Lesson 1.13 · Unit 1 · CED topic 1.6 (part 1)
Sensation: thresholds, adaptation, and sensory interaction
Right now your eyes, ears, skin, nose, and tongue are collecting more information than your brain could ever use. Sensation is how that information gets in: sensory receptors detect energy or chemicals from the environment and convert them into signals the nervous system can use. The exam questions on sensation lean on a handful of measurable ideas: how strong a stimulus must be to be noticed, how big a change must be to be detected, why you stop noticing constant stimuli, and how the senses work together. This lesson covers those general principles; the three lessons after it take the senses one at a time.
Learning objective.
- 1.6.A. Explain how the process of sensation is related to behavior and mental processes.
Essential knowledge.
- 1.6.A.1. Sensation is the process of detecting information from the environment that meets a certain threshold and transducing stimuli into neurochemical messages for processing (perception) in the brain. The absolute threshold occurs when a stimulus can be detected at least 50% of the time.
- 1.6.A.2. Detection of change in stimuli or diminished sensitivity to stimuli can be explained by the just-noticeable difference and sensory adaptation. Weber’s law describes the degree to which stimuli need to be different for the difference to be detected.
- 1.6.A.3. The sensory systems constantly work together in a process called sensory interaction. Synesthesia is an experience of sensation in which one system of sensation is experienced through another.
Sensation versus perception
Sensation is the process of detecting information from the environment and transducing (converting) stimuli into neurochemical messages that the brain can process. Light, sound waves, pressure, and chemicals are all physical energy that the nervous system cannot use directly, so each sense has specialized receptor cells that transduce the stimulus into neural impulses. The brain then interprets the messages. That interpretation is perception, covered in Unit 2. A helpful way to keep them apart: sensation is the raw detection of stimuli, and perception is organizing and making sense of them.
Thresholds
A stimulus must be strong enough to have any effect. The absolute threshold is the minimum stimulus energy needed to detect a stimulus. Because detecting a faint stimulus varies from moment to moment, it is defined by a probability: the absolute threshold is the intensity at which a stimulus can be detected at least 50% of the time. Under ideal conditions, humans are very sensitive. Commonly cited estimates are that you could see a candle flame from about 30 miles away on a dark, clear night and hear a ticking watch from 20 feet away in a quiet room.
The difference threshold, or just-noticeable difference (JND), is the minimum difference between two stimuli that a person can detect 50% of the time. If you are holding a 100-gram weight and a second weight is added, the JND is the smallest added amount you can notice.
Weber's law describes how the JND works: the size of the JND is proportional to the size of the original stimulus. In other words, for two stimuli to be perceived as different, they must differ by a constant proportion, not a constant amount. Adding 5 candles to a room lit by 10 candles is noticeable; adding 5 candles to a room lit by 1,000 is not. If the JND for the weight of lifted objects is 2% (a hypothetical value for practice), then a 100-gram weight needs 2 more grams for you to notice, but a 1,000-gram weight needs 20 more grams.
Sensory adaptation
Sensory adaptation is a diminished sensitivity to a stimulus that does not change. When you first put on a sweater, you feel it on your skin; a few minutes later you stop noticing it. When you enter a room with a strong smell, it fades. Adaptation helps the nervous system, which is designed to detect change, to pay attention to new events rather than to constant background stimulation. (Note the difference between adaptation, which is a decrease in sensitivity to an unchanging stimulus, and habituation, a decline in response to a repeated stimulus, covered in Unit 3.) If a stimulus changes, sensitivity returns, so that you notice if the sweater's tag scratches you.
Sensory interaction and synesthesia
The senses do not work in isolation. In sensory interaction, the senses influence one another: taste depends heavily on smell (a stuffy nose makes food bland), and what you see can change what you hear. The sensory systems constantly work together to produce a single experience of the world.
Harry McGurk and John MacDonald showed participants a video of a person's mouth forming the syllable "ga" while the audio played the sound "ba." Participants typically reported hearing a third syllable, "da," a blend of what they saw and heard. When they closed their eyes, they correctly heard "ba."
Reading the research: an experiment with the type of stimulus as the independent variable and the syllable reported as the dependent variable. It demonstrates sensory interaction, since vision changed what people heard. A limitation is that the reports were self-reported, and the participants varied in how strongly they experienced the effect.
Synesthesia is a rare condition in which stimulation of one sense is experienced through another. A person with synesthesia may see colors when hearing music, see letters or numbers as having specific colors, or taste a flavor when hearing a word. The experiences are automatic and consistent over time. It shows how closely sensory systems can be linked in the brain.
A researcher presents tones of increasing intensity (arbitrary units) 100 times each and records the percentage of times a participant says "I hear it." The results are: intensity 1, 4%; intensity 2, 18%; intensity 3, 44%; intensity 4, 62%; intensity 5, 86%; intensity 6, 97%. Estimate the absolute threshold.
Step 1, the absolute threshold is the intensity detected 50% of the time.
Step 2, the detection rate is 44% at intensity 3 and 62% at intensity 4, so 50% falls between them.
Step 3, estimate: the rate rises 18 percentage points (from 44 to 62) across one unit. To go from 44 to 50 takes 6 of those 18 points, so the intensity is about \(3+6/18\approx3.3\).
Step 4, interpretation: the threshold is about 3.3 units. Notice that detection does not jump from 0% to 100% at one exact value; it rises gradually, which is why a 50% criterion is used.
Suppose a person's JND for the loudness of a sound is 10% of its starting level. A sound is at 50 units. How much louder does it have to get for the person to notice? What if it starts at 200 units? Would an increase of 15 units be noticed in each case?
Step 1, JND = 10% of the starting level. At 50 units: \(0.10\times50=5\) units.
Step 2, at 200 units: \(0.10\times200=20\) units.
Step 3, an increase of 15 units: at 50 units, 15 is greater than 5, so the person would notice. At 200 units, 15 is less than 20, so it would likely go unnoticed.
Step 4, the lesson of Weber's law: the same absolute increase (15 units) is clear against a quiet background and invisible against a loud one, because what matters is the proportion.
A student walks into the school cafeteria and notices a strong smell of pizza, but 10 minutes later no longer notices it. Her friend arrives and says it smells strongly. Explain using the concepts of the lesson. Then explain why a stuffy nose makes the pizza taste bland.
Step 1, the student's receptors responded strongly at first, and then the nervous system reduced its response to the unchanging smell. This is sensory adaptation.
Step 2, her friend, newly arrived, has not yet adapted, so the smell seems strong to her.
Step 3, the stuffy nose blocks smell, and flavor comes from the combination of taste and smell, which is sensory interaction.
- Sensation = detecting and transducing stimuli; perception = interpreting them (Unit 2).
- Absolute threshold: minimum stimulus detected at least 50% of the time. JND: smallest detectable difference. Weber's law: the JND is a constant proportion of the starting stimulus.
- Sensory adaptation: reduced sensitivity to an unchanging stimulus. Sensory interaction: senses influence one another (taste and smell; the McGurk effect). Synesthesia: one sense experienced through another.
- Define absolute threshold. Why is it defined by a percentage?
Show answer
It is the minimum stimulus intensity that can be detected, specifically at least 50% of the time. Detection varies from trial to trial, so there is no single point where detection goes from never to always.
- According to Weber's law, the JND for a 50-gram weight is 1 gram. Predict the JND for a 500-gram weight.
Show answer
The JND is proportional to the starting weight. 1 gram out of 50 is 2%, so for 500 grams the JND is \(0.02\times500=10\) grams.
- Why do you stop noticing the hum of a refrigerator?
Show answer
Sensory adaptation: the nervous system reduces its sensitivity to a constant, unchanging stimulus so you can notice changes.
- What is synesthesia? Give an example.
Show answer
A condition where stimulation of one sense is experienced through another, such as seeing colors when hearing music or tasting flavors when hearing words.
- Which is an example of sensation and which of perception: (a) the retina converting light into neural signals, (b) recognizing a friend's face in a crowd?
Show answer
(a) is sensation (transduction of light); (b) is perception (interpreting and recognizing).
- In the McGurk effect, why do viewers hear "da" when they see "ga" and hear "ba"?
Show answer
Vision and hearing interact. The brain combines the visual information (the mouth shape) with the auditory information to produce a blended perception, which shows sensory interaction.