The Neurochemistry of Focus


1. FOCUS AND ATTENTION

  1. 1.1 What is attention?

    Our brains are constantly being bombarded with an array of sensory information from our environment. But there is a limit to how much information, on a moment by moment basis, the brain can process effectively. To overcome this bottleneck, the brain has developed an effective system for prioritizing the incoming information. A mechanism by which only the most relevant and important information is focused on, and the rest - the noise - is “ignored”. This mechanism is attention.

    Your brain is focusing on so much more than you might realize. Although you are most acutely aware of your attention when you are knowingly trying to focus on something, or when you are searching for something, your brain is actually paying attention on a millisecond basis to a whole world of information that you are not aware of - something that is happening at a subconscious level. Take the simple act of walking. At each step you brain is paying attention to the contours of the ground, the placing of each foot, or any obstacles in the way which may set you off balance, with most of this occurring automatically without us even realizing.

  2. 1.2 What are the different types of attention?

    Imagine you are walking down a busy street. There are colours, shapes, sounds all jumping out at you. Grabbing your attention. This is what is called “bottom up” attention - sometimes called exogenous attention. It is when your sensory world is driving what information you notice. Where the loudest voice or brightest color “wins” so to speak and causes you to suddenly reorient your focus.

    But this is only one half of how your attention system works. The other half is what is called “top down” attention - sometimes called endogenous attention. This is where it is your brain which is dictating and directing how you should be focusing your attention. It does so based on what you are trying to achieve at that particular moment in time - your goals - as well as prior knowledge and experiences arising from your memory.

    Your brain’s attention system also helps you to switch your focus back and forth as required. It also enables you to divide your attention between two things at once when you are trying to multitask. And whilst on some occasions you only need to focus your attention for a moment, other times you need to maintain your focus over time - so called sustained attention. It is this latter process of being able to hold your attention selectively on a particular goal or task whilst blocking out the unwanted noise over a period of time, which allows to you achieve mental focus.

  3. 1.3 What are the different types of noise which can interfere with your focus?

    The unwanted “noise” can come from your external surroundings in the form of sensations - sounds, smells or visual imagery - which try and compete for your attention. But your distractions can also come from within your own mind where, instead of focusing on your immediate goal, you find your mind wandering, or even daydreaming - a form of thinking which is orchestrated by your brain’s inwardly absorbed default network, rather than its outwardly focused attentional network.

2. THE NEUROCHEMISTRY

There is no single “attention centre” in your brain. Instead your ability to focus your attention depends on a widespread network of brain regions which collectively make up your brain’s “attentional system”. It is a system which closely interfaces with your thoughts, actions and feelings to help them operate more efficiently. Engaging this network, therefore, not only improves not only your attentional focus, but also - more broadly - your mental focus.

Your attentional system includes regions located in nearly every lobe of your brain. The particular set of regions which are activated when you pay attention depends on the type of information that is being attended to, as well as the way in which you are directing your attention. However, there is generally a core attentional network which includes sensory regions to selectively process the incoming information, parietal regions to control where you should be focusing, or orienting, your attention, and prefrontal regions (particularly the upper part for top-down attention and the lower part for bottom-up attention) which form you attentional control centre.

Neuromodulators such as acetylcholine, dopamine and norepinephrine play an important role in attention and focus. Each one plays a critical role in making sure you can effectively focus your attention where needed in the face of unwanted distractions which compete for your brain’s resources.

  1. 2.1 Acetylcholine

    Acetylcholine (ACh) is an important neurochemical in the brain for paying attention, learning and memory. Although there are relatively few ACh cells in the brain compared to some of the other major neurotransmitter systems, ACh cells - which arise from collections of nuclei in your evolutionarily older brainstem and midbrain - extend out to nearly every region of the brain.

    How do you make acetylcholine in the brain?
    ACh is made in the brain by the enzyme choline acetyltransferase using two chemical compounds - choline and acetyl-CoA. The enzyme first separates the acetyl part from the acetyl-CoA and then joins this with choline to create the acetylcholine - ACh.

    How does ACh act on the brain?
    ACh is released at synapses - the bits of neural machinery which form the connections between brain cells. Within these synapses there are two major types of “receptors” that ACh can bind to. One family of receptors are called “nicotinic”, and it is at these receptors where the drug nicotine, commonly found in tobacco, acts to mimic the effect of ACh. The other family of receptors are called “muscarinic”. Muscarinic receptors are most often associated with their peripheral role of controlling your body muscles as you move about, but are also found in the brain. As with most of the neurochemicals in the brain, there are also multiple variants - subtypes - of each of muscarinic receptors and nicotinic receptors which have subtle differences in terms of their brain-wide expression and precise mode of action.

    How does ACh influence focus and attention in the brain?
    Because ACh cells extends throughout the brain and because there is a widespread network of regions making up your brain’s attentional network, the specific modes of action by ACh in relation to attention are relatively complex. But in general, ACh, acting through muscarinic and nicotinic receptors, enhances attentional focus by modulating neural activity across sensory, prefrontal, parietal regions of your brain.

    In sensory regions, such as your visual cortex which is activated when you are focusing in the visual domain, ACh acts to increase the signal relative to the noise. More specifically it increases the strength of the relevant neural signal in the visual “receptive field” which represents your point of focus to make sure it is greater than the surrounding neural signals. This helps you to label which areas of your visual field are the most important, and to inhibit nearby distractions which may otherwise disturb your attentional focus.

    In your parietal context, ACh helps to support the way you orient, and reorient, your attention towards something of interest or importance by influencing top-down (brain driven), as well as bottom-up (sensation driven) attentional processes.

    In your prefrontal cortex - and your medial prefrontal cortex in particular - ACh levels are increased when you are required to sustain your attentional focus over time.

    What drugs can artificially increase ACh in the brain?
    There are various drugs which act on the ACh system to artificially boost your levels of ACh. The most well known of these is nicotine - found in tobacco - which acts by stimulating the nicotinic receptors much like ACh itself does. But there are also other ways to increase ACh levels in the brain. For example, once ACh has been released into the synapse to carry its message onto the next brain cell, it is then targeted by specific enzymes - called cholinesterases. These enzymes break down ACh back into its component parts, therefore stopping it having any effect. Drugs such as donepezil or galantamine - commonly used to treat Alzheimer's disease which disrupts the cholinergic system - selectively target, and inhibit, these cholinesterase enzymes. This means that the ACh isn’t broken down as quickly, therefore prolonging its effect.

    What happens to your attention if you increase ACh artificially in the brain?
    Artificially boosting ACh with nicotine, or blocking the action to cholinesterase enzymes have both been shown to increase attention and focus in experimental studies. For example, brain imaging studies have shown that if you increase levels of ACh by inhibiting the cholinesterase enzyme then it boosts the neural signals in sensory regions which code for information which is specifically relevant to your particular goal or task, whilst concurrently suppressing goal or task-irrelevant stimuli, therefore increasing your attentional focus.

  2. 2.2 Dopamine

    Within your brainstem and midbrain, there are small collections of cells (for example the substantia nigra, ventral tegmental area and ventral striatum which contain the neurotransmitter dopamine). Each of these cells project widely along various pathways throughout the brain to modulate various functions such as reward, novelty detection, cognitive flexibility and attention.

    How do you make dopamine in the brain?
    Dopamine is synthesized from a precursor chemical called L-DOPA by an enzyme called DOPA decarboxylase (otherwise known as Aromatic L-amino acid decarboxylase). It is an enzyme which is involved with synthesizing a variety of different neurochemicals in the brain (e.g. serotonin, histamine etc). L-DOPA is synthesized from the amino acid phenylalanine, via tyrosine (also an amino acid), both of which are sourced from your diet.

    How does dopamine act in the brain?
    There are two families of dopamine receptors - the molecules onto which dopamine binds to exert its effect in the brain. These are called D1 (which actually includes the D1 and D5 receptors) and D2 (which actually includes D2, D3 and D4 receptors). These receptors are differentially distributed around the brain and operate in slightly different ways. Within the prefrontal cortex, a key brain region which receives dopaminergic signals, dopamine doesn’t simply act by increasing activity. Instead it targets cells which are both excitatory (glutamatergic) and inhibitory (GABAergic). In doing so it helps to balance out and fine-tune the overall resulting pattern of neural activity to suit the particular mental task.

    How does dopamine influence attention?
    Although the actions of dopamine in the brain are complex, experimental studies have shown that dopamine helps to enhance attention, especially in the context of making sure that you pay attention and shift your focus in a flexible and appropriate manner based on information you have learned previously. In other words, knowing what to focus by using your experience. If you aren’t able to do this then you end up wasting much more time analyzing irrelevant information. In this way, dopamine helps to make your attention more efficient in a dynamic and ever-changing environment.

    How do you artificially increase dopamine levels in the brain and what effect does it have?
    Drugs such as L-Dopa - used as a treatment for Parkinson’s disease - increase levels of the dopamine precursor, leading to increased levels of dopamine in the brain, and therefore improvements in attention. Other clinical or recreational interventions with psychostimulant drugs such as amphetamine and methylphenidate (ritalin) - a treatment for ADHD - act on the catecholamine synapses (namely dopamine and norepinephrine) and target various chemical components of the signalling pathways which ultimately results in widespread changes in cognitive function including, in some instances, improvements in memory and attentional focus.

  3. 2.3 Norepinephrine

    Norepinephrine (noardrenaline) is a neurotransmitter found in the brain which has very similar in structure to the hormone epinephrine (adrenaline). It is a chemical involves in wakefulness, memory, alertness and generally readying the brain, and therefore the body, for action when it is being challenged or threatened.

    How is norepinephrine made in the brain?
    Norepinephrine is synthesized from dopamine by the enzyme dopamine beta-hydroxylase. In the brain it is synthesized within cells originating from brainstem nuclei such as the locus coeruleus, However, norepinephrine also has body-wide effects (e.g. in the peripheral parts of the nervous system) and is released directly into the bloodstream from a region called the adrenal medulla.

    How does norepinephrine act in the brain?
    In the brain, noradrenaline acts through two main families of receptors - alpha and beta. Different parts of the locus coeruleus project to different brain regions, including the medial prefrontal cortex, anterior cingulate cortex - a region involved in mental flexibility - and your motor cortex which oversees the planning and execution of movements.

    How does norepinephrine influence focus and attention?
    Norepinephrine has a diverse set of actions in the brain which result in both the activation and inhibition of specific brain regions. One of its key functions is to promote arousal - or mental vigilance - and wakefulness,so it acts on systems which support this function. For example, noradrenaline activates wake-promoting cholinergic (ACh) cells and inhibits sleep-promoting GABAergic cells. Increases in brain arousal, or vigilance generally leads to improvements in cognitive functioning, including attentional focus, and a speeding on your reaction times.

    Beyond its general role of wakefulness and arousal, Norepinephrine is also important in mediating the attention effect when you find your attention suddenly “grabbed” by an unexpected, novel or salient stimulus or event which occurs or appears in your vicinity.

BRAIN ENERGY

Energy is essential for all the ongoing metabolic activity within cells. One of the main sources of energy comes from the chemical molecule ATP (adenosine triphosphate). ATP is generated as part of the conversion of glucose (usually with oxygen) into water and carbon dioxide. Glucose and oxygen therefore act as “brain fuel” for ongoing neural activity. Any behaviours which you perform which increase the levels of these substances, such as eating or exercising, can therefore help to increase the levels of ATP available for ongoing metabolic activity - including neural activity in your brain - and can improve cognitive performance on tasks requiring focused attention.