How long can a monkey multitask?

You know the situation all too well. Your partner is speaking in the background, while you are attempting to scan your phone, furtively, beneath the table. A text message has buzzed in and you're scrolling through the text while nodding along and trying to fathom why your partner's colleague was so annoying during their last Zoom meeting.

Multitasking is difficult. But how long can we really pay attention to two things at once? Researchers from the University of Copenhagen and Oxford have studied this question in a new interdisciplinary study–by way of monkeys.

Statisticians and psychologists used Rhesus monkeys to look at images on a computer screen while measuring their brain activity. First, the monkeys had to look at a picture of a mailbox, and thereafter, two pictures at once.

"We wanted to study how the monkeys perceived these images in their brains. Do they only see one at a time? Or are they able to process several images at once? If so, for how long? We were able to conclude that the monkeys could process two images at once in less than 80 milliseconds," says Susanne Ditlevsen, a professor at the University of Copenhagen’s Department of Mathematical Sciences.

Both images at first and then individually

The result contributes to solving a psychological dilemma that arises as to whether monkeys—and perhaps humans too—perceive impressions in the brain in parallel or serially, i.e., if they are being processed in the brain at once, or individually, one at a time.

"What we find is that as soon as the monkeys are presented with two images at once, their brains react in parallel, meaning that the brain focuses on both images with the same intensity. However, after about 80 milliseconds, attention is paid to one of the two images. In other words, their perception becomes serial," explains Professor Ditlevsen.

As such, the research demonstrates that it is not an “either-or”, but a “both-and” situation. The monkeys' brains alternate between both ways of perceiving things. 

"We were surprised that their way of registering the pictures was so consistent. It is clear that they initially registered the two pictures in a single hodge-podge, prior to focusing on one picture at a time," says  Ditlevsen.

Big Brother or Follow the Leader

Susanne Ditlevsen, together with her colleagues, produced precise statistical models to measure what happened in the monkeys' brains during the first 500 milliseconds after seeing different numbers of images, well before the monkeys were even aware of what they were looking at.

"A large neuronal response is provoked as soon as the monkeys are presented with images. We placed this neuronal response into a mathematical probability model called a point process. This allowed us to observe when brain activity increased or decreased, within a given time interval," explains the math professor.

The model of brain activity was then placed into a larger model, which either assumes that neurons act according to a kind of control center, a la Big Brother, which constantly knows what they are doing and steers them in a certain direction in relation to their appreciation of the images, or that conversely, neurons affect each other and follow suit, one by one - in a kind of Follow the Leader manner.

"Our results demonstrate that regardless of which of the two models we use, the result is identical. First, the monkeys record both images at once, and then individually. We see this in our measurements of neurons, which first work separately to decode both images, and then begin doing the same thing, but focusing on one image together," explains Professor Ditlevsen.

Although the study's findings cannot be generalized across the board, and human and monkey brains are different after all, the professor contends that there are some exciting trends in the study which contribute to our understanding of both ourselves and the world.