Brain hemispheres

The left and right hemispheres of the brain are not merely “safety backups” for the function of the other half. They have different functions or functional focuses. They are connected by the corpus callosum.
In most people, the left hemisphere is more dominant. The right hemisphere is more strongly associated with emotional processing, the recognition of emotions and stress reactions.
Although separation of the hemispheres by surgery has little effect on functioning in daily life, it can lead to a loss of emotional abilities (including alexithymia) or symbolic imagination and is often accompanied by a reduced cortisol stress response.
Post-traumatic stress disorder (PTSD) often causes a loss of information exchange between the hemispheres. In borderline personality disorder, it is assumed that the inability to integrate positive and negative perceptions is due to impaired communication between the hemispheres.
A disturbed bond between infant and mother or early child abuse can lead to an impairment of the right hemisphere of the brain and have a negative impact on the ability to bond and social skills.

The following article is currently based largely on a review by Henry.¹

• 1. Right and left hemispheres of the brain
  • 1.1. The right hemisphere of the brain
  • 1.2. The left hemisphere of the brain
• 2. Brain hemispheres and cortisol
• 3. Brain hemispheres and catecholamines
• 4. Brain hemispheres and ADHD
• 5. Early attachment disorder impairs self-organization of the right hemisphere of the brain

1. Right and left hemispheres of the brain¶

People with a functional or surgical separation of the connection between the cerebral hemispheres (surgical: commissurotomy, callosotomy, a last resort in epilepsy) function inconspicuously in most everyday situations. Communication between the cerebral hemispheres is helpful, but not vital. Most people are characterized by a dominance of one of the two hemispheres.²
In addition to alexithymia, a separation of the cerebral hemispheres causes a subtle loss of emotional abilities, e.g. with regard to symbolic imagination. PTSD is often accompanied by alexithymia and a loss of information exchange via tactile stimuli between the cerebral hemispheres.³
It has been postulated that the inability to integrate positive and negative perceptions about people (black/white thinking) in borderline could be the result of PTSD, which impedes communication between the cerebral hemispheres.⁴ In PTSD, differences in the perception of the right and left auditory hemispheres have been found, which are attributed to a disorder in the exchange between the brain hemispheres caused by the traumatic experience.³⁵

The brain hemispheres have different tasks in relation to attention:⁷

• Right hemisphere:
  • Maintaining alertness
  • Processing new stimuli
• Left hemisphere
  • Focused attention
  • Selective attention

With age, some asymmetries become weaker, others stronger. Some even reversed direction. The total number of regions with significant asymmetries was greater in women than in men, while the total number of significant asymmetries (within regions) and the extent of significant asymmetries was greater in men.
Overall, the effect sizes for both age effects and gender differences were small. Asymmetries are an inherent organizational pattern of the brain that manifests early in life and appears to be relatively stable throughout childhood and adolescence.⁸

A distinction must be made between

• Laterality (preferred use of one side of the body (brain/hand/foot etc.) for certain tasks or activities)
• Asymmetry (different sizes of organs or organ parts - here: brain regions)

1.1. The right hemisphere of the brain¶

The right hemisphere of the brain is⁹

• Mostly the non-dominant hemisphere
• Seemingly more vulnerable to traumatic influences than the left¹
  • Probably due to their closer connection with the limbic system
• More closely connected to the autonomic nervous system
• Predominant role in
  • Physiological and cognitive aspects of emotional processing
  • Attention^{1011 12}
  • Cognitive control
    • Inferior PFC (especially right hemisphere)¹³
  • Inhibition / impulse control
    • Inferior PFC (especially right hemisphere)¹³
  • Motor inhibition¹⁴
  • Interference control
    • Inferior PFC (to the same extent as inferior PFC of the left hemisphere)¹³
  • Cognitive flexibility
    • Inferior PFC (to the same extent as inferior PFC of the left hemisphere)¹³
  • Emotions
    • Recognition of emotions¹⁵
      • Monkeys that were socially deprived in early childhood showed a deficit in recognizing the emotions of others, as did children with right hemisphere damage.¹¹⁶ Among other things, this manifests itself in a deficit in recognizing emotions in facial expressions.
      • Emotion recognition is impaired in ADHD
    • Understanding emotions¹⁷
    • Recognition of subtle emotional expressions¹⁵
    • Recognition of emotional gestures¹⁵
    • Richness of the emotional life¹⁸
    • Control of facial expression¹⁸
    • Control of speech melody (prosody)¹⁸
    • Understanding music¹⁷
    • Understanding body language¹⁷
    • Processing negative emotions¹⁵
  • Control of the HPA axis (stress axis)¹⁹ and the autonomic nervous system (the second important stress regulation system)²⁰²¹
  • Disgust (right frontal and anterior)²²
    • While satisfaction correlates with anterior temporal left hemisphere activity
  • Recognize connections (forest, not just trees)¹⁷
  • Visualization of perceptions¹⁷
  • Non-verbal understanding¹
  • Flashes of inspiration¹⁷
  • Leaps of thought¹⁷
  • Daydreams¹⁷
  • In establishing the feeling that objects such as family possessions and pets are personally familiar. Recording and maintaining personal relevant entities such as “familiar” faces, people, voices, names, linguistic expressions, handwriting, topography.²³
  • Consolation²⁴
    • People with alexithymia use comforting strategies less often than people without ADHD, whereas people with PTSD use them more often.
  • Affective reactions to proverbs and idioms, paraphrases, emotional expressions, swearing and slang²³
• More dominant in terms of activation of conditioned fear response via amygdala
  • Right amygdala is more strongly involved in the storage of frightening faces and in the expression of emotionally influenced memories of aversive experiences²⁵
• Stress reactions and emotional processes appear to be closely linked to the right hemisphere of the brain.²¹
• Early abuse in the first 2 years of life often leads to disorganized-insecure attachment, which manifests psychologically as an inability to develop a coherent strategy for coping with relationship stress. Early abuse has a negative impact on the development of the right hemisphere of the brain, which is responsible for attachment, affect regulation and stress modulation, thus laying a template for the coping deficits of mind and body that characterize PTSD symptomatology²⁵
  • The right hemisphere of the brain is involved in the infant’s attachment to the mother and later in life to other people.²³
• Social skills
  • Children with right hemispheric damage lose important social skills¹
    In our opinion, this also applies to ADHD
  • Adults with right hemispheric damage lose the sense of familiarity that is crucial for bonding.¹
    We consider whether this could explain the symptom of impaired attachment constancy.
  • Systems that promote right hemisphere activity appear to be resilience-enhancing and protective against socially dysfunctional behavior, substance abuse, HPA axis failure, and some aspects of chronic disease pathophysiology.¹
  • Damage to the right hemisphere of the brain caused a loss of ability to grasp, store and process personally relevant and familiar elements of the environment, impairing connection to, response to and interaction with other people.²³

Stress reactions and emotion-related processes are also processed in the mPFC. Prelimbic and infralimbic regions of the mPFC influence visceral motor regions, autonomic functions and emotional expression and represent an important region for the integration of neuroendocrine and autonomic activity with behavioral states and cognitive processes,¹⁹²⁶

Childhood abuse and other traumatic stress experiences caused by inescapable adverse physical, emotional or social events are significant triggers in the pathophysiology of several psychiatric disorders including ADHD.⁹²⁷²⁸

Early stress can cause developmental abnormalities in brain structures that play a crucial role in mediating the response to stress, e.g:⁹²⁷

• Amygdala
• Hippocampus
• Cerebellum
• Anterior cingulate cortex
• Corpus callosum
• And more

Stress reduces BDNF in the hippocampus, among other places, which can influence depression and neurodegenerative processes.²⁸²⁶

1.2. The left hemisphere of the brain¶

The left hemisphere of the brain is more strongly associated with

• Coping²²
• Flight/Fight²²
• Verbal understanding
• Verbal expression¹
• Cognitive inhibition¹⁴

Immediately after birth, rats showed large differences in dopamine turnover between the cerebral hemispheres, which decreased with increasing age.²⁹

2. Brain hemispheres and cortisol¶

A reduced exchange between the brain hemispheres (also in PTSD) appears to be the cause of¹

• Reduced cortisol stress response
• Alexithymia.

Furthermore, a coincidence of reduced basal cortisol levels (despite normal cortisol response to ACTH) and alexithymia is reported in some persons with ADHD, from which the authors concluded that ACTH-independent mechanisms reduce cortisol levels.³⁰ For example, the hippocampus is involved in inhibitory regulation of the HPA axis.

Monkeys with greater right hemispheric activation showed a higher CAR (cortisol awakening response) and higher basal cortisol levels. In contrast, monkeys with greater left hemispheric activation showed lower CAR and lower basal cortisol levels, while monkeys with balanced right/left hemispheric activation showed intermediate CAR. The CRH levels correlated with the basal cortisol levels.³¹

3. Brain hemispheres and catecholamines¶

It is possible that interhemispheric regulation of the amygdala and hippocampus by dopaminergic ascending systems helps to keep the right and left sides of the hippocampus in harmony.¹
Catecholamine asymmetries in the brain could therefore have a concrete benefit.³² Disorder of catecholamine asymmetry could lead to failure of interhemispheric communication and isolation of the HPA axis. The HPA axis appears to be more lateralized to the non-dominant hemisphere.
This could explain why corticoids do not remain elevated in alexithymic states (despite arousal of the fight/flight response).¹

A reduction in the size of the corpus callosum correlates with reduced communication between the hemispheres.³³
In a study using auditory evoked potentials in adults who had been abused in childhood, all of whom had neither acute mental health problems nor an acute Axis I diagnosis, they were first asked to actively recall a neutral or work-related memory and then a disturbing memory from childhood with impairment. In unaffected individuals, both hemispheres appeared to be equally involved in the recall. In adults with childhood trauma, there was a clear suppression of evoked potentials in the left hemisphere during recall of the neutral memory, indicating increased processing in the left hemisphere. During recall of the distressing event, there was a robust shift in laterality in evoked potentials. These were suppressed in the right hemisphere, indicating increased activation of the right hemisphere. Apparently, early maltreatment is associated with increased hemispheric laterality and decreased hemispheric integration.³⁴³⁵

In states of strong emotional or cognitive stress, events in the right hemisphere are functionally separated from the left hemisphere by inhibiting transmission between the brain hemispheres, even in neurologically intact people.³⁶ This can trigger alexithymic states, such as reduced emotional empathy with unchanged cognitive empathy.

See also ⇒ Empathy in ADHD In the article ⇒ Complete list of ADHD symptoms according to manifestations in the chapter ⇒ Symptoms.

4. Brain hemispheres and ADHD¶

One hypothesis assumes that ADHD (in addition to delayed cortical maturation) is due to³⁷³⁸
Overactivity of the left hemisphere can lead to hyperactivity of motor skills and behavior.
Hypoactivity of the right hemisphere, which is largely responsible for (sustained) attention, explains the attention deficit.³⁹¹²⁴⁰

ADHD patients showed

• significantly increased lateralization to the left in the inferior OFC. The lateralization indices correlated with ADHD symptoms⁴¹
• a lower lateralization to the right in the mOFC and in the rectus gyrus. The lateralization indices correlated with ADHD symptoms.⁴¹
• significantly increased lateralization to the right in the inferior frontal gyrus (opercular), in the precuneus and in the paracentral lobule⁴²
• less lateralization to the right in the insula⁴²
• Women with ADHD showed
  • a significant lateralization to the right in the putamen and in lobule VII of the cerebellar hemisphere⁴²
• Compared to ADHD-I, ADHD-C patients showed⁴²
  • increased lateralization to the right in the inferior frontal gyrus (opercular)
  • reduced lateralization to the right in the inferior temporal gyrus.
  • lateralization correlated with the values for inattentive and hyper/impulsive behaviors
• SHR showed left lateralized brain connectivity⁴³

Hyperreactivity of one hemisphere of the brain in combination with underdevelopment of the contralateral hemisphere corresponds to the nature of many neurobehavioral disorders.⁴⁴

Differences in the size of the same brain regions in the two hemispheres were not significant in a very large study (n = 3,762).⁴⁵
In a smaller study, asymmetries in mean diffusivity were found in the cingulum, inferior and superior longitudinal fascicles, and corticospinal tracts (n = 104). Stimulant treatment tended to reduce this asymmetry. Caudate, hippocampal, thalamic, and amygdala gray matter volumes were more asymmetric in ADHD patients, and asymmetry differences were more significant than lateralized comparisons (n = 849).⁴⁶

Diffusion imaging studies in attention-deficit/hyperactivity disorder (ADHD) have revealed changes in anatomical brain connections, such as the frontoparietal junction known as the superior longitudinal fiber (SLF). Studies in neurotypical adults have shown that the three SLF branches (SLF I, II, III) support different brain functions such as attention and inhibition and that their lateralization pattern is related to attentional performance. However, most studies on ADHD have examined the SLF as a single bundle and in children, so the possible contribution of lateralization of the SLF branches to the pathophysiology of ADHD in adults has not yet been clarified.

Altered asymmetry, particularly of the dorsal branch of the frontoparietal junction, which is the superior longitudinal fiber (SLF), may contribute to the pathophysiology of ADHD in adults.
Diffusion-weighted spherical deconvolution tractography was used to examine the three SLF branches (SLF I, II, III) in 60 adults with ADHD-I (including 26 responders and 34 non-responders to methylphenidate, MPH) and 20 control subjects. All three SLF branches were lateralized in adults with ADHD, but not in the control subjects. Lateralization of the SLF I HMOA was associated with line bisection performance, but not with SLF II volume, as previously reported in control subjects. In addition, increased left lateralization of the SLF I HMOA was associated with higher levels of hyperactivity in the ADHD-I group.⁴⁷

5. Early attachment disorder impairs self-organization of the right hemisphere of the brain¶

In infancy, the homeostatic structures between the “lower” autonomic and “higher” central brain systems develop in the right hemisphere of the brain⁴⁸, which serve to generate, regulate and stabilize psychobiological states.
The right hemisphere is much more strongly connected to the limbic system and the mechanisms of autonomic and behavioral arousal than the left hemisphere. The maturation of the right hemisphere is dependent on experience.⁴⁹

Disorders in the bond between infant and mother can therefore cause developmental disorders in the right hemisphere of the brain, affecting the modulation of arousal and the regulation of the neurotransmitters dopamine and noradrenaline. Among other things, dopamine and noradrenaline are responsible for
The brain’s maturation processes are responsible for this.⁵⁰⁵¹