Chapter 3: The Biological Bases of Behaviour
Nervous System, Brain Researching Methods, Parts of the Brain, Evolutionary Bases
Communication in the Nervous System
What is the Nervous System?
A living tissue composed of cells. Cells in nervous systems receive, integrate & transmit information.
2 Major Categories of Cells in the Nervous System
- Neurons: Individual cells in the nervous system that receive, integrate & transmit information. Basic link that permit communication in the nervous system. Vast majority communicate only with other neurons, but some can receive signals from sensory organs or carry messages to muscles.
- Soma (Greek for body): Contains the cell nucleus & much of the chemical machinery common to most cells.
- Dendrites: Parts of a neuron that are specialized to receive information. They’re branched feeler-like structures.
- Axon: A long, thin fibre that transmits signals away from the soma to other neurons or to muscles or glands.
- Myelin Sheath: Insulating material derived from glial cells that encases some axons, speeding up movement.
- Terminal Buttons: Axons end in a cluster of small knobs that secrete chemicals called neurotransmitters. These chemicals may activate neighbouring synapses.
- Synapses: A junction where information is transmitted from one neuron to another. It’s where neurons connect.
- Glia (Glue) Cells: Cells found throughout the nervous system that provide various types of support for neurons. Tend to be much smaller & outnumber neurons about 10-to-1. Account for over 50% of the brain’s volume.
- Supply nourishment to neurons.
- Help remove neurons’ waste products.
- Provide insulation around many axons.
- Plays a role in the development of the nervous system in the human embryo.
- Some types of glia can detect neural impulses & send signals to other glia cells.
- May play an important role in memory formation, & deterioration may contribute to Alzheimer’s disease.
- May play a crucial role in the experience of chronic pain.
- Impaired neural-glial communication may contribute to disorders, such as schizophrenia & mood disorders.
The Neural Impulse - Using Energy to Send Information
Alan Hodgkin & Andrew Huxley found neural impulses to be a complex electrochemical reaction. Both inside & outside the neuron are fluids containing electrically charged atoms & molecules called ions. Cell membranes are semipermeable (permitting movement of some ions).
The Neuron at Rest - A Tiny Battery
Positively charged sodium & potassium ions and negatively charged chloride ions flow back and forth across the cell membrane, but they don’t cross at the same rate. The difference in flow rates leads to a slightly higher concentration of negatively charged ions inside the cell. The resulting voltage means that the neuron at rest is a tiny battery, a store of potential energy.
The resting potential of a neuron is its stable, negative charge when the cell is inactive. This charge is ~270 millivolts.
What is the Action Potential?
If voltage is constant, the cell is quiet & no messages are sent. When the neuron is stimulated, channels in its cell membrane briefly open, allowing positively charged sodium ions to rush in. For an instant, the neuron’s charge is less negative (or even positive) creating an action potential.
An action potential is a very brief shift in a neuron’s electrical charge that travels along an axon. The absolute refractory period is the minimum length of time after an action potential during which another action potential cannot begin, usually 1 or 2 milliseconds. Basically, it’s a cooldown! It’s followed by a brief relative refractory period, where neurons require more intense stimulation to fire.
The All-or-None Law
The neural impulse is an all-or-none proposition. However, they do convey strength of stimulation intensity by the rate at which they fire action potentials.
The Synapse - Where Neurons Meet
What is the Synaptic Cleft?
A microscopic gap between the terminal button of one neuron & the cell membrane of another. Signals cross this gap when neurons communicate. Neurons that send a signal are called presynaptic neurons & neurons that receive the signal are called postsynaptic neurons.
How do signals travel across the synaptic cleft?
The arrival of an action potential at an axon’s terminal buttons triggers release of neurotransmitters (chemicals that transmit information from one neuron to another). Within the buttons, most of these chemicals are stored in the synaptic vesicles. Once released, they may bind with special molecules in the postsynaptic cell membrane at receptor sites. These sites are tuned to recognize to certain neurotransmitters.
Postsynaptic Potentials (PSP)
A voltage change at a receptor site on a postsynaptic cell membrane. These do not follow the all-or-none law, causing them to be graded. They vary in size & they increase/decrease the probability of a neural impulse in the receiving cell in proportion to the amount of voltage change. Eventually, neurotransmitters drift away or are inactivated.
Two Types of PSPs
- Excitatory PSP: A positive voltage shift that increases likelihood postsynaptic neuron will fire action potentials.
- Inhibitory PSP: A negative voltage shift that decreases likelihood postsynaptic neuron will fire action potentials.
What is Reuptake?
Process where neurotransmitters are sponged up from the synaptic cleft by the presynaptic membrane.
Integrating Signals - Neural Networks
The state of a neuron firing its action potential is a weighted balance between excitatory & inhibitory PSP influences. Rita Carter says millions of neurons must fire to produce even the simplest action. Neurons are interlinked in complex chains, pathways, circuits & networks. Our perceptions, thoughts & actions depend on patterns of neural activity in elaborate neural networks. Neurons in these networks frequently fire together or sequentially to perform functions. The links in these networks are fluid as new synaptic connections are made while old connections wither away.
The elimination of old synapses seems to play a larger role in the sculpting of neural networks than the creation of new synapses. The nervous system normally forms more synapses than needed & then gradually eliminates the less-active synapses. This is called synaptic pruning.
What were Donald Hebb’s contributions to Neural Networks?
McGill University’s Donald Hebb realized that neurons don’t act alone in influencing behaviour but that they’re linked in cell assemblies (complex networks).
He came up with the Hebbian Learning Rule to specify how these linkages might operate/occur. He famously said that a neuron stimulating another neuron repeatedly produces changes in the synapse; learning has taken place. His ideas are often referred to as the Hebb Synapse.
What is Long-Term Potentiation?
Refers to a long-lasting increase in neural excitability in synapses along a specific neural pathway. Repeated synaptic activity leads to a strengthening of the synapse. Work by Kandel on this topic not only supports Hebb’s initial conceptualizations, but it even won him a Nobel Prize in 2000.
Neurotransmitters & Behaviour
There are 9 classic (small-molecule) transmitters, about 40 additional neuropeptide chemicals that function (at least part-time) as neurotransmitters, and a handful of ‘novel’ neurotransmitters. Specific neurotransmitters work at specific kinds of synapses. Such specialization reduces cross talk between densely packed neurons, making the nervous system’s communication more precise.
What is an Agonist?
A chemical that mimics the action of neurotransmitter by being able to bind in the receptor sites.
What is an Antagonist?
A chemical that opposes the action of a neurotransmitter by filling a receptor site & blocking it.
What are Monoamines?
Monoamines include 3 transmitters: dopamine, norepinephrine & serotonin.
Amino Acid-based Neurotransmitters
Two of them are GABA & Glycine. These seem to only produce inhibitory PSPs. GABA receptors are widely distributed in the brain & may be present at 40% of all synapses.
What is Schizophrenia?
A severe mental illness that’s marked by irrational thought, hallucinations, poor contact with reality, & deterioration of routine adaptive behaviour. Afflicting ~1% of the population, schizophrenia requires hospitalization more often than any other psychological disorder. Studies suggest (albeit with many complications) over-activity in dopamine circuits constitutes the neurochemical basis for schizophrenia because drugs treating symptoms are DA antagonists.
What does Endogenous mean?
It means “internally produced”.
Organization of the Nervous System
Traumatic Brain Injury (TBI) & Acquired Brain Injury (ABI)?
Traumatic Brain Injury (TBI) is a brain injury that is caused from an external mechanical force.
Acquired Brain Injury (ABI) is a brain injury that is caused by events after birth, rather than genes.
What are Nerves?
Bundles of neuron fibres (axons) that are routed together in the peripheral nervous system.
What is the Peripheral Nervous System?
The peripheral nervous system is made up of all nerves that lie outside the brain & spinal cord. This is the part of the nervous system that is outside the central nervous system. The peripheral nervous system is divided into 2: the somatic nervous system & the autonomic nervous system.
What is the Somatic Nervous System?
The somatic nervous system is made up of nerves that connect to voluntary skeletal muscles & to sensory receptors. These nerves carry information from receptors in the skin, muscles & joints to the CNS & that carry commands from the CNS to the muscles.
These functions require 2 kinds of nerve fibres. Afferent nerve fibres are axons that carry information inward to the CNS from the periphery of the body. Efferent nerve fibres are axons that carry information outward from the CNS to the periphery of the body. Each body nerve contains many axons of each type. Thus, somatic nerves are “two-way streets”.
What is the Autonomic Nervous System?
The autonomic nervous system (ANS) is made up of nerves that connect to the heart, blood vessels, smooth muscles & glands. It’s ultimately governed by the CNS. The ANS controls automatic, involuntary, visceral functions that people don’t normally think about, such as heart rate, digestion & perspiration. It mediates much of the physiological arousal that occurs experiencing emotions.
The ANS can be divided into 2 branches. The sympathetic division is the branch of the ANS that mobilizes the body’s resources for emergencies. It creates the fight-or-flight response. Activation of the sympathetic division slows digestive processes & drains blood from the periphery, lessening bleeding in the case of an injury. Key sympathetic nerves send signals to the adrenal glands, triggering the release of hormones that ready the body for exertion. In contrast, the parasympathetic division is the branch of the ANS that generally conserves bodily resources. For example, actions by parasympathetic nerves slow heart rate, reduce blood pressure & promote digestion.
It also mediates the fight-or-flight response, which was coined by Walter Cannon.
What is the Central Nervous System (CNS) Made Up Of?
The central nervous system (CNS) is the portion of the nervous system that lies within the skull & spinal column, consisting of the brain & the spinal cord. It’s protected by enclosing sheaths called the meninges.
What is the Cerebrospinal Fluid (CSF)?
It’s a fluid that the CNS is bathed in which nourishes the brain & provides a protective cushion for it.
What is the Spinal Cord?
The spinal cord connects the brain to the rest of the body through the peripheral nervous system, & is an extension of the brain. It’s also enclosed in meninges & bathed in CSF. It runs from the base of the brain to just below the level of the waist.
What is the Brain?
The brain is the part of the central nervous system that fills the upper portion of the skull. It weighs ~1.5 kilograms & contains billions of interacting cells that integrate information from inside & outside the body, coordinate the body’s actions, and enable humans to talk, think, remember, plan, create & dream.
Looking Inside the Brain - Research Methods
Who are Neuroscientists?
Investigators who conduct research on the brain or other parts of the nervous system. Brain research involves collaboration by neuroscientists from several disciplines, including anatomy, physiology, biology, pharmacology, neurology, neurosurgery, psychiatry & psychology.
What is an EEG & What Does it Output?
A device that monitors the electrical activity of the brain over time by means of recording electrodes attached to the surface of the scalp.
The resulting EEG recordings are translated into line tracings, commonly called brain waves. Different brain-wave patterns are associated with different states of mental activity.
What is Lesioning?
Involves destroying a piece of the brain. It’s typically done by inserting an electrode into brain structures & passing a high-frequency electric current to burn the tissue & disable the structure.
How is Electrical Stimulation Used to Study Brain Function?
Electrical stimulation of the brain (ESB) involves sending a weak electric current into a brain structure to stimulate (activate) it. The current is delivered through an electrode, but the current doesn’t exactly duplicate normal signals in the brain but is close enough.
Most ESB research is conducted with animals.
What is Transcranial Magnetic Stimulation?
Transcranial magnetic stimulation (TMS) is a new non-invasive technique that permits scientists to temporarily enhance or depress activity in a specific area of the brain. In TMS, a magnetic coil mounted on a small paddle is held over a specific area of a subject’s head. The coil creates a magnetic field that penetrates to a depth of 2 centimetres. By varying the timing & duration of the magnetic pulses, a researcher can either increase or decrease the excitability of neurons in the local tissue. This technology allows scientists to create ‘virtual lesions’ in human subjects for short periods of time.
Limitations of TMS
It can’t be used to study areas deep within the brain.
Other Brain-Imaging Procedures
- Computerized Tomography (CT) Scan: A computer-enhanced X-Ray of brain structure. X-Rays are shot from multiple angles & the computer combines the readings to create an image of a horizontal slice of the brain.
- Least expensive & widely used in research.
- Research has uncovered an association between schizophrenic disturbance & enlargement of the brain’s ventricles.
- Positron Emission Tomography (PET) Scanning: PET scans examine brain function, mapping activity in the brain over time. Radioactively tagged chemicals are introduced into the brain, which serve as markers of blood flow/metabolic activity in the brain which is monitored by X-Ray.
- Can be used to study neurotransmitters.
- Researchers have determined how cocaine affects activity in dopamine circuits in the human brain.
- Magnetic Resonance Imaging (MRI) Scan: Uses magnetic fields, radio waves & computerized enhancement to map out brain structure that provide better images than CT scans by producing 3D pictures.
- Provided useful insights about depressive disorders. For example, depression is associated with shrinkage of the hippocampus.
- Function Magnetic Resonance Imaging (fMRI): A variation on MRI that monitors blood flow & oxygen consumption in the brain to identify areas of high activity. This is more precise than PET scans.
- Researchers have identified patterns of brain activity associated with cocaine craving in addicts, the contemplation of a loved one, the visual recognition of shapes & textures, the decision making required by risky gambles, relations between memories & future expectations.
- Adrian Owen asked “locked-in” patients to imagine doing routine activities such as playing tennis & found that brain activity was indistinguishable to those who actually played tennis.
The Brain & Behaviour
What is the Hindbrain?
It includes the cerebellum & 2 structures in the lower part of the brainstem: the medulla & the pons.
The medulla, attaches to the spinal cord, is in charge of vital unconscious functions, like breathing, circulating blood, maintaining muscle tone & reflexes (such as sneezing, coughing, salivating).
The pons (literally “bridge”) includes a bridge of fibres that connects the brainstem with the cerebellum. The pons also contains several clusters of cell bodies involved with sleep & arousal.
The cerebellum (literally “little brain”) is a large & deeply folded structure located adjacent to the back surface of the brainstem. It’s critical to movement coordination & sense of physical balance. The cerebellum is one of the structures first depressed by alcohol.
What is the Midbrain?
The segment of the brainstem that lies between the hindbrain & the forebrain. It’s concerned with integrating sensory processes, such as vision & hearing. An important dopamine-releasing system of neurons originates in the midbrain. A decline in this system causes Parkinson’ disease.
Running through the hindbrain & midbrain is the reticular formation. Lying at the central core of the brainstem, the reticular formation contributes to the modulation of muscle reflexes, breathing, pain perception, regulation of sleep & arousal.
What is the Forebrain?
The largest & most complex region of the brain, encompassing a variety of structures, including the 3 core structures of the thalamus, hypothalamus & limbic system which are located near the top of the brainstem. It also includes the cerebrum, which sits above the 3, & is the seat of complex thought. The surface of the cerebrum is the cerebral cortex.
What is the Thalamus?
A structure in the forebrain through which all sensory information (except smell) must pass to get to the cerebral cortex. This way state is made up of clusters of somas (cell bodies), which relay sensory information to particular part of the cortex.
What is the Hypothalamus?
A structure found near the base of the forebrain that help regulates of basic biological needs. Hypo means under, making it lie under the thalamus. It contains various clusters of cells that have many key functions such as controlling the autonomic nervous system, serving a vital link between the brain & the endocrine system.
It plays a major role in the regulation of basic biological drives related to survival, including the so-called “four Fs”: fighting, fleeing, feeding & mating.
What is the Limbic System?
A loosely connected network of structures that was first described by Paul MacLean located along the border between the cerebral cortex & deeper subcortical areas. Broadly defined, the system includes parts of the thalamus & hypothalamus, the hippocampus, the amygdala, etc. The system is involved in the regulation of emotion, memory, motivation & optimism.
What is the Cerebrum?
The cerebrum is the largest & most complex part of the brain. It includes the brain areas that are responsible for the most complex mental activities, including learning, remembering, thinking & consciousness.
The cerebral cortex is the convoluted outer layer of the cerebrum that is folded & bent.
The cerebrum is divided into two halves called hemispheres; right & left halves of the cerebrum. The hemispheres are separated in the centre of the brain by a fissure. This fissure descends to a thick band of fibres called the corpus callosum, which connects the two cerebral hemispheres. Each hemisphere is divided into 4 lobes:
- Occipital Lobe: At the back of the head, includes the cortical area, where most visual signals are sent & visual processing is begun. This area is called the primary visual cortex.
- Parietal Lobe: Includes the area that registers the sense of touch, called the primary somatosensory cortex.
- Temporal Lobe: Includes an area devoted to auditory processing, called the primary auditory cortex.
- Frontal Lobe: Largest lobe & contains areas that control muscle movement, called the primary motor cortex.
- Houses mirrors neurons that are activated by performing an action or watching someone perform that action. Mirror neurons have also been found in the parietal lobe.
- Houses the prefrontal cortex which accounts for ~1/3 of the cerebral cortex. Theorists suggest that the prefrontal cortex houses a executive control system that monitors, organizes & directs thought processes.
What is Brain Plasticity?
According to Bryan Kolb, it’s “the brain’s ability to change structure & function”. He suggests that experience is an important stimulant of brain plasticity, which affects dendritic length, synapse formation & altered metabolic activity.
This plasticity helps adjust to damage, where neurons previously responsible for certain functions re-organize once that function is no longer feasible. It also shows that neurogenesis (formation of new neurons) does occur in adults, not just infants, which may facilitate in learning.
What are Stem Cells?
“Unspecialized” cells that renew themselves through cell division & that can, under special circumstances, be “induced” to become replacements for other purposes (ex: for a heart).
Cerebral Laterality -: Right Brain/Left Brain
What is Broca’s Area?
Paul Broca, a French surgeon in 1861, was treating a patient who had been unable to speak. He showed that the probable cause of the speech deficit was a localized lesion on the left side of the frontal lobe. This area is now known as Broca’s area, which plays an important role in the production of speech.
What is Wernicke’s Area?
Identified in the temporal lobe of the left hemisphere in 1874. Damage in Wernicke’s area leads to problems with comprehension of language.
What is the Left Hemisphere of the Brain Known For?
It’s known for usually processing language, thus being seen as dominant as most thoughts are in terms of language. The left hemisphere was given the credit for handling the higher mental processes, such as reasoning, remembering, planning, problem solving, etc.
What is Split-Brain Surgery?
In split-brain surgery, the bundle of fibres that connects the cerebral hemisphere (corpus callosum) is cut to reduce the severity of epileptic seizures. Patients who go through this are heavily researched.
Each hemisphere’s primary connections are to the opposite side of the body. Both eyes deliver information to both hemispheres, but there still is a separation of input. Auditory inputs to each ear also go to both hemispheres.
Roger Sperry & Michael Gazzaniga presented visual stimuli in a single visual field (the left or the right), so that the stimuli would be sent to only one hemisphere. Images were flashed to the right or the left of a fixation point for only a split second. When pictures were flashed in the right visual field and thus sent to the left hemisphere, the split-brain subjects were able to name & describe the objects. However, the subjects were not able to name & describe objects when they were flashed in the left visual field & sent to the right hemisphere.
What is the Right Hemisphere Known For?
It’s superior to the left hemisphere in assembling little puzzles and copying drawings. The right hemisphere is better on a variety of visual–spatial tasks, including discriminating colours, arranging blocks & recognizing faces.
Hemispheric Specialization in the Intact Brain
It’s possible to study hemispheric specialization in intact brains by virtue of perceptual asymmetries, which are left-right imbalances between the cerebral hemisphere in the speed of visual or auditory processing.
For instance, when verbal stimuli are presented to the right visual field (sent to the left hemisphere first), they are identified more quickly & accurately. In contrast, the right hemisphere is faster than the left on visual–spatial tasks, such as locating a dot or recognizing a face.
The left hemisphere usually is better on tasks involving verbal processing, such as language, speech, reading, and writing. The right hemisphere exhibits superiority on many tasks involving nonverbal processing, such as most spatial, musical, and visual recognition tasks and tasks involving the perception of others’ emotions.
The Endocrine System
What Does the Endocrine System Consist Of?
Glands that release hormones into the bloodstream; hormones help to control bodily functioning.
Some neurotransmitters act as hormones when released in the endocrine system, like norepinephrine. However, there are some important differences between hormones and neurotransmitters. Neural messages are transmitted short distances in milliseconds along very specific pathways, whereas hormonal messages often travel to distant cells at a much slower speed (measured in seconds & minutes) & tend to be less specific.
Hormones regulate changing body conditions. For example, hormones released by the stomach & intestines help control digestion. Kidney hormones help control blood pressure. The pancreatic hormone (insulin) is essential for cells to use sugar from the blood.
Hormone release tends to be pulsatile (released several times per day in pulses that are a few minutes).
How is the Endocrine System Controlled?
Much of it is controlled by the nervous system through the hypothalamus.
What is the Master Gland of the Endocrine System?
The pituitary gland releases a variety of hormones, stimulating actions in other endocrine glands.
What is Oxytocin?
A hormone that regulates reproductive behaviours. Oxytocin triggers contractions when a woman gives birth & stimulates the mammary glands to release milk for breast-feeding. It may facilitate the development of a sense of security, feelings of safety, empathy & trust.
What is Gonadotropins?
A hormone which affects the gonads (sexual glands). It directs the formation of the external sexual organs in the developing fetus.
Heridity & Behaviour
What is Behavioural Genetics?
An interdisciplinary field that studies the influence of genetic factors on behavioural traits.
What are the Basic Principles of Genetics?
What are Chromosomes?
Chromosomes are strands of DNA molecules (deoxyribonucleic acid) that carry genetic information. Every human cell, except sex cells (sperm & eggs), contains 46 chromosomes. These chromosomes operate in 23 pairs, with 1 chromosome of each pair being contributed by each parent.
Parents make this contribution when fertilization creates a zygote, a single cell formed by the union of a sperm & an egg. The sex cells that form a zygote each have 23 chromosomes; together they contribute the 46 chromosomes in all of the body cells that develop from the zygote.
What are Genes?
Each chromosome in turn contains thousands of biochemical messengers called genes. Genes are DNA segments that serve as the key functional units in hereditary transmission.
Like chromosomes, genes operate in pairs, with one gene of each pair coming from each parent. In the homozygous condition, the 2 genes in a specific pair are the same. In the heterozygous condition, the 2 genes in a specific pair are different. In the simplest scenario, a single pair of genes determines a trait. Attached versus detached earlobes provide a nice example. When both parents contribute a gene for the same type of earlobe (the homozygous condition), the child will have an earlobe of that type. When the parents contribute genes for different types of earlobes (the heterozygous condition), one gene in the pair—called the dominant gene—overrides or masks the other, called the recessive gene. Thus, a dominant gene is one that is expressed when paired genes are different. A recessive gene is one that is masked when paired genes are different. In the case of earlobes, genes for detached earlobes are dominant over genes for attached earlobes.
Why don’t family members look like identical clones, then?
The reason is that parents can produce an extraordinary variety of combinations of chromosomes. When sex cells form in each parent, it’s a matter of chance as to which member of each chromosome pair ends up in the sperm or egg. Each parent’s 23 chromosome pairs can be scrambled in 8+ million (223) different ways, yielding ~70 trillion (246) possible configurations when sperm & egg unite. This is a conservative estimate. It doesn’t consider complexities such as mutations (changes in the genetic code) or crossing over during sex-cell formation (an interchange of material between chromosomes). Thus, genetic transmission is a complicated process, and everything is a matter of probability. Except for identical twins, each person ends up with a unique genetic blueprint.
What do Genotype & Phenotype refer to?
Genotype refers to a one’s genetic makeup. Genotype is determined at conception & is fixed forever.
Phenotype refers to the ways in which a person’s genotype is manifested in observable characteristics. Phenotype characteristics may change over time, and may be modified by environmental factors.
Not all gene pairs operate according to the principles of dominance. In some cases, when paired genes are different, they produce a blend, an “averaged-out” phenotype. In other cases, paired genes that are different strike another type of compromise, & both characteristics show up phenotypically. In the case of type AB blood, for example, one gene is for type A and the other is for type B.
What are Polygenic Traits?
Characteristics that are influenced by more than one pair of genes. For example, 3-5 gene pairs are thought to interactively determine skin colour. Most psychological characteristics that appear to be affected by heredity seem to involve complex polygenic inheritance.
Research Methods for Investigating Hereditary Influence
Researchers assess hereditary influence by examining blood relatives to see how much they resemble one another on a specific trait. If heredity affects the trait under scrutiny, researchers should find phenotypic similarity among relatives. Furthermore, they should find more similarity among relatives who share more genes. For instance, siblings should exhibit more similarity than cousins.
However, this correlation doesn’t provide conclusive evidence that the trait is influenced by heredity. This is because family members generally share not only genes but also similar environments.
Researchers assess hereditary influence by comparing the resemblance of identical twins & fraternal twins with respect to a trait. Identical (monozygotic) twins emerge from 1 zygote that splits for unknown reasons. Thus, they have exactly the same genotype; their genetic relatedness is 100%. Fraternal (dizygotic) twins result when 2 eggs are fertilized simultaneously by different sperm cells, forming 2 separate zygotes. Fraternal twins are no more alike in genetic makeup than any 2 siblings.
Adoption studies assess hereditary influence by examining the resemblance between adopted children and both their biological & adoptive parents. If adopted children resemble their biological parents in a trait, even though they were not raised by them, genetic factors probably influence that trait. In contrast, if adopted children resemble their adoptive parents, even though they inherited no genes from them, environmental factors probably influence the trait.
Molecular geneticists, who study the biochemical bases of genetic inheritance, have made spectacular advances in their efforts to unravel the genetic code. Genetic mapping is the process of determining the location & chemical sequence of specific genes on specific chromosomes. The Human Genome Project, a huge international enterprise, has produced a working draft of the sequence of all 3 billion letters of DNA in the human genome, and the chromosomal location of almost all human genes has been identified. It has revealed that there are ~20,500 human genes.
What is Epigenetics?
Epigenetics is the study of heritable changes in gene expression that do not involve modifications to the DNA sequence.
The Evolutionary Bases of Behaviour
What is Evolutionary Psychology?
Evolutionary psychology is a major new theoretical perspective in the field that analyzes behavioural processes in terms of their adaptive significance.
Charles Darwin’s 4 Key Insights
- Organisms vary in endless ways, such as size, speed, strength, aspects of appearance, visual abilities, hearing capacities, digestive processes, cell structure, etc.
- Some of these characteristics are heritable & are passed down from one generation to the next.
- Organisms tend to produce offspring at a pace that outstrips the local availability of food supplies, living space & other crucial resources.
- Darwin argued that if a specific heritable trait contributes to an organism’s survival or reproductive success, organisms with that trait should produce more offspring than those without the trait (or those with less of the trait), and the prevalence of that trait should gradually increase over generations—resulting in evolutionary change.
What does Fitness mean?
Fitness refers to the reproductive success (number of descendants) of an individual organism relative to the average reproductive success in the population.
What does Natural Selection mean?
The principle of natural selection posits that heritable characteristics that provide a survival or reproductive advantage are more likely than alternative characteristics to be passed on to subsequent generations and thus come to be “selected” over time.
Subsequent Refinements to Evolutionary Theory
What is a Genetic Drift?
Genetic drift consists of random fluctuation in gene frequencies over generations, as a result of chance alone.
What does Mutation mean?
A mutation is a spontaneous, heritable change in a piece of DNA that occurs in an individual organism. Mutations are unpredictable errors in DNA replication. Although infrequent, mutations increase the variability in a gene pool and give natural selection new genetic material to work with. Most mutations are not beneficial, but the minority that prove adaptive are increasingly passed on to subsequent generations.
What does Gene Flow mean?
Gene flow occurs when gene frequencies in a population shift because some individuals leave the population (emigration) and others enter it (immigration). Gene flow operates to keep neighbouring populations genetically similar. It can counterbalance gene pool differences between populations that have developed because of genetic drift, mutation, and natural selection. Conversely, when the gene flow between populations is minimal, the populations may evolve in divergent directions. This divergence can eventually contribute to the emergence of new species.
What’s an Adaptation?
An adaptation is an inherited characteristic that increased in a population (through natural selection) because it helped solve a problem of survival or reproduction during the time it emerged. Because of the gradual, incremental nature of evolution, adaptations sometimes linger in a population even though they no longer provide a survival or reproductive advantage.
Behaviours as Adaptive Traits
Many behavioural adaptations are designed to improve organisms’ chances of reproductive success or aid in survival.
Featured Study - The Neuroscience of Time Travel
What is Episodic Memory?
Episodic memory is made up of chronological, or temporally dated, recollections of personal experiences.
What is the Constructive Episodic Simulation Hypothesis?
Donna Addis & Daniel Schacter proposed this hypothesis which suggests that remembering the past & simulating the future should draw on similar kinds of information from episodic memory & utilize similar types of neural processes.
Participants included 16 right-handed healthy adults, with 7 males & 9 females, between the ages of 18-33.
Procedure involved an fMRI machine, where participants were presented with lists of concrete nouns (e.g., dress, star). In some trials, they were asked to use the words as a cue to recall an event in the past (last week, last year, last 5 years, last 20 years). In other trials, they were asked to imagine a future event that would occur (in the next week, next year, next 5 years, next 20 years). One an event was in mind (the construction phase), they notified the experimenters & began detailing the event (the elaboration phase).
The principal findings from the brain imaging indicated that there was considerable overlap in the brain regions that were active in remembering the past and imagining the future, both in the construction and elaboration phases. For example, some medial temporal, parietal, and prefrontal regions showed increases in activity.
Personal Application - Evaluating the Concept of “Two Minds in One”
- Split-brain research stimulated speculation about relationships between cerebral specialization and cognitive processes. Some theorists believe that each hemisphere has its own stream of consciousness and mode of thinking, which are applied to specific types of cognitive tasks.
- Some theorists also believe that people vary in their reliance on the right and left halves of the brain and that schools should work more to exercise the right half of the brain.
- The cerebral hemispheres are specialized for handling different cognitive tasks, but only to a degree, as most tasks engage both hemispheres. Moreover, people vary in their patterns of hemispheric specialization.
- Evidence for duality in consciousness divided along hemispheric lines is weak. Evidence on whether people vary in brainedness and whether the two hemispheres vary in cognitive style is inconclusive.
- There is no way to teach only one hemisphere of the brain, so a “right-brain curriculum” is pointless. Popular ideas about the right and left brain have gone far beyond the actual research findings.
Critical Thinking Application - The Perils of Extrapolation
Although some education and child-care reformers have used research in neuroscience as the basis for their campaigns, research has not demonstrated that birth to age three is a critical period for human neural development or that specific enrichment programs can enhance brain development. These assertions are highly conjectural overextrapolations from existing data.
What is a Critical Period?
A critical period is a limited time span in the development of an organism when it is optimal for certain capacities to emerge because the organism is especially responsive to certain experiences.