Psych 1000-001 - Sensation and Perception

Chapter 5

Sensation

The process through which our sensory receptors and nervous system detect and interpret energy or stimuli from the environment.

Transduction

The process of converting physical energy from the environment into electrical signals that can be interpreted by the nervous system.

Sensory Receptor Cells

Specialized cells responsible for transforming specific types of stimuli into neural impulses.

Olfactory

Odorants (Smell)

Somato Sensory

Pressure or damage to the skin (Touch, pain heat)

Gustatory

Chemicals in food (Taste)

Auditory

Sound waves (Hearing)

Visual

Light, protons (Sight)

Psychophysics

Helps us understand how physical stimuli interact with our sensory systems, perceptions, and mental states.

Threshold

The limit of the senses.

The minimum amount of stimulus energy required to detect a particular stimulus 50% of the time is known as the absolute threshold.
• A stimulus that is not detected 50% of the time is called a subliminal stimulus.

Absolute Threshold

The minimum intensity of a stimulus that a person can detect 50% of the time.

Signal Detection Theory

A method used to assess an individual’s ability to distinguish actual signals from background noise.

Subliminal Stimuli

Subtly influences our behavior, such as affecting our judgments or responses to advertising.

Difference Threshold

The smallest detectable difference between two stimuli that allows a person to perceive that they are not the same.

Weber’s Law

To function effectively, we need an absolute threshold low enough to notice important sights, sounds, textures, tastes, and smells.
We also need the ability to detect small differences between stimuli in order to respond accurately to changes in our environment.

Perception

The process through which the brain organizes and interprets sensory information, allowing us to recognize and make sense of objects and events.

Bottom-Up Processing

Perception that begins with the detection of environmental stimuli, which are converted into neural signals and progressively processed in more complex brain areas.
• For example, when you see someone you know, your eyes transform light into neural impulses that travel to the visual cortex for interpretation.

Top-Down Processing

Perception guided by cognitive factors such as memory, knowledge, or expectations.
• For example, when you see someone you know, your brain uses stored information about familiar faces to help interpret and recognize the visual stimuli.

Sensory Adaptation

The process by which repeated stimulation of a sensory receptor results in a decreased response over time.
• Examples:

• No longer noticing the sirens near a hospital.

• The tag on your shirt that bothered you earlier now goes unnoticed.

• Smell is especially prone to adaptation.

Perceptual Sets

Our tendency to perceive a stimulus in a particular way based on expectations, experiences, and context.
• Ambiguous stimuli can be interpreted differently depending on these factors.
• Both bottom-up and top-down processes contribute to perception.

Light Energy / Eye Structure

Vision is a crucial sense for humans.
• Light is a form of electromagnetic radiation composed of particles called photons.
• The visible spectrum ranges from approximately 380 nm to 750 nm, with different wavelengths perceived as different colors.
• Objects reflect or absorb light:

• Reflected light appears brighter to our eyes.

Short Light Wavelength

High frequency

Long Light Wavelength

Low frequency

Great Light Amplitude

Brighter

Low Light Amplitude

Duller

Iris

Light enters the eye, and muscles in the iris adjust the size of the pupil to let in more or less light.

Retina

Light entering the eye triggers chemical reactions in rods and cones in the back of the retina. Chemical Reaction then activates bipolar cells. Bipolar cells then activate ganglion cells, whose combined axons form the optic nerve. This nerve transmits information (Via hypothalamus) to the brains visual cortex.

Pathway From The Eyes to The Visual Cortex

Axons of ganglion cells form the optic nerve, which transmits signals to the thalamus. There, they synapse with neurons that carry information to the visual cortex.
• The retina’s neural layers process and relay electrical impulses, encoding and analyzing sensory information.
• Approximately half of the sensory input from each eye crosses at the X-shaped optic chiasm, reaching the opposite side of the brain.

Ganglion Cells

Transmit visual information to the brain through the optic nerve.
• Vision is also subject to sensory adaptation: for example, when you first enter a dark room, you may not see clearly, but your eyes gradually adjust.

Visual Pathway

Visual information can be processed through parallel pathways in the brain:

• “Where” pathway: processes information about an object’s location and movement.

• “What” pathway: processes information about an object’s identity, such as shape, color, and features.

Colour Processing

The perception of colour can be described along three dimensions:

1. Hue: the color type determined by wavelength (e.g., red, green, yellow).

2. Saturation: the purity or intensity of the color (e.g., red is highly saturated, pink is less saturated).

3. Brightness: the amount of light reflected or emitted; its intensity (e.g., white is the brightest, black is the least bright).

Trichromatic Theory

• Proposes that the retina contains three types of color receptors (cones), each sensitive to a different range of wavelengths: red, green, and blue.

• Proposed by Young and Helmholtz

• Any color can be produced by combining these three primary colors.

• For example, the retina has no specific receptor for yellow, but stimulating both red- and green-sensitive cones creates the perception of yellow.

• Limitation: This theory doesn’t fully explain why we can perceive many more colors beyond combinations of just red, green, and blue, or why certain colors like yellow appear as a pure, distinct color.

Opponent Process Theory

• Suggests that colors are perceived in opponent pairs, where one color inhibits the perception of its opposite.

• Proposed by Ewald Hering, who used afterimages to demonstrate the effect.

• The primary opponent pairs are: green–red, blue–yellow, and black–white, meaning these pairs cannot be seen simultaneously in the same location.

Colour Blindness

A condition in which a person cannot perceive the full range of colors.

• Most commonly affects red–green discrimination.

  • Occurs in about 1 in 12 men and 1 in 200 women.

• Monochromatic vision: the ability to see only black, white, and shades of gray.

  • Extremely rare, affecting about 1 in 33,000 people.

Depth Perception

• The ability to perceive the world in three dimensions, even though the images on the retina are two-dimensional.

• Enables us to judge distances accurately.

• Cues for depth perception:

  • Binocular cues: require both eyes, such as convergence (eyes turning inward for close objects) and retinal disparity (slight differences in images between the two eyes).

  • Monocular cues: rely on one eye (covered later).

Convergence

The inward movement of the eyes when focusing on a nearby object.

• For close objects, the eyes angle toward each other to maintain a single, clear image.

Retinal Disparity

The slight difference in images captured by the retinas of each eye, providing a binocular cue that helps the brain perceive depth.

Monocular Views

Visual signals about depth and distance that can be perceived using only one eye.

• Artists often use these cues to create the illusion of depth on a flat, two-dimensional surface.

Muller-Lyer Illusion

A visual illusion in which two lines of equal length appear different because of the arrow-like fins at their ends.

• In the classic example, the line with outward-facing fins appears longer, even though both lines are the same length.

Ponzo Illusion

A visual illusion in which converging lines (like railroad tracks) make the upper horizontal bar appear larger, even though both bars are the same length.

Form Perception

Gestalt psychologists argue that perception allows us to make sense of visual information by organizing it into meaningful patterns. They proposed several principles that explain how the visual system groups elements into coherent images.

Figure-Ground

Determines whether something is seen as the main object or as part of the background.

Proximity

Elements that are close to each other are more likely to be perceived as belonging to the same group.

Closure

We often mentally complete incomplete shapes or patterns so they appear whole.

Similarity

Elements that look alike are more likely to be seen as part of the same group.

Perceptual Constancy

A top-down process in which we perceive objects as stable and unchanging, even when the sensory information reaching us varies.

Colour Constancy

We see an object as having the same color even when lighting conditions change.

Size Constancy

We perceive an object as the same size even when its distance from us changes.

Shape Constancy

We recognize an object as having the same shape no matter the angle from which it is viewed.

Agnosia

Damage to the temporal lobe, where the “what” pathway is located, can lead to visual agnosia, a condition in which a person cannot identify objects by sight.
• A more specific form, prosopagnosia, involves the inability to recognize faces.

Strabismus

A condition in which the eyes are misaligned and send different visual information to the brain. If untreated, it can result in vision problems.

Amblyopia

Partial or total vision loss caused by abnormal development of the visual cortex during infancy, often because one eye focuses more effectively than the other.

Sound

The auditory system transforms vibrations in the air into neural signals the brain can interpret.

Frequency

The number of wave cycles per second, measured in Hertz (Hz). It determines pitch. Humans can hear roughly 20 to 20,000 Hz.

Amplitude

The height or strength of each sound wave, measured in decibels (dB). It determines loudness. A whisper is about 30 dB, while a loud concert can reach 120 dB.

Short Sound Wavelength

High frequency, high pitched sounds.

Long Sound Wavelength

Low frequency, low pitched sounds.

Great Sound Amplitude

Loud

Small Sound Amplitude

Quiet

Sound waves to Sound

1. Sound waves enter the outer ear and cause the eardrum (tympanic membrane) to vibrate.

2. These vibrations move the three tiny bones in the middle ear the malleus (hammer), incus (anvil), and stapes (stirrup).

3. The stapes pushes on the oval window, creating waves in the fluid inside the cochlea. These waves bend the basilar membrane, which is lined with rows of hair cells that act as auditory receptors.

4. The hair cells stimulate nerve fibers in the cochlea, which convert the mechanical movement into neural signals.

5. These signals are sent to the brainstem, then to the thalamus, and finally to the auditory cortex. The auditory cortex is arranged in a tonotopic map, meaning different frequencies are processed in specific locations.

6. The information then moves to auditory association areas, where sound is linked to meaning and language comprehension.

Frequency (Temporal) Theory

Different sound frequencies lead to different firing rates of neurons. Higher frequency sounds cause faster firing of action potentials.

Place Theory

Different frequencies stimulate specific locations along the basilar membrane. The brain interprets the point of maximum activity as the pitch being heard.

Deafness

Partial or complete hearing loss.

Conduction Deafness

Occurs when something blocks or disrupts the pathway that carries sound waves to the inner ear.

• Possible causes include earwax buildup, infection, eardrum damage, or water in the ear.

• Can be temporary or permanent

Sensorineural Hearing Loss

Results from damage or problems in the cochlea, auditory nerve, or areas of the brain involved in hearing.

• Can be present from birth (congenital).

• Tinnitus, a persistent ringing or buzzing sound, is often associated with this type of hearing loss.

Smell and Taste

Although distinct, they are grouped as chemical senses because they respond to specific chemicals.

Olfaction (smell) and gustation (taste)

Evolutionarily ancient senses.

• Smell is less critical for humans than for many other animals.

• Both senses play important protective roles, such as detecting spoiled food, toxic gases, or smoke.

Odorants

Airborne chemical molecules that are perceived as smells.

Olfactory Receptor Neurons

Specialized cells that bind odorant molecules, convert them into neural impulses through transduction, and transmit these signals to the brain.

Papillae

Small bumps covering the surface of the tongue.

Taste Buds

Clusters of sensory receptor cells located within the papillae that detect food molecules dissolved in saliva and convert them into neural impulses through transduction.

• Each taste bud contains 60–100 receptor cells for detecting different tastes.

5 Taste Receptors

1. Sweet

2. Sour

3. Bitter

4. Salty

5. Umami – the savory taste associated with glutamate (e.g., MSG).

Tactile Sense

Influences food preferences.

Spicy Food

Stimulates pain receptors through capsaicin, producing a burning sensation. Combining this pain with texture and flavour can create a pleasurable experience, sometimes leading to what feels like an addictive enjoyment.

How We Process Smell and Taste

• Signals from olfactory receptor neurons travel to the olfactory bulb, which processes and relays information to other regions of the cerebral cortex for odor recognition and discrimination.

• The olfactory bulb also sends signals to the amygdala and indirectly to the hippocampus, which is why smells can trigger memories.

• Taste receptors transmit signals to the thalamus and then to the gustatory cortex in the frontal lobe.

• Taste and smell information converge in the prefrontal cortex, allowing integrated perception of flavor.

Ageusia

The complete inability to taste, while dysgeusia—a distorted or unpleasant taste sensation—is more common.

Anosmia

The inability to smell; people with anosmia can still detect basic tastes but lose the combined perception of flavour.

Tactile (Somatosensory) System

• Integrates multiple skin senses, including pressure, touch, temperature, vibration, and pain.

• Relies on various receptors distributed across different areas of the skin.

• Enables us to perceive physical sensations, such as the weight of a laptop on your lap.

Merkel’s Discs

Respond to light pressure.

Meissner’s Corpuscles

Detect lower pressure and low-frequency vibrations.

Pacinian Corpuscles

Respond to transient pressure and high-frequency vibrations.

Ruffini Corpuscles

Sensitive to skin stretch.

Pain

• Unlike other sensations, pain is not triggered by a single type of stimulus.

• It is detected by nociceptors, a group of sensory receptors.

• Free nerve endings in the skin, muscles, joints, and organs sense pain.

• Nociceptors respond to temperature, pressure, and chemical signals (e.g., capsaicin from spicy foods).

• Protects us from injury

• Is both top-down and bottom-up processing

Biological Influences on Pain

• Genetics play a role in how individuals experience pain and other physical traits.

• Women generally have a lower pain threshold and report experiencing pain more intensely.

• Men may experience pain similarly but can be less likely to acknowledge it.

Gate Control Theory

• The spinal cord has a neurological “gate” that regulates the transmission of pain signals to the brain.

• Small nerve fibers carry most pain signals. An injury activates these fibers, opening the gate and sending pain signals to the brain.

• Large fiber activity, such as from massage or pressure, can close the gate and reduce pain perception.

• Chronic pain can be managed with gate-closing methods like massage or acupuncture.

• Endorphins are released in response to severe pain, helping to modulate it.

• The brain can also generate pain without physical injury, as seen in phantom limb sensations.

Touch

• Neural signals from tactile receptors and nerve endings travel to the medulla, then the thalamus, and finally to the somatosensory cortex for processing.

• Some individuals experience neuropathic pain, a type of chronic pain resulting from nerve damage or dysfunction