One of the largest and most highly contested debates in psychology is the nature versus nurture issue, arguing which plays a greater role in psychological traits or behaviors - genes and heredity or environment?

A favorite topic of psychology students and researchers is serial killers. Evil is fascinating. But are evil people born that way or made that way? 

The biological approach to psychology focuses on how the body and brain enable emotions, memories, and sensory experiences, as well as, how our genes and heredity influence our individual differences. 

Everything psychological is biological. 

If Beyonce’s children grow up to be superstars, can we attribute their talent to genes or environment?

Behavior geneticists are those that study the relative power and limits of genetic and environmental influences on behavior.

• Heredity: the genetic transfer of characteristics from parents to offspring

• Environment: every nongenetic influence, from prenatal nutrition to the people and things around us

Behavior geneticists explore the genetic and environmental roots of human differences. Evolutionary psychologists, however, focus on what makes us alike as humans, primarily by exploring the principles established by Charles Darwin. 

• Natural selection: the principle that inherited traits that better enable an organism to survive and reproduce in a particular environment will (in competition with other trait variations) most likely be passed on to the succeeding generations

• Mutation: a random error in gene replication that leads to a change

Evolutionary psychologists presume all human behaviors reflect the influence of physical and psychological predispositions that helped human ancestors survive and reproduce. To study this, researchers observe twins.

Identical vs. Fraternal Twins

• Identical or monozygotic twins develop from a single fertilized egg that splits, meaning all the genes are the same.

• Fraternal or dizygotic twins develop from different eggs that were fertilized simultaneously, so they share the same genes as siblings but are not identical.

Twin studies are a favorite of psychologists because they provide the perfect opportunity to test the nature vs. nurture debate. Identical twins raised in the same household should be the same, right? Not always. Just as identical twins separated at birth and raised in different environments often have many similarities. 

The nervous system is the body’s speedy, electrochemical communication network, consisting of all nerve cells and is divided into several smaller systems based on function.

• Central nervous system (CNS): brain and spinal cord

• Peripheral nervous system (PNS): sensory and motor neurons that connect to brain and spinal cord\

  • Autonomic nervous system: controls involuntary functions such as heartbeat, digestion, breathing, etc.

    • • Sympathetic nervous system → fight or flight response; Automatically accelerates heart rate, breathing, dilates pupils, slows down digestion

      • Parasympathetic nervous system → rest and digest; Automatically slows the body down after a stressful event

  • Somatic nervous system: controls voluntary functions

    • • Afferent Nerves: sensory neurons

      • Efferent Nerves: motor neurons

• Interneurons: neurons in the brain and spinal cord that serve as an intermediary between sensory and motor neurons; carry info around the brain for processing. 

• Reflexes: automatic responses to stimuli; sensory neurons take info up through spine to the brain → Some reactions occur when sensory neurons reach just the spinal cord.

• Enteric Nervous System: directly controls the gastrointestinal tract, pancreas and gallbladder; contains efferent, afferent and interneurons

The "fight-flight-freeze" response is a physiological reaction to perceived threats or stress, preparing the body for action. It encompasses three primary responses: fighting, fleeing, or freezing. This instinctive reaction is triggered by the amygdala, the brain's alarm system, and is a survival mechanism developed over time. 

The nervous system is made up of billions of neurons, or nerve cells. While there are several different types of neurons, their key components are the same:

• Cell body: contains nucleus & DNA

• Dendrites: receives signals from other neurons

• Axon (covered in Myelin Sheath: insulates & protects axon): carry signals from one end to the other

• Axon Terminals/Terminal Buttons: send signals to next neuron

• Schwann cells produce myelin for the myelin sheath and the Nodes of Ranvier are the spaces between Schwann cells.

• Glial cells (glia) are support cells for the nervous system, providing extra protection and nourishment to neurons. If nerve cells are queen bees, glial cells are the worker bees.

Neurons transmit messages when stimulated by our senses or by neighboring neurons. Although this process has several steps, it happens in a fraction of a second.

1. When not firing (or at rest), a neuron has a slightly negative charge → resting potential. The ions are aligned or polarized. 

2. If stimulation reaches the threshold (minimum stimulation needed to trigger a neural impulse), a neuron will fire. When firing an impulse, a neuron is active and ions are exchanged → action potential. The ions are scrambled or depolarized. All-or-Nothing Law/Response: a neuron’s reaction of firing or not firing is not determined by the strength of stimulation, as long as the threshold is met

3. Before a neuron can fire again, the ions need to return to their original position, or repolarized. This brief pause between firings is called the refractory period.

4. Once polarized, the neuron is back to resting potential.

Communication between neurons is chemical, while communication within a neuron is chemical, so a conversion from electrical energy to chemical and back again has to take place with each message. Axon terminals convert electrical signals to chemical messengers or neurotransmitters. These neurotransmitters are released into the synapse or synaptic gap, a small space between the neurons. Once they reach the dendrites of the next neuron, the neurotransmitters are absorbed and the chemical signal is converted back to an electrical one, which is then carried down the axon. Once the conversion is complete, the neurotransmitters are released by the neurons and travel back across the synapse to be reabsorbed by the axon terminals, in a process called reuptake.

Neurotransmitters are the chemical messengers of the nervous system. Different neurotransmitters serve a different function, and incorrect amounts can cause difficulties with movement, memory, and mood. Many drugs work because they either increase/mimic a neurotransmitters action (agonists) or block receptor sites (antagonists). Reuptake inhibitors block the reuptake process, leaving the drug/neurotransmitter in the synaptic gap longer.

The oldest part of the brain is the brain stem, found at the base of the skull above the spinal cord. Often referred to as the "reptilian brain", the structures within the brain stem are responsible for automatic survival functions.

• Reticular Formation: a nerve network that travels through the brainstem into the thalamus and plays an important role in controlling arousal

  • If severed or damaged, you could be in a state of permanent sleep or wakefulness.

• Medulla: the base of the brainstem that controls breathing and heartbeat

  • If severed or damaged, you’ll most likely die or be on life support.

• Pons: Connects hindbrain, midbrain, forebrain together; involved in respiration and REM sleep, also serves as a communications and coordination center between the two hemispheres of the brain

Next to the brainstem is the cerebellum or “Little brain”, which coordinates movement and balance, processes sensory input, judgment of time, and enables nonverbal learning and memory. 

• • One of the first parts of the brain impacted by alcohol, thus explaining why people stagger and struggle to react quickly when intoxicated.

Above the brainstem is the thalamus, which serves as the sensory “switchboard”, directing messages to the sensory receiving areas in the cortex and transmitting replies to the cerebellum and medulla. 

• • Receives sensory information from all senses except smell

At the core of the brain is the limbic system, a series of structures associated with emotions and drives.

• • Amygdala: responsible for survival emotions of fear and aggression

  • Hippocampus: responsible for processing and storing explicit memories of facts and events

  • Hypothalamus: below the thalamus; direct several maintenance behaviors, like eating, drinking, and maintaining optimal body temperature, helps regulate the endocrine system via the pituitary gland, and is linked to emotion and reward

Connecting the two hemispheres together is the corpus callosum, a network of fibers allowing for communication between both sides of the brain and body.

• • Can be severed as a treatment for epilepsy, but has some interesting effects (see split-brain studies)

Any areas of the cerebral cortex that are not involved in primary motor or sensory functions, but are rather involved in higher mental functions such as learning, remembering, thinking, and speaking are known as association areas. These areas help us produce a meaningful perceptual experience of the world, enable us to interact effectively, and support abstract thinking and language. 

The cerebral cortex is the intricate fabric of interconnected neural cells covering the cerebral hemispheres that functions as the body’s ultimate control and information-processing center. It plays a key role in memory, attention, perceptual awareness, thinking, speaking, and consciousness, and is organized into lobes based on function:

• • Frontal

  • Parietal

  • Temporal

  • Occipital

Your brain is not only sculpted by your genes but by your life. Your brain is constantly changing, reorganizing itself for your needs without your awareness. Neuroplasticity refers to the brain’s ability to change, build, and reorganize after damage or experience. This process is easier for children than it is for adults. The whole “you can’t teach an old dog new tricks” thing. 

Although the brain will try to mend itself by reorganizing existing tissue, it sometimes has to produce new neurons, a process known as neurogenesis.

Complications among brain structure/chemistry, damage to the nervous system, and genetic anomalies can lead to neurological disorders such as multiple sclerosis or myasthenia gravis. 

• Multiple Sclerosis (MS): chronic autoimmune disease in which the immune system mistakenly attacks the myelin sheath

• Myasthenia Gravis: chronic autoimmune neuromuscular disorder characterized by weakness and rapid fatigue of the voluntary muscles caused by a breakdown in the normal communication between nerves and muscles

Only in recent years have scientists had the ability to view the brain in an non-invasive and painless way. Prehistoric skulls show evidence of trephination, or drilling holes into the skull to “release evil spirits”. In some cases today, skulls will be drilled into and brain tissue destroyed (lesioned), but that is for the purpose of behavior modification and only as a last resort. Otherwise, research on the brain involves the following techniques:

• Electroencephalogram (EEG)

• Magnetoencephalography (MEG)

• Computed tomography (CT)

• Positron Emission tomography (PET)

• Magnetic resonance imaging (MRI)

  • Functional Magnetic Resonance Imaging (fMRI)

The endocrine system is the body’s slow chemical communication system, which consists of a series of glands that secrete hormones into the bloodstream. Neurotransmitters are the chemical messengers of the nervous system, and hormones are the chemical messengers of the endocrine system. The nervous system has a quicker reaction, but the sensation quickly fades, while the endocrine has a slower response but lingers longer. 

• Hypothalamus: brain region responsible for maintaining homeostasis, regulating hunger, thirst, body temperature, and sex drive

• Pituitary gland: “master gland”; regulates growth and controls all other endocrine glands (under the influence of the hypothalamus)

• Adrenal glands: secrete epinephrine and norepinephrine that help the body’s response in times of stress 

  • Adrenaline is both a neurotransmitter and a hormone.

• Thyroid gland: regulates metabolism

• Parathyroids: regulate calcium in blood

• Pancreas: regulate sugar in blood

• Ovary: female reproductive organ, secretes female sex hormones

• Testes: male reproductive organ, secretes male sex hormones

Most psychologists define consciousness as our subjective awareness of ourselves and our environment. The strengths and limitations of this awareness are largely debated, and can be altered by outside influences.

• Hypnosis

• Meditation

• Psychoactive Drugs

• Sleep & Dreams

Consciousness offers a reproductive advantage by helping us cope with novel situations, read the emotions of others, and follow through on long-term goals. Cognitive neuroscience is the study of the brain activity linked with cognition, including language, perception, memory, and thinking. 

At any moment, we are aware of little more than what is in our conscious awareness, but beneath the surface, unconscious information processing takes place simultaneously. This is known as dual processing. If you see a bird in flight, you’ll be consciously aware of which type of bird it is if you know, but unconsciously be taking in information about its color, size, movement, etc. Our ability to take in information about the look and movement of the bird simultaneously is known as parallel processing. 

This alternative to this is sequential processing, in which we process one aspect of a problem at a time. This is used for processing new information or solving difficult problems. Move your dominant foot in a counterclockwise direction while writing the number 3 repeatedly at the same time. This difficult task will often result in your foot going in the direction of the writing the number 3 or your hand drawing circles.

Blindsight is a condition in which a person can respond to a visual stimulus without consciously experiencing it. This is used to help explain how those without sight can sense objects in their environment.

Sleep is a periodic, natural loss of consciousness - as distinct from unconsciousness resulting from a coma, general anesthesia, or hibernation. In other words, even in sleep, you still have some level of awareness, that’s why you don’t roll off the top bunk, the music from your headphones might end up in your dreams, or why that prank of putting a sleeping friend’s hand in warm water works. 

Why do we sleep?

• Protection

• Recuperation

• Restore/rebuild memory

• Feeds creative thinking

• Growth

What causes problems with sleep?

• Age

  • Newborns need 16 hours of sleep, teenagers need 9, while most adults can survive on 6.

• Genetics

• Environment

• Brain

  • Suprachiasmatic nucleus (SCN): a pair of cell clusters in the hypothalamus that control circadian rhythm 

  • Pineal gland: adjust melatonin levels

  • Reticular formation: monitors sleep and wake cycle

• Exposure to Light

Sleep is part of our circadian rhythm - our biological clock that regulates our mood, temperature, and arousal through a 24-hour cycle. As morning approaches, our body temperatures rise and we begin to wake up. Our temperature and arousal peak around midday, then start to dip as we approach dusk. This is why most people prefer to sleep in colder temperatures - it helps you fall asleep faster.

Circadian rhythms can be thrown off by a change of time zones (jetlag) or a certain work schedule. They can also vary by age and individual (night owls vs. morning larks).

• Other biological rhythms include:

• 90-minute sleep cycles

• 28-day menstrual cycles

• Annual → Hibernation/Migration

• Mating seasons

About every 90 minus, we cycle through distinct sleep stages: NREM or non- rapid eye movement sleep and REM or rapid eye movement sleep.

• NREM-1: light sleep, easily awoken → alpha waves

  • Can experience hallucinations (false sensory experiences) and hypnagogic sensations (bizarre experiences such as jerking with a sensation of falling)

• NREM-2: fully asleep → theta waves with sleep spindles (random short bursts of activity) & k-complexes (random tall bursts of activity)

• NREM-3: deep sleep → delta waves

• REM: body relaxed as if paralyzed but brain as active as if it was awake (paradoxical sleep) → beta waves

  • Becomes longer & more frequent as night progresses

  • Vivid dreams occur

  • REM Rebound: If deprived of REM sleep, a person will spend more time in REM the next time they go to sleep. 

After oxygen, food, and water, the thing our body needs most is sleep. If we do not sleep, or do not get enough sleep, our body makes it known to us how badly it needs a break. Research shows that inadequate sleep can make you more likely to:

• Struggle with focus and concentration

• Gain weight

• Get sick

• Be irritable and/or depressed

• Feel old

  • Not-so-fun fact: Driving sleep deprived is as dangerous as driving drunk. 

• Insomnia: recurring problems falling or staying asleep

• Narcolepsy: a sleep disorder characterized by uncontrollable sleep attacks; sufferer may lapse directly into REM sleep

• Sleep Apnea: a sleep disorder characterized by temporary cessations of breathing during sleep and repeated momentary awakenings 

• Night Terrors: a sleep disorder characterized by  high arousal and an appearance of being terrified; unlike nightmares, night terrors occur during NREM-3 sleep and are not remembered

• Somnambulism: sleepwalking

• Somniloquy: sleep talking

Nightmares are emotional dreams, occurring during REM sleep with our bodies still and you are able to remember it. Night terrors affect the body very strongly, causing the heart to pound, breathing rate to increase, and blood pressure rises. You are unable to remember the event, able to move during the episode, and occurs in the first 3 hours of sleep. 

Dreams are sequences of images, emotions, and thoughts passing through a sleeping person’s mind. Although dreams can occur at any stage of sleep, they are most vivid during REM sleep. 

Why do we dream? 

• Wish-Fulfillment 

• Information Processing 

• Activation Synthesis 

• Physiological Function

• Cognition

• Negative Emotional Content: 8 out of 10 dreams have negative emotional content.

• Failure Dreams: People commonly dream about failure, being attacked, pursued, rejected, or struck with misfortune.

• Sexual Dreams: Contrary to our thinking, sexual dreams are rare. Sexual dreams in men are 1 in 10; and in women 1 in 30.

• Dreams of Gender: Women dream of men and women equally; men dream more about men(their buddies or co-workers) than women.

• Lucid dreaming: knowing that you are dreaming and occasionally being able to control aspects of your dream

• Sleep paralysis a feeling of being conscious but unable to move, occurring when a person passes between stages of wakefulness and sleep. During these transitions, you may be unable to move or speak for a few seconds up to a few minutes. 

Psychoactive drugs are chemical substances that alter perceptions and moods. While some psychoactive drugs can be used in moderation to help with concentration or pain relief, others can lead to addiction and substance use disorder (a disorder characterized by continued substance craving and use despite significant life disruption and/or physical risk. 

When is drug use a disorder?

• Diminished control & social functioning

• More is needed to get the desired effect

• It is hazardous to use

Dependence/Addiction

• Many psychoactive drugs can be harmful to the body.

• Psychoactive drugs are particularly dangerous when a person develops an addiction or becomes  physically and/or psychologically dependent on the substance.

Factors related to addiction: 

• Tolerance: diminished psychoactive effects after repeated use

• Withdrawal: painful symptoms of the body re-adjusting to the absence of the drug.

• impact on daily life of substance use

• physical and psychological dependence

Types of Drugs:

• Stimulants - excite neural activity

  • Increase energy, decrease appetite, brief feelings of euphoria

  • Speed up (stimulate) bodily processes

  • Ex. caffeine, nicotine, cocaine, amphetamines (meth)

• Depressants - reduce neural activity

  • Increase relaxation and pain relief, decrease mood and arousal

  • Slow down (depress) bodily processes

  • Ex. alcohol, barbiturates, opiates (heroin)

• Hallucinogens - distort perception

  • Cause false sensory hallucinations, impair memory, feelings of relaxation and/or euphoria

  • Ex. marijuana, mushrooms, LSD, ecstasy/MDMA (also a stimulant)

Psychoactive drugs primarily work by stimulating, inhibiting, or mimicking neurotransmitter activity. The prolonged and excessive use of alcohol (also known as alcoholism or alcohol use disorder) can shrink the brain, disrupt memory formation, reduce inhibitions, and slow functioning. 

Another state of consciousness is a near-death experience, when individuals report strange sensations or vivid visions following a close brush with death (similar to drug-induced hallucinations). 

Hypnosis is a social interaction in which one person (the hypnotist) suggests to another (the subject) that certain perceptions, feelings, thoughts, or behaviors will spontaneously occur. Hypnosis is also described as a systematic procedure that typically produces a heightened state of suggestibility. 

In psychology, hypnosis has been used to unlock repressed memories, overcome traumatic experiences, treat phobias, and cure addiction. Some phenomena associated with hypnosis include:

• Analgesia: treatment of acute and chronic pain (used by dentists/doctors as a substitute for anesthetic)

• Sensory distortions and hallucinations: sensing things that are not present, or failing to sense things that are

• Disinhibition: effect of hypnosis that removes boundaries that would typically prevent subject from performing specific actions (social norms, illegal acts, etc.)

• Posthypnotic amnesia: supposed inability to recall what one experienced during hypnosis; induced by the hypnotist’s suggestion

• Posthypnotic suggestion: a suggestion, made during a hypnosis session, to be carried out after the subject is no longer hypnotized; used by some clinicians to control undesired symptoms or behaviors

Meditation refers to a family of practices that train attention to heighten awareness and bring mental processes under greater voluntary control. There are two types of meditation:

• Focused attention: attention is concentrated on a specific object/image/sound/sensation

• Open monitoring: attention is directed to the contents of one’s moment-to-moment experience in a nonjudgmental and nonreactive way

Sensation is the process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment. All of our senses receive sensory stimulation, transform that information into neural impulses, and deliver information to the brain. Perception is the process of organizing and interpreting sensory information, enabling us to recognize meaningful objects and events. 

Under normal conditions, sensation and perception blend into one continuous process, working together to help you decipher the world around you.

• Bottom-up processing: analysis that begins with the sensory receptors and works up to the brain’s integration of sensory information (sensation first)

• Top-down processing: information processing guided by higher-level mental processes , as when we construct perceptions drawing on our experiences and expectations (experience first)

While walking through an art museum, you come to the abstract art section. If you used bottom-up processing, you would stare at the piece and try to figure out what the artist was trying to create. If you used top-down processing, you would read the title of the work and look for elements of that in the painting. 

The process of converting one form of energy to another (or, in this case, converting sensory stimulation into neural signals) is called transduction. 

The field of psychophysics studies the relationships between the physical characteristics of stimuli and our psychological experience of them. In order for a neuron to fire, the threshold to trigger an impulse must be reached. This principle continues with sensation. The absolute threshold is the minimum amount of stimulus energy needed to detect a particular stimulus 50% of the time. The concept of absolute thresholds was studied by Gustav Fechner. 

Detection of a stimulus not only relies on the strength of the stimulus, but also our psychological state - our experience, expectations, motivation, alertness, etc. Signal detection theory predicts how and when we  detect the presence of a faint stimulus amid background stimulation, thus assuming there is no single absolute threshold and that detection depends on the person. Absolute thresholds deal with detection of one stimulus, while difference thresholds (a.k.a. just noticeable difference) note the minimum difference between two stimuli required for detection 50% of the time. 

Ernest Weber described this phenomenon with a new principle - Weber’s law. Weber’s law states that, to be perceived as different, two stimuli must differ by a constant percentage rather than a constant amount. 

Consider playing with light dimmers. How much do you need to move the dimmer switch to see something as darker or lighter than before?

Earlier, we stated that if our brain was giving equal attention to all the information it received, we’d go crazy. This is one of the reasons our brain is primed to detect change and ignore constants. Sensory adaptation is diminished sensitivity as a consequence of constant stimulation or exposure.

When you know the water is cold, you are hesitant to jump in, but all the adults say the same thing - “you’ll get used to it after a few minutes”. You jump in, are frozen for a moment, and then start to acclimate to the water. 

Up until now, you have ignored much of your senses because they’ve been exposed to constants, but now that I’m bringing it to your attention (the smell of the room or your perfume, the feeling of the clothes on your skin), you notice it again.

Sensory interaction refers to the ability of one sense to influence or interact with another. Two senses that commonly interact with each other are taste and smell. So, the taste of strawberry interacts with its smell and its texture on the tongue to produce flavor.

Stimuli you cannot consciously detect 50% of the time are subliminal, or below your absolute threshold. 

Can we be controlled by subliminal messages? Some advertisers sneak subliminal messages into their marketing in hopes of increasing sales. 

Priming is a phenomenon in which exposure to one stimulus influences how a person responds to a subsequent, related stimulus. Through experience, we come to expect certain results. Those expectations may give us a perceptual set - a mental predisposition to perceive one thing and not another.

A visual illusion involves an apparently inexplicable discrepancy between the appearance of a visual stimulus and its physical reality.

• Müller-Lyer: illusion of line length that is distorted by inward-turning or outward-turning corners on the ends of the lines, causing lines of equal length to appear to be different.

• Ames Room: Due to the shape of the room-trapezoid, the person standing to the right side of the room appears much larger.

• Moon Illusion: moon always appears larger on the horizon than it does overhead

• Ponzo Illusion: an optical illusion in which two identical figures are made to appear of different sizes because of the effect of perspective

Light travels in waves, and the shape of those waves influences what we see. Light’s wavelength (distance from the peak of one wave to the next) determines the hue (color). The wave’s amplitude (height) determines the intensity (the amount of energy the wave contains) or brightness. The purity of the wave determines how vivid the color appears. 

Light enters the cornea (the eye’s clear, protective layer) and then the pupil (adjustable opening of the eye allowing light to pass through) whose size is determined by the iris (colored ring of muscle tissue). Light is then focused by the lens (transparent structure which changes shape to focus images) on the retina (the light sensitive inner surface of the eye with layers of neurons to convert light into neural impulses). The process of focusing these images is called accommodation. These neural impulses are carried to the brain via the optic nerve.

• Rods: retinal receptors that detect black, white, and gray, are sensitive to movement and are necessary for peripheral vision

• Cones: retinal receptors concentrated near the center of the retina responsible for daylight/color vision

• Fovea: the central focal point in the retina, around which the eye’s cones cluster → where visual acuity is greatest

• Blind Spot: the point at which the optic nerve exits the eye so there is an absence of receptor cells 

If you have perfect vision, you will have a perfectly spherical eyeball where all images fall perfectly on the retina. Nearsighted/myopic people (those who see things clearly up close, but struggle with things farther away) will have a longer eyeball while farsighted/hyperopic people (those who can clearly see things from far away but struggle with things up close) will have a taller eyeball. 

About 1 person in 50 is “colorblind.” That person is usually male, because the defect is genetically sex linked. Most people with color-deficient vision are not actually blind to all colors. They simply lack functioning red- or green-sensitive cones, or sometimes both. Their vision—perhaps unknown to them, because their lifelong vision seems normal—is monochromatic (one-color) or dichromatic (two-color) instead of trichromatic, making it impossible to distinguish the red and green. Dogs, too, lack receptors for the wavelengths of red, giving them only limited, dichromatic color vision.

Feature detectors are nerve cells in the visual cortex respond to specific features, such as edges, angles, and movement. For humans, we have specialized feature detectors for faces.

Damage to these feature detectors or the area of the temporal lobe responsible for facial recognition could lead to prosopagnosia (also known as face blindness or facial agnosia)- a neurological disorder characterized by the inability to recognize faces.

Blindsight is a condition in which a person can respond to a visual stimulus without consciously experiencing it. This is used to help explain how those without sight can sense objects in their environment.

This apple is obviously red, right? Of course! But actually, the apple is everything but red, because it rejects the wavelengths we process as red and absorbs the rest. Light waves are colorless but our brain perceives them in color.

Color vision is largely a mystery, but we have some theories to explain how we see a world in color.

• Young-Helmholtz Trichromatic Theory states that the retina contains three different types of color receptors for three basic colors - red, green, and blue - which when stimulated can come together to form every color. 

Why not yellow? Because when both red and green receptors are stimulated, we see yellow. 

But why do people blind to red and green often still see yellow? And why does yellow appear to be a pure color and not a mixture of red and green, the way purple is of red and blue? 

• Opponent-process theory states that color vision depends on three sets of opposing retinal processes—red-green, blue-yellow, and white-black. As impulses travel to the visual cortex, some neurons in both the retina and the thalamus are turned “on” by red but turned “off” by green. If exposed to one color for an extended period, the opposite will appear in the afterimage. 

Stare at the red circle for 30 seconds. Look at nothing else. After the 30 seconds are up, move your eyes to the X. You should see an afterimage of a red background with a green spot. 

Audition is our sense of hearing. Like light, sound travels in waves. Sound waves are composed of compression and rarefaction of air molecules. The height of the wave, or amplitude, determines the volume of the sound, measured in decibels. The frequency (number of wavelengths that pass a point in a given time) determines the pitch (highness or lowness of tone). 

Because we have two ears, sounds that reach one ear faster than the other ear cause us to localize the sound, or determine the direction of the sound’s source. 

When high frequency sounds are experienced too frequently for a single neuron to adequately process and fire for each sound event, the organ of Corti  combines the multiple stimuli into a "volley" in order to process the sounds. The volley principle states that groups of neurons of the auditory system respond to a sound by firing action potentials slightly out of phase with one another so that when combined, a greater frequency of sound can be encoded and sent to the brain to be analyzed.

Sound waves are funneled into the auditory canal by the pinna (exterior part of ear). Once in the ear canal, sound waves vibrate the eardrum (tight membrane preceding the middle ear), then the hammer/malleus, anvil/incus, and stirrup/stapes (also known as the ossicles), finally vibrating the oval window of the cochlea (the coiled, bony, fluid-filled tube of the inner ear responsible for transduction of sound). The cochlea is lined with a basilar membrane (a layer of hair cells which convert the sound waves into neural impulses). Neural impulses are carried to the brain via the auditory nerve. 

Although it has been established that the hair cells along the basilar membrane of the cochlea are responsible for transduction of sound waves, how that occurs is not fully known. Two theories propose how hair cells work:

• Place theory states that the pitch of a sound we hear is due to activation of specific hair cells on the basilar membrane (like a piano).

• Frequency theory states that the entirety of the basilar membrane vibrates in response to sound, and the speed of the vibration is how we perceive pitch (like a drum). 

Touch, our tactile sense, is vital to our development and survival. Contact comfort helps us establish bonds with caregivers, and premature babies have a better chance of survival if they are held. The tactile sensations include pain, pressure, touch, and temperature, and are processed by our parietal lobe.

Pain tells the body that something has gone wrong, usually resulting from damage to the skin and other tissues. Pain begins at sensory neurons known as nociceptors. Melzak and Wall (1965, 1983) proposed that our spinal cord contains neurological “gates” that either block pain or allow it to be sensed. Gate-control theory states that the spinal cord acts as a buffer between pain and the brain, deciding which signals will pass through; pain is a function of the balance between the information traveling into the spinal cord through large nerve fibers and information traveling into the spinal cord through small nerve fibers.

Pain is not merely a physical phenomenon of injured nerves sending impulses to a definable brain or spinal cord area. The brain can also create pain, as it does in phantom limb sensations after a limb amputation. Without normal sensory input, the brain may misinterpret and amplify spontaneous but irrelevant central nervous system activity. As the dreamer may see with eyes closed, so 7 in 10 such people feel pain or movement in nonexistent limbs.

Gustation, or our sense of taste, is a chemical sense. There are five identified taste sensations:

• Sweet - helps us identify sugary foods for energy

• Sour - helps us identify foods that have gone bad or could make us sick

• Salty - sodium is essential for physiological functioning

• Bitter - helps us identify poison or foods that could make us sick

• Umami (savory) - helps us identify foods high in protein which help grow/repair tissue

• Oleogustus - carbs/fats for energy, insulation, & cell growth

The small bumps on the surface of the tongue are called papillae. They serve as our taste receptors.

Olfaction, or our sense of smell, is also a chemical sense and works closely with taste through a process called sensory interaction (when one sense influences another). This is why if you plug your nose or have a bad sinus infection and can’t smell, you also lose your sense of taste.

Odorants enter the nasal cavity to stimulate 5 million receptors in the olfactory bulb to sense smell, and then it bypasses the thalamus and goes straight to the temporal lobe to be processed. Scientists suspect this is an evolutionary trait, as smell is our first indication that food has spoiled and will likely make us ill if consumed. This could also explain why smell is closely connected to memory; if something made us ill in the past, its smell will be a reminder not to eat it again (taste aversion). 

The vestibular sense monitors the head and body position, as well as, our sense of balance.

• Receptors in the semicircular canals and vestibular sacs of the ear

• Works with cerebellum

The sense of our individual body parts’ position and movement is called kinesthetic sense. 

• Receptors in the muscle tissues and joints

Sensation and perception have to work together to give us a complete understanding of our surroundings, thus each side works in a continuous loop where we begin to think within the body. This is known as embodied cognition - the influence of bodily sensations, gestures, and other states on cognitive preferences and judgments. 

Consider the phenomenon of synesthesia, where sensory stimulation triggers more than one sense. Have you seen the TikToks of people who see sounds, taste numbers, or hear colors?