical associations?
To test how quickly people could process numbers in different arrangements, we set up an experiment in which people were shown pairs of numbers 1 to 9 on a monitor and used a joystick to indicate where the big numbers are located?
If the screen shows 6 and 8, for example, the correct answer would be 8. A participant will indicate this by moving the joystick towards 8 as quickly as possible.
To measure participants’ reaction times as accurately as possible, we used a fast-refreshing 120Hz monitor and a high-performance, zero-lag arcade joystick.
what we found
When numbers were separated vertically and horizontally, we found that only vertical order affected reaction time.
This suggests that, given the opportunity to use horizontal or vertical mental representations of numbers in space, the participants used only vertical representations.
When the large number was above the small number, people responded much faster than in any other number arrangement.
It turns out that our mental number line actually goes from bottom (small numbers) to top (large numbers).
Why is this important?
Numbers affect almost every part of our lives (and our security). Pharmacists need to accurately measure drug doses, engineers need to determine the pressure on buildings and structures, pilots need to know their speed and altitude, and we all need to know what an elevator is, but which button to press.
How we learn to use numbers and how designers choose to show us numerical information can have significant effects on how quickly and accurately we make decisions.
In fact, in some real-time decision-making environments, such as airplane cockpits and stock market floors, numbers are often displayed vertically.
Our findings, and another recent study, may have implications for designers looking to help users quickly understand and use numerical information.
Modern devices allow for very advanced number display options, which can help people use technology more efficiently and safely.
There are also implications for education, suggesting that we should teach children to use vertical mental number lines from bottom to top, as well as familiar lines that run from left to right.
Bottom-up seems to be the way our brains are most efficient at using numbers, and it can help our heads figure out
