Mcat Reddit Law of Common Fate Continuity

Learn key MCAT concepts about consciousness, sensation, and perception, plus practice questions and answers

(Note: This guide is part of our MCAT Psychology and Sociology series.)

Table of Contents

Part 1: Introduction to consciousness, sensation, and perception

Part 2: Consciousness

a) EEGs and waveforms; beta, theta, alpha, delta, etc

b) Sleep cycles

c) Circadian rhythms

d) Consciousness-altering drugs

Part 3: Sensation

a) Sensory thresholds

b) Visual sensation

c) Auditory sensation

d) Physical sensation

e) Chemical sensation

Part 4: Perception

a) Adaptation to stimuli

b) Visual processing

c) Proprioception

Part 5: High-yield terms

Part 6: Passage-based questions and answers

Part 7: Standalone questions and answers

Part 1: Introduction to consciousness, sensation, and perception

Psychology is the study of interactions between the mind, body, and environment. The body receives sensations from the environment, encodes them in a specific way, and sends these signals to the brain to be formed into conscious perception. The combination of multiple simultaneous sensations and perceptions leads us to a clearer understanding of the world around us.

Studying this topic for the MCAT can be very daunting as it appears to cover a lot of material. The human body has many different mechanisms of sensation and perception, and understanding each of these mechanisms is no simple task. In this guide, we will break down the main concepts you will need to know for the MCAT and will provide real-life examples similar to those you will see on exam day.

Several important terms are scattered throughout this guide and are bolded. While we provide some definitions, feel free to create your own terms, definitions, and examples that serve you best! At the end of this guide, there is an MCAT-style passage and standalone questions that will test your knowledge on these topics.

Let's begin!

Part 2: Consciousness

What is consciousness? Many philosophers and psychologists have drawn different conclusions in response to this question. Dualists like Rene Descartes suggest that the mind and body are two separate entities that work together to create an internal picture of the self and the environment. Physicalists believe that there is no such separate entity as the mind and that every thought, perception, and sensation that we experience is a direct result of the physical being, namely, the body and brain. Still, others believe that consciousness arises from a mix of dualism and physicalism.

Though there is much disagreement, one thing has been made clear. Consciousness is an innate and inseparable property of living beings that describes an awareness of ourselves and the environment we live in. It requires the reception of sensations from the environment, encoding these sensations into signals that travel to the brain, and decoding these signals to create an understanding of both ourselves and the surroundings.

a) EEGs & waveforms

An EEG, or electroencephalogram, is a non-invasive test that uses electrodes to detect the brain's electrical activity and determine the active state of the brain at any given time. The brain's neurons are constantly firing, even when we are asleep. By observing the waveforms detected by the EEG, the brain's state of activity can be observed at any time.

There are several key waveforms that appear on a typical EEG. Each of these waveforms provides insight into what state of consciousness the brain is in.

• Alpha waves are typical of an awake and resting or drowsy state. These waves show a low amplitude and high-frequency pattern.

• Beta waves are typical of either an awake and alert state or REM sleep. These waves demonstrate a sawtooth pattern with low amplitude and high frequency.

• Theta waves are typical of the early stages of sleep (Stage 1 & 2). These waves show a moderate amplitude and moderate frequency.

• Delta waves are found in deep stages of sleep (Stage 3 & 4). These waves are characterized by large amplitude and low frequency.

Practices such as hypnosis and meditation can additionally alter these waveforms. Experienced practitioners of meditation may be able to experience theta waves during consciousness, while hypnosis can induce altered patterns of alpha waves.

b) Sleep cycles

Sleep serves many functions, including the consolidation of memory, the release of growth hormones, and the healing of injuries. While sleep clearly plays an important role in our lives, its complexities are poorly understood.

Our sleep cycle is divided into several stages. During sleep, the state of the brain cycles through a fixed pattern of waveforms. One sleep cycle lasts roughly 1.5 hours, so a night's sleep is typically composed of 5-6 sleep cycles. Let's take a closer look at the individual stages of the sleep cycle.

• Awakeness, though technically not part of the sleep cycle itself, is characterized by beta waves. During awakeness, individuals are conscious of their surroundings and go about their daily living.

• Drowsiness occurs when individuals have their eyes closed and are relaxing. This stage is characterized by alpha waves.

• Stage 1 is the first stage of sleep. This stage of light sleep is characterized by theta waves. The eyes are slowly rolling, and the muscles begin to relax.

• Stage 2 is also composed of theta waves and occurs when entering medium sleep. The heart rate begins to drop, breathing rate slows, and core temperature begins to fall due to changes regulated by the hypothalamus. This stage is characterized by unique EEG features called K complexes and sleep spindles, which are bursts of unique electrical activity interspersed among theta waves.

• Stage 3 marks the beginning of deep sleep. In this stage, which is characterized by large slow delta waves, the heart rate continues to fall.

• Stage 4 comprises the bulk of deep sleep. Characterized by delta waves, we achieve the deepest sleep in this stage. This is where the body undergoes the most repair, memory consolidation, and growth hormone release.

• REM (rapid eye movement) stage occurs after Stage 4 and is characterized by the same sawtooth beta waves that are found in the awake state. REM sleep is also referred to as paradoxical sleep, in which the mind is alert but the body is motionless. During this stage, there is also quick eye movement, vivid dreams and imagery, and high electrical activity in the brain. To prevent the body from receiving the brain's signals and acting out any harmful actions in dreams, the body's muscles enter a temporarily paralyzed state.

During sleep, we dream. What are dreams? Some, like Freud, say that dreams are a form of "manifest content" that reveals the "latent content"—in other words, the plotlines of the dreams show insight into the deepest secrets and unconscious desires. Others believe that dreams are a way to build problem-solving skills and improve learning during REM sleep.

Yet others believe that dreams are merely byproducts of brain activity during REM. This theory is called the activation-synthesis theory, which states that dreams serve no real purpose and are simply the result of random neural activity.

This entire sleep cycle, from stage 1 through REM sleep, lasts roughly 90 minutes. As the night progresses, the deep sleep stages (stages 3 & 4) become shorter and the REM stage becomes longer and more frequent. REM frequency decreases with aging, in addition to the total hours needed for nightly sleep. REM rebound is a phenomenon that occurs when REM sleep occurs with earlier onset and longer duration—particularly when individuals experience sleep deprivation or otherwise inadequate sleep.

Sleep disorders can also affect the duration or quality of sleep that a patient receives. Insomnia refers to an inability to fall asleep that is not attributed to a relevant cause. As a result, insomnia may be considered a symptom of another psychological disorder.

Narcolepsy is a sleep disorder in which patients fall asleep and often at inappropriate times during the day.  It is often associated with cataplexy, a sudden loss of muscle tone. Patients with these disorders may pose harm to themselves or others, as they are unable to control when they fall asleep and may be unable to protect themselves in dangerous situations.

Parasomnias are a group of sleep disorders that refer to abnormal behavior during, directly before, or directly after sleep. They are most commonly found in children and often spontaneously resolve by adolescence. Parasomnias include sleepwalking, night terrors, and sleep paralysis.

For more information on related disorders, be sure to refer to our guide on psychological disorders.

c) Circadian rhythms

Circadian rhythms are the body's natural and internal regulation of a sleep-wake cycle that repeats roughly every 24 hours. However, it has been shown that humans' internal clock possibly runs on a schedule closer to 25 hours, as was observed when an experimental subject stayed in a dark place with no light or sense of time. His sleep cycle shifted from a 24-hour to a 25-hour cycle over the course of many days.

How can circadian rhythms shift in the dark? Internal circadian rhythms compete with external cues—such as light—that regulate consciousness.

The suprachiasmatic nucleus (SCN) is the brain's internal clock. It receives signals of light from the eyes in the morning and sends a signal to the rest of the brain to increase wakefulness and alertness.  As the day progresses and night falls, the pineal gland produces melatonin, a compound that triggers drowsiness and decreases alertness. As the sleep cycles are initiated, melatonin is produced throughout the night until morning arrives, and the SCN is stimulated again.

As core body temperature rises before wakefulness, a hormone called cortisol is steadily released. In response to cortisol production, melatonin production falls, and the body is signaled to wake in the morning.

d) Consciousness-altering drugs

There are many different classes of drugs, and each affects the body and the brain differently. Let's take a deeper look into the 4 major classes of drugs you are likely to come across in your MCAT studies.

Stimulants increase physiological function. They activate the sympathetic nervous system, thus stimulating the "fight-or-flight" response. Breathing rate increases, the heart rate increases, the pupils dilate, and alertness is heightened. Many stimulants increase the release and reduce the reuptake of neurotransmitters such as dopamine and serotonin. This allows neurotransmitters to remain in the synaptic cleft in higher quantities for longer periods of time, allowing for the resulting "high" to be stronger and last longer.

Notable examples of stimulants include caffeine, nicotine, cocaine, amphetamines, and MDMA.

Opiates act at the opiate receptors, which are responsible for alleviating pain. The human body produces naturally occurring compounds called endorphins (endogenous morphine) that act at these receptors in response to pain. They also slow the breathing rate and increase dopamine levels to create a "feel-good" sensation. (This is why exercise and other endorphin-releasing activities can result in a "runner's high" and help to reduce physical or emotional pain.) In fact, most pain-relieving medications are opiates or opioids (synthetic opiates) that act at these receptors. Unfortunately, because of their extremely addictive potential and risk of respiratory suppression, these prescription medications are monitored extremely closely. Opiate overdose is a serious epidemic that has caused millions of deaths in this country, and extensive research is being done to discover new medications that can treat pain just as effectively without addictive potential or respiratory failure.

Notable examples of opiates include morphine, fentanyl, heroin, oxycodone, hydrocodone, and methadone.

Hallucinogens are drugs that act at serotonin receptors to amplify the senses and distort the perception of both external and internal reality. These drugs affect levels of the neurotransmitter serotonin, which normally regulates mood, hunger, sensory perception, sleep, sexual behavior, body temperature, and digestion. All hallucinogens cause hallucinations, or sensations and images that appear to be real (even though they are not). Hallucinogens frequently cause sensations of dissociation and separation from the physical body.

Notable examples of hallucinogens include LSD, psilocybin ("shrooms"), mescaline (peyote), DMT, and PCP.

Depressants, unlike stimulants, are drugs that decrease physiological function. Depressants tend to cloud judgment, decrease the activity of the frontal lobe of the brain, and decrease heart rate. Additionally, the time spent in REM sleep is decreased and memory consolidation decreases. These drugs increase GABA and dopamine in the brain. GABA is an inhibitory neurotransmitter that inhibits the activity of other neurons and brain regions. By increasing GABA, the regular "control mechanisms" are inhibited. Together, increases in GABA and dopamine result in feelings of reward and euphoria, disinhibition, and a decrease in pain.

Notable examples of depressants include alcohol and barbiturates (also known as tranquilizers).

Part 3: Sensation

Sensation is the process through which information is received from the external and internal environments. This information is vital in producing the correct responses to maintain homeostasis. Let's take a closer look at the components of sensation.

a) Sensory thresholds

If every single stimulus of light, sound, or touch were perceived, the brain would be flooded with information. Our centers of consciousness wouldn't be able to distinguish any significant stimuli from non-significant stimuli. For this reason, the body operates using sensory thresholds: a minimum stimulus intensity that is required from a sensation before activating the sensory signal to be sent to the brain for processing.

There are two kinds of thresholds: absolute thresholds and difference thresholds.

An absolute threshold is the minimum intensity of a stimulus needed to activate a sensory receptor at least 50% of the time. This threshold can change due to aging or other factors. For example, the absolute threshold of hearing decreases over time as hair cells die, either due to age or constant exposure to extremely loud sounds (e.g., concerts, etc.)

A difference threshold, or the just-noticeable difference (JND), is the minimum noticeable difference between two stimuli that can be recognized. A commonly used application is in the determination of the two-point threshold, a test where two pinpoints are placed simultaneously on the patient's skin. When the two points are very far apart, the patient can easily identify them as two separate stimuli. However, as the pinpoints are placed closer and closer, they reach a point when they can no longer distinguish between the two different stimuli. This distance is the difference threshold.

Note that different forms of stimuli will be subject to different forms of difference thresholds. For instance, the just-noticeable difference between two sounds will be in the form of a decibel or sound intensity level.

Weber's law explains the difference threshold in further detail. Weber's law states that two stimuli must differ by a constant proportion—rather than a constant difference—to be subject to the same perceived change.

To place this in a real-world context, let's consider the number of light bulbs in a room. If a room initially has 4 light bulbs and 2 are taken away, 50% of the light in the room would be lost. Contrast that with another scenario: if a room has 100 lightbulbs and 2 are taken away, only 2% of the light in the room would be lost. To generate the same amount of change in perceived light, 50 light bulbs would need to be removed.

Any meaningful signals that are generated by the environment and perceived must be separated from background noise. Signal detection theory describes how and when we detect signals among noise. The efficiency of signal detection may be quantified through SNR, or the signal-to-noise ratio.

	YES	NO
YES	hit (correct)	false alarm (incorrect)
NO	miss (incorrect)	rejection (correct)

b) Visual sensation

Visual sensation is produced through light stimulus. As light passes through each of the structures of the eye, it activates photoreceptors, or light receptors in the retina. These photoreceptors then encode the light signal into an electrical signal that can be transmitted to visual processing centers of the brain, located in the occipital lobe.

Figure: The anatomy of the eye.

Light enters the eye through the cornea, a clear and curved structure that refracts light and focuses it on the pupil (an opening to let light in). The lens fine-tunes the refraction angle of the light so that it lands on the retina, a sheet of photoreceptors and cells located in the back of the eye. The lens itself can be stretched or pulled by the ciliary muscle.

Figure: The structure of the retina.

Broadly speaking, the retina consists of 3 major cell layers: photoreceptors, bipolar cells, and ganglion cells. Light travels from the lens all the way to the rearmost ganglion layer and then is received sequentially by the ganglion cells, bipolar cells, and finally the photoreceptor cells. The ganglion cells group together to form the optic nerve. Each eye possesses its own optic nerve, which travels toward the brain. Importantly, the two optic nerves "cross over" each other at the optic chiasm—sending signals from the right eye to the left side of the brain, and vice versa.

Photoreceptors are classified into two types, rods and cones. These two types of photoreceptors process and transduce different kinds of light.

Rods	Cones
Black and white vision	Color vision
Respond to dim light	Respond to bright light
Low-acuity vision	High-acuity vision
Concentrated in periphery	Concentrated in fovea
Detection based on motion	Detection based on frequency of light

Photoreceptors possess specialized molecular compounds retinal and opsin. In the default dark state, retinal is in a cis-formation, which keeps the Na⁺ channel open and the photoreceptor cell depolarized at rest. The depolarized photoreceptor releases glutamate onto bipolar cells, which inhibits ON bipolar cells and in turn reduces ganglion cell firing.

Phototransduction, or the conversion of light into electrical signals, begins when light strikes a retinal molecule. The retinal responds by converting to trans-formation, causing the Na⁺ channel to close and the photoreceptor to hyperpolarize. This hyperpolarization stops the release of glutamate from the photoreceptor, allowing downstream ON bipolar cells to excite ganglion cells—thus creating an action potential in the optic nerve.

There are multiple kinds of bipolar cells, including ON and OFF cells, but many of these specifics are out of the scope of the MCAT. For the purposes of the MCAT, be sure to understand how each step affects its downstream targets.

c) Auditory sensation

Compared to our evolutionary cousins of reptiles and amphibians, mammals have extraordinarily developed middle and inner ears. Jawbones that were found in our shared ancestors were modified over time and have specialized into fine-tuned instruments that transduce sound very precisely.

The perception of sound begins as sound waves enter the outer ear and are channeled into the external canal. After traveling down the ear canal, sound waves strike the eardrum. The eardrum marks the beginning of the air-filled middle ear.

The vibrations of the eardrum are transmitted to 3 small ossicles, or ear bones: the malleus, incus, and stapes. The vibrations of these ossicles transmit pressure waves to the oval window. The oval window marks the beginning of the fluid-filled inner ear.

The vibrations from the oval window are transmitted to the perilymph and endolymph fluids in the cochlea, a coiled structure in the inner ear that separates sounds based on frequency or pitch. Place theory states that different locations within the cochlea register differing frequencies: the base of the cochlea registers high-frequency sounds, while the apex (or tip) registers low-frequency sounds.

Figure: An overview of auditory sensation.

Cochlear vibrations are transmitted to the adjacent basilar membrane, which is embedded with hair cells. When these hair cells vibrate and brush against the tectorial membrane, they transmit action potentials through the auditory nerve to the temporal lobe for auditory processing. The louder the sound, the higher the sound wave's amplitude and the greater the number of rapid action potentials.

d) Physical sensation

Much like how auditory sensation uses vibrations to transmit information, a similar method is used in transmitting information about physical sensation. This can include touch, pressure, temperature, pain, etc. Below we will go over some of the important types of receptors involved in physical sensation, or somatosensation.

Mechanoreceptors, like hair cells in the inner ear, or Pacinian corpuscles in the fingers, are activated by touch, vibration, or physical movement. In these receptors, the action potential is initiated by the mechanical movement of the receptor, for example, how the movement of the hair cell from the cochlear vibration initiates an action potential in the auditory nerve.

Nociceptors detect pain, either on the inside or outside of the body (referred to as somatic or autonomic pain). The initiation of a pain response releases a slew of chemicals. When some of these chemicals bind to the nociceptors, an action potential is initiated and a pain signal is sent to the brain.

Thermoreceptors detect changes in temperature and can distinguish between cold, warm, and hot.

Baroreceptors respond to changes in pressure. Baroreceptors are located in the carotid arteries and aorta, where they sense blood pressure and relay this information to the brain for negative feedback and proper maintenance of regular blood pressure.

e) Chemical sensation

The senses of taste and smell are facilitated by chemoreceptors, or receptors that can detect the presence of certain chemical molecules. Chemoreceptors are present in taste buds on the tongue and olfactory (smelling) pathway.

The olfactory pathway begins with the wafting of molecules through the nares, or nostrils of the nose. Scent molecules are carried on ambient air and travel upward through the nasal cavity. Olfactory receptors are found embedded within the nasal cavity, which are nerve projections containing G-protein coupled receptors, or GPCRs, on their surface. (For more information on the mechanism and function of GPCRs, be sure to refer to our guide on protein function.)

Odorous molecules in the nasal cavity can bind to a corresponding GPCR on a particular olfactory receptor, which transduces a signal sent to the brain. The olfactory receptors send a signal through the dendrite directly into the olfactory bulb of the brain. There, the resulting signal can be processed and perceived.

Since olfactory receptors are direct extensions of the olfactory bulb, olfactory information is the only sensory information that is not passed through the thalamus.

Part 4: Perception

While sensation is a process that allows us to receive environmental stimuli, perception is the process of understanding these stimuli and converting stimuli to useful information.

a) Adaptation to stimuli

It may seem advantageous to perceive every small stimulus we sense. However, filtering out certain sensory information will allow us to attentively focus on more important problems at hand.

Sensory adaptation is a natural process that allows us to filter out unnecessary sensory information from the brain. As the sensory receptors are constantly exposed to a certain stimulus—such as a strong perfume or a loud siren—for a long enough period of time, the receptors physically become less sensitive to the stimuli, and the perception of these stimuli decreases. In other words, the stimuli become less noticeable.

Desensitization, on the other hand, is often used as a treatment technique that is used to reduce the body's sensitivity to a stimulus. For example, desensitization might be used in patients with hyperalgesia, which describes extremely sensitive response to pain. By consistently and repeatedly stimulating the area, the corresponding neural circuits are flooded with sensory input. Eventually, the brain will desensitize in response to the constant input and decrease its pain response output, thus decreasing the patient's perception of pain.

b) Visual processing

Our brain makes sense of the sensory images that reach our retinas through visual processing. Many aspects of visual images (e.g., color, depth, movement, size, distance, length, shadow, etc.) must be integrated flawlessly in order to create an accurate depiction of the world.

Feature detection theory states that different areas of the brain are activated when different features of the image are processed. This is an example of parallel processing, in which many processing tasks are performed simultaneously.

Visual features, or cues, can be perceived with either a single eye or require visual input from both eyes. These cues are called monocular and binocular cues, respectively, and provide information about the depth, form, motion, and constancy of visualized objects. Monocular cues allow us to perceive depth from a 2D retinal image. Though they are great for detecting motion from close by, monocular cues often provide poor depth information from a distance. Examples of monocular cues include the following:

• Relative size & height of objects

• Interposition: in which one object blocks another

• Relative clarity: in which a more distant object appears blurry

• Relative motion: in which a more distant object moves more slowly

• Parallel lines converging

• Light & shadow

Binocular cues, on the other hand, offer the best possible depth perception and are able to detect motion at a distance. Examples of binocular cues include the following:

• Retinal disparity: in which a greater difference between the two retinal images means the object is closer

• Convergence: in which the eyes facing at a more inward angle indicates the object is closer

Parallax is a visual phenomenon that occurs when the position or movement of an object appears to be different based on where the observer is viewing it from. Think to your own experience in a train or other fast-moving vehicle: objects in the background appear to move more slowly than objects in the foreground. This is a feature of depth perception that allows the brain to perceive the 2D images on the retinas as a 3D representation of the world.

Gestalt theory is a visual philosophy that states that the whole is greater than the sum of its parts. The essence of this psychology lies in the idea that images are perceived from the top-down. Bottom-up processing means that we use and piece together sensory information to create a big picture, whereas top-down processing means that we apply past experiences and existing mental images to interpret incoming sensory information.

Figure: Bottom-up versus top-down processing

Gestalt principles guide much of our understanding of perception, especially visual perception. These rules allow us to perceive distinct objects as belonging to a common category. Let's take a look at each of these rules.

• Proximity: If objects are close together, they are considered to be a group.

• Similarity: If objects have similar characteristics, they are considered to be a group.

• Continuity: If objects are connected by smooth continuous lines, they are considered to be a group.

• Closure: If there are narrow gaps between objects, the brain will automatically fill these gaps to perceive them as a group.

• Common fate: if objects appear to move in the same direction or in the same manner, they are considered to be a group.

• Connectedness: if objects are linked to each other, they are considered to be a group.

Figure: Depictions of some Gestalt principles.

c) Proprioception

You may have realized that our body has additional senses to the five senses of smell, touch, taste, hearing, and sight. These senses include a sense of balance (the vestibular sense) and a sense of bodily position (the kinesthetic sense).

Proprioception is how we can sense and perceive the self in relation to the environment. This is achieved with proprioceptors, or various receptors in the body that send signals to the brain about the position and placement of the body parts. Proprioceptors are located all over the body, including in tendons, muscles, and joint capsules. Additionally, there are proprioceptors in the inner ear that sense the relative position of the head in correlation to the fluid in the cochlea. The cochlear fluid allows us to maintain balance; when the body is off-balanced, the cochlear fluid is disturbed and the proprioceptors detect a difference in the position of the fluid in relation to the gravitational environment. Excessive motion by the cochlear fluid causes dizziness.

_{Acknowledgements: Snigdha Nandipati}

Part 5: High-yield terms

Consciousness: an awareness of ourselves and the environment we live in

Electroencephalogram (EEG): non-invasive test that uses electrodes to detect the brain's electrical activity and determine the active state of the brain at any given time

K complexes and sleep spindles: bursts of unique electrical activity interspersed among theta waves during stage 2 sleep

REM (rapid eye movement) sleep: also known as paradoxical sleep; a late stage of the sleep cycle characterized by quick eye movement, vivid dreams and imagery, and high electrical activity in the brain

Activation-synthesis theory: states that dreams serve no real purpose and are simply the result of random EEG waves

Circadian rhythms: the body's natural and internal regulation of a sleep-wake cycle that repeats roughly every 24 hours

Suprachiasmatic nucleus (SCN): the brain's internal clock

Melatonin: a compound that triggers drowsiness and decreases alertness

Stimulant: a consciousness-altering drug that increases physiological function and stimulates the sympathetic nervous system

Opiates: consciousness-altering drugs that act at the opiate receptors, which are responsible for alleviating pain

Hallucinogens: drugs that act at serotonin receptors to amplify the senses and distort the perception of both the external and internal reality

Depressants: drugs that decrease physiological function and suppress the sympathetic nervous system

Sensory threshold: a minimum stimulus intensity that is required from a sensation before activating the sensory signal to be sent to the brain for processing; includes absolute thresholds and difference thresholds

Weber's law: states that two stimuli must differ by a constant proportion—rather than a constant difference—to be subject to the same perceived change

Photoreceptors: light receptors in the retina

Retinal: a specialized molecular compound that triggers the phototransduction cascade when struck by a photon, converting from cis- to trans-formation

Mechanoreceptors: receptors that are activated by touch or vibration or physical movement

Nociceptors: receptors that detect pain, either on the inside or outside of the body

Thermoreceptors: receptors that detect changes in temperature

Baroreceptors: receptors that detect changes in pressure

Sensory adaptation: a natural process that allows us to filter out unnecessary sensory information from the brain

Desensitization: a technique that is used to reduce the body's sensitivity to a stimulus

Feature detection theory: states that different areas of the brain are activated when different features of the image are processed

Monocular cues: visual cues that allow us to perceive depth from a 2D retinal image

Gestalt theory: a visual philosophy that states that images are perceived from the top-down

Proprioception: the means by which we are able to sense and perceive the self in relation to the environment

Part 6: Passage-based questions and answers

Exposure to bright light in the morning (bright light treatment or therapy, BLT) is a well-established treatment for mood disorders such as seasonal affective disorder (SAD), major depressive episodes from unipolar or bipolar disorders, and sleep disorders, including shift work and sleep phase disorders. Despite the use of this effective treatment for decades, the underlying biological mechanism of its beneficial effect remains unclear. Based on the observed effects of BLT on affective disorders and the hypothesis it partially acts on the circadian system, researchers investigate the effect of BLT on mood and insulin sensitivity in patients with T2DM and depression.

Researchers conducting a study hypothesized that 1 hour of morning BLT over the long term will prevent the development of circadian syndrome. Using a diurnal animal model naturally exposed to daylight, researchers concluded that therapy consisting of daily morning exposure to 3000 lux, full-spectrum electric light has beneficial health effects. Control animals and BLT animals were each maintained under NP (neutral photoperiod, aka light exposure) or SP (short photoperiod). The activity of individual animals is shown in Figure 1.

Figure 1: Percent of activity conducted during the day under various experimental conditions

_{CREATOR AND ATTRIBUTION PARTY: BILU, C., EINAT, H., ZIMMET, P. ET AL. BENEFICIAL EFFECTS OF DAYTIME HIGH-INTENSITY LIGHT EXPOSURE ON DAILY RHYTHMS, METABOLIC STATE AND AFFECT. SCI REP 10, 19782 (2020). THE ARTICLE'S FULL TEXT IS AVAILABLE HERE: HTTPS://WWW.NATURE.COM/ARTICLES/S41598-020-76636-8. THE ARTICLE IS NOT COPYRIGHTED BY SHEMMASSIAN ACADEMIC CONSULTING. DISCLAIMER: SHEMMASSIAN ACADEMIC CONSULTING DOES NOT OWN THE PASSAGE PRESENTED HERE. CREATIVE COMMON LICENSE: HTTP://CREATIVECOMMONS.ORG/LICENSES/BY/4.0/. CHANGES WERE MADE TO ORIGINAL ARTICLE TO CREATE AN MCAT-STYLE PASSAGE.}

Question 1: Which of the following conclusions is best supported by the results shown in Figure 1?

A) Animals not exposed to bright light therapy under a short photoperiod were mostly diurnal.

B) Animals exposed to bright light therapy under a short photoperiod were mostly diurnal.

C) Animals exposed to bright light therapy under a neutral photoperiod were mostly diurnal.

D) Animals exposed to bright light therapy under a neutral photoperiod were mostly nocturnal.

Question 2: Which of the following best describes the type of study described in the passage?

A) Cohort study

B) Survey study

C) Experimental study

D) Observational study

Question 3: If the study were repeated with an additional medium photoperiod (MP) condition, how might the results change?

A) Both control and BLT-MP (medium photoperiod) animals would show diurnal activity.

B) All three BLT animals would show diurnal activity.

C) None of the MP animals would show diurnal activity.

D) None of the above

Question 4: Which structure in the brain is active in maintaining subjects' sleep-wake cycle?

A) Optic nerve

B) Suprachiasmatic nucleus

C) Hypothalamus

D) Pineal gland

Question 5: How would someone living in higher latitudes react to bright light therapy, compared to an individual living in equatorial latitudes?

A) Those in higher latitudes would exhibit diurnal activity, and those in equatorial latitudes would exhibit primarily nocturnal activity.

B) Those in equatorial latitudes would show diurnal activity, while those in higher latitudes would exhibit primarily nocturnal activity.

C) Both individuals would exhibit primarily diurnal activity regardless of geographical location

D) Both individuals would exhibit primarily nocturnal activity regardless of geographical location

Answer key for passage-based questions

1. Answer choice C is correct. According to the data presented in Figure 1, the only animals that show diurnal activity are BLT-NP animals. This is indicated by the high percentage of activity taking place during the daylight hours (choice C is correct). Control-SP animals show nocturnal activity rather than diurnal activity (choice A is incorrect). BLT-SP animals show nocturnal activity rather than diurnal activity (choice B is incorrect).

2. Answer choice C is correct. Because the researchers are actively manipulating independent variables (BLT administration and photoperiod length) and measuring a dependent variable (percent activity during the day), this is an experimental study (choice C is correct). A cohort study involves multiple people who share a common experience or feature (choice A is incorrect). A survey study involves the administration of a survey and retrospective analysis of the received results (choice B is incorrect). An observational study involves observation of the subject(s) without any intervention (choice D is incorrect).

3. Answer choice D is correct. Diurnal activity implies that over 50% of activity takes place in daylight hours. Based on an extrapolation of the results, any BLT-MP condition would result in individuals exhibiting approximately 50 to 60 percent of activity during the day (choice D is correct). Control animals show nocturnal activity, regardless of the photoperiod (choice A is incorrect). Further, it has already been established that BLT-SP animals show nocturnal activity (choice B is incorrect). Based on the pattern with control-NP and control-SP animals, control-MP animals would similarly show nocturnal activity (choice C is incorrect)

4. Answer choice B is correct. The suprachiasmatic nucleus is responsible for the maintenance of circadian rhythms, even in the absence of light. It does so through the use of endogenous clock proteins (choice B is correct). The optic nerve is responsible for carrying visual signals to the occipital lobe (choice A is incorrect). The hypothalamus is involved in releasing hormones to maintain homeostasis throughout the body (choice C is incorrect). The pineal gland plays a role in maintaining sleep-wake cycles, but it does so by receiving light cues to release melatonin which induces sleep (choice D is incorrect).

5. Answer choice B is correct. In the wintertime, higher latitudes experience shorter daylight hours than equatorial latitudes. Therefore, individuals in higher latitudes are considered to have short photoperiods (SP), while individuals in equatorial latitudes have neutral photoperiods (NP). Based on the study, it is reasonable to conclude that BLT administration in SP conditions results in nocturnal activity, while BLT administration in NP conditions results in diurnal activity (choice B is correct).

Part 7: Standalone questions and answers

Question 1: Which of the following drugs is a hallucinogen?

A) LSD

B) Alcohol

C) Codeine

D) Caffeine

Question 2: Paula walks into a room that smells strongly of cologne. She is overwhelmed by the scent at first, but after 15 minutes, she barely notices the scent. Which of the following terms best describes this situation?

A) Desensitization

B) Detection

C) Sensory adaptation

D) Mitigation

Question 3: A researcher finds that infants can hear a just-noticeable difference between pitch at 2 Hz and 5 Hz. If an additional sound is played at 6 Hz, to what frequency should the sound be changed such that a similar difference is perceived?

A) 5 Hz

B) 9 Hz

C) 12 Hz

D) 15 Hz

Question 4: Which brain waves are present in the least amounts during sleep in the elderly population compared to the younger population?

A) Alpha waves

B) Beta waves

C) Theta waves

D) Delta waves

Question 5: Which receptor is NOT involved in proprioception?

A) Muscle spindle

B) Golgi tendon organ

C) Vestibular apparatus

D) Baroreceptor

Answer key to standalone questions

1. Answer choice A is correct. LSD is a type of hallucinogen, a drug that produces hallucination-like side effects through its activity on serotonin. Alcohol is a depressant (choice B is incorrect). Codeine is an opioid (choice C is incorrect). Caffeine is a stimulant (choice D is incorrect).

2. Answer choice C is correct. Sensory adaptation is the process through which the body's receptors naturally become less sensitive to a particular sensation to reduce the unwanted information being processed by the brain (choice C is correct). Desensitization, however, describes an unnatural process used to diminish an unwanted sensation and is used frequently in minimizing pain (choice A is incorrect). Detection is a general term describing the ability to detect and sense signals from the environment (choice B is incorrect). Mitigation is not a term relevant in this context (choice D is incorrect).

3. Answer choice D is correct. Weber's law states that the just-noticeable of a stimulus is based on a constant proportion of the change from initial to final stimulus, not the absolute difference between the initial and final. In other words, the proportion must stay the same. A change of 2 Hz to 5 Hz is a change by a factor of 2.5. A similar change from an initial frequency of 6 Hz would result in a final frequency of 15 Hz (choice D is correct).

4. Answer choice B is correct. REM sleep consists of beta waves (similar to an awake state), and elderly people spend less time in REM sleep than younger people. As a result, elderly people will show fewer beta waves during sleep (choice B is correct). Alpha waves are present in an awake but drowsy state (choice A is incorrect). Theta waves are present in light sleep (choice C is incorrect). Delta waves are present in deep sleep (choice D is incorrect).

5. Answer choice D is correct. Muscle spindles and Golgi tendon organs, located in skeletal muscles, are both involved in proprioception and relay information to the brain about the body's position in space (choices A and B are incorrect). The vestibular apparatus is a proprioceptor located in the inner ear that relays information about the position of the head by detecting the movement of cochlear fluid (choice C is incorrect). Baroreceptors are mechanical receptors that detect changes in pressure. Thus, they are NOT involved in proprioception (choice D is correct).

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Source: https://www.shemmassianconsulting.com/blog/consciousness-sensation-perception-mcat