Color & Sound Lab

AP Psychology, Unit 3. Work through the stations in order with your guided notes open. Every station has something to try before you write anything down.

Print the student guide (PDF) and fill it in as you go.

Today’s route
  1. The afterimage (do this first)
  2. Trichromatic theory and the cone mixer
  3. Opponent-process theory: how both theories fit
  4. Color vision deficiency
  5. Check: color
  6. The ear, step by step, plus the hearing test
  7. Pitch, location, and deafness
  8. Check: hearing
1

The afterimage

Don’t read ahead. Press start, stare at the black dot for the full 30 seconds without moving your eyes, then keep staring at the dot when the flag disappears.

Tip: get your face close to the screen so the flag fills your view.

What did you see? Open after you try it.

You saw a normal red, white, and blue flag on a blank white screen. Nothing on the screen was red or blue.

Staring wore out the cells signaling cyan, black, and yellow. When the image vanished, their partners took over: cyan gave way to red, black to white, yellow to blue. That rebound only makes sense if colors are processed in opposing pairs, which is Station 3.

2

Trichromatic theory

Trichromatic theory (Young–Helmholtz): the retina has three types of cones, each most sensitive to a different range of wavelengths: long (red), medium (green), and short (blue).

Every color you see comes from the ratio of activity across those three cones. There is no “yellow cone.” Yellow is what you see when red and green cones fire together.

Cones are not equal in number. Red and green cones make up the large majority; blue cones are rare (well under 10%).

Cone mixer

Each slider is how hard one cone type is firing. Try the challenges.

80
30
20

what you’d see

Your TV and phone screen use this exact trick: every pixel is just red, green, and blue lights.

Exam trap: trichromatic theory is about the cones in the retina. It cannot explain afterimages, so if a question mentions an afterimage, it is pointing you to opponent-process.
3

Opponent-process theory

Opponent-process theory (Hering): after the cones, color signals are handled in opposing pairs. A cell excited by one color in a pair is inhibited by the other.

When you stare at one color, that side of the pair fatigues. Look away and the opposite side rebounds, so you see the complementary color as an afterimage.

It also explains why you can picture a reddish-yellow, but never a reddish-green: one pair can’t signal both at once.

So who’s right? Both, at different stages.

Stage 1: retina

Three cone types detect light.

Trichromatic theory

Stage 2: after the cones

vs
vs
vs

Opponent-process theory (ganglion cells and the thalamus)

Say it this way on an FRQ: “Trichromatic theory explains color detection by three cone types in the retina; opponent-process theory explains how those signals are then processed in opposing pairs, which accounts for afterimages.”
4

Color vision deficiency

Most “color-blind” people still see color. The most common form is anomalous trichromacy: all three cone types are there, but one of them, usually red or green, responds to the wrong wavelengths, so red and green are harder to tell apart. In dichromatism, one cone type is missing or doesn’t work, so red and green get confused.

Monochromatism (true color blindness, seeing only shades of gray) is very rare.

Red-green deficiency is inherited on the X chromosome, so it is far more common in males (around 1 in 12 males of Northern European ancestry, compared with about 1 in 200 females).

It supports trichromatic theory: lose one cone type and you lose exactly the colors that cone helps separate.

See it the other way

stop light
go light
orange
lime
berry
olive
blue
yellow

Notice which pairs collapse together and which stay distinct. Blue and yellow survive. Why?

5

Check: color

Pick an answer. If you miss one, read the feedback before moving on.

6

The ear, step by step

Hearing is a chain of vibrations until the very end, when hair cells do the transduction: turning vibration into neural signals.

  1. Outer ear funnels sound waves inThe visible ear and the ear canal
  2. Eardrum vibrates
  3. Hammer → anvil → stirrupThe three tiny middle-ear bones (ossicles) amplify the vibration
  4. Oval window passes the vibration into the cochlea
  5. Cochlea: fluid inside ripples the basilar membraneThe cochlea is the snail-shaped tube in the inner ear
  6. Hair cells bend and fire. Transduction happens here.The hair cells sit in the organ of Corti, on the basilar membrane
  7. Auditory nerve → thalamus → auditory cortexThe auditory cortex is in the temporal lobe
Outer ear Middle ear Inner ear to the brain Outer ear Eardrum Oval window Hair cells Hammer, anvil, stirrup Cochlea Auditory nerve Hair cells line the basilar membrane inside the cochlea.
Sound moves left to right: funneled in, vibrating, amplified, then turned into a neural signal by the hair cells.

How high can you hear?

Humans hear roughly 20 to 20,000 Hz, but the top of that range shrinks with age and loud noise. Start low and work up. Stop when you can’t hear it anymore.

Volume: keep your device volume low. Headphones work best; many laptop speakers can’t play the highest tones at all, so silence on those may be your speaker, not your ears.
1,000 Hz
Why do high pitches disappear first?

High-pitched sounds are sensed by hair cells near the entrance (base) of the cochlea. Every sound wave passes through that spot, so those hair cells take the most wear from loud noise and aging. Damaged hair cells don’t grow back. That is sensorineural hearing loss, and it’s why adults usually can’t hear the top presets.

7

Pitch, location, and deafness

Loudness comes from the wave’s amplitude (height). Pitch comes from its frequency (waves per second, in hertz).

Three theories explain how we hear pitch, and each wins in a different range:

Place theory: different pitches vibrate different places on the basilar membrane. Best for high pitches.

Frequency theory: the auditory nerve fires at the same rate as the sound wave (100 Hz tone, about 100 firings per second). Best for low pitches.

Volley theory: a single neuron can’t fire faster than about 1,000 times a second, so groups of neurons take turns firing to keep up with mid-range pitches.

Place theory: different pitches vibrate different places on the basilar membrane high pitch peaks here low pitch peaks here Basilar membrane(unrolled) Base, by the oval window Tip of the cochlea Frequency theory: the nerve fires at the same rate as the sound wave Sound wave Nerve firing Volley theory: neurons take turns, so together they keep up with faster waves Faster wave Neuron A Neuron B Neuron C Together
Three ways the ear codes pitch. Each theory explains a different part of the range.

Sound localization: a sound off to one side reaches the nearer ear slightly sooner and slightly louder. Your brain uses those tiny differences to locate it. Sounds directly in front or behind are the hardest to place.

Conduction deafness

Damage to the parts that carry vibration: eardrum or middle-ear bones.

Often helped by a hearing aid or surgery.

Sensorineural deafness

Damage to the hair cells or the auditory nerve. Loud noise and aging are the big causes.

Permanent. A cochlear implant can bypass damaged hair cells.

Exam trap: high pitch means place theory, low pitch means frequency theory. Students flip these constantly.
8

Check: hearing

Exit ticket

On paper, in three sentences or fewer: explain why you saw a red, white, and blue flag in Station 1 using both color theories. Then name the hearing theory that explains your top frequency in Station 6.