Lecture Notes: Thresholds, Subliminal Perception, ESP, and Vision Concepts
Sensing, Thresholds, Subliminal Perception, ESP, and Vision
Course goals (overview)
Understand absolute threshold and the just noticeable difference (JND).
Consider subliminal perception and extrasensory perception (ESP).
Explore physical properties of light (wavelength, amplitude) and the visible spectrum.
Examine the eye’s anatomy, retina layers, and how visual information is processed in the brain.
Discuss light/dark adaptation, color vision, and early brain processing of visual signals.
Note: the brain’s interpretation of signals will be covered more in depth later; today is a foundation.
Basics of sensing and transduction
Sensory receptors are specialized neurons that respond to specific environmental stimuli.
Sensation occurs when a sensory receptor detects a stimulus.
Transduction is the conversion of stimulus energy into neural signals (action potentials) that the CNS can interpret.
Example (vision): light energy causes chemical changes in photoreceptor cells at the back of the eye, which then relay messages as action potentials to the brain.
The classic five senses: vision, hearing (audition), smell (olfaction), taste (gestation), touch (somatosensation).
Additional sensory systems: vestibular (balance), proprioception (body position/movement), nociception (pain), thermoception (temperature).
Absolute threshold (the threshold of sensation)
Definition: the minimum energy of a stimulus that must be present for detection 50% of the time.
Why 50%? Provides a practical, trial-to-trial and population-average benchmark due to variation across people and moments.
Interpretation: how dim can light be or how soft can a sound be and still be detected about half the time.
Factors that influence detection (signal detection theory): attention, motivation, fatigue, and other contextual factors.
Illustrative thresholds (examples of sensory limits):
On a clear night, a candle flame can be detected from up to 30\,\text{miles} under flat-horizon conditions.
The ticking of a clock can be detected from up to 20\,\text{feet} away in quiet conditions.
One drop of perfume in a 3\,\text{room} apartment.
One teaspoon of sugar dissolved in 2\,\text{gallons} of water.
The flap of a bumblebee’s wing can be felt from up to 1\,\text{cm} away from the cheek.
Just noticeable difference (JND) and Weber’s Law
JND: the minimum change in stimulus intensity required to detect a change, measured at 50% detection.
Weber’s Law: the size of the JND is proportional to the initial stimulus intensity.
Formal idea (typical formulation): \frac{\Delta I}{I} = k where \Delta I is the smallest detectable change in intensity, I is the initial intensity, and k is a constant (depends on the sense).
Concrete intuition: adding one more spoonful of salt to an extremely salty pot may go unnoticed, whereas the same amount added to a unsalted pot is noticeable.
Subliminal perception (perception without awareness)
Subliminal perception: unconscious perception of stimuli.
Historical example: James Vickery flashed messages (e.g., eat popcorn, drink Coca‑Cola) between frames of a movie to boost concession sales by about areported 18\%, but this was not a rigorous experiment and data were sparse.
Scientific stance: most controlled studies fail to show convincing evidence that subliminal messages reliably persuade.
Specific conditions where subliminal effects might occur (rare):
When subliminal cues address dimensions the person is already evaluating (e.g., product attributes they care about).
When subliminal cues align with a person’s personality traits or situational factors (e.g., risk-taker traits or staying up late).
Subliminal perception and social judgments: some evidence that threat-related cues (e.g., angry faces) can be processed subliminally and can influence behavior (e.g., pausing responses before acting).
Developmental notes: infants as young as seven months may subliminally perceive cues related to trustworthiness in faces, suggesting early evolutionary advantages.
Extrasensory perception (ESP): skepticism and evidence
ESP definition: purported ability to acquire information without using the five senses (e.g., mind reading, precognition).
Scientific stance: overall, ESP has been cast into serious doubt by rigorous tests.
Ganzfeld procedure: one person acts as sender; a receiver tries to perceive a target image; many studies show no reliable evidence beyond chance.
Neuroimaging attempts: modified Ganzfeld with fMRI; receivers guessed correctly about the target image only at chance level (≈ 50\%); no brain activity patterns reliably differentiated correct vs. incorrect guesses.
Bottom line: current body of evidence does not support ESP as a real, replicable phenomenon.
Vision: light as energy, color, and wavelength
Light properties: wavelength, amplitude, and saturation.
Hue comes from wavelength; shorter wavelengths (toward the blue end) correspond to cooler colors; longer wavelengths (toward the red end) correspond to warmer colors.
Brightness corresponds to amplitude (the height of the wave): higher amplitude = brighter light.
Saturation: purity of light; single-wavelength (monochromatic) light is most saturated; mixtures of wavelengths reduce saturation.
Visible spectrum for humans: approximately 360\,\text{nm} \le \lambda \le 750\,\text{nm}
Other species can sense beyond human-visible ranges (e.g., snakes detect infrared; some spiders, butterflies, and rats detect ultraviolet).
Eye as a translator: converting electromagnetic waves into neural signals for the brain to interpret.
Anatomy of the eye and the path of light
Outer structures:
Cornea: clear outer surface; begins the focusing process by bending light.
Sclera: white, supportive outer layer that holds the eye together (not directly involved in processing).
Iris: colored part containing circular muscles; controls pupil size (constriction in bright light, dilation in dim light).
Pupil: opening that lets light into the eye (not a tissue but a hole).
Lens: clear structure behind the iris; focuses light further via accommodation (shape change controlled by ciliary muscles).
Light path (simplified):
Light from an object (e.g., a butterfly) is reflected and enters the cornea → passes through the pupil (size controlled by the iris) → lenses focus the light to form a sharp image on the retina.
Accommodation adjusts lens shape to keep the image in focus as distance changes.
Retina and initial processing:
Light is converted to neural signals by photoreceptors (rods and cones) after passing through several retinal layers.
The optic nerve carries signals from the retina to the brain; there is a blind spot where the optic nerve exits (no rods or cones at that point).
Each eye has a blind spot, but since both eyes work together, the brain fills in gaps and the blind spots are not usually noticed.
Rods, cones, and photoreceptors
Rods:
Sensitive to light; provide black-and-white vision; do not convey color.
High density in the periphery of the retina; around ~10^8 rods per eye.
Cones:
Responsible for color vision; three types corresponding to different photopigments.
Require more light (less sensitive in dim conditions); high density in the fovea (center of gaze).
Approximately ~5\times 10^6 cones per eye; greatest concentration at the fovea; density falls toward the periphery.
Photopigments and transduction:
Rod photopigments (e.g., rhodopsin) are broken down by light, triggering a cascade of chemical reactions that modulate the rate of neurotransmitter release and stimulate downstream neurons.
Cones have three types of photopigments, enabling color vision (red, green, blue sensitivity).
In rods, photopigments are shared across rods; in cones, each cone type has a distinct photopigment tuned to a broad region of the spectrum, enabling color discrimination.
Fovea and peripheral vision:
The fovea is the region of highest visual acuity with dense cone packing; color vision is best when objects are directly in front of us.
Peripheral retina contains more rods, enhancing sensitivity to light intensity and motion in peripheral vision.
Photopigments beyond vision (circadian relevance)
Photopigments may influence non-visual processes such as sleep regulation and pupillary reflexes.
In some blind individuals, photoreceptive light energy can still influence circadian rhythms even if conscious vision is absent.
Dark adaptation and light adaptation
Dark adaptation: the process by which the eye becomes more sensitive in the dark as photopigments accumulate; can take about 30\text{ to }45\text{ minutes} after exposure to bright light.
Light adaptation: the process by which the eye becomes less sensitive when moving into bright light; can occur quite rapidly, about 1\text{ minute} after a sudden change.
Mechanisms beyond pupil size:
Photopigment regeneration and breakdown in photoreceptors contribute significantly to adaptation.
The balance of photopigment availability affects how quickly we adjust to changing light levels.
Practical implication: after leaving a bright environment (e.g., a sunny day) entering a dark theater, it can take around 30\text{ to }45\text{ minutes} for full dark adaptation; returning to bright light can cause a brief, painful glare due to leftover photopigments.
Color vision: mechanisms and theories
Trichromatic theory (three cones):
There are three types of cones, each most sensitive to a different portion of the spectrum (roughly long/red, medium/green, short/blue).
Color perception arises from the relative activity of these three cone types.
Color matching experiments historically supported this theory; explains how we see colors by comparing activity across the red, green, and blue cones.
Color blindness (red-green) often results from lacking red or green cones; approximately 8\% of males in the U.S. are affected; prevalence varies by population and is sex-linked (likely genetic).
Opponent-process theory:
Proposes cells that respond in opposing pairs: red/green, yellow/blue, black/white.
These cells can only signal one member of each color pair at a time, contributing to color perception after initial photoreceptor processing.
Contemporary view: Both theories are valid but operate at different stages of visual processing:
Trichromatic theory explains color detection at the level of cones (photoreceptors).
Opponent-process theory explains subsequent processing in neural pathways beyond the receptors.
Color vision and diversity of color experiences:
Color mixing for light uses additive color mixing (red, green, blue). All three together produce white light.
Color mixing for pigments (paints) uses subtractive mixing and tends toward darker colors (e.g., red + green paint yields brownish/blackish results).
Visual pathways: from retina to cortex
Retina to brain: rod and cone signals are gathered and transmitted via retinal neurons to the optic nerve.
Optic nerve and chiasm: signals travel through the optic nerves; at the optic chiasm, fibers from the nasal (inner) halves cross to the opposite hemisphere, while temporal fibers stay on the same side.
Post-chiasm pathways: most information goes to the thalamus (lateral geniculate nucleus, LGN) and then to the visual cortex in the occipital lobe, where interpretation and conscious perception occur.
Hemispheric mapping: information from the right visual field is processed in the left hemisphere, and information from the left visual field is processed in the right hemisphere.
Blind spot and binocular vision:
The blind spot is the optic disc where the optic nerve exits the retina; there are no photoreceptors there.
With two eyes, the brain fills gaps and combines information to form a cohesive percept.
Sex differences in vision and color preferences
General tendencies observed in some studies:
Females: better at discriminating objects in the visual field, color naming, and processing facial expressions.
Males: better at processing moving objects and certain spatial aspects of objects; could be better at judging relative positions of locations.
Developmental observations:
Infants show early preferences that may reflect broad tendencies (e.g., female infants showing more interest in dolls; male infants in some studies more interested in motion-oriented toys). Note: language in class can be sensitive; some phrasing in the transcript reflects gendered language; current research emphasizes the influence of social and cultural factors as well as biology.
Newborns have shown a general early preference for the color blue across sexes, suggesting that some color preferences are not solely innate and may be shaped by culture and experience.
Interpretation: color and visual preferences likely reflect a combination of biological predispositions and cultural/situational contexts.
Connections to broader themes and real-world relevance
Thresholds and perception affect real-world tasks (driving at night, detecting hazards, interpreting warning signals).
Subliminal perception has practical and ethical considerations in advertising and media; robust evidence for broad effects is weak, emphasizing the importance of critical evaluation.
ESP findings illustrate the value of rigorous experimental design and skepticism in evaluating extraordinary claims.
Understanding color vision and lighting is important for design, safety, marketing, and accessibility (e.g., color blindness considerations).
The brain’s interpretation of sensory information highlights top-down versus bottom-up processing and why perception can differ between individuals.
Key equations, numbers, and constants (quick reference)
Visible spectrum: 360\,\text{nm} \le \lambda \le 750\,\text{nm}
Absolute threshold (definition): detection at least 50\% of trials.
Just noticeable difference (definition): minimum detectable change in stimulus intensity; measured at 50\% threshold.
Weber’s law (concept): \frac{\Delta I}{I} = k where \Delta I is the JND, I is initial intensity, and k is a constant dependent on sense.
Photoreceptor counts (approximate):
Rods: \approx 10^8 per eye
Cones: \approx 5\times 10^6 per eye
Primary colors of light (additive color mixing): red, green, and blue; combining all yields white light.
Time scales for adaptation:
Dark adaptation: up to 30\text{ to }45\text{ minutes} after bright exposure
Light adaptation: about 1\text{ minute} after a sudden increase in brightness
Common color-blindness statistic: red-green color blindness affects approximately 8\% of males in the United States (and varies by population; often sex-linked).
Quick study tips from the lecture material
Focus on understanding the distinction between sensation (detection) and perception (interpretation).
Practice applying Weber’s Law with simple examples (e.g., how much salt must change before you notice differencem, given different baseline saltiness levels).
Use the eye as a model of information processing: from light waves to neural signals to perception in the visual cortex.
Remember the key differences between rods and cones and how their distributions shape color vision and sensitivity in different lighting.
Be prepared to discuss how context (motivation, fatigue, attention) can alter a person’s perceptual thresholds.
Ethical, philosophical, and practical implications discussed in class
Subliminal messaging: despite appealing anecdotes, rigorous evidence is lacking; consider the ethics of attempting to influence behavior without awareness.
ESP and other extraordinary claims: emphasize skepticism and the importance of replicable, well-controlled experiments.
Real-world applications of threshold concepts: design of warnings, safety thresholds, color accessibility, and environmental sensing rely on understanding human perceptual limits.
Quick recall prompts
What is an absolute threshold and why is it defined at 50% detection? How does this relate to individual differences?
Explain Weber’s Law and give a real-world example.
Distinguish between the trichromatic theory and the opponent-process theory of color vision; how do they complement each other?
Describe the role of rods and cones in vision and where the fovea fits into color vision.
What is a blind spot and why don’t we typically notice it?
How do dark adaptation and light adaptation differ in time scale and mechanism?
Connections to prior and upcoming topics
Builds on basic neural signaling (action potentials) from last week by showing transduction leading to perception.
Sets up deeper discussion of perception (top-down and bottom-up processing) in the next class when analyzing how the brain interprets visual input.
Prepares for broader psychophysics topics (sensation, perception, and thresholds) and practical applications in technology and design.