Difficulty: Introductory to Intermediate | Prerequisites: Neuroscience notes recommended (action potentials, neural pathways)
Tags: sensation, perception, attention, bottom-up processing, top-down processing, absolute threshold, difference threshold, just noticeable difference, JND, Weber's law, signal detection theory, trichromatic theory, opponent-process theory, perceptual set, sensory adaptation, visual system, auditory system, olfactory system, gustatory system, somatosensory system, rods and cones, retina, cochlea
Sensation is the raw input: how your sensory receptors detect light, sound, chemicals, and pressure. Perception is the interpretation: how your brain organises and makes sense of that input. This unit bridges the neuroscience material (neurons converting stimuli into signals) with higher-level cognitive topics (attention, memory, learning). Understanding the difference between bottom-up and top-down processing is especially important, as it shows up across the entire course.
Your sensory organs convert physical stimuli into neural signals (transduction), which travel to the brain for interpretation. Bottom-up processing builds perception from raw data; top-down processing uses prior knowledge to shape what you perceive. Key principles include thresholds (how much stimulation you need to notice something), Weber's law (bigger stimuli need bigger changes to be noticed), signal detection theory (perception involves decision-making, not just sensitivity), and sensory adaptation (you stop noticing constant stimuli).
Attention
Focusing awareness on a specific stimulus or set of stimuli while filtering out irrelevant information. It can be voluntary (choosing to focus) or involuntary (a loud noise grabs your attention).
Think of it as: your brain's spotlight, deciding what gets processed and what gets ignored.
Bottom-up processing
Perception that starts with sensory receptors and works upward to the brain's integration of information. It is data-driven and relies on no prior knowledge.
In simple terms: you build the picture from the raw details, like assembling a jigsaw puzzle without seeing the box.
Top-down processing
Perception guided by prior knowledge, expectations, and experience. It helps you interpret ambiguous stimuli and fills in gaps.
In simple terms: you use what you already know to make sense of what you are seeing or hearing.
Difference threshold (just noticeable difference, JND)
The minimum difference between two stimuli that a person can detect. It varies depending on the intensity of the original stimulus and follows Weber's law.
Weber's law
The just noticeable difference between two stimuli is proportional to the magnitude of the original stimulus. In practical terms: if you are holding a 1 kg weight, you might notice an extra 100 g. If you are holding 10 kg, you would need an extra 1 kg to notice the change.
Signal detection theory (SDT)
A framework for understanding how people distinguish between a real stimulus and background noise under conditions of uncertainty. It accounts for sensitivity and decision-making factors like motivation and expectations, producing four outcomes: hit, miss, false alarm, correct rejection.
Think of it as: perception involves judgement, not just sensory ability.
Sensory adaptation
The reduction in sensitivity of sensory receptors to unchanging stimuli over time. Example: you stop noticing the smell of a room after being in it for a few minutes.
Perceptual set
A mental predisposition to perceive things in a particular way, shaped by expectations, experience, and context. It can cause perceptual biases or illusions.
Trichromatic theory (Young-Helmholtz theory)
The theory that three types of cone photoreceptors in the retina, each sensitive to red, green, or blue light, combine their activity to produce the full spectrum of colour vision. Explains most colour blindness.
Opponent-process theory
The theory that colour vision depends on three pairs of opposing retinal processes: red-green, yellow-blue, and black-white. Cells are stimulated by one colour and inhibited by the other in the pair. Explains afterimages.
In simple terms: trichromatic theory explains how cones detect colour at the retina; opponent-process theory explains how the brain processes those signals further along the pathway. Both are correct at different stages.
Light enters the eye through the cornea, passes through the pupil (opening regulated by the iris), and is focused by the lens onto the retina.
The retina contains two types of photoreceptors:
Rods: sensitive to low light, responsible for peripheral and night vision. No colour information.
Cones: responsible for colour vision and fine detail. Concentrated in the fovea. Three types correspond to red, green, and blue sensitivity (trichromatic theory).
Signals from rods and cones are processed by ganglion cells, whose axons form the optic nerve.
Visual information travels from the optic nerve to the thalamus (lateral geniculate nucleus) and then to the primary visual cortex in the occipital lobe.
Colour vision is explained at two levels:
Trichromatic theory: three cone types at the retina level.
Opponent-process theory: opposing colour pairs processed at the ganglion cell and thalamus level. Explains why you see afterimages.
Sound waves are collected by the outer ear (pinna).
Waves travel through the ear canal to the eardrum (tympanic membrane), which vibrates.
Vibrations pass through three tiny bones in the middle ear (the ossicles: hammer, anvil, stirrup), amplifying the signal.
In the inner ear, the cochlea converts mechanical vibrations into neural signals via hair cells on the basilar membrane.
Signals travel via the auditory nerve to the brain for perception.
Odorant molecules enter the nasal cavity and stimulate olfactory receptors in the olfactory epithelium.
These receptors convert chemical stimuli into neural signals.
Uniquely, olfactory signals go directly to the olfactory cortex, bypassing the thalamus. This is why smells can trigger vivid memories so quickly.
Chemical molecules from food interact with taste receptors in taste buds on the tongue's papillae.
Five basic tastes: sweet, sour, salty, bitter, and umami (savoury).
Signals travel primarily via the facial nerve to the thalamus, then to the primary gustatory cortex.
Receptors in the skin detect pressure, temperature, and pain.
Signals travel via peripheral nerves to the spinal cord and up to the somatosensory cortex in the parietal lobe.
Bottom-up processing: building perception from raw sensory data, no expectations involved. Dominant when encountering novel or unfamiliar stimuli.
Top-down processing: using knowledge and context to interpret sensory input. Dominant when stimuli are ambiguous or familiar.
Sensory adaptation: prolonged exposure to a constant stimulus reduces your sensitivity to it. You stop noticing the hum of a refrigerator or the feel of clothes on your skin.
Signal detection theory: whether you detect a stimulus depends on both your sensory sensitivity and your response bias (shaped by motivation, expectations, and consequences of being wrong).
Weber's law: the JND is a constant proportion of the original stimulus. Heavier objects need bigger absolute changes to notice a difference than lighter objects do.
Signal detection theory is used in radiology (detecting tumours on scans), airport security screening, and quality control on production lines. Weber's law explains why you notice a price increase on a cheap item more than the same increase on an expensive one. Sensory adaptation is why you need to step outside and come back in to smell whether your house has an odour.
Students often think trichromatic theory and opponent-process theory are competing explanations. They are complementary: trichromatic theory explains colour detection at the retina; opponent-process theory explains further processing in the visual pathway.
Students sometimes believe that the tongue has distinct "taste zones" (sweet at the tip, bitter at the back). This is a persistent myth. All areas of the tongue can detect all five basic tastes.
Students often confuse sensation and perception. Sensation is the raw detection of a stimulus; perception is the brain's interpretation and organisation of that sensory input.
Students sometimes think sensory adaptation means your receptors stop working. They do not stop; they simply become less responsive to an unchanging stimulus. A new or changing stimulus will still be detected.
⚠️ Be able to distinguish bottom-up from top-down processing and give an example of each.
⚠️ Know the pathway for vision: cornea, pupil, lens, retina (rods and cones), ganglion cells, optic nerve, thalamus, visual cortex.
⚠️ Understand Weber's law well enough to apply it to a novel scenario (e.g. "Why would you notice 5 g added to 100 g but not 5 g added to 10 kg?").
⚠️ Be able to explain the four outcomes of signal detection theory: hit, miss, false alarm, correct rejection.
⚠️ Smell (olfaction) is the only sense that bypasses the thalamus. This is a common exam fact.
True or false: Bottom-up processing relies on prior knowledge to interpret stimuli. (False. That is top-down processing.)
Fill in the blank: The ______ is the only sense that bypasses the thalamus on its way to the cortex. (Smell / olfaction)
True or false: Weber's law states that the JND is a fixed amount regardless of stimulus intensity. (False. The JND is a constant proportion of the original stimulus.)
Fill in the blank: The two types of photoreceptors in the retina are ______ and ______. (Rods and cones)
True or false: Sensory adaptation means you can no longer detect any stimuli in that modality. (False. You stop noticing the unchanging stimulus, but you still detect new or changing ones.)
Q: You walk into a bakery and immediately smell fresh bread, but after 10 minutes you barely notice it. What principle explains this?
A: Sensory adaptation. Your olfactory receptors have reduced their sensitivity to the constant, unchanging smell of bread.
Q: A radiologist examining hundreds of chest X-rays per day sometimes misses a small tumour. How does signal detection theory explain this?
A: The radiologist's sensitivity (ability to detect the tumour) may be adequate, but fatigue, expectation of normal results, and the base rate of actual positives affect their response bias. A "miss" occurs when the stimulus (tumour) is present but goes undetected.
Q: Explain why both the trichromatic theory and opponent-process theory are needed to fully account for colour vision.
A: Trichromatic theory explains the initial detection of colour by three types of cones (red, green, blue) in the retina. Opponent-process theory explains further processing at the ganglion and thalamic level, where opposing colour pairs (red-green, blue-yellow, black-white) are processed. Together, they account for normal colour perception and phenomena like afterimages.
Q: Give one example of bottom-up processing and one of top-down processing.
A: Bottom-up: hearing an unfamiliar sound and trying to work out what it is based solely on its acoustic properties. Top-down: reading a sentence with a misspelled word and understanding it because context fills in the gap.
Sensation and perception connect directly to the neuroscience unit (transduction relies on action potentials and neural pathways) and to the attention and consciousness unit. Top-down processing is central to memory and learning topics later in the course, since prior knowledge shapes what we perceive. Signal detection theory also appears in clinical and applied psychology contexts.
sensation vs perception, transduction, attention selective and divided, bottom-up processing data-driven, top-down processing concept-driven, absolute threshold, difference threshold, just noticeable difference JND, Weber's law, signal detection theory hit miss false alarm correct rejection, sensory adaptation habituation, perceptual set, trichromatic theory Young-Helmholtz, opponent-process theory Hering, rods and cones, fovea, retina, optic nerve, visual cortex, auditory system cochlea basilar membrane, olfactory system bypasses thalamus, gustatory system five basic tastes, somatosensory cortex, General Psychology OSU