Attention, the ability to focus on a part of the sensory information available to us, involves filtering and selecting specific sensory input from the environment. For example, we can focus our gaze on a small portion of the surrounding world while the rest of our visual field plays a subordinate role. There are two types of attention to distinguish between: stimulus-driven attention and controlled attention.

Stimulus-driven attention is automatic and occurs when unexpected stimuli capture our focus—for instance, when a sudden noise makes us turn our head or body to locate the source of the sound. This connection develops in infants during their first four months of life.

Moving objects particularly attract attention. This can be observed, for example, when you're engaged in an interesting conversation at a bar with a TV showing a football game above the bar counter. The movements on the screen draw your attention, potentially making you appear disinterested to your conversation partner.

Controlled attention involves actively choosing what to focus on. It's like using a flashlight to direct your attention to different parts of a dimly lit house. For instance, in a noisy environment like a cocktail party, you can choose to focus on a single voice, making the other voices fade into the background, and you won't perceive what they're saying.

According to traditional theory, attention is necessary because the brain has limited processing capacity, so some sensory information needs to be filtered out. Experiments that support this theory involve subjects listening to speech from two different people in their left and right ears simultaneously. These experiments show that individuals have a limited ability to perceive what is being said and usually need to concentrate on one voice. They also tend to remember only what is said by the voice they are attending to.

But now, there's an alternative to the traditional theory of attention. It suggests that attention is needed as part of preparing for action.

When we're preparing to perform a task, our attention is directed toward the relevant information needed to carry out that task effectively. In this perspective, attention is an integral part of the action, and brain overload doesn't play a central role. Instead, attention is about taking in the necessary information for action, becoming part of the process.

A common misconception is that the visual process works like a camera, but it's easy to show that this is not the case. For example, when we look at something, we perceive it as sharp, but in reality, only a small portion of our visual field is in focus (approximately two degrees of visual angle or about the width of two fingers at arm's length). The periphery appears blurry, and it's harder to perceive colors. We get the illusion that everything is sharp when we fix our gaze on it. It's similar to the idea that the light in the refrigerator is always on when we look inside.

Another illusion is that we don't perceive gaps in our visual field. There's an area on the retina with no receptors called the blind spot, and it creates a part of the visual field where we can't see anything. This could explain why we sometimes miss things in our surroundings even though we think we've been looking for them. "Looking without seeing" is said to be the third most common cause of traffic accidents.

By studying eye movements with a special camera, researchers can determine what is being attended to and what the individual is not perceiving in a given situation. Sudden movements or perceived threats immediately capture attention.

For example, if someone sees a scene where someone is holding a weapon, their attention is immediately drawn to it, and they may not notice much else. In emotionally charged situations, tunnel vision occurs. This kind of focus is stimulus-driven and happens without voluntary control.

Experimental evidence shows that we miss a lot when our attention is focused on something else. An example is the well-known video where six people pass a basketball to each other. Three of them are wearing white shirts, and three are wearing black shirts. The task for viewers is to count the number of passes made by the individuals in the white shirts. This requires concentration, and most people watching the video completely miss the fact that a person in a gorilla costume walks into the scene, beats their chest, and then walks off. Once you've seen the gorilla, it's hard to believe that you didn't see it during the counting task.

Tunnel vision has implications for how witness testimonies should be evaluated. If a person witnesses a situation involving a weapon, it becomes harder to trust what the person says about the rest of the situation. Because the weapon captures attention, the person is unlikely to notice the appearance of the perpetrator, for example.

Most magic tricks rely on controlling the audience's attention. Since movement captures attention, magicians often use misdirection. They may move their right hand forward and do something with an object while the audience is distracted, not noticing what the magician’s left hand is doing. Viewers must actively train themselves not to focus on the magician's right hand.

A famous example of attention control is when the magician David Berglas "made a piano disappear." During a dinner at a hotel, a pianist was playing the piano. Berglas gave a speech but was expected to perform a trick. He first announced that he would make the crystal chandeliers on the ceiling disappear. Everyone looked up and saw the lights flicker and swing ominously. A woman screamed.

Meanwhile, Berglas's assistants removed the piano, which had been pre-cut into smaller pieces, through a door. Berglas then announced that the piano would disappear, and when the audience looked back at the pianist, he pretended to start playing. He fell forward since there was no piano left. No one in the audience had noticed that the piano had been removed because they were focused on the ceiling. In reality, the pianist had not been playing for real because the piano was in pieces; there was another pianist playing behind the scenes.

A specific form of attention impairment is visual neglect. A stroke in the right parietal lobe can cause a patient to be unable to attend to the left side of their visual field, even if their visual abilities are otherwise intact (right-sided neglect is rare). These patients behave as if the left side of the world has ceased to exist. They only dress the right side of their body and don't hear people speaking on their left side.

Patients with left-sided neglect don't perceive themselves as blind in the left visual field; it just doesn't exist for them. However, if an object is thrown at them from the left, they instinctively duck, demonstrating that the visual process is still functioning. A patient with left-sided neglect might, for instance, only eat the right half of their plate and be unaware that there is food left. If you then turn the plate by 180 degrees, they will continue eating. The same principle applies when a patient is asked to draw a clock; they only draw numbers on the right half, completely ignoring the left half of the clock face.