Sensation & Perception

This is Unit Four of Psychology, Part One: sensation, and vision. The unit begins by separating two words that get used as synonyms and are not. Sensation is detection — physical energy hitting a receptor and being converted into a signal. Perception is what happens to that signal afterwards, when it gets organised into something meaningful. Once the two are separate, a set of questions opens up. How faint can a stimulus be and still register? How much does something have to change before the change is noticed? And is detection even a fixed property of a person, or partly a decision they are making? There is a myth in here as well — about messages presented below the level of conscious detection. The phenomenon is real. The claim built on top of it is not, and the entry is precise about where the one stops and the other starts. Then the eye, in order, from the light arriving to the signal leaving. And color, which needs two theories rather than one — not because they compete, but because they describe different stages of the same process. By the end you will be able to do five things. One: say what separates sensation from perception. Two: say what Weber's law claims and where it stops holding. Three: say what the subliminal myth adds to the real finding. Four: say what rods and cones do, and why there is a blind spot. Five: say why color needs two theories.

Three entries, and the distinction they draw runs through the entire unit. One process detects. A separate process interprets. And between them sits a conversion that turns something physical into something the nervous system can carry.

The first stage.

Psych 0371 Sensation

The detection of physical energy by sensory receptors and its conversion into neural signals. Detection and conversion — the stage ends before any meaning appears.

The second.

Psych 0372 Perception

The organisation and interpretation of sensory signals into a meaningful experience. The meaning is added here, by the perceiver, from signals that did not carry it.

The conversion.

Psych 0374 Transduction

Converting physical energy into the electrochemical signals the nervous system uses. Every sense does this, and the physical energy differs each time while the output format does not.

Two entries naming two directions of processing. One builds a perception up from what arrived. The other reaches down from what you already expect — and almost every real perception is both at once.

Upward.

Psych 0376 Bottom-Up Processing

Building a perception from the incoming sensory data upward, without prior expectation. What is there, assembled into what it is.

Downward.

Psych 0377 Top-Down Processing

Using existing knowledge and expectation to interpret the incoming data. It is why an ambiguous shape resolves one way in one context and another way elsewhere — the data did not change.

Five entries on limits. The smallest thing detectable, and then the smallest change detectable — which turns out not to be a fixed amount at all. It depends on where you started, and that dependence follows a law with a stated range where it works.

The smallest detectable.

Psych 0379 Absolute Threshold

The minimum stimulus intensity a person detects half of the time. Half is in the definition, because detection at low intensities is probabilistic rather than fixed.

The smallest change.

Psych 0381 The Difference Threshold

The smallest change between two stimuli that can be detected — also called the just noticeable difference. A quantity about comparison rather than about presence.

And it moves.

Psych 0382 The Difference Threshold Is Not Fixed

It changes with the starting intensity of the stimulus rather than holding at one fixed amount. The same added weight is noticeable on a light object and not on a heavy one.

The law.

Psych 0384 Weber's Law

The difference threshold is a constant proportion of the original intensity. Not a constant amount — a constant fraction, which is why it scales.

And its range.

Psych 0385 Where Weber's Law Breaks Down

The constant differs by sense, and the law holds poorly at very low and very high intensities. A law with its own domain stated alongside it, which is unusual and useful.

Two entries, and the pair is a good example of how a real finding turns into a false claim. Stimuli below the detection threshold exist and can be presented. What they cannot do is the entire content of the myth.

The real phenomenon.

Psych 0387 Subliminal Stimuli

A stimulus presented below the absolute threshold, so it is not consciously detected. Real, presentable, and measurable.

And the myth.

Psych 0388 The Subliminal Persuasion Myth

Subliminal messages cannot compel purchases, beliefs, or actions against a person's intentions. The finding is that something undetected can be presented; the myth is that it can override a decision.

Two entries about the person doing the detecting. Whether a faint signal gets reported depends on the observer's threshold for saying yes — which is a decision, and it can move without their sensitivity changing at all.

Detection as a decision.

Psych 0390 Signal Detection Theory

Detecting a faint stimulus depends on the observer's decision criterion as well as on their sensitivity. Two people with identical eyes can report differently, because they are answering a different question about when to say yes.

The unchanging signal.

Psych 0392 Sensory Adaptation

Receptors responding less to an unchanging stimulus over continued exposure. The system is built to report change, so a constant signal fades out of notice.

Two entries on the physics before the biology. Two measurable properties of a wave — and the entry is careful to say they are physical rather than perceptual, because the words we use for the experience are different words.

Two properties.

Psych 0394 Amplitude and Wavelength

Amplitude is the height of a wave and wavelength the distance between peaks — both physical measurements, not descriptions of experience.

What we can use.

Psych 0396 The Visible Spectrum

The narrow band of electromagnetic wavelengths a human eye can transduce. Narrow against the whole spectrum, and it is the entire visual world.

Seven entries following the structure in the order light passes through it. Two receptor types with different jobs, one region where acuity is highest, and one region with no receptors at all — which is a design consequence rather than a defect.

Focusing.

Psych 0398 The Cornea and Lens

The cornea and lens focus incoming light onto the retina, with the lens changing shape to focus at different distances. One fixed element and one adjustable one.

The surface.

Psych 0400 The Retina

The light-sensitive layer at the back of the eye, where light becomes a neural signal. Everything before this is optics; everything after is biology.

The first receptor.

Psych 0402 Rods

Responding to low light levels and encoding no color. Which is why a dim room has shapes and not colors — the receptors reporting are the ones without color.

The second.

Psych 0403 Cones

Responding in brighter light and encoding both color and fine detail. Two capabilities in one receptor type, and both need the light.

The sharp region.

Psych 0405 The Fovea

The small central region packed with cones, where acuity is highest. Sharp vision covers a small area, and eye movement is what conceals that.

The gap.

Psych 0407 The Blind Spot

The region where the optic nerve exits and no receptors are present. There is a hole in every visual field, and perception fills it without reporting that it did.

The route out.

Psych 0409 The Optic Nerve and Optic Chiasm

The optic nerve carries signals to the optic chiasm, where fibres from the inner halves cross to the opposite hemisphere. Each hemisphere ends up with half of each eye's view.

One procedure, and it assembles the previous topic into one route. Cornea, lens, retina, optic nerve, thalamus, occipital cortex — with every conversion named as the signal changes form.

Watch light become vision.

Psych 0411 Tracing Light to the Visual Cortex

Following light from cornea to lens to retina to optic nerve to thalamus to occipital cortex, naming each conversion. Unit Two's relay and Unit Two's lobe, both appearing in their place.

Three entries, and the first two look like rival theories and are not. They describe different stages of the same process — one at the receptors and one further along — which is why both survived.

The first.

Psych 0413 Trichromatic Theory

Color vision arising from three cone types tuned to different wavelength ranges. A claim about the receptors, at the point of transduction.

The second.

Psych 0414 Opponent-Process Theory

Color encoded in opposing pairs — red against green, blue against yellow, black against white. A claim about the coding after the receptors, which is why it does not contradict the first.

And the condition.

Psych 0416 Color Blindness

Reduced or absent discrimination between certain colors, usually from a missing or altered cone type. Which colors are affected follows from which type is involved.

Two entries on how a flat image becomes a scene with distance in it. One family of cues needs both eyes and works from the geometry between them. The other family works with one eye, which is why a photograph still has depth.

With two eyes.

Psych 0418 Binocular Depth Cues

Cues requiring both eyes, arising from the slightly different image each one receives. The difference between the two views is the information.

With one.

Psych 0419 Monocular Depth Cues

Cues requiring only one eye — relative size, interposition, linear perspective, texture gradient. Available in any flat image, which is why painting and photography work at all.

Five things you can now do, and Part Two takes up hearing and perception. You can separate sensation from perception — detection and conversion on one side, organisation and interpretation on the other — with transduction as the step between. You can say what Weber's law claims, that the difference threshold is a constant proportion rather than a constant amount, and that it holds poorly at very low and very high intensities. You can say what the subliminal myth adds: the real finding is that undetected stimuli can be presented, and the myth is that they can override a decision. You can say what rods and cones each do, and why a blind spot exists — the optic nerve exits through the retina and no receptors sit there. And you can say why color needs two theories: one describes the receptors, the other describes the coding after them.