Source: Exam 1 Study Guide | General Psychology, The Ohio State University
Difficulty: Intermediate | Prerequisites: Unit 2 (Neuroscience). You need to understand neurons and action potentials, since transduction converts stimuli into neural signals.
This unit asks two questions: how does raw physical energy (light, sound waves, pressure) get converted into neural signals your brain can use, and how does your brain interpret those signals into a coherent experience of the world? Sensation is the detection side (your eyes register light), and perception is the interpretation side (your brain tells you that light is a red car). The unit leans heavily on vision, covering the anatomy of the eye, how colour is processed, and how the brain infers depth from a flat retinal image. Understanding the difference between bottom-up and top-down processing is central here.
Sensation is detecting a stimulus; perception is interpreting it. Your sensory receptors convert physical energy into neural signals (transduction), and your brain makes sense of those signals using a mix of raw data (bottom-up) and prior knowledge (top-down). For this exam, focus on how the eye works, the two theories of colour vision, and how the brain constructs depth from two-dimensional input.
Transduction
The process of converting physical energy (light, sound, pressure, chemicals) into neural signals (action potentials) that the brain can process. Think of it as translation: the world speaks in light waves, your brain speaks in electrical impulses, and transduction is the interpreter.
Sensory adaptation
The tendency of sensory receptors to become less responsive to a constant, unchanging stimulus over time. This is why you stop noticing the hum of your refrigerator.
Bottom-up processing
Perception driven by the raw sensory data coming in from the environment. You build your understanding from the stimulus itself, piece by piece.
Top-down processing
Perception driven by your expectations, knowledge, and context. Your brain fills in gaps or interprets ambiguous stimuli based on what it already knows. In simple terms, you see what you expect to see.
Absolute threshold
The minimum amount of stimulus energy needed to detect a stimulus 50% of the time. For example, the faintest sound you can just barely hear in a quiet room.
Difference threshold (just noticeable difference, JND)
The smallest detectable difference between two stimuli. Weber's Law states this is a constant proportion of the original stimulus, not a fixed amount.
Signal detection theory
A framework for understanding how we detect stimuli amid background noise. It accounts for both the sensitivity of the sensory system and the decision-making criteria of the person (their willingness to say "yes, I detected it"). Produces four outcomes: hit, miss, false alarm, correct rejection.
Rods
Photoreceptors in the retina that detect light and dark. They work in low-light conditions but do not detect colour. Concentrated in the periphery of the retina. Think of them as your night-vision system.
Cones
Photoreceptors in the retina that detect colour and fine detail. They require more light to function. Concentrated in the fovea (the centre of the retina). Think of them as your high-definition daytime system.
Trichromatic theory (Young-Helmholtz)
The theory that colour vision depends on three types of cones, each sensitive to a different wavelength: short (blue), medium (green), and long (red). All colours we see are combinations of activity across these three cone types.
Opponent-process theory
The theory that colour is processed in opposing pairs: red vs. green, blue vs. yellow, black vs. white. This explains afterimages (stare at a red image, then look at a white wall, and you see green).
Depth cues
Visual signals that help the brain perceive distance and three-dimensionality.
Binocular cues require both eyes (e.g. retinal disparity: each eye gets a slightly different image, and the brain uses the difference to gauge depth).
Monocular cues work with one eye (e.g. relative size, linear perspective, overlap/occlusion, texture gradient).
Sensation is the process of detecting physical energy from the environment (light hits your retina).
Perception is the process of organising and interpreting that sensory information (you recognise a face).
Transduction bridges the two: it converts the physical stimulus into neural signals.
Bottom-up processing: starts with the raw data. Your brain assembles meaning from the sensory input itself. Reading unfamiliar text in a foreign alphabet is mostly bottom-up.
Top-down processing: starts with your brain. Expectations, context, and prior experience shape how you interpret the stimulus. Reading a friend's messy handwriting relies heavily on top-down processing because you already know what words are likely.
Most real perception uses both at once.
Absolute threshold: the minimum stimulus intensity detectable 50% of the time. Below this, you are not aware of the stimulus at all.
Difference threshold (JND): the smallest change in a stimulus you can detect. Weber's Law says this is a constant ratio, not a constant amount. A 1 kg difference is noticeable when you are holding 10 kg, but not when you are holding 100 kg.
Acknowledges that detection is not just about the strength of the stimulus; it also depends on the person's alertness, expectations, and motivation.
Four possible outcomes when a stimulus may or may not be present:
Hit: stimulus present, person says "yes."
Miss: stimulus present, person says "no."
False alarm: stimulus absent, person says "yes."
Correct rejection: stimulus absent, person says "no."
Real-world example: a radiologist reading a scan. Their threshold for saying "tumour present" depends on both the clarity of the image and the consequences of missing one.
Your receptors reduce their firing rate in response to an unchanging stimulus.
Useful because it frees your brain to focus on new or changing information instead of constant background noise.
Does not apply to pain in the same way (for survival reasons, pain signals tend to persist).
Cornea: the transparent front surface that bends (refracts) incoming light.
Pupil: the opening that controls how much light enters. Dilates in low light, constricts in bright light.
Lens: focuses light onto the retina by changing shape (accommodation).
Retina: the light-sensitive layer at the back of the eye containing the photoreceptors.
Fovea: the central point of the retina where cones are most concentrated. Sharpest vision.
Optic nerve: carries visual information from the retina to the brain.
Blind spot: where the optic nerve exits the eye. No photoreceptors here, so there is a gap in your visual field that your brain fills in.
Rods: about 120 million per eye. Sensitive to low light. No colour. Found mostly in the periphery. Responsible for peripheral and night vision.
Cones: about 6 million per eye. Need more light. Detect colour and fine detail. Concentrated in the fovea.
In dim conditions, you see better out of the corner of your eye (peripheral vision uses rods) than by looking directly at something (the fovea relies on cones, which need more light).
Trichromatic theory explains colour detection at the receptor level. Three types of cones (short, medium, long wavelength) combine their signals to produce the full colour spectrum.
Opponent-process theory explains colour processing further along the visual pathway. Neurons respond in opposing pairs (red-green, blue-yellow, black-white). This is why you see afterimages: fatiguing one side of the pair strengthens the other.
Both theories are correct; they operate at different stages of the visual system.
Binocular cues (need both eyes):
Retinal disparity: the slight difference between the images each eye receives. The greater the disparity, the closer the object.
Convergence: the degree to which your eyes turn inward to focus on a near object.
Monocular cues (work with one eye):
Relative size: smaller objects appear farther away.
Linear perspective: parallel lines appear to converge in the distance.
Overlap (occlusion): an object that blocks another appears closer.
Texture gradient: texture becomes finer and less detailed with distance.
Relative height: objects higher in the visual field tend to be perceived as farther away.
Students often think trichromatic theory and opponent-process theory are competing explanations. They are not. Trichromatic theory explains what happens at the receptor level (in the retina), and opponent-process theory explains what happens at later stages of processing. Both are correct.
Students confuse absolute threshold with difference threshold. Absolute threshold is about detecting a stimulus at all; difference threshold is about detecting a change in a stimulus that is already present.
Students sometimes believe sensory adaptation means you literally cannot sense the stimulus any more. Your receptors have reduced their response, but a change in the stimulus (the fridge suddenly stops humming) will be noticed immediately.
Students mix up rods and cones. A useful mnemonic: cones are for colour and are concentrated in the centre (fovea). Rods are for dim light and are in the retina's periphery.
⚠️ Know the difference between sensation and perception, and be able to give an example of each.
⚠️ Be able to distinguish bottom-up from top-down processing with examples.
⚠️ Understand signal detection theory's four outcomes (hit, miss, false alarm, correct rejection).
⚠️ Know the roles of rods vs. cones and where each is concentrated in the retina.
⚠️ Be able to explain both trichromatic and opponent-process theories and say where each operates.
⚠️ Know several monocular and binocular depth cues and be able to identify them in a scenario.
The conversion of physical energy into neural signals is called __________.
True or False: Top-down processing relies entirely on the raw sensory data.
The photoreceptors responsible for colour vision are called __________.
True or False: Trichromatic theory and opponent-process theory are mutually exclusive.
A depth cue that requires both eyes is called a __________ cue.
Q: You walk into a room with a strong smell of coffee. After 10 minutes, you no longer notice it. What process explains this?
A: Sensory adaptation. Your olfactory receptors reduced their response to the constant, unchanging stimulus.
Q: A security guard is monitoring CCTV footage late at night and reports seeing an intruder, but the footage later shows nobody was there. In signal detection theory terms, what happened?
A: This is a false alarm. The stimulus (an intruder) was absent, but the guard responded "yes." Fatigue, expectation, or a low response criterion could all contribute.
Q: Explain why you see a green afterimage after staring at a red square for 30 seconds.
A: Opponent-process theory. Staring at red fatigues the red side of the red-green opponent pair. When you look away, the green side rebounds, producing a green afterimage.
Q: Why do you see better at night using your peripheral vision rather than looking directly at an object?
A: Rods, which are sensitive to low light, are concentrated in the periphery of the retina. The fovea (centre) contains mostly cones, which need more light. In dim conditions, peripheral vision outperforms direct gaze.
Q: Give one example each of a monocular and a binocular depth cue.
A: Monocular: relative size (a smaller object appears farther away). Binocular: retinal disparity (each eye receives a slightly different image, and the brain uses the difference to judge depth).
Transduction is the sensory-system version of the neuron signalling you learnt in Unit 2: photoreceptors are specialised neurons that convert light into action potentials. This unit also connects to Unit 4 (Consciousness), where altered states of consciousness can distort perception, and drug effects on neurotransmitters can change how sensory information is processed.
Sensation, perception, transduction, sensory adaptation, bottom-up processing, top-down processing, absolute threshold, difference threshold, just noticeable difference, JND, Weber's Law, signal detection theory, hit miss false alarm correct rejection, rods, cones, retina, fovea, cornea, pupil, lens, optic nerve, blind spot, trichromatic theory, Young-Helmholtz, opponent-process theory, afterimage, colour vision, depth cues, binocular, monocular, retinal disparity, convergence, relative size, linear perspective, occlusion, texture gradient, PSYCH 1100, general psychology, Ohio State