Laterality in Parkinson's disease: A neuropsychological review

Laterality of motor symptom onset in Parkinson's disease is both well-known and under-appreciated. Treatment of disorders that have asymmetric pathological features, such as stroke and epilepsy, demonstrate the importance of incorporating hemispheric lateralization and specialization into therapy and care planning. These practices could theoretically extend to Parkinson's disease, providing increased diagnostic accuracy and improved treatment outcomes. Additionally, while motor symptoms have generally received the majority of attention, non-motor features (e.g., autonomic dysfunction) also decrease quality of life and are influenced by asymmetrical neurodegeneration. Due to the laterality of cognitive and behavioral processes in the two brain hemispheres, analysis of hemibody side of onset can potentially give insight into expected symptom profile of the patient and allow for increased predictive accuracy of disease progression and outcome, thus opening the door to personalized and improved therapy in treating Parkinson's disease patients. This review discusses motor and non-motor symptoms (namely autonomic, sensory, emotional, and cognitive dysfunction) of Parkinson's disease in respect to hemispheric lateralization from a theoretical perspective in hopes of providing a framework for future research and personalized treatment.

Implicit Motives, Laterality, Sports Participation and Competition in Gymnasts

The implicit motivational needs for power, achievement, and affiliation are relevant for sports performance. Due to their hypothesized association with functions of the right hemisphere (McClelland, 1986), they may influence lateralized perceptual and motor processes. And due to their interactions with motive-specific incentives, they may influence performance conditional on the presence of suitable incentives. This preregistered study, conducted mostly online, examines motivational needs using a standard picture-story exercise (PSE) and their associations with indicators of perceptual and motor laterality and sports performance in gymnasts (N = 67). Further it explores how implicit motives interact with suitable motivational incentives in the prediction of sports performance. Results partly confirm a link between indicators of cerebral rightward laterality and implicit motives: the implicit affiliation and achievement motives are positively associated with an indicator of emotional-perceptional laterality (chimeric-faces task), but not with an indicator of motor laterality (turning bias). Moreover, the implicit achievement motive was positively correlated with training hours. The implicit affiliation motive was negatively associated with the highest attained competition level. The presence of achievement incentives (perceived control, failure) and affiliation incentives (training together or alone) did not interact with corresponding motives to predict sports performance.

Sheep Quickstep while the Floor Rock and Rolls: Visuomotor Lateralization during Simulated Sea Travel

Unpredictable floor motions during transport disturbs animals' balance, requiring stepping to move the centre of gravity in the direction of body movement. When repeated regularly, this may be stressful, requiring involvement of the right brain hemisphere, hence we investigated the existence of behavioral laterality in sheep during prolonged floor motions. Six sheep were restrained in pairs on a programmable rocking platform, in which they were unable to turn around. They were exposed to three continuous rocking motion treatments (roll, pitch or both) in a regular or irregular pattern for 1 h periods in a changeover design. Right forelimb and left hindlimb diagonal stepping was more frequent in response to the motion treatment of irregular roll and pitch, which previous research has suggested to be the most stressful from heart rate measurements. An overall strategy to maintain balance appeared to be the use of the right hindlimb as a stabilizer, which was repositioned least often of all limbs until towards the end of the hour of experimental treatment. Of each tested pair, sheep restrained on the left side of the rocking floor stepped significantly often than its partner restrained on the right side, and we postulate the existence of visuomotor lateralization as left restrained sheep were unable to view their partner within the field of view of their left eye. We also investigated which side sheep lie down on, which if left lateralized could explain our observed bipedal diagonal control of sheep balance under stress. From the observation of 412 web-based images of sheep, there was an overall left-sided laterality to their lying, as has been observed in cattle. We conclude that stepping activity in sheep in response to a motion stressor is lateralized, providing evidence that floor motion experienced in transport may induce stress responses.

The Alpha Hypothesis: Did Lateralized Cattle-Human Interactions Change the Script for Western Culture?

Domestic cattle possess lateralized cognitive processing of human handlers. This has been recently demonstrated in the preference for large groups of cattle to view a human closely within the predominantly left visual field. By contrast, the same stimulus viewed predominantly within the right visual field promotes a significantly greater frequency of dispersal from a standing position, including flight responses. The respective sets of behaviours correspond with the traditional terms of "near side" for the left side of cattle and horses, and the "off" or "far side" for the right side. These traditional terms of over 300 years usage in the literature communicate functional practicalities for handling livestock and the recognition of lateralized cognitive processing. In this review, the possibility of even earlier recognition and the significance of laterality in cattle-human interaction was argued, from the earliest representations of the letter "A", originally illustrated from nearly 4000 years before the present time as the head of an ox as viewed not from the front or from the right, but from the left (near) side. By extension, this knowledge of lateralization in cattle may represent the earliest written example of applied ethology-the study of the behaviour of animals under human management.

Association between heart rhythm and cortical sound processing

Sound signal processing signifies an important factor for human conscious communication and it may be assessed through cortical auditory evoked potentials (CAEP). Heart rate variability (HRV) provides information about heart rate autonomic regulation. We investigated the association between resting HRV and CAEP. We evaluated resting HRV in the time and frequency domain and the CAEP components. The subjects remained at rest for 10 minutes for HRV recording, then they performed the CAEP examinations through frequency and duration protocols in both ears. Linear regression indicated that the amplitude of the N2 wave of the CAEP in the left ear (not right ear) was significantly influenced by standard deviation of normal-to-normal RR-intervals (17.7%) and percentage of adjacent RR-intervals with a difference of duration greater than 50 milliseconds (25.3%) time domain HRV indices in the frequency protocol. In the duration protocol and in the left ear the latency of the P2 wave was significantly influenced by low (LF) (20.8%) and high frequency (HF) bands in normalized units (21%) and LF/HF ratio (22.4%) indices of HRV spectral analysis. The latency of the N2 wave was significantly influenced by LF (25.8%), HF (25.9%) and LF/HF (28.8%). In conclusion, we promote the supposition that resting heart rhythm is associated with thalamo-cortical, cortical-cortical and auditory cortex pathways involved with auditory processing in the right hemisphere.

Dominant hemisphere lateralization of cortical parasympathetic control as revealed by frontotemporal dementia

The brain continuously influences and perceives the physiological condition of the body. Related cortical representations have been proposed to shape emotional experience and guide behavior. Although previous studies have identified brain regions recruited during autonomic processing, neurological lesion studies have yet to delineate the regions critical for maintaining autonomic outflow. Even greater controversy surrounds hemispheric lateralization along the parasympathetic-sympathetic axis. The behavioral variant of frontotemporal dementia (bvFTD), featuring progressive and often asymmetric degeneration that includes the frontoinsular and cingulate cortices, provides a unique lesion model for elucidating brain structures that control autonomic tone. Here, we show that bvFTD is associated with reduced baseline cardiac vagal tone and that this reduction correlates with left-lateralized functional and structural frontoinsular and cingulate cortex deficits and with reduced agreeableness. Our results suggest that networked brain regions in the dominant hemisphere are critical for maintaining an adaptive level of baseline parasympathetic outflow.

The dynamic opponent relativity model: an integration and extension of capacity theory and existing theoretical perspectives on the neuropsychology of arousal and emotion

Arousal theory as discussed within the present paper refers to those mechanisms and neural systems involved in central nervous system activation and more specifically the systems involved in cortical activation. Historical progress in the evolution of arousal theory has led to a better understanding of the functional neural systems involved in arousal or activation processes and ultimately contributed much to our current theories of emotion. Despite evidence for the dynamic interplay between the left and right cerebral hemispheres, the concepts of cerebral balance and dynamic activation have been emphasized in the neuropsychological literature. A conceptual model is proposed herein that incorporates the unique contributions from multiple neuropsychological theories of arousal and emotion. It is argued that the cerebral hemispheres may play oppositional roles in emotion partially due to the differences in their functional specializations and in their persistence upon activation. In the presence of a threat or provocation, the right hemisphere may activate survival relevant responses partially derived from hemispheric specializations in arousal and emotional processing, including the mobilization of sympathetic drive to promote heightened blood pressure, heart rate, glucose mobilization and respiratory support necessary for the challenge. Oppositional processes and mechanisms are discussed, which may be relevant to the regulatory control over the survival response; however, the capacity of these systems is necessarily limited. A limited capacity mechanism is proposed, which is familiar within other physiological systems, including that providing for the prevention of muscular damage under exceptional demand. This capacity theory is proposed, wherein a link may be expected between exceptional stress within a neural system and damage to the neural system. These mechanisms are proposed to be relevant to emotion and emotional disorders. Discussion is provided on the possible role of currently applied therapeutic interventions for emotional disorders.

Individual Differences and Hemispheric Asymmetry While Processing Emotional Words

This research examined differences in the perception of emotion words as a function of individual differences in subclinical levels of depression and anxiety. Participants completed measures of depression and anxiety and performed a lexical decision task for words varying in affective valence (but equated for arousal) that were presented briefly to the right or left visual field. Participants with a lower level of depression demonstrated hemispheric asymmetry with a bias toward words presented to the left hemisphere, but participants with a higher level of depression displayed no hemispheric differences. Participants with a lower level of depression also demonstrated a bias toward positive words, a pattern that did not occur for participants with a higher level of depression. A similar pattern occurred for anxiety. Overall, this study demonstrates how variability in levels of depression and anxiety can influence the perception of emotion words, with patterns that are consistent with past research.

Cerebral asymmetry in the control of cardiovascular functioning: evidence from lateral vibrotactile stimulation

Research has supported hemispheric specialisation in the regulation of cardiovascular functioning, with the left hemisphere being associated with parasympathetic functioning and the right hemisphere with sympathetic functioning. We sought to investigate this relationship further using vibrotactile stimulation applied to the palms. Our prediction was that vibrotactile stimulation applied to the left hand would result in increased heart rate and blood pressure, and that stimulation applied to the right hand would result in decreased heart rate and blood pressure. The results indicated significant differences in heart rate change scores in the predicted direction. No differences were noted for systolic or diastolic blood pressure. Hence the findings provide partial support for the lateralisation of autonomic functions.

Magnitude of cerebral asymmetry at rest: covariation with baseline cardiovascular activity

The cerebral regulation of cardiovascular functioning varies along both a lateral and a longitudinal axis. The parasympathetic and sympathetic nervous systems are lateralized to the left and right cerebral hemispheres, respectively. Further, the frontal lobes are known to be inhibitory in nature, whereas the temporal lobes are excitatory. However, no systematic investigation has been conducted to determine the nature and strength of the relationship between the left and right frontal and temporal lobes in regulating cardiovascular activity. The present investigation sought to examine these relationships by testing the hypothesis that negative correlations would be found between baseline heart rate and blood pressure and asymmetry of alpha magnitude across the frontal lobes. Further, positive correlations were hypothesized to exist across the temporal lobes. A total of 20 women were asked to relax with their eyes closed while heart rate and blood pressure as well as quantitative electroencephalography data were obtained. The results indicated that, as hypothesized, significant negative correlations existed across the frontal lobes and significant positive correlations existed across the temporal lobes. The results provide further support for a division of responsibility between the left and right frontal and temporal lobes in the regulation of heart rate and blood pressure.

The covariation of cortical electrical activity and cardiovascular responding

Integrating findings from research concerning the cerebral and cardiovascular effects of increasing emotional intensity suggests that changes in functional cerebral system activation and changes in heart rate and blood pressure are related. Research has indicated that the left and right cerebral hemispheres are specialized for parasympathetic and sympathetic control of cardiovascular functioning. The present investigation sought to determine whether significant correlations existed between changes in the magnitude (microV) of alpha (8-13 Hz), low beta (13-21 Hz), and high beta (21-32 Hz) EEG activity at the frontal and temporal lobes and changes in heart rate as well as systolic and diastolic blood pressure. The results indicated that changes in the magnitude of each bandwidth analyzed were significantly correlated with changes in all three cardiovascular measures at several sites across the cerebral hemispheres, including the frontal and temporal lobes. The findings are discussed in terms of explaining the cardiovascular effects of increasing emotional intensity. Clinical implications of the findings are also discussed and suggestions for future research are provided.

Hemisphere asymmetry in sympathetic control of the human myocardium

Hemisphere asymmetry in sympathetic control of myocardial performance was studied in healthy human subjects using lateralized film presentation for selective sensory stimulation of the hemispheres and impedance cardiography for the evaluation of cardiac output, systolic time intervals and myocardial contractility. Results revealed a clear and consistent right hemisphere predominance in sympathetically mediated control of various components of myocardial performance. There is reason to assume that the obtained hemisphere differences in autonomic control of the heart are self-reliant processes not depending on emotion-related hemisphere asymmetry. As far as we know, this is the first study examining the distinct roles of the cerebral hemispheres in neural control of ventricular myocardial functions.