Can More Become Too Much?

Sometimes, more is better. Picture yourself in your kitchen with friends, baking your favorite chocolate chip cookies. You cannot get enough of them, so you have decided to double the recipe today! While gathering the ingredients, your growling stomach reminds you of what you learned in your latest biology lesson, about the fascinating digestive roles of gut bacteria. You begin to share your new knowledge with your friends as you simultaneously try to calculate the doubled measurements and whisk the ingredients together. But wait—how much sugar do you need, exactly? To ensure that you are doing the math correctly, you realize that you need to pause your explanation of gut bacteria. Clearly, more is not always better: while your stomach may be able to handle the extra cookies, adding more tasks to be processed can sometimes prove too much for your brain.

You have likely done two things, such as baking or talking to friends, separately, many times, without any problems. However, if you attempt to do both at once, you might find yourself struggling to handle either task as well as you normally could. This is called the cost of multitasking, and it occurs when the same cognitive (brain) resources are required by two or more tasks at the same time.

Why are brain scientists interested in multitasking? Humans have likely always multitasked, but recently, new technologies have made it a more common feature of our daily activities. Smartphones, for example, have become our constant companions and are often used to do several things simultaneously. Increased multitasking brings up many questions and concerns.

Many parents and teachers worry that the tech-savvy younger generation divides its attention too much between different activities and may grow up to be a bunch of unfocused “scatterbrains.” Maybe you have even wondered about the effects of frequent multitasking yourself—is it “good” or “bad” for you? Before we address this question, let us look at what makes multitasking challenging and what happens in the brain.

A Bottleneck in the Brain?

Why is our ability to perform two tasks at once limited? Scientists have developed laboratory experiments comparing participants’ performances when they were performing only one task to when they performed two tasks simultaneously (Figure 1, based on the task described in [1]). In two separate tasks, participants concentrated on a series of single letters. By pressing a button, they had to indicate whether the letter was a consonant or a vowel in the first task, and whether the letter was capitalized or not in the second task. In the so-called single-task condition, participants performed only one task at a time, whereas in the dual-task condition they had to complete both tasks simultaneously. So, while just one button press was required for a response to each letter in the single-task condition, the dual-task condition required two button presses for each letter: one for the vowel/consonant decision and one for the capital letter task. In this experiment and others like it, participants tend to respond more slowly to each of the two tasks in the dual-task condition compared to the single-task condition.

To explain this finding, scientists have developed several theories. One of the most influential is called the bottleneck theory [2]. Think of the neck of a bottle filled with marbles: only one marble at a time can pass through. Similarly, just one task at a time can move through the bottleneck of limited processing capacity in the brain. The other task must wait until the first one is completed.

In our earlier example of baking, measuring sugar and telling your friends about your biology class can be pictured as two separate marbles attempting to move through the bottleneck. Mentally doubling the recipe measurements then adds a third marble that needs to pass. Some of these tasks are more complicated and thus need more of your capacity, represented by the larger size of the marble (Figure 2). The task marbles cannot move through the bottleneck at the same time, so they are dealt with one after another. After partially completing one task, you rapidly switch to the next one and then back again to the first, repeatedly. Thus, you feel like you are doing multiple tasks simultaneously.

Are There Multitasking “Hotspots” in the Brain?

With a technology called fMRI [3], scientists can measure how active the various regions of the brain are while someone performs a task. Using fMRI, scientists have identified two main brain areas where this switch between tasks happens during multitasking. The prefrontal cortex and the parietal cortex are more active when participants complete two tasks at once, compared to when they process a single task. These brain regions thus appear to be essential for multitasking. Using clever experiments, scientists have developed an idea of what these brain areas do when we multitask (Figure 3). The parietal cortex stores the relationship between an external event, such as a picture or a sound (called a stimulus), and the response that should happen because of that stimulus. This is called a stimulus-response-mapping or S-R-mapping. In our example experiment, the task rules of pressing the left yellow button if the letter is capitalized and the right blue button if it is a vowel are examples of S-R-mappings stored in the parietal cortex. The prefrontal cortex is essential for selecting the correct S-R-mappings and responding appropriately. So, if you see a capital “A” in the yellow single-task condition, the prefrontal cortex ensures that you press the left yellow button in response [4]. The prefrontal cortex may be the location of the bottleneck. Practicing multitasking intensively can increase the speed with which the prefrontal cortex processes multiple tasks [5] and can improve multitasking even for tasks that were not practiced, but we still do not know how long-lasting these effects are and whether practice also helps us improve multitasking skills in our daily lives.

Interestingly, some people can switch between tasks or sources of information more easily than others. While many factors influence the ability to multitask, age is especially important [6]. Perhaps you have observed that your younger siblings have a harder time doing multiple things at a time than you do, or that your own multitasking ability has improved since you were little. That is because the brain regions involved in multitasking and their connections with one another need time to develop, becoming the strongest in late adolescence or early adulthood. Just as your brain keeps changing after adolescence, so does your ability to multitask. As we approach our grandparents’ age, the brain regions responsible for task switching function less effectively, making multitasking more difficult again [6].

We Live in the Age of Media Multitasking

Multitasking can be challenging for people regardless of age, but it is also the “new normal” of our time. Even when the situation does not require it, our electronic devices can tempt us to multitask. You may, for example, enjoy listening to music and keeping an eye on social media while completing your homework. In general, research suggests that media multitasking like this can scatter attention and worsen memory, which may impair learning and performance on a task. Additionally, trying to do multiple things at once does not save more time than doing tasks one after the other—multitasking actually slows us down! This seems to be mostly true even for frequent media multitaskers with much “practice” and confidence in their ability to switch between tasks [7]. However, there may be some exceptions to this rule. Experienced video gamers appear to be able to switch between tasks better than non-gamers, at least if the tasks are not very similar [8]. This does not necessarily mean that video games improve multitasking skills—perhaps people who like to play them are better task switchers to begin with. Finally, researchers are still working to find out if frequent multitasking has any long-term effects, positive or negative, on our cognitive abilities or our brains [6].

Be Smart About Multitasking!

Although the bottleneck theory has received much support from scientific experiments, there are also competing explanations, and gaps still exist in our knowledge. More research is needed to reveal exactly how multitasking works and why it is challenging to our brains. Multitasking is here to stay, and concerns about its effects will likely persist until more scientific evidence becomes available. In the meantime, it is important to remember that multitasking is a human ability, neither “good” nor “bad.” Still, try to be mindful about what kinds of activities you combine. For example, if a task is important and requires your attention, such as studying for an exam, it is better to make it your sole focus and avoid multitasking. In contrast, if time is not of the essence and there is little at stake, multitasking will not do much harm. So, while you cannot trick your brain into processing more than one task simultaneously, keeping the bottleneck theory in mind can help you multitask smartly!

Glossary

Cognitive: ↑ Relating to mental processes like learning, thinking, problem-solving, and memory, which are carried out by the brain.

Bottleneck Theory: ↑ It states that the brain has a limited capacity for processing multiple tasks simultaneously, just like only one marble can pass through the narrow neck of a bottle at once.

Functional Magnetic Resonance Imaging (fMRI): ↑ fMRI uses a strong magnetic field to take images of the brain. For example, when human participants work on a task, it can show the brain regions that are activated for that task (learn more here: https://kids.frontiersin.org/article/10.3389/frym.2019.00086).

Prefrontal Cortex: ↑ A region at the front of the brain, which is involved in multitasking by selecting the correct S-R mapping for a task and thus for responding appropriately.

Parietal Cortex: ↑ A region at the upper back of the brain, which is involved in multitasking by storing the S-R mapping of each task.

Stimulus: ↑ An event or a cue, such as a picture or a sound, that activates sensory receptors and causes a response from an organism.

Stimulus-Response-Mapping (Short: S-R-mapping): ↑ The relation between a stimulus and the response that is appropriate for this stimulus; it can be imagined as a roadmap from stimulus to response.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

We thank our fellow members of the project Mechanisms and Sequential Progression of Plasticity for their insights and feedback on this article.