Comprehensive Study Notes on Psychophysics, Signal Detection, and Sensory Systems (CHAPTER 7)

Historical Context of Light Measurement and Psychophysics

• Early Measurement Units: In the early $1800s$, candle light was established as one of the original measures of light. By approximately $1870$, this measurement system was formally established.

• Accuracy of Historical Tools: While modern technology like LEDs and dimmers allows for much smaller units of light, the measurement of the "least amount of difference" detectable by the human eye recorded in the $1800s$ remains fairly accurate by modern standards.

• Early Photography and Physics: Photography began between $1815$ and $1818$, with early significant work appearing between $1820$ and $1830$.

  • Physicists used photographic plates as early measurement systems for light before they were used for taking portraits.

  • The plates detected light levels in a room by shifting color.

  • Substantive detail: Internet records show the earliest photographs on Earth from around $1830$, including a "selfie" and a man getting his shoes shined on a street corner.

  • Historical Anecdote: In the $1830s$, photographers found surviving Revolutionary War soldiers in their $80s$ and $90s$ to document them; the speaker notes they looked "scraggly."

Evolutionary Assumptions of the Nervous System

• Spector's Assumption ($1860s$): Spector (and others like Ivan Pavlov in the $1890s$) operated on the assumption that the nervous system worked like a light switch.

  • The theory posited that presenting a stimulus "turned on" the nervous system, and removing the stimulus caused it to "shut off."

  • The implication was that the nervous system was inactive during sleep, which we now know is false; if the nervous system shuts off, the organism dies.

• The $1950s$ Junction: This decade represents a major turning point in neurobiology due to the development of miniaturized technology.

  • Laboratory work revealed that the nervous system never turns off.

  • Detection of external stimuli is actually a process of competing with background noise inside the nervous system itself.

Signal Detection Theory (SDT)

• Origin: Developed by S. S. Stevens around $1950$.

• Core Principle: Any detected stimulus is recognized only if it can be distinguished from background noise. If the signal is significantly stronger than the noise, an experience is established.

• The Four-Cell Matrix: Scientists categorize all stimulus detections into four possibilities based on whether the stimulus is Present/Absent and whether the observer believes they detected it.

  • Correct Detection (Hit): The stimulus is present, and the observer believes they saw it.

  • Correct Rejection: The stimulus is not present, and the observer correctly states it was not there.

  • Miss: The stimulus was present, but the observer believed it was not.

  • False Alarm (False Positive): The stimulus was not present, but the observer believed it was.

• Real-World Application (COVID Testing): Biological tests use the same logic.

  • A "False Positive" occurs when the test indicates COVID is present when it is not.

  • A "Miss" occurs if the sample (e.g., "boogers") was insufficient for the test to detect the actual virus.

• Social Application: SDT applies to complex social behaviors, such as noticing a best friend acting differently (e.g., combing hair differently, being "too giddy"). When you first notice this change, it constitutes a "Just Noticeable Difference" (JND).

Internal Background Noise and Sensory Deprivation

• Physical Background Noise: Most sensory systems cannot be entirely "shut off."

  • Audition: You can hear your own heartbeat, your bones creaking, or the sound of blood running through the capillaries near the cochlea (red blood cells bouncing in tiny vessels).

  • Touch: You always experience the weight of the skeletal system, clothes on skin, or air moving across the skin.

  • Anecdote on Quiet Rooms: Engineers build sound-proof rooms for acoustic research. The legend that people "go crazy" from the silence is dismissed as "total nonsense," but in those rooms, internal sounds (heartbeat/blood) become audible.

• Visual Background Noise (Phosphenes): Light is the only sense we can completely deprive.

  • In a total blackout, a person will still see "little dots."

  • These are caused by metabolic activity and pressure in the eyeball causing random action potentials.

• Point of Subjective Equality: This is the border where an individual cannot distinguish whether an experience is internal (noise) or external (signal).

Psychophysical Methodology and Thresholds

• Measuring Thresholds: Scientists use a "Sigmoidal Graph" (a curvilinear function) to determine the least amount of stimulus a person can detect.

• The $50\%$ Rule: The Just Noticeable Difference (JND) is established at the point where a person notices a stimulus $50\%$ of the time.

  • Justification: Because the SDT table has four possibilities (two correct, two incorrect), a random guess would result in $50\%$. Therefore, $50\%$ is the only statistically justifiable methodology limit to prove detection is better than chance.

• Relative Judgment (The Schwarzenegger Example): If a person with a $20$-pound arm lifts a $3$-ounce laser pointer, they must judge that weight in the context of their own limb. As a muscle grows (e.g., Arnold Schwarzenegger), the sensory system adapts slowly. If someone's arm size changed instantly, their sensory system would be "off," and they would need to relearn weight perception.

Sensory Capacity and National Standards

• Hearing Tests in Schools: Headphones play sounds of varying frequency and volume to find the "envelope" (outer range) of a student's hearing.

• Comparison Standards: Every $20$ years, the Center for Disease Control (CDC) conducts studies (e.g., with $5,000$ fifth graders) to establish national standards. An individual's results are compared against this to identify hearing problems.

• Peripheral Vision Testing: Using a sphere where a patient presses a button when white flashes appear. This is a computer-operated version of Signal Detection Theory.

Advanced Applications of Signal Detection Theory

• Medical Imaging ($1970s$): SDT was used to evaluate how accurately radiologists could identify breast cancer tumors in X-rays or brain lesions in CAT scans.

• AI vs. Human Judgment: While computer systems can evaluate pixels more efficiently and without emotion (not being "pissed off at their spouse" or "missing lunch"), human judgment is still legally required because "you can't sue a computer."

• Complex Stimuli: SDT is used to evaluate:

  • The complexity of a rock-climbing wall.

  • The annoyance level of aircraft noise.

  • Consumer choices (e.g., choosing between $\$5$ and a $\$7$ bag of M&Ms).

  • Subjective social stressors (e.g., a breakup vs. two tire blowouts during rush hour on $I-95$).

  • Foreign policy threats (e.g., comparing Iran, Russia, and China).

Principles of Sensory Modality and Stimulus Properties

• Dimensions of Detection:

  • Modality: The six sensory systems (Vision, Audition, Smell, Taste, Touch, and the sixth sense: Balance).

  • Intensity: The strength of the stimulus.

  • Extension: The size and location of a stimulus.

  • Duration: How long the stimulus lasts.

• Space and Time: The speaker notes that Extension and Duration represent space and time, appearing in three consecutive chapters (Memory, Learning, and Psychophysics).

• Philosophical Query: Space and time are the only things humans are evolved to detect, or they are the only things that exist. There are things we cannot detect, such as magnetic waves, which migratory birds use for navigation.

Specific Sensory Systems: Vision

• Anatomy of the Eye:

  • Cones: Wavelength detectors. They ignore amplitude (brightness) and detect the spatial repetition of the wave. Humans have three types: Blue, Green, and Red (the RGB model).

  • Rods: Amplitude detectors. They detect brightness but ignore wavelength (color).

• Perception of Color: Color does not exist objectively in the universe; it is "entirely subjective."

  • Visible light ranges from $380$ to $720$ nanometers.

  • The universe is described as "colorless."

  • Synthetic Organ: The eye is a synthetic organ because it mixes inputs to create a new experience (e.g., mixing blue and green to see "blue-green").

• Trichromatic Theory (Young-Helmholtz Theory): Formulated by Hermann von Helmholtz. It posits that all colors are combinations of blue, green, and red.

• Opponent Process Theory: Based on biological opposites in the eye:

  • Blue vs. Yellow

  • Red vs. Green

  • Black vs. White

  • Example: Staring at a yellow flashbulb results in a blue dot afterward due to cell fatigue (habituation). Staring at a red "Hot Doughnuts Now" sign results in a green afterimage.

• Ganglion Cells: Circular sets of receptors wired together.

  • On Cells: Activate when light hits the center of the circle.

  • Off Cells: Activate when light hits the outside of the ring.

  • Application: These cells accentuate borders. If you place two gray boxes next to each other, a border appears that is non-existent; this is a perceptual phenomenon.

  • Cave Drawings: Ancient cave drawings ($30,000$ to $70,000$ years ago) use lines to draw faces, proving ancient humans had the same ganglion cell structure as modern humans.

Specific Sensory Systems: Audition (Hearing)

• Sound Definition: Waves moving through a medium (like air).

• Human Range: $30$ to $20,000$ cycles per second (Hz).

• Dimensions:

  • Pitch: Determined by wavelength.

  • Loudness: Determined by amplitude.

  • Timbre (Tamper): The mixture of sounds.

• Analytical Organ: Hearing is an analytical organ. Unlike light (which blends), the ear can distinguish multiple sounds simultaneously (e.g., hearing a bass, guitar, and drums as separate instruments).

• Sound Localization: Achieved through four factors:

  1. Arrival Time: The brain detects the difference in when sound reaches the left vs. right ear (even at $700$ miles per hour across an $8$-inch head).

  2. Phase Differences: Offsetting sound waves.

  3. Attenuation: The signal is stronger in the ear closer to the source.

  4. Sonic Shadow: The head blocks sound, creating a "dead space" for the far ear.

Mental Workload and Attention

• Intentional Resources: Humans have only one "intentional resource." We cannot pay attention to two things at once; we instead cycle through tasks rapidly like a computer.

• NASA Research ($1960s$): Psychologists worked with NASA engineers for the Apollo and Mercury capsules to determine the "mental workload" limit for astronauts to prevent errors.

• Mathematical Distribution of Attention:

  • $1$ task = $100\%$ attention.

  • $2$ tasks = $50\%$ attention each.

  • $3$ tasks = $33\%$ attention each.

  • At $10$ tasks, only $1/10$th of attention is given to each, making errors guaranteed.

• Texting and Driving Exercise: If you are texting, eating, and driving ($3$ tasks), there is a $66\%$ chance you will fail to notice the driver in front of you slowing down because you are distracted by two other tasks.

Perceptual Ambiguity and Interpretation

• The Necker Cube: A box on a screen can be perceived with the blue wall in the back or the front. You can switch between them, but you cannot see both at the same time.

• The 12/B Ambiguity: An object represents a "$13$" when viewed horizontally ($12, 13, 14$) but a "B" when viewed vertically ($A, B, C$).

• Scientific Implication: Research biologists often view the same data differently based on their underlying theories (horizontal vs. vertical "theory"), leading them to interpret the same results as supporting different arguments.

Cultural Perception: Food and Disgust

• Bizarre Foods (Andrew Zimmern): A show focused on "shock value" where the host eats foods Westerners find disgusting (e.g., testicles, which he calls "creamy," stinky fruit, or aged eggs).

• Vietnam Case Study: Silkworms are served alongside pork and eel. Perception of what is "food" is entirely subjective based on cultural and personal history.

Questions & Discussion

• Question: Which of the following represents background noise in the sensory system?

  • Response: Heartbeat, blood flowing near the cochlea (red blood cells bouncing), random light flashes (phosphenes), and the weight of the skeletal system.

• Question: Which parts of the universe have color?

  • Response: None. Objectively, color is not in the universe; it is a perceptual experience in the brain.

• Question: If you are texting, eating, and driving, what is the chance you will miss a car slowing down?

  • Response: $66\%$ (since each of the $3$ tasks takes $33\%$ of your attention).