Human Asymmetry

Cross references: Amphioxus Asymmetry  Lamprey Asymmetry Music

Lateralization of brain function (Wiki) 
    "The longitudinal fissure separates the human brain into two distinct cerebral hemispheres, connected by the corpus callosum. The hemispheres exhibit strong, but not complete, bilateral symmetry in both structure and function. For example, structurally, the lateral sulcus generally is longer in the left hemisphere than in the right hemisphere, and functionally, Broca's area and Wernicke's area are located in the left cerebral hemisphere for about 95% of right-handers, but about 70% of left-handers.[1]"

Brain function lateralization is evident in the phenomena of right- or left-handedness and of right or left ear preference[citation needed], but a person's preferred hand is not a clear indication of the location of brain function. Although 95% of right-handed people have left-hemisphere dominance for language, 18.8% of left-handed people have right-hemisphere dominance for language function. Additionally, 19.8% of the left-handed have bilateral language functions.  Even within various language functions (e.g., semantics, syntax, prosody), degree (and even hemisphere) of dominance may differ.[2] "  


A left-ear advantage for identifying the emotional quality of tonal sequences (PubMed)  
Only abstract available online. 
Tonal sequences differing in emotional quality were presented dichotically. Subjects listened to a specified ear and judged the emotional tone of the stimulus heard at that ear.  
    Accuracy was better for identifying the emotional tone of stimuli presented to the left ear. This left ear advantage was greatest where the target and competing stimuli were of different affect.  
   The findings provide further evidence for the role of the right hemisphere in processing emotional information.

Hemisphere memory of concrete and abstract information determined with the intracarotid Sodium Amytal test. - PubMed   
The specific data pattern obtained, that abstract pictorial information is most efficiently processed by an intact right hemisphere and that verbal information is processed best by an intact left hemisphere, demonstrates the reliability of the approach taken here to present abstract and concrete to be remembered information."  
    196 Related citations

The right cerebral hemisphere: emotion, music, visual-spatial skills, body-image, dreams, and awareness (PubMed)  
Only abstract available online. 
Based on a review of numerous studies conducted on normal, neurosurgical and brain-injured individuals, the right cerebral hemisphere appears to be dominant in the perception and identification of environmental and nonverbal sounds; the analysis of geometric and visual space (e.g., depth perception, visual closure); somesthesis, stereognosis, the maintenance of the body image; the production of dreams during REM sleep; the perception of most aspects of musical stimuli; and the comprehension and expression of prosodic, melodic, visual, facial, and verbal emotion.  
    When the right hemisphere is damaged a variety of cognitive abnormalities may result, including hemi-inattention and neglect, prosopagnosia, constructional apraxia, visual-perceptual disturbances, and agnosia for environmental, musical, and emotional sounds. Similarly, a myriad of affective abnormalities may occur, including indifference, depression, hysteria, gross social-emotional disinhibition, florid manic excitement, childishness, euphoria, impulsivity, and abnormal sexual behavior. Patients may become delusional, engage in the production of bizzare confabulations and experience a host of somatic disturbances such as pain and body-perceptual distortions.  
    Based on studies of normal and "split-brain" functioning, it also appears that the right hemisphere maintains a highly developed social-emotional mental system and can independently perceive, recall and act on certain memories and experiences without the aid or active reflective participation of the left. This leads to situations in which the right and left halves of the brain sometime act in an uncooperative fashion, which gives rise to inter-manual and intra-psychic conflicts.

Anger and frontal brain activity: EEG asymmetry consistent with approach motivation despite negative affective valence.     
Full length PDF available online for free.  Must download to copy-and-paste.
from the abstract 
    "The anterior regions of the left and right cerebral hemispheres have been posited to be specialized for expression and experience of approach and withdrawal processes, respectively.   Much of the evidence supporting this hypothesis has been obtained by use of the anterior asymmetry in electroencephalographic alpha activity.
    In the present research, we tested the hypothesis that dispositional anger,  an approach related motivational tendency with negative valence, would be associated with greater left- than right-anterior activity.  
    Results supported the hypothesis, suggesting that the anterior asymmetry varies as a function of motivational direction rather than affective valence"     

Asymmetric frontal brain activity, cortisol, and behavior associated with fearful temperament in rhesus monkeys  
Full length PDF available online for free.  Must download to copy-and-paste. 
from the abstract 
    "The authors examined the hypothesis that rhesus monkeys with extreme right frontal electroencephalographic activity would have higher cortisol levels and would be more fearful compared with monkeys with extreme left frontal activity.
    The authors first showed that individual differences in asymmetric frontal electrical activity are a stable characteristic.  
    Next, the authors demonstrated that relative right asymmetric  frontal activity and cortisol levels are correlated in animals 1 year of age. Additionally, extreme right frontal animals had elevated cortisol concentrations and more intense defensive responses.  
    At 3 years of age, extreme right frontal animals continued to have elevated cortisol concentrations."  

Right hemisphere control of visuospatial attention: line-bisection judgments evaluated with high-density electrical mapping and source analysis   
The "line-bisection" task has proven an especially useful clinical tool for assessment of spatial neglect syndrome in neurological patients."   

Clarifying the emotive functions of asymmetrical  frontal cortical activity.
Full length PDF available online for free.  Must download to copy-and-paste.
from the abstract 
    "Asymmetrical activity over the frontal cortex has been implicated in the experience and expression of emotions and motivations. Explanations of the research have suggested that relatively greater left frontal activity is associated with positive affect and/or approach motivation, and that relatively greater right frontal activity is associated with negative affect and/or withdrawal motivation.  
    In past research, affective valence and motivational direction were confounded, as only positive (negative) affects that were associated with approach (withdrawal) motivation were examined. Consequently, this research is unable to address whether asymmetrical frontal activity is associated with affective valence, motivational direction, or some combination of valence and motivation.  
    In this article, I review research on the emotion of anger, a negative emotion often associated with approach motivation, that suggests that asymmetrical frontal cortical activity  is due to motivational direction and not affective valence.  Methodological and theoretical implications for the study of the frontal asymmetry specifically, and for emotion and motivation more generally, are discussed."    

Contributions from research on anger and cognitive dissonance to understanding the motivational functions of asymmetrical frontal brain activity.  
Full length PDF available online for free.  Must download to copy-and-paste. 
from the abstract 
    "Research has suggested that approach-related positive emotions are associated with greater left frontal brain activity and that withdrawal-related negative emotions are associated with greater right frontal brain activity. 
    Different explanations have been proposed. One posits that frontal asymmetry is due to emotional valence (positivity/negativity), one posits that frontal asymmetry is due to motivational direction (approach/withdrawal), and one posits that frontal asymmetry is due to a combination of emotional valence and motivational direction (positive-approach/negative-withdrawal)." 
    "Solely supporting the motivational direction model, recent research has revealed that anger and cognitive dissonance, emotions with negative valence and approach motivational tendencies, are related to relatively greater left frontal activity."  

2005    236<1217 
Different pontine projections to the two sides of the cerebellum.   
This study analyzed the projections of the basilar pontine nuclei (BPN) and of the nucleus reticularis tegmenti pontis (NRTP) to the two sides of the cerebellum in the rat. It showed that the two sides of the cerebellar cortex were innervated by different percentages of BPN (about 82% of the cells project to the contralateral cortex and 18% to the ipsilateral) and NRTP cells (some 60% project to the contralateral cortex and 40% to the ipsilateral). In comparison to projections traced to the cortex, only a few fibers were traced to the nuclei of the same animals.  
    Most of the projections of the BPN to the cerebellar nuclei were traced to the lateralis and posterior interpositus nucleus of the contralateral side (95%), while a few were traced to homologous nuclei of the ipsilateral side (5%). Thus, the BPN principally control the activity of the contralateral cerebellum, with a much less important control over the activity of the ipsilateral cerebellum.  
     Vice versa, the NRTP, which project to the lateralis, interpositus, and medialis nuclei of the two sides, with percentages (64% contra- and 36% ipsilateral) similar to those reported for the projections to the cortex, is more concerned in the bilateral control of the cerebellum, although with a moderate contralateral prevalence.  
    The fact that projections of the BPN were principally traced to the contralateral nuclei, from which the efferent projection fibers from the cerebellum originate, suggests that the BPN are principally involved in the motor control of the contralateral body. Conversely, the bilateral projections of the NRTP to the cerebellar nuclei suggest that the NRTP is mainly involved in bilateral motor activities. The comparison of the projections to the cortex and nuclei of the cerebellum of single animals supports the co-existence of coupled (i.e., projections to the cortex and the corresponding nuclei) and uncoupled (i.e., projections to the cortex but not to the nuclei) projection patterns, from both the BPN and the NRTP. These features of the pontocerebellar projections open new vistas on the functional architecture of this pathway."  

    My comments
1.  "
Most of the projections of the BPN to the cerebellar nuclei were traced to ... the contralateral side (95%)"  
2.  "
Vice versa, the NRTP, which project to ... the two sides, with percentages (64% contra- and 36% ipsilateral)"  
3.  Rats, not humans, but probably similar. 
4.  See:   2005    237<1217 
    Postural control in man: the phylogenetic perspective.  

Role of NMDA receptors in the lateralized potentiation of amygdala afferent and efferent neural transmission produced by predator stress.  

Finally, consistent with cat and human studies, the right hemisphere appears particularly important in long term response to stress."  

Complementary and lateralized forms of processing in Bufo marinus for novel and familiar prey.
    Also:  Locomotion Sequence   
from the Abstract:   
We report that specialized processes associated with the left eye (right hemisphere) of the Bufo marinus toad carry out decisions to view and strike at complex prey stimuli recognized as 'novel.'' This was demonstrated in the toads' preferences when provided a choice between identical novel insect models presented simultaneously into the left and right lateral, monocular visual fields. In a second experiment, videotaped trials of toad groups competing in an open field for live crickets were analysed for lateralized prey-catching behaviour. Concomitant with a preference for directing agonistic strikes at conspecifics within the left visual hemifield, toads were found to possess a significant preference for directing predatory responses at the familiar prey viewed in the right visual hemifield."      

2006    208    from:  Cerebellar Efferent Pathways      
Motor learning in man: a review of functional and clinical studies   
This chapter reviews results of clinical and functional imaging studies which investigated the time-course of cortical and subcortical activation during the acquisition of motor a skill.  
    During the early phases of learning by trial and error, activation in prefrontal areas, especially in the dorsolateral prefrontal cortex, is has been reported. The role of these areas is presumably related to explicit working memory and the establishment of a novel association between visual cues and motor commands.  
    Furthermore, motor associated areas of the right hemisphere and distributed cerebellar areas reveal strong activation during the early motor learning. Activation in superior-posterior parietal cortex presumably arises from visuospatial processes, while sensory feedback is coded in the anterior-inferior parietal cortex and the neocerebellar structures.  
    With practice, motor associated areas of the left-hemisphere reveal increased activity. This shift to the left hemisphere has been observed regardless of the hand used during training, indicating a left-hemispheric dominance in the storage of visuomotor skills.  
     Concerning frontal areas, learned actions of sequential character are represented in the caudal part of the supplementary motor area (SMA proper), whereas the lateral premotor cortex appears to be responsible for the coding of the association between visuo-spatial information and motor commands. Functional imaging studies which investigated the activation patterns of motor learning under implicit conditions identified for the first, a motor circuit which includes lateral premotor cortex and SMA proper of the left hemisphere and primary motor cortex, for the second, a cognitive loop which consists of basal ganglia structures of the right hemisphere.  
    Finally, activity patterns of intermanual transfer are discussed. After right-handed training, activity in motor associated areas maintains during performance of the mirror version, but is increased during the performance of the original-oriented version with the left hand. In contrary, increased activity during the mirror reversed action, but not during the original-oriented performance of the untrained right hand is observed after left-handed training.  
    These results indicate the transfer of acquired right-handed information which reflects the mirror symmetry of the body, whereas spatial information is mainly transferred after left-handed training. Taken together, a combined approach of clinical lesion studies and functional imaging is a promising tool for identifying the cerebral regions involved in the process of motor learning and provides insight into the mechanisms underlying the generalisation of actions."  

Human The Random-handed, Testosterone, Estrogen and Human Evolution (Goog) 

2014    142<354 
Aversive learning in adolescents: modulation by amygdala-prefrontal and amygdala-hippocampal connectivity and neuroticism.  
higher levels of neuroticism were associated with a stronger interaction between the right amygdala and the right hippocampus as well as the right amygdala and prefrontal cortical regions"  

2014    47<1217 
Handedness and effective connectivity of the motor system.   
Free PMC Article   
In conclusion, the data show that handedness is associated with differences in effective connectivity within the human motor network with a prominent role of SMA in right-handers. Left-handers featured less asymmetry in effective connectivity implying different hemispheric mechanisms underlying hand motor control compared to right-handers."