Types of ADHD

Is ADHD a Spectrum Disorder?

New research suggests that ADHD comprises several meaningful subgroups — each one tied to a weak connection in the brain’s neural networks. Here, Joel Nigg, Ph.D. maps the regions of the brain that control attention, impulsivity, and emotion, and explains why scientists are studying the “white matter” connections between these circuits.

A brain made out of molecules on the ADHD spectrum
A brain made out of molecules on the ADHD spectrum



According to the Diagnostic and Statistical Manual of Mental Disorders (DSM-V), ADHD comprises two predominant sub-types: inattentive and hyperactive/impulsive. Joel Nigg, Ph.D., professor of psychiatry at Oregon Health & Science University, and other researchers studying the neuroscience of ADHD believe the condition is far more nuanced.

“Some kids are anxious. Some are angry. Some don’t have problems with emotions, but can’t pay attention. ADHD is variable,” Nigg says. “Children with ADHD appear to exhibit different profiles of emotional regulation and attention problems, perhaps associated with different patterns of maturation of brain networks.”

Brain networks, of course, are numerous and complicated. At a cellular level, neurons transmit messages and make connections between and within different brain regions — the frontal lobe, the temporal lobe, the parietal lobe, and the occipital lobe — as well as subcortical structures. Brain scans show us that ADHD brains are, on average, about 10% smaller than neurotypical brains. Other scans show that connections in the brain networks are underdeveloped, suggesting problems with the quality of the connections between neurons, which are called axons.

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“As a child matures, the white matter — or the myelin sheath — around the axons continues to develop and mature until the 20s. For people with ADHD, the brain could continue to mature until the 30s. This growth in myelin is like replacing a dial-up telephone Internet connection with a fiber optic cable. It makes neural transmissions faster and more efficient,” Nigg says. “Recent research has shown that there are changes to axon growth in the ADHD brain as well. Recent neuropsychological studies suggest that people with the condition process information more slowly, and there is more ‘noise’ during their processing. This may be related to immaturity of myelin fibers, which makes the axon’s neural transmission between certain brain circuits less efficient.”

In his research, Nigg and his collaborators have concentrated specifically on the connections between and within the frontal cortex, the parietal cortex, the basal ganglia, the thalamus, and the nucleus accumbens. Underdeveloped axonal fibers in the connections between these regions may help to explain the inattention, impulsivity, and emotional regulation problems so common in people with ADHD, Nigg says.

“The fronto-cerebellar network links the frontal cortex to the cerebellum. The executive function network links the frontal cortex, the parietal cortex, and the subcortical areas (basal ganglia). The attentional network links the frontal cortex to the supplementary motor cortex and the parietal cortex. Each network could be a locus of dysfunction for people with ADHD.”

Two fundamental kinds of brain signaling must be considered to understand ADHD.

[Free Guide: Inattentive ADHD Explained]

Bottom-Up Signaling: “The signaling from the back of the brain to the front of the brain, and from the interior of the brain to the outer part of the brain is bottom-up signaling. Those signals respond to sensory input — what you see and hear — and immediately trigger attentional capture or emotional reaction.”

Top-Down Signaling: “In response to these bottom-up signals, top-down signals come from neurons projecting either from the prefrontal cortex backward towards the back of the cortex or downward into the interior of the brain to modulate the spontaneous bottom-up signals. The top-down modulatory signals are based on your goals, your learning, or what you want to be doing. They respond to internal signals instead of external signals.”

In a neurotypical brain, he says, “There is a good balance of bottom-up and top-down signaling. Bottom-up systems appropriately interrupt attention when something important happens (e.g., someone physically draws near, a loud sound, or if you are a child – the teacher frowns). These are occurrences that your brain recognizes as something unexpected, not-supposed-to-happen in the moment, and makes you notice so you can modify your top-down response.”

But in ADHD brains, these top-down signals are relatively weak. One hypothesis is that they are overpowered by the much more powerful bottom-up signals. And that imbalance manifests in several different ways, depending the area of the brain impacted. Nigg and his team have focused on three common manifestations of this imbalance: inattention, impulsivity, and emotional regulation.

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1. Inattention

The problem: A child with ADHD becomes so hyperfocused on a video game that it is not easy for him to stop playing. Or he can’t focus on his homework when siblings are watching TV or playing nearby.

The explanation: The “automatic attention capture system” in the brain is activated by the stimulation of the video game or the enticing distractions nearby. It sends a bottom-up signal to the parietal lobe, which should reply with a top-down signal reminding the brain of its long-term goals and obligations. In ADHD brains, the axonal fibers in this top-down reply are underdeveloped, so the message to ignore the environment and refocus on the goals is lost. There is not enough top-down control.

“Studies that observe the brain using an fMRI scanner while children work on an attention task (like a math problem), show the frontal-parietal attention network functions poorly,” Nigg says. “Additionally, in research that examines the axon fibers connecting the attention circuits of the brain, it’s found that certain fibers are underdeveloped, which could explain the under-functioning of the front and back areas of the attention network. It’s as if they are not well-connected, so they are not talking to each other. Because the front of the brain can’t capture attention, the behavior is not suppressed.”

2. Impulsivity

The problem: A child with ADHD blurts out answers in class, says something hurtful to a friend without stopping to consider the consequences, or literally leaps without looking and ends up injured.

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The explanation: The thalamus is the interior area of the brain that helps to signal the need for response inhibition; in other words, it helps stops you from performing a behavior that is not in your best interest. It operates like a gate, sending signals to allow and stop behaviors as is appropriate. In ADHD brains, the limbic-hippocampal connections relaying these warning signals from the thalamus to the frontal cortex are impaired. It’s as if the gate is broken, and behavior doesn’t get suppressed when it should.

“People without ADHD have the ability to stop, mid-stream, if they recognize a person is not smiling or responding well to something they are saying,” Nigg says. “The average adult needs only 200 milliseconds of warning to interrupt something they are about to do, even if they started doing it. The average child needs about 280 milliseconds. The child with ADHD needs 20 to 30 milliseconds longer warning, which is an eternity when it comes to behavior control because behavior is so fluid.”

3. Emotional Control

The problem: A child with ADHD responds in overblown, extremely emotional ways to small setbacks or challenges that most children would shrug off. Perhaps she suffers from anxiety or a mood disorder due to school frustrations, or she throws temper tantrums that last hours because she can’t regulate her anger. Long-term rewards are meaningless; immediate gratification is everything.

The explanation: The amygdala are two internal brain regions that are involved in emotional reactions and decision making. When flooded with anger or worry, these regions deploy bottom-up signals to the cerebral cortex. The insula, a region of the cerebral cortex, should then respond with top-down strategies and goals designed to inhibit an individual’s emotional response in line with the goal. This is what helps you take a deep breath and think before acting on a sudden emotion. In ADHD brains, this insula-amygdala connection is weak, which “can lead to a breakdown in regulating negative emotions,” Nigg says. “Emotion regulation is a big part of ADHD that has been traditionally ignored.”

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“At the same time, people with ADHD over-respond to rewards when they are immediate, and don’t remember or value future rewards, which indicates a potential breakdown in the regulatory system,” Nigg says. “When comparing ADHD brains to people without ADHD, we see that the connection between the prefrontal cortex and the reward system (which is partly in the nucleus accumbens) has reduced activation, especially in the dorsal part of the prefrontal cortex. This could explain overexcitement, frustration & anger, and inability to respond to delayed rewards.”

Not all Kids with ADHD Are the Same

“ADHD is not a breakdown of the brain in one spot. It’s a breakdown in the connectivity, the communication networks, and an immaturity in these networks,” Nigg says. “These brain networks are interrelated around emotion, attention, behavior, and arousal. People with ADHD have trouble with global self-regulation, not just regulation of attention, which is why there are attentional and emotional issues.”

In the future, brain imaging may lead us to classify ADHD according to various valid sub-types like those explained above. Currently, though, “brain types determined by brain scans are currently just speculation by physicians,” Nigg says. Due to variability in brain-imaging equipment and analysis procedures, Nigg says he does not recommend pursuing a brain scan to help diagnose symptoms of ADHD in anyone.

“My own view of this [brain-imaging] data is [that] it’s not worth the money that it’s going to cost you to have this test,” he says. “It may give the clinician a little more confidence in their ADHD diagnosis, but that may not be valid; it’s important to note there’s an improvement in accuracy from simply using a standardized rating scale too, and that’s a lot less expensive.”

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  1. I keep suggesting a discussion group about ADHD as a symptom of FASD (Fetal Alcohol Spectrum Disorders). I think that it might be very helpful to some parents to consider whether their child was exposed to alcohol in utero. The reason I advocate for addressing the connections between the two issues is that it can be very difficult to get an FASD diagnosis because most doctors are 1) not very well educated on FAS/FASD and 2) hesitant to bring up the possibility that a parent might have unintentionally harmed her child by drinking during pregnancy. We need to get over the taboo of discussing this health issue if kids with ADHD resulting from an FASD aren’t properly diagnosed. But my suggestions for an ADHD as part of FASD discussion group on the ATTITUDE page just disappear.

    I am the parent of adopted twins with FASD. I’ve become quite well versed on the issue since their diagnosis and I”m fortunate to have a couple of different groups (a state chapter of NOFAS that has a closed Virtual Family Center on Facebook, my state chapter of Adoption Support Network has a FB page as well, and the FB page Red Shoes is another space where parents can share experiences and find pointers, advice, and references to medical professionals who understand FASD.

    Of course, most kids with ADHD don’t have an FASD. But rather extreme FASD combined with impulsivity, dysmaturity, memory deficits, perseveration, and trouble grasping abstract concepts are all symptoms of FASD. And too many kids are misdiagnosed with ADHD, ASD, Tourettes, OCD, and Oppositional-Defiant Disorder. It’s important to families and to individuals to get a correct diagnosis is the underlying issue is FASD. A correct diagnosis allows parents to seek out information that can help with raising a child, teen, and young adult who has this, often invisible condition that results to physical damage to the developing fetal brain due to exposure to alcohol, which is a teratogenic, meaning that it interrupts some aspect/s of fetal development. Alcohol inhibits formation of the cerebral cortex in the fetus, and so results in permanent brain damage. People with an FASD can have normal, or even high IQ’s, but not really have access to their native intelligence due to damaged or missing neurons in the cerebral cortex. A child who might otherwise be denied an IEP at school (or any kind of support beyond the regular classroom), will qualify under “other health issues” with a diagnosis of FASD. FASD is not rare, and all school teachers deal with kids who have it, whether they know it or not. Being able to help teachers understand and to advocate to for my kids because I can explain that they appear to have typical verbal skills when you talk with them, but they don’t process language as quickly or as well as a child who does not have FASD. And even when they grasp an idea, they might not remember it by the next day. They need far more repetition than typical kids.

    My apologies for posting a diatribe. I am very devoted to the cause of moving FASD out of the shadows. More families will have opportunities to learn about best parenting practice with their child if there is a diagnosis of FASD. We can better advocate for our kids at school if we know how their brain functions differently. In my home state, my twins can go from their Senior year of high school into a bridging program that teaches life and job skills. They will stay in that program until age 21, when they might have the emotional maturity of a typical 15 or 16-year-od.

    Please visit http://www.nofas.org for more information and resources if you think that your child might have had any pre-natal exposure to alcohol. It is not only alcoholic women who give birth to kids with FAS/FASD. Different mothers and different fetuses seem to be more or less susceptible to FASD. In some cases, one or two incidents of binge drinking (partying) can r cause FASD. In others, and no one knows why, a fetus tolerates fairly frequent and sometimes high-level exposure to alcohol without developing FASD.

  2. My granddaughters are similar to FAS but because of drugs and tobacco use, caffeine/energy drinks and malnutrition. the older child has impulsivity and emotional problems, along with ADHD. The younger one has severe ADHD and neurocognitive disorder. She also shows signs of ODD (Oppositional-Defiant Disorder. I still think because of all the behavioral problems she is still having with all the extra help, that she has some autism going on. I feel your pain and would also like to bring these awesome children into the light to help the get the correct help they need to succeed.

  3. I have always believed that ADHD is a spectrum disorder. It might even be the “same” spectrum as autism/Aspbergers. Every brain, every individual, is unique; thus the way ADHD affects us will be unique as well.

  4. Julia is really onto something here. I would suggest that FASD is one of several key predispositions that affect biochemistry in our brains. Age of the parents, smoking, exposure to breathable toxins. I suspect most of my generation is a bit poisoned by lead. I grew up in a world where everyone used leaded gasoline, ate tons of canned and processed foods, and was exposed to second-hand smoke. Drinking during pregnancy was OK! We played on grass treated with poisonous chemicals. This on top of hereditary factors.

    Let’s also remember that an interest-based brain was an advantage in many things before the mid-20th century.

  5. What a fun article to read about. No sarcasm intended. I am definitely a top-down processor struggler (whatever you’d call it) so sensory input from the environment is always distorted and skewed. I’m 22 and I’m likely not going to see the max growth of my brain until I’m 30 as my brother has had the same sorts of symptoms as well as my dad. Father is a smoker and drank alcohol frequently and mom gave birth to two boys with bad ADHD but I never had the ODD symptoms my brother had, but I have severe hyperactivity more than inattentive (although I have both). The hyperactivity is made worse for me by caffeine and energy drinks yet I find caffeine to be a good mood booster for me short-term, so all the talk about avoid artificial sweeteners and caffeine/sugar I find to be total nonsense. I can throw back 4 Red Bulls and feel much happier than if I don’t have any stimulant, however I do prefer amphetamines more than caffeine, methylphenidates, and everything else. The amphetamines leave me drained as hell after they wear off, but they work SO MUCH BETTER than the methylphenidates as long as I don’t take too much otherwise cannot sleep. Methylphenidates are better with the mental drain afterwards for me but they give me a much more subpar focus than amphetamine salts. I didn’t like Vyvanse although I found it to be quite effective despite how it gave me Raynaud’s Phenomenon (look if up if you don’t know and want to know about that). I love Dexedrine tablets more than any other medicines I’ve tried, but my brother only likes Adderall XR and that’s about it. I am a Dexedrine fan to the moon and back so it’s totally just your own brain chemistry and how you’ve developed neurologically, socially, and your environment has a HUGE ROLE. Super important to consider your environment if you have top-down processing issues. Car honks outside and I about have a heart attack if I’m not expecting it and I”m only 22 hehe 🙂

  6. I’ve “naturally” believed it/can be a spectrum disorder just like I have self discovered it is related with ASD. Yes ADD/ADHD does vary but a little less than ASD. Who has not caught themselves (of course those diagnosed) saying the ADHD spectrum? I certainly have since ADHD covers both such as ASD now covers Autism and asperger’s. There is much research still in progress for ADHD and for tools out there to at least control it, we should evolve to a very impressive society.

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