6 Perception · 6.1 Perception
6.1.1 Sensation, attention and perceptual processing
Sensation is the detection of a physical stimulus by the sense receptors and its passage to the brain; attention selects which of the many stimuli will be processed; perception is the process by which the brain organises and interprets sensations into meaningful experience, working both bottom-up (from the stimulus) and top-down (from knowledge and expectation). Perceptual organisation (Gestalt laws), perception of form, colour, depth and time, constancy, perceptual readiness, and the role of motivation and culture are in the next note of this unit.
- APPSC Paper I
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Core facts
- Sensation = the raw detection of energy (light, sound, pressure, chemicals) by receptors; perception = organising and interpreting that input. Perception is how we make sense of the world around us.
- Transduction = changing physical energy into nerve impulses in the receptor.
- Sense modalities: vision (eye), hearing (ear), smell (nose), taste (tongue), touch / skin senses (skin), plus kinaesthetic (position and movement of body parts; receptors in muscles and joints) and vestibular (balance; semicircular canals of the inner ear).
- Rods (about 120 million): dim light, black-and-white, side vision. Cones (about 6–7 million, packed in the fovea): bright light, colour and fine detail. Normal eyes have three types of cones, which give colour vision.
- Psychophysics = the study of the relation between physical stimuli and the sensations they produce. Gustav Fechner founded it (Elements of Psychophysics, 1860).
- Absolute threshold = the smallest intensity of a stimulus detected 50% of the time.
- Difference threshold or just noticeable difference (JND) = the smallest change in a stimulus detected 50% of the time.
- Weber's law (Ernst Weber, 1834): the JND is a constant fraction of the original stimulus (ΔI / I = k). Example: for lifted weights the fraction is about 1/50 (2%): 1 g is noticed on 50 g, 2 g on 100 g.
- Fechner's law: sensation grows with the logarithm of stimulus intensity (S = k log I). Stevens' power law: S = k Iⁿ.
- Signal detection theory (Green and Swets, 1966): detection depends on both sensitivity and the person's response bias (criterion); outcomes are hit, miss, false alarm, correct rejection.
- Sensory adaptation = reduced sensitivity to an unchanging stimulus (you stop smelling your own room, stop feeling your watch). Dark adaptation takes about 30 minutes to be complete; light adaptation is quicker.
- Attention = focusing awareness on some stimuli while ignoring others. NCERT names three properties: alertness, concentration, search.
- Types (NCERT): selective (choose one input), sustained (vigilance over a long time) and divided (two tasks at once). Span of attention = the number of objects grasped in one brief glance (about 5–7).
- Classic textbook types: voluntary (by effort and will), involuntary / non-voluntary (captured by a strong stimulus), habitual (from long interest or habit).
- Sub-processes of perception: stimulus → sensory receptors → transmission to the CNS (brain) → perception (interpretation).
- Processing approaches: bottom-up (data-driven, from features to the whole) and top-down (concept-driven, from knowledge and expectation to the parts).
- James J. Gibson: theory of direct (ecological) perception; information in the environment (the optic array) is rich enough for perception without inference; key terms affordances, texture gradients, optic flow.
- Constructivist (indirect) theory: Hermann von Helmholtz (unconscious inference, 1860s) and Richard Gregory (perception as a hypothesis; Eye and Brain, 1966; Perceptions as Hypotheses, 1980): the brain builds perception from incomplete data plus knowledge; illusions show the hypothesis going wrong.
Explained
1. Sensation vs perception
| Point | Sensation | Perception |
|---|---|---|
| What | detecting the stimulus | organising and interpreting it |
| Where | sense organs, sensory nerves | brain (cortex) |
| Meaning | none, raw data | meaningful object or event |
| Depends on | stimulus and receptor | stimulus plus past experience, motives, set, culture |
The same sensation can give two perceptions (an ambiguous figure seen as a vase or two faces), and different sensations can give the same perception (a door seen as rectangular from any angle).
2. Thresholds and the methods of psychophysics
- Method of limits: the stimulus is raised (or lowered) in small steps until the person just reports it (or stops reporting it).
- Method of constant stimuli: fixed intensities are given in random order many times; the threshold is the one detected 50% of the time. Most accurate, most time-consuming.
- Method of adjustment (average error): the person adjusts the stimulus until it just appears or matches a standard.
- Subliminal stimuli are below the absolute threshold; they can have small, short-lived effects (priming), but claims that hidden advertising changes buying are not supported.
- Textbook examples of absolute thresholds (Galanter, 1962): a candle flame seen about 48 km away on a dark, clear night; a watch tick heard about 6 m away in a quiet room; one teaspoon of sugar tasted in about 7.5 litres of water.
3. Signal detection theory
| Says "yes, signal" | Says "no signal" | |
|---|---|---|
| Signal present | hit | miss |
| Signal absent | false alarm | correct rejection |
A radar operator, a security screener or a doctor reading an X-ray may miss signals because of fatigue (lower sensitivity) or may say "yes" too often because missing is costly (a liberal criterion). The theory replaced the idea of one fixed threshold.
4. Adaptation vs habituation. Adaptation happens in the receptors (they fire less to a constant stimulus). Habituation happens in the brain (we stop responding to a familiar, harmless stimulus such as the sound of a fan, though the ears still detect it). A change in the stimulus brings the response back (dishabituation).
5. Determinants (factors) of attention
| External (stimulus) factors | Internal (person) factors |
|---|---|
| intensity and size (loud, bright, big) | motives and needs (a hungry person notices food) |
| contrast and change | interests and attitudes |
| movement | expectation, mental set |
| novelty | emotional state |
| repetition | habit, past training |
| position (top-left of a page) | physical state (fatigue, drugs) |
Advertising, road signs and warning lights use the external factors; the internal factors explain why two people in the same room notice different things.
6. Theories of selective attention
- Colin Cherry (1953): the cocktail party effect: we can follow one conversation in a noisy room; in his tests listeners took in almost nothing of the unattended message except physical features (a voice, a change from male to female). Neville Moray (1959) then showed that one's own name in the unattended message is often noticed. Studied by dichotic listening (a different message in each ear) and shadowing (repeating one message aloud).
- Filter theory (Donald Broadbent, 1958): an early filter selects one channel by physical features (ear, voice); the rest is blocked.
- Attenuation theory (Anne Treisman, 1960, 1964): the unattended message is only weakened, not blocked, so important words (one's name) can still get through.
- Late selection (J.A. Deutsch and D. Deutsch, 1963): all messages are analysed for meaning; selection happens late, near response.
- Multimode theory (Johnston and Heinz, 1978): selection can be early or late as needed; later selection costs more mental effort.
- Capacity (resource) model (Daniel Kahneman, 1973): attention is a limited pool of mental effort shared between tasks; this explains divided attention.
- Feature integration theory (Anne Treisman and Garry Gelade, 1980): simple features (colour, shape) are picked up pre-attentively, but focused attention is needed to bind them into one object.
7. Sustained and divided attention. Vigilance tasks (radar watching, long-distance driving, quality inspection) show a vigilance decrement: detection falls after about 20–30 minutes (Norman Mackworth's clock test with radar operators, 1948). Divided attention is easier when tasks are practised and automatic and use different senses; talking on a phone while driving still slows braking. Inattentional blindness: people focused on one task can fail to see an unexpected object in plain view (Simons and Chabris, 1999).
8. The perceptual process and processing approaches
- Many textbooks describe perception as selection → organisation → interpretation: attention selects, the brain groups the input into patterns, and meaning is given using memory and context.
- Bottom-up: starts from the stimulus; feature detectors in the visual cortex (David Hubel and Torsten Wiesel) respond to edges, lines and angles that are combined into objects.
- Top-down: starts from what we know and expect; we read a smudged word from its sentence, and the same shape is read as "B" among letters and "13" among numbers.
- Perceptual set = a readiness to perceive in a certain way, created by expectation, context, motives and emotion. (Perceptual readiness is treated in detail in the next note.)
- Both approaches work together in normal perception.
9. Theories of perception
| Theory | Key person | Direction | Main idea |
|---|---|---|---|
| Direct / ecological | J.J. Gibson | bottom-up | the environment gives all the information needed; affordances |
| Constructivist / indirect | Helmholtz, Gregory, Bruner | top-down | perception is built and tested like a hypothesis |
| Gestalt | Wertheimer, Köhler, Koffka | organisation | the whole is different from the sum of its parts |
| Perceptual cycle | Ulric Neisser | both | schemas direct exploration; new data change schemas |
- Gibson's terms: optic array (the pattern of light reaching the eye); invariants (features that stay constant as we move); texture gradient (texture looks denser with distance); optic flow (the scene streams outward from the point we move towards, used by pilots in landing); affordances (what an object offers for action: a chair affords sitting).
- Constructivist evidence: visual illusions (Müller-Lyer), ambiguous figures and the effect of expectation and culture. Jerome Bruner's "New Look" studies (1940s) showed that values and needs change perception (poorer children judged coins as larger).
Common traps
- ✗ "Sensation and perception are the same." ✓ Sensation detects; perception organises and interprets.
- ✗ "Absolute threshold = the stimulus detected every time." ✓ Detected 50% of the time.
- ✗ "Fechner's law says the JND is a constant fraction." ✓ That is Weber's law; Fechner's law is logarithmic (S = k log I).
- ✗ "Perception theory (direct perception) was given by Watson or Köhler." ✓ J.J. Gibson gave the direct (ecological) theory; Köhler is a Gestalt psychologist.
- ✗ "Gibson's theory is top-down." ✓ Gibson is bottom-up; Gregory and Helmholtz are top-down / constructivist.
- ✗ "Broadbent's filter weakens unattended messages." ✓ Broadbent's filter blocks them; Treisman said they are attenuated (weakened).
- ✗ "Rods give colour vision." ✓ Cones (three types) give colour; rods work in dim light.
- ✗ "Sustained attention means doing two tasks at once." ✓ That is divided attention; sustained = vigilance over time.
- ✗ "Sub-processes of perception: sensation → experience → stimulus." ✓ Stimulus → sensory receptors → CNS → perception.
- ✗ "Sensory adaptation happens in the brain." ✓ Adaptation is in the receptors; ignoring a familiar sound is habituation.
Quick revision
- Sensation = detect; perception = organise + interpret (make sense of the world).
- Fechner: father of psychophysics, 1860; Fechner's law S = k log I.
- Weber's law: JND / I = constant (weights about 1/50).
- Absolute threshold and JND are both defined at 50% detection.
- Signal detection: hit, miss, false alarm, correct rejection; sensitivity + criterion.
- Attention: alertness, concentration, search; selective, sustained, divided.
- Broadbent filter (early), Treisman attenuation, Deutsch and Deutsch (late), Cherry cocktail party.
- Sub-processes: stimulus → receptors → CNS → perception.
- Bottom-up = data-driven; top-down = knowledge-driven.
- Gibson: direct, ecological, affordances; Gregory / Helmholtz: constructivist, hypothesis, unconscious inference.
Go deeper: Deeper (beyond what APPSC Paper I usually asks)
Deeper Advanced
1. Weighing Gibson against the constructivists. Gibson studied perception in real, rich environments (his early work trained Second World War pilots), so he found little need for inference. Gregory studied brief, impoverished or ambiguous displays in the laboratory, where knowledge must fill gaps. Neisser's perceptual cycle and modern accounts treat the two as working at different times: bottom-up when input is clear, top-down when it is weak or ambiguous.
2. Weber fractions differ by sense. The fraction is small for sensitive judgements and larger for others; Weber's law holds well in the middle range of intensities and breaks down near the absolute threshold and at very high intensities. Stevens showed that some sensations grow faster than the stimulus (electric shock) and others slower (brightness), which a single logarithmic law cannot explain.
3. Pattern recognition models. Template matching compares input with stored copies (fails with variety in handwriting); feature analysis (Oliver Selfridge's Pandemonium model, 1959) detects features and combines them; prototype models compare with the best typical example. Feature analysis fits the feature detectors found by Hubel and Wiesel.
4. Application to skills and jobs. Pilots, drivers and machine operators need selective attention (ignoring distractions), sustained attention (vigilance), divided attention (scanning instruments while steering) and fast signal detection. Job design uses this: rest breaks to limit the vigilance decrement, alarms using the external factors of attention (intensity, movement, contrast), and simulator training that makes routine actions automatic so attention is free for hazards.
Practise this topic. Questions on Sensation, attention and perceptual processing, with answers explained.
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