Source: General Psychology, Ch. 3 (through p. 109)
Tags: sensation, perception, bottom-up processing, top-down processing, absolute threshold, difference threshold, just noticeable difference, JND, Weber's law, perceptual set, sensory adaptation, transduction, psychophysics
Difficulty: Introductory Prerequisites: None. This is foundational material for the rest of the sensation and perception unit.
This chapter introduces how we take in raw physical energy from the world (sensation) and convert it into meaningful experience (perception). It sits right at the boundary between biology and psychology: you need to understand how sensory organs detect stimuli before you can make sense of how the brain interprets them. If you have not reviewed basic neuron structure and signalling from Chapter 2, a quick refresher will help, but it is not strictly required. Everything in later chapters on learning, memory, and cognition assumes you are comfortable with the sensation-to-perception pipeline covered here.
Your sensory organs pick up physical energy (light, sound, pressure) and convert it into neural signals. Your brain then organises and interprets those signals based on context, expectations, and experience. The key debate is how much of perception is driven by raw data coming in versus what your brain already expects to find.
Sensation
The process by which sensory receptors and the nervous system receive and represent stimulus energies from the environment. In simple terms, this is the "detection" step: your eyes, ears, skin, nose, and tongue picking up what is out there.
Perception
The process of organising and interpreting sensory information, enabling you to recognise meaningful objects and events. Think of it as your brain making sense of the raw data your senses have collected.
Transduction
The conversion of one form of energy into another. In sensation, this refers to transforming physical stimuli (light waves, sound waves, chemical molecules) into neural impulses. In simple terms, it is the translator between the physical world and the language your brain speaks.
Bottom-up processing (data-driven processing)
Analysis that begins with the sensory receptors and works up to the brain's integration of sensory information. Think of it as building your perception from the ground up, starting with the raw stimulus. You are assembling the picture piece by piece from the incoming data, with no prior expectations guiding you.
Top-down processing (conceptually driven processing)
Information processing guided by higher-level mental processes, such as expectations, experiences, and context. In simple terms, your brain uses what it already knows to fill in gaps and speed up interpretation. Reading messy handwriting is a good example: you use the surrounding words to guess a letter you cannot quite make out.
Absolute threshold
The minimum stimulation needed to detect a particular stimulus 50% of the time. Think of it as the quietest whisper you can just barely hear in a silent room, or the faintest star you can just about see on a clear night. The 50% criterion matters: it is a statistical definition, not a hard on/off switch.
Difference threshold (just noticeable difference, JND)
The minimum difference between two stimuli required for detection 50% of the time. In simple terms, it is the smallest change you can notice. If you are holding a 1 kg weight and someone adds 1 gram, you will not feel it. They would need to add roughly 20 grams before you reliably notice the change.
Weber's law
The principle that, to be perceived as different, two stimuli must differ by a constant minimum percentage (not a constant amount). In simple terms, the heavier the original weight, the more you need to add before you notice a difference. The ratio stays the same even as the absolute numbers change. This is why you notice a £1 price increase on a £5 coffee but barely register it on a £500 appliance.
Signal detection theory
A theory predicting how and when we detect the presence of a faint stimulus (signal) amid background noise. Detection depends not just on the stimulus strength but also on the person's experience, expectations, motivation, and alertness. Think of it as the framework that explains why a new parent wakes at the faintest whimper but sleeps through traffic noise.
Perceptual set
A mental predisposition to perceive one thing and not another. Shaped by schemas, context, motivation, and emotion. In simple terms, you tend to see what you expect to see. If you believe an expensive medication works better, your expectations (perceptual set) colour how you experience its effects, even when the active ingredient is identical to a cheaper version.
Sensory adaptation
Diminished sensitivity as a consequence of constant stimulation. Think of it as your senses "tuning out" unchanging input. You stop noticing the hum of your refrigerator after a few minutes, or the feeling of your watch on your wrist. It frees up your attention for new or changing stimuli, which are more likely to matter.
Sensation is the raw input stage: energy from the environment hits a receptor.
Transduction converts that energy into neural signals.
Perception is the interpretation stage: the brain organises the signals into something meaningful.
The entire pipeline runs so quickly and seamlessly that most people never notice it has separate stages.
Bottom-up processing is data-driven. You start with sensory data and build upward toward a recognition.
Example: identifying an unfamiliar object by carefully examining its shape, colour, and texture.
Top-down processing is conceptually driven. Your brain applies prior knowledge to incoming data.
Example: reading a sentence with a missing letter and barely noticing because context fills it in.
In practice, perception uses both simultaneously. Bottom-up provides the raw material; top-down provides the interpretive framework.
Absolute threshold answers: "Can I detect this stimulus at all?"
Classic examples: a candle flame visible 30 miles away on a dark, clear night; a watch ticking 20 feet away in a quiet room.
Difference threshold (JND) answers: "Can I tell these two stimuli apart?"
Example: detecting which of two drinks is slightly sweeter.
Weber's law formalises the JND: the ratio of the just noticeable difference to the original stimulus is constant.
For weight, the Weber fraction is roughly 1/50. For brightness, roughly 1/60.
This means larger base stimuli require proportionally larger changes to be noticed.
Subliminal stimulation occurs below the absolute threshold. Research suggests subliminal stimuli can prime faint reactions but cannot reliably control complex behaviour. The idea that subliminal advertising can compel purchases is largely a myth.
Goes beyond simple thresholds by accounting for psychological factors.
Four outcomes when detecting a stimulus:
Hit: stimulus present, you detect it.
Miss: stimulus present, you fail to detect it.
False alarm: stimulus absent, you think you detect it.
Correct rejection: stimulus absent, you correctly say "not there."
Your response criterion (how willing you are to say "yes, I detect it") shifts with motivation, fatigue, and expectations.
Your expectations actively shape what you perceive.
Sources of perceptual set include schemas (mental frameworks), cultural context, motivation, and emotional state.
The medication-price example from the worksheet: if you expect the expensive pill to work better, your perceptual set biases you toward interpreting your symptoms as improved, even when the drug is chemically identical to the cheaper one.
Receptors are tuned to detect change, not constancy.
When a stimulus is unchanging, receptor firing decreases over time.
Everyday examples: you stop smelling your own home after a few minutes; you stop feeling the pressure of clothing against your skin.
Sensory adaptation does not occur with pain to the same degree, which makes biological sense: ongoing pain signals potential tissue damage that you should not ignore.
Perceptual set explains the placebo effect in medicine: patients who expect a treatment to work often report feeling better, which is why clinical trials use double-blind designs. Sensory adaptation is why air fresheners seem to "stop working" after a while, even though visitors can still smell them when they walk in.
Students often confuse absolute threshold with difference threshold. Absolute threshold is about detecting a stimulus at all; difference threshold is about detecting a change between two stimuli. They are answering different questions.
Students sometimes think subliminal messages can control behaviour. They can prime fleeting, weak responses, but the evidence for meaningful behavioural control (such as buying a product) is not there.
Sensory adaptation is sometimes confused with habituation. Adaptation is a sensory-level process (receptors reduce firing). Habituation is a brain-level learning process (you learn to ignore a repeated stimulus). They operate at different levels of the nervous system.
Weber's law describes a constant ratio, not a constant amount. Students often slip into thinking you always need to add, say, 10 grams to notice a difference, regardless of the starting weight. The ratio is what stays the same.
⚠️ Be prepared to distinguish bottom-up from top-down processing and give examples of each. This is a near-certain exam question.
⚠️ Know the exact definitions of absolute threshold and difference threshold, including the "50% of the time" criterion. Exam questions often test whether you remember that 50% benchmark.
⚠️ Weber's law: know that it is a constant ratio, and be ready to apply it to a novel example (e.g., "If the Weber fraction for weight is 1/50, how much must be added to a 100 g weight to notice a difference?").
⚠️ Perceptual set: expect a scenario-based question (like the medication example) where you need to explain behaviour in terms of expectations shaping perception.
⚠️ Sensory adaptation: be ready to provide your own example and explain the mechanism (receptor firing decreases with constant stimulation).
True or False: Bottom-up processing relies on prior knowledge and expectations. (False – that is top-down processing.)
The minimum stimulation needed to detect a stimulus 50% of the time is called the ________. (Absolute threshold.)
True or False: Weber's law states that the just noticeable difference is a constant amount regardless of the original stimulus. (False – it is a constant ratio, not a constant amount.)
Fill in the blank: The decrease in sensitivity to an unchanging stimulus is called ________. (Sensory adaptation.)
True or False: Subliminal messages have been proven to reliably control purchasing behaviour. (False.)
Q: Define bottom-up processing and give an example.
A: Bottom-up processing is analysis that begins with sensory receptors and works up to the brain's integration of sensory information. Example: tasting an unfamiliar food and identifying the individual flavours before recognising the dish.
Q: Define top-down processing and give an example.
A: Top-down processing is information processing guided by higher-level expectations, context, and experience. Example: reading a text message full of typos and still understanding it because your brain fills in what it expects the words to be.
Q: What is the difference between absolute threshold and difference threshold?
A: Absolute threshold is the minimum stimulation needed to detect a stimulus 50% of the time. Difference threshold (JND) is the minimum difference between two stimuli needed to detect a change 50% of the time. One asks "Is anything there?", the other asks "Has something changed?"
Q: People believe an expensive medication works better than a cheap one, even when the active ingredient is identical. Explain this using the concept of perceptual set.
A: Perceptual set is a mental predisposition to perceive things in a certain way. The higher price creates an expectation that the medication is more effective. This expectation biases the person's perception of their own symptoms, making them more likely to interpret any change as improvement. The stimulus (the drug's chemical action) is the same; the perception differs because the interpretive framework differs.
Q: Give an example of sensory adaptation and explain why it occurs.
A: Jumping into a cold swimming pool: after a few minutes, the water feels less cold. Sensory adaptation occurs because receptors decrease their firing rate in response to constant, unchanging stimulation. This frees up neural resources for detecting new or changing stimuli, which are more likely to be behaviourally relevant.
Q: State Weber's law and explain what it means in practical terms.
A: Weber's law states that the just noticeable difference between two stimuli is a constant proportion of the original stimulus, not a constant amount. Practically, this means you need a bigger absolute change to notice a difference when the starting stimulus is large. Adding 5 grams to a 500 g package is undetectable, but adding 5 grams to a 50 g letter is noticeable.
This material connects directly to the next section of Chapter 3, which applies these principles to specific senses (vision, hearing, touch, taste, smell). Understanding transduction here sets up the discussion of how rods and cones convert light into neural signals in the vision section. Signal detection theory reappears in research methods discussions about how participants respond in experiments, and perceptual set links to cognitive psychology topics like schemas and confirmation bias in later chapters.
sensation, perception, transduction, bottom-up processing, data-driven processing, top-down processing, conceptually driven processing, absolute threshold, difference threshold, just noticeable difference, JND, Weber's law, Weber fraction, signal detection theory, hit, miss, false alarm, correct rejection, subliminal perception, subliminal stimulation, perceptual set, schemas, sensory adaptation, habituation, psychophysics, General Psychology, PSY 1100, Chapter 3, Ohio State