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Content Category 6A: Sensing the Environment

by Med School Tutors on Jan 23, 2020 3:20:00 PM

Sensory Processing Vision Hearing (PSY, BIO) Other Senses (PSY, BIO)
Somatosensation (e.g. pain perception)
Taste (e.g. taste buds/chemoreceptors that detect specific chemicals)
Perception (PSY)

Please see our Resource Abbreviation Key

“Psychological, sociocultural, and biological factors affect our sensation and perception of the world. All sensory processing begins with first detecting a stimulus in the environment through sensory cells, receptors, and biological pathways.
"After collecting sensory information, we then interpret and make sense of it. Although sensation and perception are distinct functions, they are both influenced by psychological, social, and biological factors and therefore become almost indistinguishable in practice. This complexity is illuminated by examining human sight, hearing, touch, taste, and smell.
"The content in this category covers sensation and perception across all human senses.”

~AAMC Official Guide

Sensory Processing


Definition: Sensation is how a person is able to experience the world around them. It is caused by the relaying of information from sensory receptors to our brain, and how our brain interprets these messages. 


Absolute threshold: this is the minimum stimulus you can detect 50% of the time. 

Difference threshold: also known as the “just noticeable difference (JND).” This is the minimum difference between two stimuli that you can detect 50% of the time. 

For example, pretend you are holding a rock. If the rock weighs almost nothing, you might not even know it’s there. If you increase the weight of the rock, eventually you will notice it. That is the absolute threshold. Now pretend you are holding two rocks. If they weigh almost the same, you might think they weigh the same. As the difference in weight between the two rocks increases, eventually you will notice it- that is the difference threshold. 

Online Resources: KHN/AAbsolute Threshold of Sensation, Weber’s Law and Thresholds

Other Notes: This pertains to sensory threshold; for neuronal threshold for action potential, see Neurons.

Weber’s Law

Definition: Weber’s Law states that the difference threshold is proportional to the magnitude of the stimulus. 

For example, let’s say you have two rocks. If you have one rock that weighs 5 ounces, and you find your difference threshold is with another rock that weighs 6 ounces, your difference threshold would be 1.2x your original mass (5 * 1.2 = 6). If you were to have two heavier rocks, with one weighing 5 pounds, Weber’s law states that your difference threshold will be seen with another rock that weighs 6 pounds, while a rock that weighs 5 pounds and 1 ounce will be unnoticed even though the absolute magnitude difference was the same as with the 5 and 6 ounce rocks. 

Online Resources: KHN/AWeber's Law and Thresholds

Signal Detection Theory:

Definition: Signal detection theory is based on the idea that all decisions based on stimuli occur with uncertainty. In a nutshell, SDT boils down to the idea that your ability to detect a stimulus is lessened when put in a distracting environment.  

Online Resources: KHN/ASignal Detection Theory Part 1, Signal Detection Theory Part 2

Note: many consider the KA videos to be slightly confusing for this topic- try this page instead
Sensory Adaptation:

Definition: Sensory adaptation describes how your ability to respond to a certain stimulus will decrease as you are constantly exposed to the stimulus. 

Context: When you first put on a hat, you will know exactly when you put on the hat, because you feel yourself put it in. After a couple hours, you might forget you are even wearing the hat since you have gotten used to it. 

Definition: the study of the relationship between physical stimuli and cerebral responses.

Online Resources: Fechner's Law (YouTube)

Sensory Pathways:

For each sense (vision, hearing, taste, touch, smell), there is a specific pathway between stimuli and brain. These pathways will be covered in greater detail further down this page.
Types of Sensory Receptors:

Receptor Types:
Thermoreceptor: receptor for temperature changes 

Chemoreceptor: receptor for chemicals 

Photoreceptor: receptor for light and electromagnetic waves (like for vision)

Baroreceptor: receptor for pressure 

Nociceptor: receptor for pain 

Mechanoreceptor: receptor for touch and sound

Four major mechanoreceptors in mammalian skin

Meissner’s corpuscles: sensitive to light touch

Pacinian corpuscle: sensitive to vibration and pressure

Merkel’s disk: sensitive to light tough

Ruffini corpuscle: sensitive to stretching

Online Resources: KHN/A  Somatosensation


Online Resources: KHN/A — Visual sensory information


Basic Principle: The eye is the main sensory organ of the visual system. Light rays are detected and transduced into electrical and chemical signals to create images, which are what we see.


Image from BOU/P

Retina: the back of the eyeball where light is absorbed by photoreceptors and converted into neural signals sent to the brain

Cornea: the transparent, front part of the eye. The cornea protects the rest of the eye while also bending light to focus on the retina

Iris: a thin, circular structure in the eye responsible for controlling the size of the pupil. This governs how much light reaches the retina

Lens: curved structure in the eye that can change its shape to focus light rays onto the retina (when you look at objects that are far away or close by, and your eye is able to focus differently, this is because of your lens changing shape)

Optic Chiasm: an X-shaped structure formed from the crossing over of the two optic nerves, which connect the brain to the eye. This means that visual information form the left eye is processed by the right hemisphere of the brain and vice versa.

Optic Nerve: carries visual information from the eye to the brain

Pupil: an opening found at the center of the iris that allows light to reach the retina

Phototransduction: the process by which the retina converts light into electrical signals 

Image from OPE/P

Photoreceptor: neurons capable of responding to light

Cone: a specialized photoreceptor that is most active during daytime and other bright light conditions. It is able to detect color.

Rod: a specialized photoreceptor that is most active in low light conditions

Fovea: a small pit in the retina that provides central vision. It is very dense in cones.


Visual Pathways in the Brain

Image from OPE/P

Note that the right hemisphere of the brain gets the input (green in above figure) from the right half of EACH eye’s retina, which detect the left field of vision.  Hence, information from either side of the visual field crosses over to the opposite hemisphere of the brain, regardless of eye (see optic chiasm above).
Online Resources: KHN/A — Visual field processing

Parallel Processing (PSY):

Definition: Parallel processing described when multiple mental processes are executed simultaneously

Online Resources:   KHN/A — Feature detection and parallel processing  

Feature Detection (PSY):

Definition: Feature detection is the ability to respond to more specific aspects of stimuli

Online Resources:  Sparknotes Psychology Study Guide; KHN/AFeature detection and parallel processing

Hearing (PSY, BIO)


Basic Principle: Human hearing is caused by the ears’ ability to turn physical sound waves into electrical impulses that can be processed by the brain to perceive and localize sounds.

Image from BOU/P

Image from BOU/P

Basilar membrane: a small organ within the cochlea that contain the hair cells which serve as the auditory system’s sensory receptors.

Cochlea: The main sensory organ of the auditory system. It is snail-shaped and contains structures within it that convert sounds into electrical impulses.

Hair cell: sensory receptor for hearing. Hair cells vibrate due to sound waves and convert the vibrations into electrical impulses.

Incus: One of the three bones found in the middle ear. The incus transmits vibrations from the malleus to the stapes. The incus is also called the “anvil.”

Malleus: One of three bones found in the middle ear. The malleus receives vibrations from the tympanic membrane and transmits them to the incus.

Pinna: when you think “ear,” you are thinking of the pinna. This is the visible, external portion of the ear that collects sound waves.

Stapes: One of three bones found in the middle ear. The stapes receives vibrations from the incus and transmits them to the oval window.

Oval window: The oval window connects vibrations transmitted from the middle ear to the inner ear.

AUDITORY PROCESSING (e.g. auditory pathways in the brain)

Image from MISC

Online Resources: KHN/AAuditory processing


Basic principle: Sound waves hit the tympanic membrane --> tympanic membrane's vibration causes ossicles (e.g. stapes) to move --> stapes presses into the oval window --> fluid inside cochlea begins to move --> fluid mechanically stimulates hair cells, i.e. "auditory receptor cells of the inner ear embedded in the basilar membrane" --> hair cell mechanical activation stimulates generation of neural impulses that travel through the auditory nerve to the brain.

Online Resources: KHN/AAuditory processing, Cochlear implants

Other Senses (PSY, BIO)

SOMATOSENSATION (e.g. pain perception)

Definition: The processes that convey information from the body’s surface to the nervous system. Somatosensation involves a vast variety of sensory organs, and involves pain, pressure, temperature, proprioception, position, and muscle contraction.

TASTE (e.g. taste buds/chemoreceptors that detect specific chemicals)

Basic Principle: There are five conventionally recognized tastes: sweet, salty, bitter, sour, and umami. They each have different mechanisms of recognition by taste buds.

Taste bud: grouping of taste receptor cells with hair-like extensions that protrude into the central pore of the taste bud.

Bitter, sweet, and umami: receptors are GPCRs 

Sour: Hydrogen ion channels (sour is usually caused by acidic foods) 

Salty: Sodium channels

Online Resources: KHN/AGustation - structure and function


Olfactory Cells/Chemoreceptors that Detect Specific Chemicals :

Definition: Smell (olfaction) is caused by chemoreceptors in olfactory cells that are able to detect certain chemicals and transduce them into electrical impulses. 


Image from OPE/P

Online Resources: KHN/A — Olfaction - structure and function

Pheromones (BIO):
Definition: A chemical that animals secrete that affects the behavior of other members of the same species. Pheromones typically function as a means of attracting members of the opposite sex. Pheromones are picked up by the olfactory system however often not consciously recognized.

Online Resources: KHN/A — Pheromones

Olfactory Pathways in the Brain (BIO):
Definition: Like with the other senses, there is a specific pathway in the brain that processes olfactory information. Olfaction is the sense most closely tied to memory due to the close neural connections between olfaction and memory centers of the brain.

Online Resources: KHN/A — Olfaction - structure and function


Kinesthesia: perception of the position of the body

Proprioceptors: receptor cells in the body that respond to position and movement

Proprioception: perception of movement of the body

Online Resources: KHN/A — Proprioception and kinesthesia


Vestibular system: the sensory system that is responsible for body balance and orientation (for example, when things aren’t level, we just know. The part of our body that is telling us we are not level is the vestibular system).

Muscle spindle: receptors found in muscles that detect changes in the length of the muscle 

Golgi tendon organ: sensory receptor organ that can sense change in muscle tension. Found between tendons and muscles 

Perception (PSY)

Definition: Perception is how we process and perceive sensory information


Top-down processing: processing begins in the brain and used to govern a reaction. For example, you might avoid turkey because the last time you ate it, you threw up. 

Bottom-up processing: processing begins with sensory information. For example, the first time you eat turkey, you find it disgusting, and throw up.

Online Resources: KHN/A — Bottom-up vs. Top-down processing


PERCEPTUAL ORGANIZATION (e.g. depth, form, motion, constancy)

Depth perception: Our ability to perceive how far away something is in 3D space (why we can tell when something is a picture and when something is real life). 

Binocular cues: when we use both of our eyes to establish a sense of depth (typically this is our body subconsciously processing the difference between visual information between the two eyes. This is why people’s depth perception is lessened when blind in one eye).

Monocular cues: Depth perception can be perceived with only one eye, but to a lesser degree. An example of a monocular cue is motion parallax, which is the ability to perceive depth by using relative motion between the observer and the object. 

Online Resources: KHN/A — Visual cues


Definition: a set of principles in psychology that describes how humans tend to perceive stimuli in specific patterns or objects

The Gestalt principles include similarity, continuation, closure, proximity, figure/ground, and symmetry/order.

Images from OPE/P
MCAT PS Outline Order and Symmetry
Image from MISC

Online Resources: KHN/A — Gestalt Principles

*This is a living document, so it will likely be added to as we evaluate further resources.  Remember: You're an integral part of the process!  Did we get something wrong?  Is there a resource you found helpful?  Is there something you don't understand or a question you'd like to pose?  Contribute to the community by posting it below.  We'll respond as promptly as possible and give credit where credit is due.  No trolling, advertising, or trash talking, please!*

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Welcome to our Expansion of the AAMC's Outline

We’ve built on the AAMC’s MCAT outline for the Psychological, Social, and Biological Foundations of Behavior Section with links, resources, page references, definitions, images and more.

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6A: Sensing the Environment
6B: Making Sense of the Environment - Part 1 (Attention, Cognition & Consciousness)
6B: Making Sense of the Environment - Part 2 (Memory & Language)
6C: Responding to the World
7A: Individual Influences on Behavior - Part 1 (Biological Bases of Behavior & Personality)
7A: Individual Influences on Behavior - Part 2 (Psychological Disorders, Motivation & Attitude)
7B: Social Processes that Influence Human Behavior
7C: Attitude and Behavior Change
8A: Self-Identity
8B: Social Thinking
8C: Social Interactions - Part 1  (Elements of Social Interaction & Self-Presentation and Interacting with Others)
8C: Social Interactions - Part 2 (Social Behavior & Discrimination)
9A: Understanding Social Structure - Part 1 (Theoretical Approaches & Social Institutions)
9A: Understanding Social Structure - Part 2 (Social Institutions Cont'd & Culture)
9B: Demographic Characteristics and Processes
10A: Social Inequality


MISC = Misc. resources

KHN = Khan Academy video
KHN/A = Khan Academy/AAMC collaborative video
BOU = Boundless
BOU/P = Boundless Psychology
BOU/S = Boundless Sociology
OPE = OpenStax
OPE/P = OpenStax Psychology
OPE/S = OpenStax Sociology
APA = American Psychological Association

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