The brain hemispheres (cerebral hemispheres) are not merely “safety backups” for the function of the other half. They have different functions or focal points of function. They are connected by the corpus callosum.
This post is currently based largely on a review by Henry.
1. Right and left brain hemisphere
People with functional or surgical disconnection of the brain hemispheres (surgical: commissurotomy, callosotomy, an ultima ratio in epilepsy) function inconspicuously in most everyday situations. Communication of the brain hemispheres is helpful but not vital. Most people are characterized by a dominance of one of the two hemispheres.
A separation of the cerebral hemispheres causes a subtle loss of emotional abilities, e.g. regarding symbolic imagination, in addition to alexithymia. PTSD is often accompanied by alexithymia as well as a loss of exchange of information about tactile stimuli between the cerebral hemispheres.
It has been postulated that the inability to integrate positive and negative perceptions about people (black/white thinking) in borderline may be the result of PTSD, which impedes communication between the cerebral hemispheres. In PTSD, differences in the perception of the right and left hemispheres have been attributed to a disruption in the exchange of brain hemispheres due to the traumatic experience.
Gott et al describe a subject who could switch between two states volitionally. In a left-hemispheric state evidenced by EEG data and task performance scores, she was a businesswoman who enjoyed business planning and meeting people, whereas in a right-hemispheric state she enjoyed gardening, being relaxed, or experiencing an erotic mood with an intimate friend.
The brain hemispheres have different tasks in relation to attention:
- Right hemisphere:
- Maintaining alertness
- Processing new stimuli
- Left hemisphere
- Focused attention
1.1. The right brain hemisphere
The right brain hemisphere is
- Mostly the non-dominant hemisphere
- Seemingly more vulnerable to traumatic influences than the left
- Presumably due to their closer connection with the limbic system
- More closely associated with the autonomic nervous system
- Predominant role in
Physiological and cognitive aspects of emotional processing
- Emotion recognition
- Understanding emotion
- Recognition of subtle emotional expressions
- Emotional gesture recognition
- Richness of the emotional life
- Control of facial expression
- Control of speech melody (prosody)
- Music appreciation
- Understanding body language
- Processing negative emotions
- Control of the HPA axis (stress axis) and the autonomic nervous system (the second major stress regulatory system)
- Disgust (right frontal and anterior)
- While satisfaction correlates with left hemisphere activity anterior temporally
- Recognize connections (forest, not only trees)
- Visualization of perceptions
- Nonverbal understanding
- Mind Leaps
- In establishing the sense that objects such as family possessions and pets are personally familiar. Capturing and maintaining personal relevant entities such as “familiar” faces, people, voices, names, linguistic expressions, handwriting, topography.
Alexithymia sufferers are less likely to use comforting strategies than non-affected individuals, whereas PTSD sufferers use them more frequently.
- Affective reactions to proverbs and idioms, circumlocutions, emotional expressions, curses and slang
- More dominant in terms of activation of conditioned fear response via amygdala
- Right amygdala is more involved in the storage of fearful faces and in the expression of emotionally influenced memory of aversive experiences
- Stress responses and emotionality-related processes appear to be closely linked to the right hemisphere of the brain.
- Early abuse in the first 2 years of life often results in disorganized-insecure attachment, which manifests psychologically as an inability to develop a coherent strategy for coping with relationship stress. Early abuse negatively impacts the development of the right hemisphere of the brain responsible for attachment, affect regulation, and stress modulation, thus laying a template for the mind-body coping deficits that characterize PTSD symptomatology
- The right hemisphere of the brain is involved in the infant’s attachment to the mother and, later in life, to other people.
- Children with right hemispheric damage lose important social skills
In our opinion, this also applies to ADHD
- Adults with right hemispheric damage lose the associated sense of familiarity that is critical for bonding.
In our opinion, this is not typical of ADHD, but could represent a more common comorbidity
- Systems that promote right hemisphere activity appear to be resilience-promoting and protective against socially dysfunctional behavior, substance abuse, HPA axis failure, and some aspects of chronic disease pathophysiology.
Stress responses and emotionality-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,
Childhood abuse and other traumatic stressful experiences caused by unavoidable adverse physical, emotional, or social events are significant triggers in the pathophysiology of several psychiatric disorders including ADHD.
Early stress may condition developmental abnormalities in brain structures that play a critical role in mediating the response to stress, e.g.:
Anterior cingulate cortex
- And more
Monkeys that were socially deprived in early childhood, like children with right hemisphere damage, showed a deficit in recognizing the emotions of others. This manifested itself, among other things, in a deficit in the recognition of emotions in facial expressions.
Damage to the right hemisphere of the brain caused a loss of ability to comprehend, store, and process personally relevant and familiar elements of the environment, impairing connection to, response to, and interaction with others.
Stress decreases BDNF in the hippocampus, among others, which may influence depression and neurodegenerative processes.
1.2. The left cerebral hemisphere
The left cerebral hemisphere is more strongly associated with
- Verbal understanding
- Verbal expression
Rats showed large differences in dopamine turnover between the cerebral hemispheres immediately after birth, which decreased with age.
2. Brain hemispheres and cortisol
A reduced exchange between the brain hemispheres seems to be the cause (also in PTSD) for
- Reduced cortisol stress response
Further, a coincidence of reduced basal cortisol levels (despite normal cortisol response to ACTH) and alexithymia has been reported in some sufferers of chronic stress, from which the authors inferred ACTH-independent mechanisms that reduce cortisol levels. For example, the hippocampus is involved in inhibitory regulation of the HPA axis.
Monkeys with greater right hemispheric activation showed higher CAR (cortisol awakening response) and higher basal cortisol levels. In contrast, monkeys with greater left hemispheric activation showed decreased CAR and decreased basal cortisol levels, whereas monkeys with balanced right/left hemispheric activation showed intermediate CAR. CRH levels correlated with basal cortisol levels.
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.
Catecholamine asymmetries in the brain could thus have a concrete benefit. Disruption 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 nondominant hemisphere.
This may explain why in alexithymic states (despite arousal of the fight/flight response) corticoids do not remain elevated.
This corresponds with our impression that alexithymic states in ADHD are more likely to occur in ADHD-HI and ADHD-C than in ADHD-I and that we suspect a frequently flattened cortisol stress response in the former and a very frequently elevated cortisol stress response in the latter.
A reduction in the size of the corpus callosum correlates with decreased communication between the hemispheres.
In a study using auditory evoked potentials of adults abused in childhood, all of whom had neither acute mental health problems nor an acute Axis I diagnosis, they were asked to actively recall first a neutral or work-related memory and then a disturbing memory from childhood with impairment. In unaffected individuals, both hemispheres seemed equally involved in the memory. Adults with childhood trauma showed marked suppression of evoked potentials in the left hemisphere during recall of the neutral memory, suggesting increased processing in the left hemisphere. During recall of the distressing event, there was a robust shift in laterality in the 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.
In states of strong emotional or cognitive stress, even in neurologically intact people, events in the right hemisphere are functionally separated from the left hemisphere by inhibition of transmission between the brain hemispheres. This can trigger alexithymic states, such as reduced emotional empathy while cognitive empathy remains unchanged.
See also ⇒ Empathy in ADHD In the article ⇒ Complete list of ADHD symptoms according to manifestations in the chapter ⇒ Symptoms.
4. Early attachment disorder impairs self-organization of the right cerebral hemisphere
In infancy, the homeostatic structures between the “lower” autonomic and “higher” central brain systems develop in the right cerebral hemisphere, which serve to generate, regulate, and stabilize psychobiological states.
The right hemisphere is significantly more connected than the left to the limbic system and the mechanisms of autonomic and behavioral arousals. The maturation of the right cerebral hemisphere is experience-dependent.
Disorders of infant-mother bonding can therefore cause developmental disorders of the right cerebral hemisphere, affecting arousal modulation and regulation of the neurotransmitters dopamine and norepinephrine. Dopamine and norepinephrine are responsible for, among other things
Maturation processes of the brain.
ADHD is described as a developmental disorder of the brain in the sense of a developmental delay. Multiple studies confirm that early childhood stress experiences can cause such changes in behavior and neurotransmitters. ⇒ Stress damage due to early / prolonged stress
We do not see this as contradictory to the predominantly genetic cause of ADHD.
First, the dopaminergic and noradrenergic changes that affect brain development may be caused as much by genes as by corresponding environmental influences during developmental spurts of the affected brain regions.
Second, the assumed 75% heritability necessarily implies 25% other causes.
Third, environmental influences can mediate their effects through epigenetic changes, which in turn can be passed on for a few generations.
Fourth, gene-environment interactions are well known especially with respect to the important gene candidates for ADHD DRD4-7R, COMT and MAO-A. Early childhood attachment problems may represent one such environmental influence.