European Journal of Molecular & Clinical Medicine ISSN 2515-8260 Volume 07, Issue 10 , 2020 Human brain laterality markers and IQ:A comparative analysis

1. Vishram Singh, 2. Ruchira Sethi, 3. Rahul Rai*, 4. Yogesh Yadav

1. Professor, Department of Anatomy, Santosh Medical College and Hospital, Ghaziabad 2. Professor, Department of Anatomy, Heritage Institute of Medical Sciences, Varanasi 3. PhD Scholar, Department of Anatomy, Santosh Medical College and Hospital, Ghaziabad 4. Professor, Department of Anatomy, Santosh Medical College and Hospital, Ghaziabad Corresponding Author* Rahul Rai Department of Anatomy Santosh Medical College and Hospital, Ghaziabad Email: [email protected]

Abstract Introduction: Laterality is the preferences of individuals for use of symmetrical body parts. The intelligence of a person is associated with brain lateralization. All the markers for brain lateralization may have a correlation to the IQ level of an individual. Thus, the present study is an effort to define the correlation between IQ and hand-foot-eye preference of an individual, also included with study is the pattern of hair whorl. Materials and methods: With total of 600 participants, Edinburg Handedness Inventory, Chapman foot inventory and Miles test were used to assess handedness, footedness and eyedness respectively. Intelligence was assessed by Raven's Colored Matrices test for age group 3-11 years and Raven's Standard Progressive Matrices test for age group 12-20 years. Results: The Raven's Matrices score was significantly high in case of left handed, left footed and left eyed individuals compared to their right handed counterparts. Conclusion: In this study left handed, left footed and left eyed individuals were found to have higher IQ levels.

Key words: Laterality, IQ, Handedness, Preference

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Introduction

Human brain is divided into two hemispheres which are very similar anatomically but differ in their specialized functions. The left hemisphere controls the right while the right hemisphere controls the left side of the body. Each hemisphere performs very precise and complex activities such as speech, cognitive thinking, writing, depending upon their own coordination and combined effort. For this co-operative work to happen there is necessity of structured and automatized bilateral integrity which had given rise to the concept of brain lateralization that is termed as Laterality [1].

Laterality can be defined as the individual's preference on the use of symmetrical body parts, such as hand, foot, eye and ear [2]. For instance, if an individual prefers to use his/her right hand, the brain hemisphere that is more dominant in such individual is the left hemisphere while if one chooses to use left hand then, the brain hemisphere that is more dominant would be right hemisphere.

Handedness is the most commonly studied marker of brain laterality and the association between handedness and the cognitive abilities of an individual have been shown in previous studies. Studies have shown left handed individuals to be more creative and with higher cognitive abilities compared to right handed individuals, thereby indicating more specialized right hemisphere in left handers [3,4].

Despite of having higher intellectual abilities, left handed individuals are overlooked in our society due to persisting cultural and social norms like for performing any religious act, eating, social greeting and interactions. The use of left hand in such activities is considered to be rude, undisciplined and unfavorable. Unfortunately, use of left hand has been restricted to the maintenance of personal hygiene [5].

Due to such traditions and false beliefs, people in our community consider use of left hand to be unclean and untraditional. In addition to this misery, the technologies that are flourishing more rapidly in this 21st century are also for right side dominant. As for example, designing of machineries, cars, gadgets etc have been made keeping focus on the right handers. When the left handed individuals are forced to change their handedness to right, it affects the intellectual development there by creating physiological, educational and social burden during interim development of children [6].

For every dominant hand there is a dominant foot which is also an important marker of cerebral laterality. However, not much literature is available correlating footedness with cognitive abilities. Footednees can serve as better indictors compare to handedness in terms of brain lateralization as it is less affected by social and cultural factors. Of course there exists variation in social belief with regards to use of right and left foot too. For instances, it is considered good sign to enter holy place, auspicious ceremony or a new house with the right foot first [7].

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Another important marker of laterality is eyedness. Many researchers have suggested eyedness to be more fundamental measure of cerebral laterality as it is not influenced by cultural pressure. Also the proper coordination between eyedness and handedness is important for intellectual development as in case of reading and speech. Like eyedness, hair whorl direction can also serve as important marker of lateralization and is not influenced by social or cultural beliefs [8,9].

Till date there exists a debate on whether left brain dominant individuals are more intelligent or vice versa. Most of the previous studies have focused only on handedness as the markers of laterality and IQ, while use of foot, eye and hair whorl direction have been neglected. Also such types of studies have been conducted rarely in India. Therefore, in this study we used foot, eye and hair whorl in addition to hand as brain laterality markers and targeted to find out level of intelligence in both right and left lateralized individuals.

Materials and Methods

In this study, 600 participants were selected from different schools and colleges of Jaunpur and Varanasi districts. Equal number of right and left handed participants were selected randomly.

Inclusion criteria

• Volunteers of age group between 3-20 years • Volunteers who consented to participate

Exclusion criteria

• Participants with any gross deformity, anomaly or mental retardation • Participants not willing to give consent

Prior to study ethical approval was obtained from Institutional ethical committee and consent was obtained from either the participants or their parents in case of minors. Each was informed about the study and the procedures to be used.

For the assessment of handedness, Edinburg Handedness Inventory was used [10] while for assessment of footedness Chapman foot preference inventory [11] was used. Miles test was used to assess the dominant eye [12]. To assess the individual level of intelligence, Raven's Colored Progressive Matrices Test (RCPMT) for the age group between 3-11 years and Raven's Standard Progressive Matrices Test (RSPMT) for age group between 12-20 years were used.

RCPMT have 36 questions divided into 3 sets, each with 12 questions. RSPMT have 60 questions divided into 5 sets, each having 12 questions [13]. The participants were briefed about the test and instructed to choose correct option. After completion, the time taken by the participants was noted and a score was generated (vide infra) which was later analyzed in terms of hand, foot, eye preference and hair whorls. The score was used to categorize subjects into following categories:

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1. Intellectually superior (score >95th percentile) 2. Above average intellectually ability (score ≥ 75th percentile)) 3. Average intellectual ability (score between-75 th25th-75th percentile) percentile) 4. Below average (score ≤ 25th percentile) 5. Intellectually impaired (score ≤ 5th percentile) percentile) Results- The distribution of subjects with reference to handedness, footedness, eyedness and hair whorl pattern (HWc=clockwise and HWa=anticlockwise) is shown in figure 1.

Figure 1: Distribution of participants based on markers of laterality

80 70 67.2 60 53.3 55.8 50 50 50 46.7 44.2

40 32.8 30

Percentage (%) Percentage 20 10 0 RH LH RF LF RE LE HWc HWa

Laterality markers Figure 2: Distribution of participants based on Gender

80 Male 69.6 70 Female 64.4 60 56.2 55.8 55.9 52.3 50 51 49 48 47.7 50 43.8 44.2 44.1 40 35.6 30.4 30 20 10 0 RH LH RF LF RE LE HWc HWa Laterality Markers The gender distribution of study subjects with respect to laterality markers is shown in figure 2.

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Table- 1: Comparison of intelligence based on RSPMT score and laterality markers

Laterality Score markers Side Mean±sd p Right 32.53±15.35 Handedness 0.029* (S) Left 35.28±15.58 Right 31.97±15.31 Footedness 0.001* (S) Left 36.12±15.48 Right 32.26±15.1 Eyedness 0.003* (S) Left 35.98±15.6 Clockwise 33.16±16.3 0.422 Hair whorl Anti clockwise 34.27±15.14 (NS) S= significant, NS= Non significant

The RSPMT scores of left handed, left footed and left eyed individuals were found to be significantly (p<0.05 at 95% of CI) higher than that of their right handed counterparts. In terms of hair whorl direction the scores were not different significantly (table 1)

Table- 2: Comparison of intelligence based on RSPMT score, age and laterality markers

Laterality RSPMT score (mean±sd) marker Side Age 3-7 yrs Age 8-11 yrs Age 12-15 yrs Age 16-20 yrs Right 12.8±5.26 25.68±5.04 43.18±5.51 48.78±6.16 Handedness Left 15.26±5.57 27.13±5.96 46.01±4.99 52.25±5.42 P 0.006* (S) 0.1(NS) 0.002* (S) <0.001* (S) Right 13.48±5.42 25.44±5.15 44.04±5.55 48.68±6.05 Footedness Left 14.83±5.65 27.42±5.8 45.31±5.15 52.28±5.52 P 0.19 (NS) 0.028* (S) 0.073 (NS) <0.001* (S) Right 13.65±5.73 25.48±5.63 43.97±5.04 49±6.3 Eyedness Left 15.0±5.23 27.21±5.57 45.78±5.81 52.12±5.32 P 0.379 (NS) 0.063 (NS) 0.184 (NS) <0.001* (S) Clockwise 14.15±5.68 26.62±5.64 44.05±5.17 50.35±6.27 Hairwhorl Anti clockwise 14.80±5.35 25.34±5.63 45.6±5.73 50.86±5.49 P 0.621 (NS) 0.208 (NS) 0.089 (NS) 0.61 (NS) In table 2, the intelligence of participants is compared based on the RSPMT score and age. In the age group of 3-7 years, left handed children had significantly high score compared to right handed children. However, the score did not differ significantly in terms of footedness, eyedness and hair whorl direction. In case of age group of 8-11 years, the scores were significantly high in left footed children compared to right footed children while for handedness, eyedness and hair whorl direction, the mean difference of score was insignificant. Similarly, for the age group of 12-15 years, left handed participants had significantly high score compared to right handed

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participants but the mean score did not differ significantly in terms of footedness, eyedness and hair whorl direction. The scores were significantly high for left handed, left footed and left eyed individuals in the age group of 16-20 years while for hair whorl, the difference was insignificant. The RSPMT score for IQ was compared between genders for each markers of laterality (table 3).

Table -3: Comparison of intelligence based of RSPMT score, gender and laterality markers

Laterality RSPMT Score (mean + sd) marker Side Male Female p Handedness Right 32.53±15.07 32.43±15.7 1.19 (NS) Left 34.21±15.89 36.68±15.10 0.33 (NS) Footedness Right 31.43±15.31 32.52±15.34 1.04 (NS) Left 35.44±15.48 36.98±15.49 0.87 (NS) Eyedness Right 32.19±15.5 32.33±15.08 0.93 (NS) Left 34.93±15.41 37.18±15.74 0.24 (NS) Clockwise 31.81±16.32 34.47±16.19 0.25 (NS) Hairwhorl Anti clockwise 34.10±15.11 34.48±15.21 0.8 (NS)

NS= Non significant

Discussion

Intelligence in the simple term may be defined as the ability of an individual to respond to the problems for the survivability in their social and natural environment. Individual difference in the intelligence can be assessed considering preferential use of hand, foot and eye; and hair whorl direction which are also the markers of human brain lateralization. In this study, left handed, left footed and left eyed individuals were found to have high RSPMT score compared to their right dominant counter parts. This indicated that, individuals with right brain dominance are more intelligent. However, significant difference in RSPMT score based on hair whorl direction was not obtained.

In the right hemisphere of the brain, blood flow, glucose utilization and brain wave activities increase during performance of perpetual tasks. The results of the study by Ghayas S et al showed superiority of left hander over right handers in terms of cognitive abilities as observed in present study [14]. Similarly, Nicholls et al in their study involving 825 participants concluded right side asymmetry to be associated with higher cognitive abilities [15]. However, Somers et al did not show any significant difference between right and left handed individuals in terms of overall IQ [16].

In this study 50% males and 47.7% of females were left handed. Likewise 49%, 44.2% of males and 43.8%, 44.1% females were left footed and left eyed respectively. In study of Syuichi O et al 13.9-14.7% of males were left handed while 13.9-14.7 % of females were right handed. A study

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Past evidences show that babies can reliably exhibit right hand preference by the age of seven months during object manipulation [19] or at reach to eat behaviors by age of 11 months [20]. Fairly accurate identification of dominant hand can be done at the age of 3 [21]. A crucial point to note is that motor skills are associated with hand preference. Functional asymmetries are more evident in right handers during the early phase of learning [22]. Children of preschool ages and especially left handers, show weak hand preference and during the transition period to older ages like 10 years or more, they undergo a stage of refinement of their motor skills with the increase in their reliance on the hand preferred [23].

In terms of gender variations in cognitive abilities, a significant difference in RSPMT score was not obtained. However, Denny et al reported that left handed males are more intelligent than left handed females [24].

The hormonal theory states that hormones play a vital role in determining brain lateralization. According to this theory males prefer to use left hand more than the females. It is also inferred that the function of right parts of brain like spatial skill and visual skills are more superior in males thus making males to excel in areas like music. Technical skills required for jobs, engineering works etc is more effective in males while females were superior in linguistic and normal skill [25].

Males also have greater right lateralization in superior temporal, inferior frontal and on inferior occipital cortices in terms of brain connectivity. In case of females, they exhibit greater left lateralization in inferior fontal cortex [26].

Conclusion

This study shows that individuals with dominant right cerebral hemisphere are more intelligent than those with dominant left cerebral hemisphere. Thus the result of this study may be useful in educating parents and society to abolish existing false belief regarding use of left side of body especially left hand and left foot. This study would also be helpful in changing negative attributions to left handed or left footed individuals so that the intellectual development of a left lateralized child is not suffered in this right dominant world.

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