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Sensation
How We Detect the World

Explore how sensory receptors convert physical energy into neural signals, the thresholds that govern detection, and the processes that shape raw sensory experience.

AP Psychology

What Is Sensation?

Sensation is the process by which our sensory receptors and nervous system receive and represent stimulus energies from our environment. It is the bottom-up process that begins with raw physical energy and converts it into neural signals the brain can interpret.

The Sensory Pathway

Physical Energy Sensory Receptors Transduction (Neural Signal) Brain

Types of Sensory Receptors

Photoreceptors
Detect light energy in the retina (rods and cones)
Mechanoreceptors
Detect pressure, vibration, and movement (touch, hearing)
Chemoreceptors
Detect chemical substances (taste and smell)
Thermoreceptors
Detect changes in temperature (warmth and cold)

Absolute Threshold

The absolute threshold is the minimum stimulus intensity needed for a person to detect a stimulus 50% of the time. Below this level, the stimulus is subliminal: present but undetected.

% Detected Stimulus Intensity 0 50 100 50% Absolute Threshold

Difference Threshold and Weber's Law

The difference threshold (or just noticeable difference, JND) is the minimum difference between two stimuli needed for detection 50% of the time. Weber's Law states that this difference is a constant proportion of the original stimulus:

Light
\( k \approx \frac{1}{60} \): a small ratio means high sensitivity
Weight
\( k \approx \frac{1}{50} \): you need ~2% more weight to notice
Tone (pitch)
\( k \approx \frac{1}{333} \): hearing is very sensitive to pitch change

Signal Detection Theory

Signal Detection Theory (SDT) predicts when we will detect a faint stimulus amid background noise. Detection depends not only on stimulus strength but also on psychological factors: experience, expectations, motivation, and alertness.

Signal Present Signal Absent Respond "Yes" Respond "No" Hit False Alarm Miss Correct Rejection

Sensation vs. Perception

Sensation Perception
Process Type Bottom-up Top-down
Focus Detecting stimuli Interpreting stimuli
Starting Point Sensory receptors Higher brain areas
Example Light hits retina Recognizing a face
Influenced By Stimulus intensity Experience, context, expectations

Sensory Adaptation

Sensory adaptation is the diminished sensitivity to an unchanging stimulus. After constant exposure, sensory receptors fire less frequently: you stop noticing the stimulus even though it is still present.

Why It Happens
Receptors reduce their firing rate when stimulation is constant, freeing neural resources for novel changes in the environment
Everyday Example
Entering a room with a strong odor: after a few minutes, you no longer smell it, but a newcomer notices it instantly

Analyzing a Sensation Question

1
Identify the stimulus type: Is it light, sound, chemical, pressure, or temperature?
2
Name the receptor: Which receptor class detects this energy (photoreceptors, mechanoreceptors, etc.)?
3
Determine the threshold: Is the question about absolute threshold (detection) or difference threshold (comparison)?
4
Check for adaptation or SDT: Does the scenario involve diminished sensitivity over time (adaptation) or psychological factors influencing detection (signal detection)?
5
Distinguish sensation from perception: Is the question asking about raw detection (sensation) or interpretation (perception)?

Example: Absolute Threshold

1
Stimulus type: Light (electromagnetic energy)
2
Receptor: Photoreceptors (rods in the retina, sensitive in low light)
3
Threshold: This measures the absolute threshold: the minimum intensity detected 50% of the time

Example: Weber's Law

1
Apply Weber's Law: \( \Delta I = k \times I = \frac{1}{50} \times 50 = 1 \text{ gram} \)
2
Interpretation: You need at least 1 gram more to notice a change
3
Scale it up: If you held 500 grams, the JND would be \( \frac{500}{50} = 10 \text{ grams} \). The proportion stays constant, but the required change grows with intensity

Example: Signal Detection

1
The signal: A faint tumor shadow on the X-ray
2
The noise: Normal tissue patterns that can look like abnormalities
3
Expectation shift: Knowing the family history lowers her response criterion: she is more likely to say "yes, I see something." This increases hits but also false alarms

The Five Senses at a Glance

Sense Stimulus Receptor Organ
Vision Light waves Rods, Cones Eye (retina)
Hearing Sound waves Hair cells Ear (cochlea)
Touch Pressure, temperature Skin receptors Skin
Taste Chemicals Taste buds Tongue
Smell Chemicals Olfactory neurons Nose

Common Mistakes

✕ Confusing Sensation with Perception
Sensation is detection of stimuli. Perception is interpretation. Recognizing a friend's voice is perception, not sensation.
✕ Mixing Up Thresholds
Absolute threshold is the minimum to detect anything. Difference threshold is the minimum change between two stimuli.
✕ Forgetting Adaptation Exceptions
Students often claim we adapt to all stimuli. We do not fully adapt to pain, as it signals ongoing danger.

Question 1 of 5

The process by which sensory receptors convert physical energy into neural signals is called:

Question 2 of 5

According to Weber's Law, if you are holding a 100-gram weight and the Weber fraction for weight is \( \frac{1}{50} \), the just noticeable difference is:

Question 3 of 5

After swimming in a cold pool for several minutes, you no longer feel the water as cold. This is an example of:

Question 4 of 5

In signal detection theory, a person says "Yes, I hear it" when no sound is present. This response is classified as a:

Question 5 of 5

Which of the following best illustrates the difference between sensation and perception?