Sensation and perception together account for about 7% of the CLEP Psychology exam. Sensation is the raw detection of stimulus energy by your sense organs; perception is the brain's organization and interpretation of it. The bridge between them is transduction — the conversion of physical energy (light, sound, pressure, chemicals) into neural impulses. Eyes transduce light, ears transduce sound waves, taste buds transduce chemical molecules. Without transduction the outside world never enters the nervous system.
Psychophysics studies the relationship between physical stimuli and psychological experience.
| Concept | Definition | Key point |
|---|---|---|
| Absolute threshold | Minimum intensity to detect a stimulus 50% of the time | Not "the faintest ever"; the 50% is non-negotiable |
| Difference threshold (JND) | Minimum change needed to notice a difference 50% of the time | About detecting change between two stimuli |
| Weber's law | Two stimuli must differ by a constant proportion, not a constant amount | Weight ≈ 2%, but each sense has its own fraction |
| Sensory adaptation | Reduced receptor firing to a constant, unchanging stimulus | Frees attention to notice change |
| Signal detection theory | Detecting a faint signal depends on the stimulus and the observer's alertness, expectations, and decision criterion | Yields hits, misses, false alarms, correct rejections |
A stimulus below the absolute threshold is subliminal. Subliminal stimuli can briefly prime associations, but there is no good evidence they control behavior.
Light passes through the cornea and lens (which focuses by changing shape — accommodation) and lands on the retina, where transduction happens.
This is why a faint star is clearer when viewed slightly off-center — its light falls on the rod-rich periphery. Signals leave the eye through the optic nerve, creating a blind spot. In the visual cortex, feature detectors (Hubel & Wiesel) respond to specific edges, angles, and movements.
Two color theories describe consecutive stages, so both are correct: - Trichromatic (Young-Helmholtz) theory: three cone types (red, green, blue) operate at the retina. - Opponent-process theory: red-green, blue-yellow, black-white pairs operate after the cones and explain afterimages.
Sound waves vibrate the eardrum, then the hammer, anvil, and stirrup, then the cochlea, where bending hair cells transduce the signal. Two pitch theories both hold: - Place theory: pitch depends on where on the basilar membrane hair cells are most stimulated — best for high pitches. - Frequency theory: pitch depends on the rate of firing matching the wave — best for low pitches (the volley principle covers the middle range).
Touch, taste (five basic tastes), smell (the only sense not routed through the thalamus), and the vestibular (balance) and kinesthetic (body position) senses round out the sensory systems.
Bottom-up processing builds a perception from raw sensory data; top-down processing uses prior knowledge, context, and expectations to interpret it. Context effects — reading the same mark as "B" or "13" depending on its neighbors — are top-down.
Gestalt principles describe how the brain groups sensation into wholes: figure-ground, proximity, similarity, closure (filling gaps to see a whole), continuity, and common fate.
Depth perception uses two cue types: - Binocular cues (need both eyes): retinal disparity (the two eyes' images differ; bigger difference = closer) and convergence (inward eye rotation). - Monocular cues (one eye): linear perspective, interposition, relative size, texture gradient.
Perceptual constancies keep objects stable — a door swinging open still looks rectangular (shape constancy); a friend walking away still looks normal-sized (size constancy). A perceptual set is a readiness, built from experience and schemas, to perceive one thing rather than another.
Q1 — C (transduction). - Correct: Transduction is the conversion of physical stimulus energy into neural impulses. - A) Perception is interpretation, not conversion. B) Accommodation is the lens changing shape to focus. D) Sensory adaptation is reduced sensitivity to a constant stimulus. E) Parallel processing is analyzing several features at once, not converting energy. - Fix rule: Transduction = the doorway turning physical energy into neural signals.
Q2 — D (50 percent of the time). - Correct: The absolute threshold is the minimum intensity detected on half of trials. - A) and B) add conditions that are not part of the definition. C) "100 percent" is the classic misconception — it is not "always detectable." E) confuses it with a comparison to the difference threshold. - Fix rule: Absolute threshold = detected 50% of the time, not "the faintest ever."
Q3 — A (88). - Correct: The just-noticeable difference at level 40 was 4, a proportion of 10%; Weber's law keeps that proportion, so at level 80 the JND is 8, giving 88. - B) 84 wrongly adds the same absolute amount (4). C) 96 uses a 20% fraction. D) 82 uses 2%. E) 120 uses 50%. - Fix rule: Weber's law scales by constant proportion — find the percentage, then apply it to the new base.
Q4 — B (sensory adaptation). - Correct: Reduced sensitivity to a constant, unchanging stimulus is sensory adaptation. - A) Signal detection concerns deciding about a faint signal, not fading of a steady one. C) The difference threshold is about noticing change. D) Perceptual set is a readiness to perceive. E) Priming involves below-threshold activation of associations. - Fix rule: Stop noticing an unchanging stimulus = sensory adaptation (receptors fire less).
Q5 — C (more liberal decision criterion, signal detection theory). - Correct: More "abnormal" calls and more false alarms with no gain in true detections is a looser (liberal) criterion, a signal-detection concept. - A) The absolute threshold is a fixed-threshold idea that ignores the decision-maker. B) Adaptation is receptor fatigue. D) The difference threshold is unrelated. E) Bottom-up processing is not what shifts here. - Fix rule: More false alarms without more hits = a looser (liberal) criterion.
Q6 — D (rods, more sensitive in dim light). - Correct: The peripheral retina is dominated by rods, which excel in dim light. - A) Cones need bright light and give color/detail. B) The fovea is the center of gaze, not the periphery. C) Bipolar cells relay signals; they do not code color. E) Feature detectors are cortical neurons, not retinal light-sensors. - Fix rule: Dim light + periphery = rods; color + detail + fovea = cones.
Q7 — C (flawed; opponent-process theory explains afterimages). - Correct: Afterimages come from fatigued opposing color pairs downstream of the cones, the signature evidence for opponent-process theory. - A) and E) misassign afterimages to trichromatic theory. B) Afterimages arise after the cones, not at them. D) The blind spot is where the optic nerve exits and has nothing to do with color afterimages. - Fix rule: Afterimages = opponent-process (a fatigued color pair firing its opposite).
Q8 — E (location on the basilar membrane). - Correct: Place theory says pitch is coded by where hair cells are most stimulated, best for high pitches. - A) Firing rate matching the wave is frequency theory. B) Loudness comes from amplitude. C) Alternating volleys is the volley principle. D) Damage to hair cells causes hearing loss, not pitch coding. - Fix rule: Place theory = pitch mapped to LOCATION; frequency theory = pitch mapped to firing RATE.
Q9 — B (retinal disparity). - Correct: Presenting each eye a slightly different image exploits retinal disparity, a binocular cue. - A) Linear perspective, C) interposition, and E) texture gradient are monocular cues. D) Convergence is binocular but concerns eye rotation, not two different images. - Fix rule: Two slightly different images to the two eyes = retinal disparity.
Q10 — B (closure). - Correct: Filling in gaps to perceive a complete figure that is not actually drawn is closure. - A) Proximity groups nearby items. C) Similarity groups look-alike items. D) Figure-ground separates object from background. E) Continuity favors smooth continuous lines. - Fix rule: Mentally completing a figure the eye does not actually receive = closure.
Q11 — E (context / top-down processing). - Correct: The same mark is perceived differently because surrounding context steers interpretation — top-down processing. - A) Transduction is energy conversion. B) Adaptation is fading of a steady stimulus. C) The absolute threshold is about detectability. D) Retinal disparity is a depth cue. - Fix rule: Same stimulus, different perception from its surroundings = context effect (top-down).
Q12 — A (flawed; shape constancy). - Correct: Shape constancy lets us perceive a stable rectangle despite the changing retinal trapezoid. - B) Perception does not slavishly match the retinal image — that is the whole point of constancy. C) Bottom-up processing alone would not stabilize the shape. D) The retina does register shape. E) Retinal disparity is a depth cue, not a constancy. - Fix rule: Stable perceived shape despite a changing retinal image = shape constancy.
Q1 — C (transduction). - Correct: Transduction is the conversion of physical stimulus energy into neural impulses. - A) Perception is interpretation, not conversion. B) Accommodation is the lens changing shape to focus. D) Sensory adaptation is reduced sensitivity to a constant stimulus. E) Parallel processing is analyzing several features at once, not converting energy. - Fix rule: Transduction = the doorway turning physical energy into neural signals.
Q2 — D (50 percent of the time). - Correct: The absolute threshold is the minimum intensity detected on half of trials. - A) and B) add conditions that are not part of the definition. C) "100 percent" is the classic misconception — it is not "always detectable." E) confuses it with a comparison to the difference threshold. - Fix rule: Absolute threshold = detected 50% of the time, not "the faintest ever."
Q3 — A (88). - Correct: The just-noticeable difference at level 40 was 4, a proportion of 10%; Weber's law keeps that proportion, so at level 80 the JND is 8, giving 88. - B) 84 wrongly adds the same absolute amount (4). C) 96 uses a 20% fraction. D) 82 uses 2%. E) 120 uses 50%. - Fix rule: Weber's law scales by constant proportion — find the percentage, then apply it to the new base.
Q4 — B (sensory adaptation). - Correct: Reduced sensitivity to a constant, unchanging stimulus is sensory adaptation. - A) Signal detection concerns deciding about a faint signal, not fading of a steady one. C) The difference threshold is about noticing change. D) Perceptual set is a readiness to perceive. E) Priming involves below-threshold activation of associations. - Fix rule: Stop noticing an unchanging stimulus = sensory adaptation (receptors fire less).
Q5 — C (more liberal decision criterion, signal detection theory). - Correct: More "abnormal" calls and more false alarms with no gain in true detections is a looser (liberal) criterion, a signal-detection concept. - A) The absolute threshold is a fixed-threshold idea that ignores the decision-maker. B) Adaptation is receptor fatigue. D) The difference threshold is unrelated. E) Bottom-up processing is not what shifts here. - Fix rule: More false alarms without more hits = a looser (liberal) criterion.
Q6 — D (rods, more sensitive in dim light). - Correct: The peripheral retina is dominated by rods, which excel in dim light. - A) Cones need bright light and give color/detail. B) The fovea is the center of gaze, not the periphery. C) Bipolar cells relay signals; they do not code color. E) Feature detectors are cortical neurons, not retinal light-sensors. - Fix rule: Dim light + periphery = rods; color + detail + fovea = cones.
Q7 — C (flawed; opponent-process theory explains afterimages). - Correct: Afterimages come from fatigued opposing color pairs downstream of the cones, the signature evidence for opponent-process theory. - A) and E) misassign afterimages to trichromatic theory. B) Afterimages arise after the cones, not at them. D) The blind spot is where the optic nerve exits and has nothing to do with color afterimages. - Fix rule: Afterimages = opponent-process (a fatigued color pair firing its opposite).
Q8 — E (location on the basilar membrane). - Correct: Place theory says pitch is coded by where hair cells are most stimulated, best for high pitches. - A) Firing rate matching the wave is frequency theory. B) Loudness comes from amplitude. C) Alternating volleys is the volley principle. D) Damage to hair cells causes hearing loss, not pitch coding. - Fix rule: Place theory = pitch mapped to LOCATION; frequency theory = pitch mapped to firing RATE.
Q9 — B (retinal disparity). - Correct: Presenting each eye a slightly different image exploits retinal disparity, a binocular cue. - A) Linear perspective, C) interposition, and E) texture gradient are monocular cues. D) Convergence is binocular but concerns eye rotation, not two different images. - Fix rule: Two slightly different images to the two eyes = retinal disparity.
Q10 — B (closure). - Correct: Filling in gaps to perceive a complete figure that is not actually drawn is closure. - A) Proximity groups nearby items. C) Similarity groups look-alike items. D) Figure-ground separates object from background. E) Continuity favors smooth continuous lines. - Fix rule: Mentally completing a figure the eye does not actually receive = closure.
Q11 — E (context / top-down processing). - Correct: The same mark is perceived differently because surrounding context steers interpretation — top-down processing. - A) Transduction is energy conversion. B) Adaptation is fading of a steady stimulus. C) The absolute threshold is about detectability. D) Retinal disparity is a depth cue. - Fix rule: Same stimulus, different perception from its surroundings = context effect (top-down).
Q12 — A (flawed; shape constancy). - Correct: Shape constancy lets us perceive a stable rectangle despite the changing retinal trapezoid. - B) Perception does not slavishly match the retinal image — that is the whole point of constancy. C) Bottom-up processing alone would not stabilize the shape. D) The retina does register shape. E) Retinal disparity is a depth cue, not a constancy. - Fix rule: Stable perceived shape despite a changing retinal image = shape constancy.