Individual Differences and Differences in the Functioning of Brain

It is irrefutable that people differ considerably from one another, with this variability arising from a unique combination of hereditary, biological factors and lived environmental experiences. Comprising persistent individual factors differentiating individuals from one another to establish distinctiveness, individual differences (namely those of intelligence and personality) are correlated with inherent differences observed in the cerebral cortex - the outer layer of grey matter covering the cerebral hemispheres - providing invaluable insights into explanations for behavioural variability between people. This essay aims to evaluate empirical evidence suggesting apparent relationships between observed individual differences.

Constituting characterological behaviour patterns or thinking distinguishing one person from another, personality differences have long been associated as being underpinned by neurobiological mechanisms, a theory of which, concording with the preceding ‘Personality Theory’, is the ‘Arousal Theory’, devised by Eysenck. As one of the foremost proponents of the neurobiological predisposition to developing personality traits, Eysenck posited that variability in the reticulothalamic-cortical arousal system contributes to extraversion-introversion dimension differences, with high scores on extraversion scales correlating with deficient functioning of the Ascending Reticular Activating System (ARAS) - a system associated with balancing excitatory and inhibitory mechanisms in cortical arousal - that underreacts to neural inputs. 

Resultantly, it is theorised that individuals exhibiting greater dispositional extraversion experience diminished internal basal cortical arousal, therefore these perpetually under-aroused individuals require significantly elevated stimulation in order to achieve adequate cortical arousal, propelling them to seek arousal externally from behaving impulsively and engaging with high sociability. Conversely, those scoring highly on the introvert personality dimension are theorised to possess an overactive ARAS. 

With an elevated arousal potential, subjecting them to high, chronic levels of cortical arousal, introverts engage in frequent asociality and an evasion of external arousal which would evoke painful levels of overarousal, reducing arousal levels and sensory stimulation to make them feel calmer. This hypothesised association between cortical arousability and extraversion was evidenced by Hagemann et al. which found a positive relationship between extraversion scores and EEG alpha activity, inversely indicative of cortical arousal, therefore concluding that more extraverted individuals experience reduced cortical arousal in the brain comparative to introverts. 

As such, this suggests that differential arousal in the cerebral cortex may evoke individual differences in personality. However, this finding was contradicted by a study by Korjus et al. (2015), which investigated whether such EEG resting state data is a predictor of personality traits, including introversion and extraversion, and subsequently found no support for this. This is significant as it suggests that this methodology is unreliable, therefore future studies investigating this relationship should utilise different measures of cortical arousal to draw reliable conclusions as to the nature of this linkage.

Moreover, evidence has been elucidated to indicate apparent correlations between cortical deficits and dysfunctional patterns of conduct, markedly those favouring and predisposing individuals to violence and inhibited behavioural control. Raine et al. investigated whether there were innate differences in the functioning of brain regions that may perpetuate violent predispositions and personalities. 

Using PET scans on a sample of murderers pleading not guilty by reason of insanity (NGRI) and controls, Raine et al. found that the inherent brain differences: brains of murderers exhibited reduced activity in many areas, namely the prefrontal cortex and left side of the amygdala, Resultantly, given prefrontal deficits are associated with elevated impulsivity and lack of behavioural inhibition, whilst reduced amygdala activity evokes unusual emotional regulation and reduced fear, this study provided preliminary evidence supporting a neurological theory of violence, indicating that deficits in cortical brain processes within the cerebral cortex may predispose individuals to violence, though the ‘snapshot’ nature of the study resulted in an inability to determine the causal direction of brain dysfunction: namely whether these defects were congenital or developed over life,

Furthermore, as defined by Gottfredson, intelligence is a general cognitive ability (first described as ‘g’ by Spearman, 1904) necessitating an ability to reason, adaptively learn from experience, and comprehend concepts and affiliated relations, reflecting individual differences in brain processes. Following the inception of research investigating individual differences in neurobiological substrates underlying individual differences in intelligence, the evolutionary perspective dictated that intelligence was a clear function of brain volume, with greater intelligence being conferred through anatomically larger brains. 

This evolution, alongside high densities of neurons (specifically 16.3 billion) present in the cerebral cortex, were considered to be integral to imparting the enhanced ability of the Homo Sapien brain to adapt and understand the surrounding environment, observed through the evolvement of incrementally sophisticated tools. Contrary to this widely-held belief, many studies have emerged to critique these evolutionary theories of brain size evolution and neuron density, the most renowned being that of Einstein - a denoted ‘genius’ who had an IQ of 160. 

Diamond et al  found that Einstein possessed a higher proportion of glial cells - those integral to maintaining healthy cellular environments - relative to neuron cells compared to the average person, suggesting an increased need for metabolism to sustain creative thinking, contradicting previous evidence that high neuron density conferred higher intelligence.

Moreover, morphological differences were identified in the brain of Einstein, suggesting that his lack of parietal operculum enabled the relative expansion of the parietal lobe - the region of the cerebral cortex associated with memory, as well as processing language and mathematics. This is significant as it suggests that these observed anatomical and physiological neural modifications support the ‘neural efficiency hypothesis’ proposed by Raven et al.: that intelligence is determined by the efficacy at which the brain can process information, and not by how hard it works, inferring that more intelligent individuals can more efficiently use their cortex than their less intelligent counterparts and reinforcing cortical differences as neurobiological underpinnings of intellectual differences.