There are systems in our central nervous system that enable to us to concentrate and to attend to particular things around us while excluding other things from our consciousness. For example, when a shopper looks at a particular item in a display and her central nervous system excludes the other people walking by, the overhead music, and the items in neighboring displays. Once she has seen what she wanted to see, she withdraws her attention and at the same time she notices the people walking by, the overhead music and the neighboring displays again. Or, as another example, a student concentrates his attention on an examination, and while doing so the student won’t notice the cars driving by outside, the birds singing outside the window, or the other kids whispering in the back of the class.
When these systems that control attention and concentration malfunction, we are unable to exclude the competing stimuli from our consciousness to allow us to attend to particular stimuli. That is to say, the shopper’s central nervous system does not exclude the people walking by, the overhead music or the neighboring displays from consciousness. Similarly, the young person trying to take the test notices the cars driving by, the birds singing and the other kids whispering in the back of the class. Consequently, the shopper has trouble deciding whether she wants to buy the idem in the display and the student has trouble completing his test.
There are related systems that allow us to be still while we concetrate. As an example from the animal kingdom, consider what happens when a dog perceives something new in its evironment, say a bird in its yard. The dog typically looks, listens, and smells in the direction of the bird while at the same time becoming still and quiet before rushing the bird. Something similar happens when we concentrate on something. Consider a person reading a book while waiting for a flight in the airport. While reading the person is usually still and quiet, with little physical movement and no speech. When we attend to something, in other words, our physical activity and speech is suppressed. When the systems controlling this suppression in the central nervous system malfunctions we have difficulty being still and quiet. Instead the person with ADHD fidgets and wiggles, talks too much, and in extreme cases runs and jumps at times the person is expected to be still and quiet (school, a movie, church, etc).
Note that a person with ADHD can through force of will be still and quiet and concentrate, but this force of will requires effort, work, and consequently the person tires or fatigues after a short time and the activity, talking and inattention reemerge. Sometimes parents or teachers perceive this ability to be still, quiet and concentrate for short periods and conclude that the person with ADHD is lazy or that they lack desire to be still, quiet and attentive. This is incorrect – the person with ADHD actually works very hard to be still, quiet and attentive, much harder than other people without ADHD. A person without ADHD, by contrast, does not have to work to be still, quiet and attentive.
The diagnosis of Attention-Deficit Hyperactivity Disorder (ADHD) requires symptoms from one or both of two categories: inattention and hyperactivity/impulsivity. The inattention symptoms include distactibleness, difficulty maintaining attention, forgetfulness, disorganization, difficulty following through on tasks, losing things, making hasty mistakes in tedious work. The hyperactivity/impulsiveness symptoms include difficulty sitting still, difficulty sitting for long periods of time, talking too much, being loud, impatience, blurting things out in groups, interrupting people, finishing sentences or cutting people off in conversation.These symptoms must have been present before a person is 7 yo (some experts say 10 yo and some are now urging the consideration of adult onset ADHD). The ADHD is categorized as inattentive type when inattention predominates, hyperactive/impulsive type when there is little inattention, and combined type when there is both inattention and hyperactivity/impulsiveness.
Note that ADHD of any type is different from intelligence. Many people who suffer from ADHD are very bright and as a result the ADHD symptoms may not become problematic until later in life even though the symptoms have been present since early childhood. Patients who are of the inattentive type are often not identified until later in life for several reasons. The division of ADHD into these subtypes was not made until 1994 with the publication of the current diagnostic critieria in the Diagnostic and Statistical Manual, fourth edition. More importantly, most people suffering from ADHD, inattentive type, are not trouble makers and consequently in school they were perceived as lazy or too social or uninterested in school rather than suffering from a disorder of attention.
And finally, we find that many people suffering from ADHD are visual and imaginative so they enjoy reading fiction and do so for hours on end, from which they conclude they do not have problems with attention. The real test comes, however, when they are required to read non-fiction that does not lend itself to visualization. This task is often painfully difficult, revealing the problem with attention.