Top-down vs. bottom-up processing is widely discussed as one of the reasons why people with ADHD struggle with executive functioning. As our racing brain sends signals from the bottom up our prefrontal cortex sends braking signals down. It is these executive function braking signals that help keep us on track and focused on the tasks at hand.
Ned Hallowell and John Ratey note that “people ADHD with have the power of a Ferrari engine with bicycle brakes“. It’s a great analogy that is both easy to explain and neurologically correct (although not the entire story).
The thing is that the mismatch between engine and braking strength is true for everyone.
David Rock, in his (non-ADHD) book “The Brain at Work” uses a similar car and with poor brakes analogy. Rock says “Our braking system is the most fragile, temperamental, energy-hungry region of the brain.
The key is difference is signal strength and timing. Bottom-up messages are faster and stronger than top-down down messages. For neurotypicals the time gap between a bottom-up signal and top-down braking signal is about 200 milliseconds – just enough time to stop you from an impulsive response. The time gap for ADHDers is about 20 milliseconds slower – a mere fraction slower but just long enough for you to say or do something impulsive.
Two things impact on the strength and speed of the signal as it travels from one neuron to the next.
- First, amount of dopamine and norepinephrine in the space between each neuron (the synapse), and the speed of reuptake of these neurotransmitters. ADHDers have less and/or a faster reuptake system, essentially one way or another there’s less of these neurotransmitters in the synapse to carry the message across as fast as neurotypical brains. Stimulants slow the reuptake of dopamine in the synapse. Other factors influence the amount of dopamine available. For example, vigorous exercise boosts dopamine, and having sufficient protein in your diet helps to build up dopamine stores.
- The second is the amount of insulation wrapped around each neuron. Just like an electric cable, the more insulation the better the signal. Within the brain, a fatty layer called the myelin sheath acts as insulation around the neurons. In ADHD the myelin sheath appears to be develop slower during childhood (taking two to three years longer). Since it is a fatty layer, it is often suggested that Omega 3 supplements may help improve ADHD symptoms in children.
That said, it is helpful to remember that everyone struggles with this overtaxed braking system to some degree, especially as our brains become fatigued. “Your breaking system only works at its best very now and then” notes David Rock.
For our always on brains, our brakes are under constant strain. Recognising that we need regular periods of rest and recovery, and not making important decisions when our brains are tired and we’re more likely to be impulsive are just a couple of things we can do to keep our race car brains on the track.
