Psy 341 Neuropsych: Exam 2: Ch 12 Individual Differences in Cerebral Organization

Handedness and Functional Asymmetry

• Hand preference appears by about 10 weeks of fetal development

• Most people are right-handed

• 90% of the population are right-handed in terms of writing

• 70% to 90% are right-handed when judged more broadly

Anatomical Studies

• Few studies have identified reproducible differences in the brains of left-handed versus right-handed individuals

• One study looked at the difference in the angles of the left and right lateral fissure, also called the Sylvian fissure, and found that the difference was greater in individuals with language on the left side of the brain

• In people who are right-handed, the pyramidal tract descending to the right hand contains more fibers than the tract to the left hand

• Individuals who are left-handed show less lateralized hand dominance

Cerebral Organization in Left-Handers

• Cognitive functions may be more bilateral in left-handers, but the difference is small

• Left-handed individuals have left-hemispheric specialization for language in 70% of cases, right-hemispheric specialization in 15% of cases, and bilateral language in 15% of cases

• Right-handed individuals have left-hemispheric specialization for language in more than 95% of cases

• Individuals with a family history of left-handedness may show a different pattern of cerebral organization compared with left-handed individuals with no family history

Environmental Theories of Hand Preference

• Many theories exist, but it is difficult to evaluate whether they might cause hand preferences or just be random correlations

• Mothers are more likely to hold an infant in their left hand to be soothed by the mother's heart, leaving the mother's right hand free to do other things

• For centuries, children were taught to write with their right hands, but modern educational practices allow writing with either hand, and the prevalence of left-handedness has not changed

• There may be some genetic bias toward being right-handed, suggesting that being left-handed is the result of a genetic anomaly or stressful in-utero development

Anatomical Theories of Hand Preference

• The left hemisphere generally shows enhanced maturation and greater development, which may influence handedness

• A modified version of this theory points out that the heart is on the left side of the chest, the temporal lobe is larger on the left, and the areas that influence birdsong are found in the left side of the brain

• Left-hemisphere dominance for language and motor control may just be another manifestation of this trend

Hormonal Theories of Hand Preference

• Exposure to different levels of testosterone early in life could impact cerebral organization and asymmetry

•Testosterone is generally inhibitory on development

• Geschwind and Galaburda proposed that testosterone acts on the left hemisphere, leading to greater development of the right hemisphere and left-handedness in some individuals

• After much research, there does not seem to be much support for the theory

Genetic Theories of Hand Preference

• There could be a dominant gene for right-handedness

• A more widely researched variant suggests the dominant gene encodes left-hemisphere speech

• The genes that favor left-hemisphere speech also favor an advantage in motor control of the right hand

• The dominant form of the gene favors left-hemisphere specialization; the recessive form confers no preference

• People with two copies of the recessive gene divide evenly between left- and right-hemisphere specialization

• This model suggests 12.5% of the population would be left-handed, close to the population average of 10%

• However, no identified genes are associated with such lateralization

Sex Differences in Cerebral Organization

• Researchers have characterized cognitive differences between males and females

• Such differences may be due to differences in brain organization between men and women

• In general, men tend to be better at spatial analysis and women tend to be better at using language

• There is significant variation within each sex, and the variation within each sex is actually greater than the average variation between the sexes

Sex Differences in Children's Behavior

• Sex differences can be observed in the play choices of children, which influence brain development

• Boys prefer toy vehicles and more physical play

• Girls prefer dolls

• Children prefer to play with others of their same gender, reinforcing their play behaviors

• Toy and play preferences may be influenced by socialization, but developmental studies show that early testosterone exposure influences toy choices and play behaviors

• Monkeys show similar differences in toy choice based on sex

Sex Differences in Adult Behavior

• There are differences between male and female performance, but the question is how significant they are

• Effect size quantifies how big the difference is between the means of two groups

• For most sex differences, the effect size is on the order of 0.5, suggesting significant overlap between males and females

Sex Differences: Motor Skills

• One of the largest effect sizes was for target throwing, indicating that men are superior at throwing and catching

• Difference is unlikely due to practice, as it is apparent in children as young as 3 years and is also present in chimpanzees

• Women have better fine motor control than men, a finding that appears in young children

Sex Differences: Spatial Analysis

• Men are generally better at spatial tasks requiring mental rotation and navigation

• Men are quicker to learn spatial navigation tasks

• Women improve on mental rotation tasks when block figures are replaced by human shapes in different postures

• Women are better than men at spatial memory

Sex Differences: Mathematical Aptitude

• Men traditionally do better on tests of mathematical reasoning

• Women traditionally do better on tests of computation

• Standardized tests of math find a ratio of high-achieving boys to girls of 4:1

• In tests of reading and math, boys do better in math and girls do better in reading, but the sex difference in reading is significantly larger

• There is greater variability in the scores of males than in females

• The difference in the number of women in STEM fields may be because females have more flexibility in career options because both math and language abilities are strong

Sex Differences: Perception

• Perception is the recognition and interpretation of sensory information

• Women are more sensitive to sensory information other than vision

• Women have lower thresholds to detect sensory stimuli

• Women detect sensory stimuli more quickly than do men

Sex Differences: Verbal Ability

• Women outperform men on tests of verbal fluency, and they have better verbal memory

• The Chicago Word Fluency test asks participants to list in a set amount of time as many words as possible with a particular characteristic

• Girls outperform boys on this word fluency test

Sex Differences: Does Experience Matter?

• Arguments have been made that the male-female differences are based on experience, but training does not seem to impact the differences

• On a water level task, where subjects have to draw the water line in tilted jars, females of all ages underperform males of the same age

• If the task is performed with manipulatable cutouts where a rod is used to simulate the water level, the difference disappears

Sex Differences in Brain Structure

• Much research has looked at the anatomical differences between male and female brains

• One study found that males have a uniform concentration of gray matter, but females have some areas of high concentration and some areas of low concentration

Brain Structure: Influence of Sex Hormones

• Differences in the distribution of estrogen and androgen receptors during development could affect brain structure

• There are sexually dimorphic regions of the brain in the prefrontal cortex, the paralimbic cortex, and the posterior parietal cortex

Brain Structure: Anatomical Asymmetries

• Women's brains are more symmetrical than are men's brains

• The asymmetry of the planum temporale and Sylvian fissure is more common in men than in women, supporting the supposition of a difference in the language-related structures of men and women

• Women have more interhemispheric connections than do men

• Women have more asymmetrical fingerprints than men do, and the patterns of ridges in the fingerprints correlate with performance on cognitive tests

Sex Differences in Functional Imaging

• Researchers cannot identify a brain as male or female by looking at any one brain image, but the patterns of connections between brain regions found in resting-state fMRI are a reliable predictor of sex

• Males show greater connectivity in the right hemisphere

• Females show greater connectivity in the left hemisphere

• A study using diffusion tensor imaging shows that the female brain has greater interhemispheric connectivity, while the male brain has greater intrahemispheric connectivity

Gender Identity and Sexual Orientation

• Sexual orientation is an individual's attraction to members of a particular gender

• Gender identity is the relationship between biological gender, internal sense of self, and external gender behaviors

• Parts of the hypothalamus are different between homosexual men, heterosexual men, and heterosexual women

• Differences have been identified in midline brain structures between homosexual men and heterosexual men and women

• Responses in the ventral striatum to erotic images differ between heterosexual, bisexual, and homosexual men as well as between men and women

Research with Neurological Patients

• Given the differences in anatomy and connectivity, one might anticipate differences in the effects of brain injury between males and females

• Men might have more asymmetrical effects of unilateral lesions than women do

• The differences in intrahemispheric organization could result in different impacts of frontal-lobe damage

• In men, left-hemisphere lesions impacted verbal IQ more than performance IQ and right-hemisphere lesions impacted performance IQ more than verbal IQ

• In women, left-hemisphere lesions impacted both forms of IQ, but right-hemisphere lesions did not impact either form

Effects of Brain Damage

• While both men and women were likely to experience aphasia after left hemisphere damage, the location of damage mattered

• For men, aphasia and apraxia resulted from posterior damage

• For women, aphasia and apraxia results from anterior damage

• Following right-hemisphere damage, there were impairments in block design and object assembly tests

• For women, anterior damage impaired test performance

• For men, anterior or posterior damage impaired performance

Hormonal Explanations for Sex Differences

• Sex hormones have an inductive effect, resulting in sexual differentiation of the brain

• Androgens (male hormones) are converted to estradiol (female hormones) in the brain

• Receptors for estradiol are found in developing brains but not mature brains, so the hormones act during development to produce permanent changes in the brain

• Effects of hormones can be seen throughout life

Hormonal Explanations

• In women, higher estrogen levels are associated with decreased spatial ability and increased verbal and motor abilities

• Estrogen impacts the level of catecholamine neurotransmitters, which may explain the impact of estrogen levels on cognitive functions

• Estrogen can directly impact the number of dendritic spines on neurons in the brain

• In men, there appears to be an optimal level of testosterone to optimize performance on cognitive tests

• Both increases above and decreases below the optimal level impair test performance

Hormonal Interventions

• Testosterone levels decrease with age in men

• Supplementing low testosterone levels boosts spatial cognition and verbal memory scores

• Men at risk for prostate cancer have blocked testosterone production, which impairs their performance on verbal memory and attention

• Estrogen treatment in postmenopausal women improves verbal fluency, verbal memory, and spatial memory

• There may be a critical period around menopause for women to receive hormone supplementation to experience the benefits

Other Explanations for Sex Differences

• Possible gene on the X chromosome is responsible for spatial ability

• Females generally mature more rapidly than males, and research suggests that those who mature sooner do better on tests of verbal ability

• Expectations from the social environment may favor boys being more independent and physical and girls being more nurturing and verbal

• There may be a difference in preferred strategies used by men and women to address problems

All these ideas have some supporters and some critics, so much more study and understanding is needed

Environment and Cerebral Organization

• For people who are bilingual, the left-hemisphere language areas are likely enlarged or organized differently from the language areas of individuals who only speak one language

• Some studies support the idea that early acquisition of a second language increases connectivity and cognitive flexibility

• There are behavioral differences in how Western subjects and Chinese subjects use language, perceive faces, and use verbal working memory

• Emerging research shows structural and functional differences in the brains of Western and Chinese subjects

• Unclear whether differences are the result of genetics or environment

Sensory and Environmental Deficits

• In deaf individuals, left-hemisphere damage results in aphasia for the use of sign language

• Research has found that deaf individuals do not show right-visual-field advantages for perceiving language stimuli

• Some visuospatial skills may shift to the left hemisphere in deaf individuals who communicate with sign language

• In blind individuals, the visual cortex is recruited to participate in nonvisual tasks such as hearing, touch, and olfaction

• Temporal-lobe visual areas are repurposed for hearing, with some blind people creating a sound map of the environment to support echolocation

Environmental Deprivation

• Orphans raised in deprived environments demonstrate long-lasting impacts of early deprivation on brain structure and function

• Children adopted into enriched environment around age 2

• At age 12, their brains remained approximately 20% smaller than age-matched control and included impoverished gray and white matter

• Significant behavioral and cognitive problems into adulthood

• Even less severe neglect can cause lasting changes in brain development and organization

• Socioeconomic status (SES) quantifies income, education, and occupation

- SES is related to cortical surface area in the frontal, parietal, and temporal lobes