Conclusion - Sensation and Perception

MCAT Behavioral Sciences Review - Kaplan Test Prep 2021–2022

Sensation and Perception

The sensory systems described in this chapter are key to your success on Test Day. Not only are the eye, ear, and other senses high-yield in their own right, but connections to topics in physics, biology, research design, and other concepts in the behavioral sciences make these key topics for passages. But sensation is only one part of the system; we must then take this raw information and process it in the brain to truly perceive the world around us. We use complex neurological pathways to integrate and sort sensory information. We then process sensory information through multiple systems, analyzing individual features and components of the environment while building expectations based on our memories and past experiences. We fill in gaps in our sensorium using Gestalt principles. And what reaches our conscious awareness is the final product: a cohesive concept of the world around us.

You’ve completed your vacation in Europe. You used your rods and cones to see the sites, your chemoreceptors to taste and smell the local food, your hair cells to listen to the local music, and your kinesthetic and vestibular senses to help navigate through physical space. As you get ready to board the plane for home, all you’re left with are your memories—a topic we’ll turn to in the next chapter.

Concept Summary

Sensation vs. Perception

· Sensation is the conversion, or transduction, of physical, electromagnetic, auditory, and other information from the internal and external environment into electrical signals in the nervous system.

· Perception is the processing of sensory information to make sense of its significance.

· Sensory receptors are nerves that respond to stimuli and trigger electrical signals.

o Sensory neurons are associated with sensory ganglia: collections of cell bodies outside the central nervous system.

o Sensory stimuli are transmitted to projection areas in the brain, which further analyze the sensory input.

o Common sensory receptors include photoreceptors, hair cells, nociceptors, thermoreceptors, osmoreceptors, olfactory receptors, and taste receptors.

· A threshold is the minimum stimulus that causes a change in signal transduction.

o The absolute threshold is the minimum of stimulus energy that is needed to activate a sensory system.

o The threshold of conscious perception is the minimum of stimulus energy that will create a signal large enough in size and long enough in duration to be brought into awareness.

o The difference threshold or just-noticeable difference (jnd) is the minimum difference in magnitude between two stimuli before one can perceive this difference.

o Weber’s law states that the jnd for a stimulus is proportional to the magnitude of the stimulus, and that this proportion is constant over most of the range of possible stimuli.

· Signal detection theory refers to the effects of nonsensory factors, such as experiences, motives, and expectations, on perception of stimuli.

o Signal detection experiments allow us to look at response bias. In a signal detection experiment, a stimulus may or may not be given, and the subject is asked to state whether or not the stimulus was given. There are four possible outcomes: hits, misses, false alarms, or correct negatives.

· Adaptation refers to a decrease in response to a stimulus over time.


· The eye is an organ specialized to detect light in the form of photons.

o The cornea gathers and filters incoming light.

o The iris divides the front of the eye into the anterior and posterior chambers. It contains two muscles, the dilator and constrictor pupillae, which open and close the pupil.

o The lens refracts incoming light to focus it on the retina and is held in place by suspensory ligaments connected to the ciliary muscle.

o The ciliary body produces aqueous humor, which drains through the canal of Schlemm.

o The retina contains rods and cones. Rods detect light and dark; cones come in three forms (short-, medium-, and long-wavelength) to detect colors.

o The retina contains mostly cones in the macula, which corresponds to the central visual field. The center of the macula is the fovea, which contains only cones.

o Rods and cones synapse on bipolar cells, which synapse on ganglion cells. Integration of the signals from ganglion cells and edge-sharpening is performed by horizontal and amacrine cells.

o The bulk of the eye is supported by the vitreous on the inside and the sclera and choroid on the outside.

· The visual pathway starts from the eye, and travels through the optic nerves, optic chiasm, optic tracts, lateral geniculate nucleus (LGN) of the thalamus, and visual radiations to get to the visual cortex.

o The optic chiasm contains fibers crossing from the nasal side of the retina (temporal visual fields) of both eyes.

o The visual radiations run through the temporal and parietal lobes.

o The visual cortex is in the occipital lobe.

· Vision, like all senses, is processed through parallel processing: the ability to simultaneously analyze and combine information regarding color, form, motion, and depth.

o Color is detected by cones.

o Form is detected by parvocellular cells, with high spatial resolution and low temporal resolution.

o Motion is detected by magnocellular cells, with low spatial resolution and high temporal resolution.

o Depth is detected by binocular neurons.

Hearing and Vestibular Sense

· The ear is divided into the outer, middle, and inner ear.

o The outer ear consists of the pinna (auricle), external auditory canal, and tympanic membrane.

o The middle ear consists of the ossicles: malleus (hammer), incus (anvil), and stapes (stirrup). The footplate of the stapes rests on the oval window of the cochlea. The middle ear is connected to the nasal cavity by the Eustachian tube.

o The inner ear contains the bony labyrinth, within which is the membranous labyrinth. The bony labyrinth is filled with perilymph; the membranous labyrinth is filled with endolymph. The membranous labyrinth consists of the cochlea, which detects sound; utricle and saccule, which detect linear acceleration; and semicircular canals, which detect rotational acceleration.

· The auditory pathway starts from the cochlea and travels through the vestibulocochlear nerve and medial geniculate nucleus (MGN) of the thalamus to get to the auditory cortex in the temporal lobe.

· Sound information also projects to the superior olive, which localizes the sound, and the inferior colliculus, which is involved in the startle reflex.

Other Senses

· Smell is the detection of volatile or aerosolized chemicals by the olfactory chemoreceptors (olfactory nerves) in the olfactory epithelium.

o The olfactory pathway starts from the olfactory nerves and travels through the olfactory bulb and olfactory tract to get to higher-order brain areas, such as the limbic system.

o Pheromones are chemicals given off by animals that have an effect on social, foraging, and sexual behavior in other members of that species.

· Taste is the detection of dissolved compounds by taste buds in papillae. It comes in five modalities: sweet, sour, salty, bitter, and umami (savory).

· Somatosensation refers to the four touch modalities: pressure, vibration, pain, and temperature.

o A two-point threshold is the minimum distance necessary between two points of stimulation on the skin such that the points will be felt as two distinct stimuli.

o Physiological zero is the normal temperature of the skin to which objects are compared to determine if they feel “warm” or “cold.”

o Nociceptors are responsible for pain perception. The gate theory of pain states that pain sensation is reduced when other somatosensory signals are present.

· Kinesthetic sense (proprioception) refers to the ability to tell where one’s body is in three-dimensional space.

Object Recognition

· Bottom-up (data-driven) processing refers to recognition of objects by parallel processing and feature detection. It is slower, but less prone to mistakes.

· Top-down (conceptually driven) processing refers to recognition of an object by memories and expectations, with little attention to detail. It is faster, but more prone to mistakes.

· Perceptual organization refers to our synthesis of stimuli to make sense of the world, including integration of depth, form, motion, and constancy.

· Gestalt principles are ways that the brain can infer missing parts of a picture when a picture is incomplete.

o The law of proximity says that elements close to one another tend to be perceived as a unit.

o The law of similarity says that objects that are similar appear to be grouped together.

o The law of good continuation says that elements that appear to follow the same pathway tend to be grouped together.

o Subjective contours refer to the perception of nonexistent edges in figures, based on surrounding visual cues.

o The law of closure says that when a space is enclosed by a group of lines, it is perceived as a complete or closed line.

o The law of prägnanz says that perceptual organization will always be as regular, simple, and symmetric as possible.

Answers to Concept Checks

· 2.1

1. Sensory receptor → afferent neuron → sensory ganglion → spinal cord → brain (projection areas)

2. 1—F, 2—A, 3—E, 4—D, 5—C, 6—G, 7—B

3. Absolute threshold is the minimum stimulus that can evoke an action potential in a sensory receptor. Threshold of conscious perception is the minimum stimulus that can evoke enough action potentials for a long enough time that the brain perceives the stimulus. The difference threshold (just-noticeable difference) is the minimum difference between two stimuli that can be detected by the brain.

4. Weber’s law explains that just-noticeable differences are best expressed as a ratio, which is constant over most of the range of sensory stimuli. Signal detection theory concerns the threshold to sense a stimulus, given obscuring internal and external stimuli.

5. Adaptation generally raises the difference threshold for a sensory response; as one becomes used to small fluctuations in the stimulus, the difference in stimulus required to evoke a response must be larger.

· 2.2

1. Cornea: gathers and focuses the incoming light

Pupil: allows passage of light from the anterior to posterior chamber

Iris: controls the size of the pupil

Ciliary body: produces aqueous humor; accommodation of the lens

Canal of Schlemm: drains aqueous humor

Lens: refracts the incoming light to focus it on the retina

Retina: detects images

Sclera: provides structural support

2. Cornea → pupil → lens → vitreous → retina (rods and cones → bipolar cells → ganglion cells) → optic nerve → optic chiasm → optic tract → lateral geniculate nucleus (LGN) of thalamus → radiations through parietal and temporal lobes → visual cortex (occipital lobe)

3. Parallel processing is the ability to simultaneously analyze color, shape, and motion of an object and to integrate this information to create a cohesive image of the world. Parallel processing also calls on memory systems to compare a visual stimulus to past experiences to help determine the object’s identity.

4. Cones are responsible for color. Parvocellular cells are responsible for form. Magnocellular cells are responsible for motion. Binocular neurons are responsible for depth.

· 2.3

1. Linear acceleration is detected by the utricle and saccule. Rotational acceleration is detected by the semicircular canals.

2. Pinna → external auditory canal → tympanic membrane → malleus → incus → stapes → oval window → perilymph in cochlea → basilar membrane → hair cells → vestibulocochlear nerve → brainstem → medial geniculate nucleus (MGN) of thalamus → auditory cortex (temporal lobe)

3. The basilar membrane is tonotopically organized: high-pitched sounds cause vibrations at the base of the cochlea, whereas low-pitched sounds cause vibrations at the apex of the cochlea.

· 2.4

1. Nostril → nasal cavity → olfactory chemoreceptors (olfactory nerves) on olfactory epithelium → olfactory bulb → olfactory tract → higher-order brain regions, including limbic system

2. Smell is sensitive to volatile or aerosolized compounds; taste is sensitive to dissolved compounds.

3. The four main modalities of somatosensation are pressure, vibration, pain, and temperature.

· 2.5

1. Bottom-up processing requires each component of an object to be interpreted through parallel processing and then integrated into one cohesive whole. Top-down processing starts with the whole object and, through memory, creates expectations for the components of the object.


Gestalt Principle



Components close to one another tend to be perceived as a unit.


Components that are similar (in color, shape, size) tend to be grouped together.

Good continuation

Components that appear to follow in the same pathway tend to be grouped together; abrupt changes in form are less likely than continuation of the same pattern.

Subjective contours

Edges or shapes that are not actually present can be implied by the surrounding objects (especially if good continuation is present).


A space enclosed by a contour tends to be perceived as a complete figure; such figures tend to be perceived as more complete (or closed) than they really are.


Perceptual organization will always be as regular, simple, and symmetric as possible.

Shared Concepts

· Behavioral Sciences Chapter 1

o Biology and Behavior

· Biology Chapter 4

o The Nervous System

· Biology Chapter 10

o Homeostasis

· Physics and Math Chapter 3

o Thermodynamics

· Physics and Math Chapter 7

o Waves and Sound

· Physics and Math Chapter 8

o Light and Optics

Discrete Practice Questions

1. A weight lifter is just able to tell the difference between 100 and 125 pounds. According to Weber’s law, the lifter would notice a difference between:

1. 125 and 150 pounds.

2. 5 and 6 pounds.

3. 25 and 35 pounds.

4. 225 and 275 pounds.

2. A man is at a party with his wife. There is loud music in the background and the location is crowded. While listening to the music, he hears what he believes to be his wife’s laughter and turns around to investigate. The man is exhibiting:

1. feature detection.

2. bottom-up processing.

3. vestibular sense.

4. signal detection.

3. A woman is at a restaurant and orders a spicy entrée. After the first bite, she experiences burning in her mouth and becomes concerned that her food is too hot for her. The next few bites are similarly uncomfortable, but after a while the spiciness seems to subside somewhat, and by the end of the meal, she doesn’t notice the spice level. The end of the meal experience is best described as:

1. adaptation.

2. signal detection.

3. a difference threshold.

4. pain perception.

4. Which sensory receptors send signals in response to tissue damage?

1. Chemoreceptors

2. Nociceptors

3. Osmoreceptors

4. Photoreceptors

5. Which part of the eye is responsible for gathering and focusing light?

1. Cornea

2. Pupil

3. Iris

4. Retina

6. A man is looking for change to do laundry. He decides to look under the seats of his car. He uses a flashlight but is still unable to get more than an obscured look at the space below. There are various items such as wrappers and papers, but the man sees the glint of silver from an object laying flat and determines it to be a coin. To make this determination, this man used:

1. signal detection.

2. sensory adaptation.

3. feature detection.

4. kinesthetic sense.

7. Upon which part of the eye are images projected and transduced into electrical signals?

1. Cornea

2. Pupil

3. Retina

4. Lens

8. The ability to sense stimuli against one’s own skin is known as:

1. somatosensation.

2. kinesthetic sense.

3. vestibular sense.

4. chemoreception.

9. Which of the following is NOT a taste modality?

1. Sweet

2. Floral

3. Savory

4. Bitter

10. Which of the following best describes the difference between endolymph and perilymph?

1. Endolymph is found in the vestibule, while perilymph is found in the cochlea.

2. Endolymph is found in the cochlea, while perilymph is found in the vestibule.

3. Endolymph is found in the membranous labyrinth, while perilymph is found in the bony labyrinth.

4. Endolymph is found in the bony labyrinth, while perilymph is found in the membranous labyrinth.

11. Chemicals that compel behavior after binding with chemoreceptors are known as:

1. pheromones.

2. olfactory receptors.

3. somatostimuli.

4. papillae.

12. Prolonged vitamin B12 deficiency can be associated with subacute combined degeneration of the spinal cord. Patients with this disease have difficulty walking because they lose the ability to feel where their feet are in space. This represents a loss of:

1. vestibular sense.

2. kinesthetic sense.

3. parallel processing.

4. feature detection.

13. A person proofreading a paper reads over a long, misspelled word in which an “e” is replaced with an “o.” The person does not recognize the error and reads the word as correct. Which of the following could explain why the proofreader read the word as correct?

1. Parallel processing

2. Feature detection

3. Top-down processing

4. Bottom-up processing

14. A corporate logo uses five unconnected angles equally spaced in a circular fashion. When viewed, it appears to be a star. Which of the following is the logo artist using to create a complete pattern to viewers?

1. Bottom-up processing

2. Top-down processing

3. Gate theory

4. Gestalt principles

15. A patient comes in with a tumor of the pituitary gland which has grown upward into the optic chiasm and caused a visual field defect. The most likely defect from compression of the optic chiasm is:

1. complete blindness in one eye.

2. loss of the upper visual fields in both eyes.

3. loss of the nasal visual fields in both eyes.

4. loss of the temporal visual fields in both eyes.

Discrete Practice Answers

1. CWeber’s law posits that thresholds are proportional. Going from 100 to 125 pounds is a 25 percent increase. (C) is a 40 percent increase while all the rest are all under 25 percent.

2. DThe man is discerning a specific noise within a field of many noises. This is the definition of signal detection. In an experimental setup, his response would be considered a hit if his wife was indeed laughing; his response would be considered a false alarm if his wife was not laughing.

3. AThe spicy food can be considered an extreme stimulus because it eclipses what the woman believes she can handle in terms of heat. However, after experiencing the stimulus over and over, the experience of spice drops to barely perceptible. This is sensory adaptation: a reduction in response to a stimulus over time.

4. BNociceptors are important for pain sensation, which would be expected during tissue damage. Chemoreceptors, (A), respond to chemicals, whether volatile or aerosolized (olfaction) or dissolved (taste). Osmoreceptors, (C), respond to changes in blood osmolarity, and photoreceptors, (D), respond to light.

5. AThe cornea is responsible for gathering and focusing light. The pupil and iris, (B) and (C), are both involved in regulating the amount of light coming into the eye but not in focusing it. The retina, (D), transduces the light into electrical signals that are sent to the brain. The lens serves a similar function to the cornea and would also be a valid answer to this question.

6. CThis man was able to distinguish the coin from other items by recognizing specific features of the coin; in this case, it was the glint of the metal surface and its position in the car. This phenomenon is called feature detection.

7. CThe retina is the part of the eye upon which images are projected. Rods and cones in the retina then convert the electromagnetic radiation into electrical signals.

8. ASomatosensation refers to the various modalities of touch: pressure, vibration, temperature, and pain. Kinesthetic sense, (B), refers to the ability to tell where one’s body is in space. Vestibular sense, (C), refers to the detection of linear and rotational acceleration in the middle ear. Finally, chemoreception, (D), refers to sensing chemicals in the environment.

9. BThe five tastes are sweet, sour, salty, bitter, and umami. Floral would be related to smell rather than taste.

10. CEndolymph is the potassium-rich fluid that bathes the hair cells of the inner ear, all of which are found within the membranous labyrinth. Perilymph is found in the space between the membranous labyrinth and the bony labyrinth. Both the membranous labyrinth and bony labyrinth contribute to the cochlea and the vestibule, eliminating (A) and (B).

11. APheromones are the volatile chemicals given off by organisms that bind with olfactory chemoreceptors and influence behavior. It is debatable if pheromones serve a role in humans, but they are known to affect foraging and sexual behavior in some animals.

12. BKinesthetic sense, or proprioception, refers to the ability to tell where body parts are in three-dimensional space. The sensors for proprioception are found predominantly in the muscles and joints. Loss of vestibular sense, (A), would also cause difficulty walking, but this would be due to a sense of dizziness or vertigo, not an inability to feel one’s feet.

13. CThe proofreader used a larger pattern to identify the word and expected to see an “e,” thus missing the error. This is related to top-down processing; the proofreader used recognition and expectations, which led to missing a detail. Bottom-up processing, (D), would be the analysis of each detail individually before creating a cohesive image.

14. DGestalt principles are the basis for many optical illusions and include the tendency of people to see continuity even when lines are unconnected. Specifically, this logo appears to rely on the law of closure to create one complete star from five nontouching angles.

15. DThe optic chiasm houses the crossing fibers from each optic nerve. Specifically, the fibers coming from the nasal half of the retina in each eye cross in the chiasm to join the optic tract on the opposite side. Remember that the lens of the eye causes inversion, so images on the nasal half of the retina actually originate in the temporal visual field. This condition is called bitemporal hemianopsia.

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