Short answer
Within our brain, different networks struggle constantly for focus, and the outcome determines whether our attention is focused or if we are easily distracted. If the balance tips too much towards distracted, we start mentioning attention disorders like ADHD.
Longer answer
Attention is our ability to filter specific information out of our environment, to prioritize certain stimuli (the teacher in the class, or the car in front of us). It allows us to focus on a single task, direct our thoughts and be involved in social interactions. Without any control over our attention, we would drown in a sea of stimuli.
Figure 1: the dorsolateral prefrontal cortex and the parietal cortex (figure: Joosje Kist).
Distraction: from disaster to life-saving. As everyone knows from experience: it is not always easy to stay focused. Distraction is always around the corner, especially in these modern times of social media and smartphones. This can have disastrous consequences, because you cannot pay an equal amount of attention to the road and to your WhatsApp feed. Then again, it is also important to allow your attention to be distracted by stimuli from the outside world, such as a fire alarm, even as you read this text with such focus. Even internal distractions, daydreams, can have its advantages, such as the expansion of your perspective. The ability to remain flexible as you switch between focus and distraction is very important in our complex and dynamic world.
What happens in the brain when I lose focus? Attention control depends on the function of various areas on the surface of the brain (cortical) and some deeper areas (subcortical) influencing each other. The dorsal prefrontal cortex, located at the front of the brain (see figure 1), and the dorsal parietal cortex, located more in the back of the brain (see figure 1), play a crucial role in attention control. These areas allow us to focus our attention on a specific task. When distracted, there is often less activity in this dorsal frontoparietal network: the task-oriented focus is diminished. The subcortical areas are also involved in attention control, such as the superior colliculus in the midbrain and the pulvinar (see figure 2). Damage to these areas also leads to issues in focusing one’s attention.
Figure 2: Subcortical areas involved with attention (figure: Scholarpedia, http://www.scholarpedia.org/article/Attention )
With external distraction from outside stimuli, such as when our attention is pulled to a sudden loud noise, areas in the front of the brain (the prefrontal and parietal cortex) that are located lower (ventrally) become more active. The activation of this ventral network acts as a circuit disruptor and makes it so that the attention is focused on the new stimulus.
Internal distraction, specifically daydreaming, goes hand in hand with the activation of another network of brain areas: the so-called ‘default mode network’ (Figure 3). This network activates when we are busy with internal thoughts. Activating the default mode network is also coupled with lower activity in the attention control network of figure 1.
Figure 3: an fMRI scan where the ‘default mode’ network (red) is active (source: Wikipedia, https://en.wikipedia.org/wiki/Default_mode_network )