Source: Psychology 1100 Key Concepts and Terms
Tags: sensation, perception, bottom-up processing, top-down processing, attention, sensory adaptation, signal detection theory, Weber's law, difference threshold, trichromatic theory, opponent-process theory, perceptual set, visual system, auditory system, olfactory system, somatosensory system, gustatory system, Ohio State PSY 1100
Difficulty: Intermediate Prerequisites: Biological psychology notes (neurons, brain divisions, the thalamus as a relay station). Understanding the action potential helps with transduction.
Sensation and perception sit at the boundary between the physical world and your psychological experience of it. Sensation is the raw input: light hitting your retina, sound waves vibrating your eardrum. Perception is the brain's interpretation of that input: recognising a face, identifying a song. This unit covers how each sensory system converts physical energy into neural signals (transduction), how those signals reach the brain, and the theories that explain how we detect and interpret stimuli. Exam questions frequently ask you to trace a stimulus through a specific sensory system from physical event to conscious experience.
Your sensory organs detect physical energy and convert it to neural signals (sensation). Your brain organises and interprets those signals (perception). Different theories explain colour vision, stimulus detection, and why your expectations shape what you perceive. Each sensory system follows the same general pattern: physical stimulus, sensation, transduction, perception.
Attention
Focusing awareness on a narrowed range of stimuli or events. In simple terms, attention is the spotlight your brain shines on certain inputs while ignoring others.
Bottom-up processing
Analysis that begins with the sensory receptors and works upward to the brain's integration of sensory information, with no prior knowledge guiding the process. Think of it as building a picture from raw data, piece by piece, without any expectations.
Top-down processing
The use of pre-existing knowledge, expectations, and context to organise individual features into a unified whole. Think of it as your brain filling in gaps based on what it already knows. Reading messy handwriting relies heavily on top-down processing.
Sensory adaptation
The tendency of sensory receptor cells to become less responsive to a stimulus that remains constant and unchanging. In simple terms, this is why you stop noticing a smell after being in a room for a few minutes. The stimulus has not changed, but your receptors have dialled down their response.
Signal detection theory
An approach to perception that focuses on decision making about stimuli under conditions of uncertainty. Think of it as the framework for understanding why you sometimes "hear" your phone ring in a noisy restaurant when it has not rung. Your detection depends on the signal strength, the background noise, and your own expectations and motivations.
Difference threshold (just noticeable difference / JND)
The minimum degree of difference that must exist between two stimuli before the difference is detected. In simple terms, how different must two things be before you can tell them apart?
Weber's law
The principle that, to be perceived as different, two stimuli must differ by a constant minimum percentage rather than a constant absolute amount. Think of it this way: you notice 1 kg added to a 10 kg bag more easily than 1 kg added to a 100 kg bag. The percentage matters, not the raw amount.
Trichromatic theory (Young-Helmholtz)
Three types of colour receptors (cones) in the retina allow us to perceive three colours: red, green, and blue. All other colours are mixtures of these three. In simple terms, your retina has three kinds of cone, each tuned to a different wavelength, and the combination of their activity produces the full colour spectrum.
Opponent-process theory
The theory that opposing retinal processes (red-green, yellow-blue, white-black) enable colour vision. Some cells are stimulated by one colour in the pair and inhibited by the other. Think of it as a colour seesaw: when one end goes up (stimulated by green), the other goes down (inhibited by red).
Perceptual set
A mental predisposition to perceive one thing and not another, shaped by expectations, context, culture, and motivation. In simple terms, you see what you expect to see.
Every sensory system follows this pattern:
Physical stimulus: energy in the environment (light, sound, chemicals, pressure).
Sensation: sensory receptors detect the energy.
Transduction: receptors convert the energy into neural signals.
Perception: the brain interprets those signals as a conscious experience.
Light waves reflect off objects, pass through the cornea, enter through the pupil, and are focused by the lens onto the retina.
Rods (dim light, peripheral vision) and cones (colour, fine detail) in the retina detect light (sensation) and convert it into neural signals (transduction).
Signals are processed by ganglion cells and sent via the optic nerve to the thalamus, then to the primary visual cortex.
Each visual field is processed on the opposite side of the brain (contralateral processing).
Trichromatic theory explains colour mixing at the receptor level (three cone types).
Opponent-process theory explains phenomena trichromatic theory cannot, such as afterimages (stare at a green square, then look at white, and you see red).
Both theories are correct, they operate at different stages of visual processing. Trichromatic at the retina, opponent-process further along in the pathway.
Sound waves enter the outer ear, vibrate the eardrum, pass through the ossicles in the middle ear, and reach the cochlea in the inner ear.
In the cochlea, pressure waves move the basilar membrane, stimulating hair cells (sensation).
Hair cells convert vibrations into neural signals (transduction), sent via the auditory nerve to the thalamus, then to the primary auditory cortex.
Odorant molecules enter the nose and stimulate olfactory receptors in the olfactory epithelium.
Olfactory receptors convert odorants into signals transmitted directly to the brain via the olfactory nerve.
The olfactory nerve carries signals to the olfactory bulb and then to the primary olfactory cortex.
Smell is the only sense that bypasses the thalamus entirely.
Sensory receptors in the skin detect warm, cold, and pressure stimuli.
These receptors convert stimulation into neural signals.
For touches above the neck: cranial nerves carry signals to the brain.
For touches below the neck: spinal nerves carry signals to the spinal cord, which transmits them to the brain.
Signals travel through the thalamus to the primary somatosensory cortex, which maps to specific body regions.
Chemical molecules combine with saliva and stimulate taste receptors in taste buds, grouped within papillae on the tongue.
Taste receptors convert chemical molecules into neural signals transmitted primarily via the facial nerve.
Signals travel to the thalamus, then to the primary gustatory cortex.
Signal detection theory accounts for the fact that detecting a stimulus is not purely about the stimulus strength. Your motivation, expectations, and the noise in the environment all influence whether you report detecting something.
Weber's law explains why a 1-decibel change is noticeable in a quiet room but not at a concert. The just noticeable difference scales with the baseline intensity.
Bottom-up: data-driven, starting from raw sensory input.
Top-down: concept-driven, starting from knowledge and expectations.
In practice, perception almost always involves both simultaneously.
Signal detection theory is used in medical imaging: a radiologist scanning for tumours must balance sensitivity (catching real tumours) against false alarms (flagging healthy tissue). Weber's law explains why marketers must make price drops large enough, proportionally, for consumers to notice. Sensory adaptation is why you stop smelling your own perfume but others still can.
Students often think trichromatic theory and opponent-process theory are competing explanations. They are complementary; both are needed to explain colour vision fully.
Smell is the only sense that bypasses the thalamus. All other senses relay through the thalamus on the way to the cortex.
Sensory adaptation is not the same as habituation. Adaptation happens at the receptor level (sensory); habituation is a learning process at the brain level (cognitive).
Weber's law is about proportional differences, not absolute ones. A common exam mistake is treating the threshold as a fixed amount.
⚠️ Be ready to trace a stimulus through any of the five sensory systems, from physical event to cortical processing.
⚠️ Know which sense bypasses the thalamus (smell).
⚠️ Expect a question on trichromatic vs. opponent-process theory, possibly involving afterimages.
⚠️ Weber's law questions often give you a scenario and ask you to apply the proportional rule.
⚠️ Distinguish bottom-up from top-down processing with an example of each.
True or False: Smell is processed through the thalamus before reaching the cortex.
Fill in the blank: The conversion of physical energy into neural signals is called __________.
True or False: Weber's law states that the just noticeable difference is a constant absolute amount.
Fill in the blank: __________ processing uses pre-existing knowledge to interpret sensory input.
True or False: Rods are responsible for colour vision.
(Answers: 1. False; 2. transduction; 3. False, it is a constant percentage; 4. Top-down; 5. False, cones handle colour vision.)
Q: Explain how trichromatic theory and opponent-process theory work together to explain colour vision.
A: Trichromatic theory operates at the retinal level, where three types of cones (red, green, blue) respond to different wavelengths. Opponent-process theory operates further along the visual pathway, where cells respond to opposing colour pairs (red-green, yellow-blue, white-black). Together, they explain both colour mixing and afterimage effects.
Q: Why do you stop noticing a constant smell after a few minutes?
A: Sensory adaptation. Olfactory receptors become less responsive to an unchanging stimulus, reducing the signal they send to the brain.
Q: A friend says they can tell when 5 grams is added to a 50-gram weight but not when 5 grams is added to a 500-gram weight. Which principle explains this?
A: Weber's law. The just noticeable difference is a constant proportion of the original stimulus, not a fixed amount. Five grams is 10% of 50 g but only 1% of 500 g.
Q: Give one example each of bottom-up and top-down processing.
A: Bottom-up: assembling the shape of a letter from lines and curves without knowing what word to expect. Top-down: reading a misspelled word correctly because the sentence context tells your brain what the word should be.
Q: Trace the path of a sound wave from the environment to conscious perception.
A: Sound waves enter the outer ear, vibrate the eardrum, pass through the ossicles (middle ear), and reach the cochlea (inner ear). Pressure waves move the basilar membrane, stimulating hair cells (transduction). The auditory nerve carries signals to the thalamus, then to the primary auditory cortex, where the sound is perceived.
Sensation and perception connect back to biological psychology (neurons and brain structures do the transduction and processing) and forward to consciousness (altered states change how we perceive). Attention, covered here, also links to the stress and coping unit, because stress narrows attentional focus. Perceptual set connects to social psychology later in the course, where expectations shape how we interpret others' behaviour.
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