Sensation and Perception: An Overview of Sensory Systems and Gestalt Principles

Our senses are super important! They help us find food, stay safe, connect with others, and avoid danger. This part of the notes explains how our body takes in information from the world and how our brain makes sense of it, shaping what we actually experience.

Sensation vs. Perception

• Sensation: This is when our body's detectors (like our eyes or ears) first pick up information from the outside world. It's the initial raw input.

• Perception: This is how our brain understands and makes sense of that raw sensory information, turning it into something we consciously experience. It's organizing and giving meaning to what our senses detect.

Physical Properties of Wave Forms

Things we see (light) and hear (sound) often travel in waves. These waves have some basic features that are key to how we perceive them.

Amplitude and Wavelength

• Amplitude: Think of this as the "height" of a wave. It's the distance from the middle line of the wave to its highest point (called a crest) or its lowest point (called a trough). A taller wave has a bigger amplitude.

• Wavelength: This is the length of one complete wave. Imagine measuring from one peak of a wave to the very next peak (Figure 5.5).

Frequency

• Frequency: This tells us how many waves pass by a certain point in a specific amount of time.

• Measurement: We usually measure frequency in hertz ($Hz$), which basically means "cycles per second." So, $10 Hz$ means $10$ waves pass by in one second.

• Relationship between Wavelength and Frequency:- If waves are long, fewer of them will pass by in a given time, so they have lower frequencies.

  • If waves are short, many more can pass by, so they have higher frequencies (Figure 5.6).

Light Waves: The Visible Spectrum

Electromagnetic Spectrum

• The electromagnetic spectrum is like a big family of all kinds of energy waves in our environment.

• This family includes things like gamma rays, X-rays, ultraviolet light, the light we can see (visible light), infrared light, microwaves, and radio waves.

• The visible spectrum is just the tiny part of this huge family that human eyes can actually detect (Figure 5.7).

Human Visible Spectrum and Associated Perception

• Wavelength Range: For humans, the light we can see has wavelengths between about $380 \text{ nm}$ and $740 \text{ nm}$. A nanometer ($\text{nm}$) is a tiny unit of length, equivalent to one billionth of a meter ($1 \text{ nm} = 10^{-9} \text{ m}$), so these wavelengths are very small!

• Perception of Color (Hue): The wavelength of light is directly connected to the color we see (Figure 5.8).

  • Longer wavelengths: These appear red to us.

  • Medium wavelengths: These appear green.

  • Shorter wavelengths: These appear blue and violet.

  • Tip to remember: The acronym ROYGBIV (Red, Orange, Yellow, Green, Blue, Indigo, Violet) helps us remember the colors in order from longest to shortest wavelength.

• Perception of Brightness/Intensity: How bright or intense a color appears depends on the wave's amplitude (its height). Taller waves (larger amplitudes) make colors look brighter.

Species Variations in Vision

• Not all animals see the same way! Different species can detect different parts of the electromagnetic spectrum.

• Honeybees: They can see ultraviolet light, which humans can't (Wakakuwa, Stavenga, & Arikawa, 2007).

• Some Snakes: Besides regular visible light, some snakes can also detect infrared radiation, which is like sensing heat (Chen et al., 2012; Hartline et al., 1978).

Sound Waves

Physical Properties and Perception of Sound

• Pitch: The frequency of a sound wave determines how high or low we perceive a sound to be.

  • High-frequency sound waves sound like high-pitched notes.

  • Low-frequency sound waves sound like low-pitched notes.

• Audible Range (Humans): Humans can hear sounds with frequencies generally between $20 \text{ Hz}$ and $20000 \text{ Hz}$. We hear best (are most sensitive) to sounds in the middle of this range.

• Loudness: The loudness of a sound is mostly determined by the amplitude (height) of its wave. Waves with larger amplitudes sound louder.

• Measurement of Loudness: Loudness is measured in decibels ($dB$). This is a special unit that describes how intense a sound is.

  • A typical conversation: $60 \text{ dB}$ (medium loud).

  • A rock concert: $120 \text{ dB}$ (very loud for most people) (Figure 5.9).

  • Whispering or rustling leaves: Very quiet sounds at the low end of our hearing ability.

  • Everyday sounds like an air conditioner or normal talking: These are in a comfortable range.

• Hearing Damage: Sounds between about $80 \text{ dB}$ and $130 \text{ dB}$ can potentially harm our hearing. About one-third of all hearing loss comes from being exposed to too much noise.

  • Examples of harmful sounds: Food processors, lawnmowers, heavy trucks nearby, subway trains, live rock music, jackhammers.

  • Listening to music loudly through earbuds ($100-105 \text{ dB}$) for just $15$ minutes can cause noise-induced hearing loss and increase the risk of hearing loss as we get older (Kujawa & Liberman, 2006).

• Pain Threshold: Sounds around $130 \text{ dB}$ are so loud they can cause pain (e.g., a jet taking off nearby or a gun firing close up) (Dunkle, 1982).

• Interaction between Frequency and Amplitude: While amplitude usually relates to loudness, it's not the only factor.

  • A very low-frequency sound ($10 \text{ Hz}$) might be inaudible no matter how high its amplitude is.

  • However, a mid-frequency sound ($1000 \text{ Hz}$) will definitely sound louder as its amplitude increases.

Species Variations in Hearing

• Just like vision, different animals hear different ranges of sound frequencies:

  • Chickens: Hear from $125 \text{ Hz}$ to $2000 \text{ Hz}$.

  • Mice: Hear from $1000 \text{ Hz}$ to $91000 \text{ Hz}$ (much higher than humans!).

  • Beluga Whale: Hear from $1000 \text{ Hz}$ to $123000 \text{ Hz}$ (even higher!).

  • Dogs: Roughly $70 \text{ Hz}$ to $45000 \text{ Hz}$.

  • Cats: Roughly $45 \text{ Hz}$ to $64000 \text{ Hz}$ (Strain, 2003).

Anatomy and Function of the Auditory System

Our ears are amazing! They turn invisible sound waves (changes in air pressure) into meaningful sounds, allowing us to enjoy music, understand speech, and know what's happening around us.

Divisions of the Ear

Our ear has three main parts:

1. Outer Ear: (Figure 5.18)

   • Pinna: This is the part of your ear you can see on the side of your head.

   • Auditory Canal: This is the tube that leads from the pinna to your eardrum.

   • Tympanic Membrane (Eardrum): A thin skin-like sheet inside your ear that vibrates when sound waves hit it.

2. Middle Ear: (Figure 5.18)

   • This tiny space contains three small bones, which are the smallest bones in your body, called ossicles:

     • Malleus (hammer)

     • Incus (anvil)

     • Stapes (stirrup)

3. Inner Ear: (Figure 5.18)

   • Semi-circular Canals: These loops help us keep our balance and sense movement.

   • Cochlea: This is a spiral-shaped, fluid-filled structure, kind of like a snail shell. It contains the special "hair cells" that are the actual sound detectors for our hearing system.

   • Basilar Membrane: A delicate strip of tissue inside the cochlea where these important hair cells are located.

Process of Hearing

Here's how sound travels through your ear and turns into something your brain can understand:

1. Sound waves travel through your auditory canal and hit your tympanic membrane (eardrum), making it vibrate.

2. This vibration then makes the three tiny ossicles in your middle ear move like a chain reaction.

3. The last ossicle, the stapes, pushes against a small opening in the cochlea called the oval window.

4. When the oval window moves, it makes the fluid inside the cochlea slosh around.

5. This fluid movement causes the tiny hair cells (which are the real sound sensors) embedded in the basilar membrane to bend and move.

6. When the hair cells move, it's a mechanical action that triggers electrical signals, called neural impulses.

7. These neural impulses then travel along the auditory nerve all the way to your brain.

8. The sound information goes through a few stops in the brain (like the inferior colliculus and medial geniculate nucleus of the thalamus) before finally reaching the auditory cortex in the temporal lobe, which is where your brain processes what you hear.

9. Interestingly, information about what a sound is (recognizing it) and where it came from (localizing it) seems to be processed at the same time by different parts of the brain (Rauschecker & Tian, 2000; Renier et al., 2009).

Pitch Perception

How high or low a sound seems (its pitch) depends on the sound wave's frequency. Low-frequency waves mean low-pitched sounds, and high-frequency waves mean high-pitched sounds. Scientists have two main ideas about how our ears and brain figure out pitch:

Temporal Theory of Pitch Perception

• This theory says that the frequency of a sound is figured out by how fast our sensory neurons (nerve cells) fire off signals.

• So, a hair cell would send out electrical bursts at a rate that matches the sound wave's frequency.

• Hold on, though: Humans can hear a really wide range of frequencies, from $20$ to $20000 \text{ Hz}$. But our nerve cells can only fire so fast because of how their tiny electrical parts work. So, one single cell can't fire fast enough to cover every single frequency we hear (Shamma, 2001). This is a limitation of this theory alone.

Place Theory of Pitch Perception

• This theory suggests that different parts of the basilar membrane (where the hair cells are) are sensitive to different sound frequencies (Shamma, 2001).

  • The part of the basilar membrane closest to the entrance (the base) responds best to high-frequency sounds.

  • The far end of the basilar membrane (the tip) responds best to low-frequency sounds.

• So, hair cells located at the base are registered by the brain as "high-pitch" detectors, and those at the tip are "low-pitch" detectors.

Combined Explanation

• It turns out both theories are important in different ways.

• For sounds with frequencies up to about $4000 \text{ Hz}$, both how fast nerve cells fire (temporal theory) and where on the basilar membrane the action happens (place theory) help us hear the pitch.

• But for much higher frequency sounds, only the "place" on the basilar membrane can explain how we hear their pitch (Shamma, 2001).

Sound Localization

Knowing where a sound comes from in our environment is just as important as seeing depth visually. Our auditory system uses clues from one ear (monaural) and clues from both ears (binaural) to pinpoint sound locations.

Monaural Cues

• The visible part of your ear (the pinna) affects incoming sound waves differently depending on where the sound is coming from relative to your body.

• This unique interaction helps your brain figure out if a sound is coming from above, below, in front, or behind you.

• Monaural cues are really useful because if a sound comes from exactly in front, behind, above, or below you, it would hit both ears at the same time and with the same intensity. Your pinna's shape helps distinguish these.

Binaural Cues

• These clues rely on comparing the sound information received by both of your ears to figure out where a sound is coming from along a horizontal line (left or right).

• If a sound isn't coming from directly in front of you, it creates two types of binaural cues (Grothe et al., 2010; Figure 5.19):

  • Interaural Level Difference: A sound coming from your left, for example, will be louder (more intense) in your left ear than in your right ear. This is because your head blocks some of the sound wave as it travels to the farther ear.

  • Interaural Timing Difference: This refers to the tiny difference in time it takes for a sound wave to reach each ear. A sound from your left will arrive at your left ear a tiny bit sooner than your right ear.

• Specific brain areas constantly check these differences to build a mental map of where the sound is coming from.

Hearing Loss

Deafness means you can't hear partly or at all.

Types of Hearing Loss

1. Congenital Deafness: This means someone is born deaf.

2. Conductive Hearing Loss: This happens when there's a problem getting sound energy to the cochlea (the inner ear's sound processor).

   • Causes: Things like an ear canal blockage, a hole in the eardrum, issues with the tiny middle ear bones (ossicles), or fluid buildup between the eardrum and cochlea.

   • Treatment: Often helped by devices like hearing aids. These amplify (make louder) incoming sound waves, which helps the eardrum vibrate and the ossicles move better.

3. Sensorineural Hearing Loss: This is the most common kind of hearing loss. It means the neural signals (electrical messages) aren't being sent correctly from the cochlea to the brain.

   • Causes: This can be due to getting older, head injuries, loud noises (noise-induced hearing loss; Figure 5.20), infections (like measles or mumps), some medicines, tumors, or even certain toxins.

   • Ménière's Disease: A specific condition related to sensorineural hearing loss where parts of the inner ear get damaged. This can cause hearing loss, ringing in the ears (tinnitus), a spinning sensation (vertigo), and increased pressure in the inner ear (Semaan & Megerian, 2011).

   • Treatment: Ordinary hearing aids usually don't help here. But some people can get cochlear implants.

     • Cochlear Implants: These are electronic devices that have a microphone, a speech processor, and a set of electrodes. They pick up sound, process it, and then directly stimulate the auditory nerve, sending signals straight to the brain.

Congenital Insensitivity to Pain (Congenital Analgesia)

• This is an extremely rare genetic condition where people are born unable to feel pain.

• They can still feel things like temperature differences and pressure, but they don't experience pain.

• Sadly, people with this condition often get serious injuries they don't notice, leading to infections and shorter lives (U.S. National Library of Medicine, 2013).

Deaf Culture

• In places like the United States, deaf people often form their own community with a unique culture. They have their own language (for example, American Sign Language - ASL), schools, and traditions.

• ASL is a visual language made up entirely of signs and gestures, with no spoken words.

• A key belief in deaf culture is preserving traditions like sign language. They often prefer this over things like teaching deaf children to speak, read lips, or get cochlear implants, viewing these as attempts to "fix" deafness rather than embrace deaf identity.

The Chemical Senses (Taste and Smell)

Taste and smell are called "chemical senses" because their detector cells react to actual tiny molecules from food or the air. These two senses work very closely together; what we call "flavor" is actually a mix of taste and smell.

Taste (Gustation)

• Basic Taste Groupings: We used to think there were just four basic tastes (sweet, salty, sour, bitter). Now, research shows there are at least six:

  1. Sweet

  2. Salty

  3. Sour

  4. Bitter

  5. Umami: A Japanese word meaning "yummy," often linked to the savory taste of things like MSG (monosodium glutamate) (Kinnamon & Vandenbeuch, 2009).

  6. Fatty Content: There's growing evidence that we might have a specific taste for fatty foods (Mizushige, Inoue, & Fushiki, 2007).

• How Taste Works:

  1. When you eat or drink, molecules from the food dissolve in your saliva.

  2. These dissolved molecules then interact with special taste receptors found on your tongue, and also a bit in your mouth and throat.

  3. Taste buds are tiny clusters of these taste receptor cells. Each taste bud has little hair-like extensions that stick out into a central opening (Figure 5.21).

  4. When taste molecules stick to these receptors, it causes chemical changes in the taste cell, which then generates electrical signals (neural impulses).

  5. These neural impulses are sent to the brain through different nerves.

  6. The taste information travels through the medulla and thalamus, then to the limbic system, and finally reaches the gustatory cortex (which is hidden under the front and side parts of your brain) for detailed processing (Maffei, Haley, & Fontanini, 2012; Roper, 2013).

• Taste Bud Lifespan: Your taste buds don't last forever! They typically live for about $10$ days to $2$ weeks and then regenerate, meaning new ones grow back.

Smell (Olfaction)

• How Smell Works:

  1. The special cells that detect smell, called olfactory receptor cells, are located in a mucus layer at the very top of your nose.

  2. Tiny hair-like parts of these receptors are where odor molecules (which have dissolved in the mucus) connect with chemical receptors (Figure 5.22).

  3. Once an odor molecule links up with a receptor, it causes chemical changes in the cell, sending signals to the olfactory bulb.

  4. Olfactory Bulb: This is a small, bulb-shaped structure at the front tip of your brain (frontal lobe) where the nerves for smell begin.

  5. From the olfactory bulb, smell information is sent to parts of the limbic system (which deals with emotions and memory) and the primary olfactory cortex, which is located near the taste cortex (Lodovichi & Belluscio, 2012; Spors et al., 2013).

• Species Variations in Olfaction:

  • Dogs have incredible senses of smell, much better than humans. They can even potentially detect tiny changes in blood sugar or cancerous tumors in people (Wells, 2010).

  • This amazing ability in dogs might be because they have many more working genes for olfactory receptors (between $800$ and $1200$) compared to humans and other primates (who have fewer than $400$) (Niimura & Nei, 2007).

• Pheromones: Many animals communicate using chemical messages called pheromones, which are released by one individual and affect others of the same species (Wysocki & Preti, 2004).

  • Pheromones often give information about a potential mate's reproductive state (e.g., female rats) (Furlow, 1996, 2012; Purvis & Haynes, 1972; Sachs, 1997).

  • There's ongoing research and debate about whether humans also release and respond to pheromones (Comfort, 1971; Russell, 1976; Wolfgang-Kimball, 1992; Weller, 1998).

The Body Senses: Touch, Thermoception, and Nociception

Our skin is packed with different kinds of detectors that help us feel the world.

Receptors in the Skin

Our skin has various special receptors spread out to respond to different kinds of touch (Figure 5.23):

• Meissner's Corpuscles: These pick up pressure and softer, lower-frequency vibrations.

• Pacinian Corpuscles: These detect fleeting pressure and faster, higher-frequency vibrations.

• Merkel's Disks: These respond specifically to light pressure, like a gentle touch.

• Ruffini Corpuscles: These sense when your skin is stretching.

Free Nerve Endings

• Besides these specialized receptors, we also have simpler free nerve endings that do sensory jobs.

• They react to many touch-related sensations.

• They are the sensory detectors for:

  • Thermoception: Our ability to feel temperature (hot and cold).

  • Nociception: This is a sensory signal that indicates potential harm or injury, and can lead to the feeling of pain (Garland, 2012; Petho & Reeh, 2012; Spray, 1986).

Neural Pathway

Information from all these skin receptors and free nerve endings travels up your spinal cord. From there, it's sent to parts of the medulla and thalamus, and finally reaches the somatosensory cortex, which is an area in the parietal lobe of your brain that processes touch, temperature, and pain.

Pain Perception

Pain is an uncomfortable feeling that has both physical sensations and emotional/psychological aspects. It's actually helpful because it tells us when something is wrong (like an injury) and motivates us to avoid further harm.

Types of Pain

1. Inflammatory Pain: This type of pain tells us that tissues in our body have been damaged (like a cut or bruise).

2. Neuropathic Pain: This pain comes from damage to the nerve cells themselves, either in the nerves throughout the body or in the brain/spinal cord. It often leads to pain signals that are stronger or more persistent than they should be.

Pain Relief Options

Ways to treat pain range from relaxation techniques and painkiller medicines to more advanced treatments like deep brain stimulation. The best treatment depends on how severe and long-lasting the pain is, and other medical or psychological conditions a person might have.

The Vestibular Sense, Proprioception, and Kinesthesia

The Vestibular Sense

• This sense helps us keep our balance and maintain our body's posture.

• Sensory Organs: The special organs for this sense are located right next to the cochlea in your inner ear. They include the utricle, saccule, and three semicircular canals (Figure 5.24).

• How it Works: These organs are filled with fluid and contain hair cells, much like your hearing system. They respond to movements of your head and the force of gravity. When they are stimulated, they send signals to your brain through the vestibular nerve.

• Importance: Even though we're usually not aware of it, this sense is crucial. You notice it when you feel carsick or dizzy from an inner ear infection (Khan & Chang, 2013).

• Function: It gathers vital information for controlling how we move and for reflexes that help us adjust our body when our position changes.

Proprioception and Kinesthesia

• Proprioception: This is your brain's awareness of where your body parts are in space, even without looking (e.g., knowing where your arm is without seeing it).

• Kinesthesia: This is your perception of your body's movement through space (e.g., knowing your arm is moving).

• How they work together: Both of these senses combine their information with what your vestibular system tells you.

• Receptors: They get their information from special receptors that respond to stretching and tension in your muscles, joints, skin, and tendons (Lackner & DiZio, 2005; Proske, 2006; Proske & Gandevia, 2012).

• Neural Pathway: Proprioceptive and kinesthetic information travels up your spinal column to your brain. Various parts of the brain, including the cerebellum, receive and exchange information with these sensory organs.

Gestalt Psychology of Perception

Started in the early $20^{\text{th}}$ century by thinkers like Max Wertheimer, Wolfgang Köhler, and Kurt Koffka, Gestalt psychology is based on the idea that when we perceive things, we see more than just individual pieces of sensory information. The German word "gestalt" means "form" or "pattern." Their main idea is that the whole thing we perceive is greater and different from just adding up its individual parts.

Gestalt psychologists identified several rules that our brain uses to organize sensory information into predictable and meaningful impressions.

Gestalt Principles of Perceptual Organization

1. Figure-Ground Relationship: When we look at something, we usually separate it into two main parts (Figure 5.25):

   • Figure: This is the main object or person that we focus on.

   • Ground: This is the background around the figure.

   • What we see as the "figure" and what we see as the "ground" can change our perception. Our ability to understand sensory information often depends on making this distinction (though some researchers have debated this idea) (Peterson & Gibson, 1994; Vecera & O’Reilly, 1998).

2. Proximity: We tend to group together things that are close to each other (Figure 5.26).

   • Example: You read "this sentence like this" because you see letters grouped into words (no spaces within words) and words grouped into sentences (spaces between words).

3. Similarity: We tend to group together things that look alike (Figure 5.27).

   • Example: In a football game, you group players by the color of their uniforms to tell the teams apart.

4. Law of Continuity (Good Continuation): Our brains prefer to see smooth, continuous lines and patterns rather than broken or choppy ones (Figure 5.28). If you see a line that seems to continue behind another object, you'll perceive it as still continuing.

5. Closure: Our brains like to see complete objects. We tend to fill in any missing gaps to perceive a whole, finished shape or object (Figure 5.29).

Pattern Perception

• According to Gestalt thinkers, our ability to tell the difference between various shapes and patterns (pattern perception) happens because our brains use these principles to organize what we see.

The Depths of Perception: Bias, Prejudice, and Cultural Factors

Perception isn't just about what our senses detect; it's also deeply shaped by our past experiences, our individual biases, any prejudices we might hold, and our culture. This means that two people can experience the exact same thing but perceive it very differently.

Implicit Bias and Stereotypes

• Research shows that unconscious racial prejudice and stereotypes can actually change how we perceive things.

• Examples:

  • In studies, people who are not Black tend to spot weapons faster, and are more likely to mistakenly identify harmless objects as weapons, when those objects are shown alongside pictures of Black individuals (Payne, 2001; Payne, Shimizu, & Jacoby, 2005).

  • In video games, White individuals tend to make quicker decisions to shoot an armed target if the target is Black (Correll, Park, Judd, & Wittenbrink, 2002; Correll, Urland, & Ito, 2006).

• Why this matters: This research is extremely important, especially considering real-world events where unarmed Black individuals have been killed by people who claimed they saw a threat.

Perceptual Hypotheses and Perceptual Set

• Our perceptions are built on perceptual hypotheses: these are like educated guesses our brain makes to interpret the sensory information it receives.

• These guesses are influenced by our personality, our past experiences, and what we expect to see or hear.

• Based on these hypotheses, we develop a perceptual set, which is a readiness or tendency to perceive a stimulus in a specific way. If you expect to see something, you're more likely to see it.

• Example: If someone tells you what to expect beforehand (verbal priming), it can make you interpret uncertain images in a biased way (Goolkasian & Woodberry, 2010).

Key Terms (Glossary)

• Absolute Threshold: Minimum amount of stimulus energy that must be present for the stimulus to be detected $50\%$ of the time.

• Afterimage: Continuation of a visual sensation after removal of the stimulus.

• Amplitude: Height of a wave.

• Basilar Membrane: Thin strip of tissue within the cochlea that contains the hair cells which serve as the sensory receptors for the auditory system.

• Binaural Cue: Two-eared cue used to localize sound.

• Binocular Cue: Cue that relies on the use of both eyes.

• Binocular Disparity: Slightly different view of the world that each eye receives.

• Blind Spot: Point where we cannot respond to visual information in that portion of the visual field.

• Bottom-Up Processing: System in which perceptions are built from sensory input.

• Closure: Organizing our perceptions into complete objects rather than as a series of parts.

• Cochlea: Fluid-filled, snail-shaped structure that contains the sensory receptor cells of the auditory system.

• Cochlear Implant: Electronic device that consists of a microphone, a speech processor, and an electrode array to directly stimulate the auditory nerve to transmit information to the brain.

• Conductive Hearing Loss: Failure in the vibration of the eardrum and/or movement of the ossicles.

• Cone: Specialized photoreceptor that works best in bright light conditions and detects color.

• Congenital Deafness: Deafness from birth.

• Congenital Insensitivity to Pain (Congenital Analgesia): Genetic disorder that results in the inability to experience pain.

• Cornea: Transparent covering over the eye.

• Deafness: Partial or complete inability to hear.

• Decibel ($dB$): Logarithmic unit of sound intensity.

• Depth Perception: Ability to perceive depth.

• Electromagnetic Spectrum: All the electromagnetic radiation that occurs in our environment.

• Figure-Ground Relationship: Segmenting our visual world into figure and ground.

• Fovea: Small indentation in the retina that contains cones.

• Frequency: Number of waves that pass a given point in a given time period.

• Gestalt Psychology: Field of psychology based on the idea that the whole is different from the sum of its parts.

• Good Continuation (Continuity): We are more likely to perceive continuous, smooth flowing lines rather than jagged, broken lines.

• Hair Cell: Auditory receptor cell of the inner ear.

• Hertz ($Hz$): Cycles per second; measure of frequency.

• Inattentional Blindness: Failure to notice something that is completely visible because of a lack of attention.

• Incus: Middle ear ossicle; also known as the anvil.

• Inflammatory Pain: Signal that some type of tissue damage has occurred.

• Interaural Level Difference: Sound coming from one side of the body is more intense at the closest ear because of the attenuation of the sound wave as it passes through the head.

• Interaural Timing Difference: Small difference in the time at which a given sound wave arrives at each ear.

• Iris: Colored portion of the eye.

• Just Noticeable Difference: Difference in stimuli required to detect a difference between the stimuli.

• Kinesthesia: Perception of the body’s movement through space.

• Lens: Curved, transparent structure that provides additional focus for light entering the eye.

• Linear Perspective: Perceive depth in an image when two parallel lines seem to converge.

• Malleus: Middle ear ossicle; also known as the hammer.

• Meissner’s Corpuscle: Touch receptor that responds to pressure and lower frequency vibrations.

• Ménière's Disease: Results in a degeneration of inner ear structures that can lead to hearing loss, tinnitus, vertigo, and an increase in pressure within the inner ear.

• Merkel’s Disk: Touch receptor that responds to light touch.

• Monaural Cue: One-eared cue to localize sound.

• Monocular Cue: Cue that requires only one eye.

• Neuropathic Pain: Pain from damage to neurons of either the peripheral or central nervous system.

• Nociception: Sensory signal indicating potential harm and maybe pain.

• Olfactory Bulb: Bulb-like structure at the tip of the frontal lobe, where the olfactory nerves begin.

• Olfactory Receptor: Sensory cell for the olfactory system.

• Opponent-Process Theory of Color Perception: Color is coded in opponent pairs: black-white, yellow-blue, and red-green.

• Optic Chiasm: X-shaped structure that sits just below the brain’s ventral surface; represents the merging of the optic nerves from the two eyes and the separation of information from the two sides of the visual field to the opposite side of the brain.

• Optic Nerve: Carries visual information from the retina to the brain.

• Ossicles: Three tiny bones in the middle ear consisting of the malleus, incus, and stapes.

• Pacinian Corpuscle: Touch receptor that detects transient pressure and higher frequency vibrations.

• Pattern Perception: Ability to discriminate among different figures and shapes.

• Peak (Crest): Highest point of a wave.

• Perception: Way that sensory information is interpreted and consciously experienced.

• Perceptual Hypothesis: Educated guess used to interpret sensory information.

• Pheromone: Chemical message sent by another individual.

• Photoreceptor: Light-detecting cell.

• Pinna: Visible part of the ear that protrudes from the head.

• Pitch: Perception of a sound’s frequency.

• Place Theory of Pitch Perception: Different portions of the basilar membrane are sensitive to sounds of different frequencies.

• Principle of Closure: Organize perceptions into complete objects rather than as a series of parts.

• Proprioception: Perception of body position.

• Proximity: Things that are close to one another tend to be grouped together.

• Pupil: Small opening in the eye through which light passes.

• Retina: Light-sensitive lining of the eye.

• Rod: Specialized photoreceptor that works well in low light conditions.

• Ruffini Corpuscle: Touch receptor that detects stretch.

• Sensation: What happens when sensory information is detected by a sensory receptor.

• Sensorineural Hearing Loss: Failure to transmit neural signals from the cochlea to the brain.

• Sensory Adaptation: Not perceiving stimuli that remain relatively constant over prolonged periods of time.

• Signal Detection Theory: Change in stimulus detection as a function of current mental state.

• Similarity: Things that are alike tend to be grouped together.

• Stapes: Middle ear ossicle; also known as the stirrup.

• Subliminal Message: Message presented below the threshold of conscious awareness.

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

• Temporal Theory of Pitch Perception: Sound’s frequency is coded by the activity level of a sensory neuron.

• Thermoception: Temperature perception.

• Timbre: Descriptive term which refers to a sound’s quality; impacted by the interplay of frequency, amplitude, and timing of sound waves.

• Top-Down Processing: Interpretation of sensations is influenced by available knowledge, experiences, and thoughts.

• Transduction: Conversion from sensory stimulus energy to action potential.

• Trichromatic Theory of Color Perception: Color vision is mediated by the activity across the three groups of cones.

• Trough: Lowest point of a wave.

• Tympanic Membrane: Eardrum.

• Umami: Taste for monosodium glutamate.

• Vertigo: Spinning sensation.

• Vestibular Sense: Contributes to our ability to maintain balance and body posture.

• Visible Spectrum: Portion of the electromagnetic spectrum that we can see.

• Wavelength: Length of a wave from one peak to the next peak.