Sensation & Perception

This is Unit Four of Psychology, Part Two: hearing, the body, and perception. Part One ended with vision. This part starts with hearing, which follows the same shape — a physical property, a route through anatomy, a receptor that transduces — and then covers the senses that get less attention and are not less important. One of those is the sense of where your own body is. You are using it constantly and it has no obvious organ, which is why it goes unnoticed until something disrupts it. Then pain, and the unit is careful here in a way worth marking. There is a signal that detects tissue damage, and there is the experience of pain — and those are two different things, with the second constructed by the brain from the first plus its context. And the last third is perception proper. Five principles that group and separate what you are looking at — running before recognition, before naming, before anything you would call thinking. By the end you will be able to do five things. One: trace a sound from the outer ear to the cortex. Two: say what separates the two kinds of hearing loss. Three: say what separates nociception from pain. Four: name the five grouping principles. Five: say which principle assigns an element to its group.

Four entries taking a pressure wave into the nervous system. Two physical properties first, then three stages of anatomy — and the middle stage exists because the transition from air to fluid loses energy that has to be made up.

Two properties.

Psych 0421 Frequency and Amplitude in Sound

Frequency is cycles per second and determines perceived pitch; amplitude determines perceived loudness. Two physical quantities, two experiences, mapped one to one.

The first two stages.

Psych 0423 The Outer and Middle Ear

The outer ear collects sound waves, and the middle ear's three bones amplify and transfer them inward. Amplification is the middle ear's whole job.

The inner structure.

Psych 0425 The Cochlea and Basilar Membrane

A fluid-filled structure whose basilar membrane vibrates in response to sound. The vibration is what the receptors read.

The receptors.

Psych 0427 Hair Cells

Cells on the basilar membrane transducing its vibration into neural signals carried by the auditory nerve. This is the transduction step, in the same position it occupied in vision.

One procedure, matching the visual one from Part One. A pressure wave followed from the outside of the head to the temporal lobe, with each handoff named — and the same relay appears in the middle that appeared in vision.

Watch a sound become hearing.

Psych 0429 Tracing Sound to the Auditory Cortex

Following a pressure wave from outer ear to eardrum to the middle ear bones to the cochlea to the hair cells to the thalamus to the temporal cortex. Seven handoffs, none of them skipped.

Five entries. Two accounts of how pitch gets coded — which, as with color, describe different parts of the range rather than competing. Then two kinds of hearing loss, distinguished by where the damage is, which is what determines what can be done about it.

The first account.

Psych 0431 Place Theory

Pitch coded by which location on the basilar membrane vibrates most. A position on a structure, read as a frequency.

The second.

Psych 0432 Temporal Theory

Pitch coded by the rate at which auditory neurons fire in step with the sound wave. Unit Two's rate coding, doing a specific job here.

The first loss.

Psych 0434 Conductive Hearing Loss

Damage to the outer or middle ear preventing sound from reaching the cochlea. The receptors are intact and nothing is arriving at them.

The second.

Psych 0435 Sensorineural Hearing Loss

Damage to the hair cells or the auditory nerve. Sound arrives and there is nothing working to transduce or carry it — which is a different problem entirely from the previous entry.

Finding the source.

Psych 0437 Sound Localization

Sound is localised by comparing the timing and intensity of its arrival at the two ears. The same logic as binocular depth — two receivers, and the difference between them is the information.

Two entries on the senses that read chemicals rather than energy. And one of them takes a route into the brain that no other sense takes — it reaches the cortex without passing through the relay everything else goes through.

Dissolved chemicals.

Psych 0439 Taste

Transduction of dissolved chemicals by receptors detecting sweet, sour, salty, bitter and umami. Five categories, and everything else people call taste is largely smell.

Airborne chemicals.

Psych 0441 Smell

Transduction of airborne chemicals, with signals reaching the cortex without passing through the thalamus. The exception Unit Two named when it introduced the relay, met from the other side.

Two entries on senses that report on you rather than on the world. There is no organ you would point to for either, and both are running continuously — which is why they are invisible until something interferes with them.

Balance.

Psych 0443 The Vestibular Sense

Detecting head position and movement using fluid-filled canals in the inner ear. Housed beside hearing, and doing something unrelated to it.

The body's own report.

Psych 0445 Proprioception and Kinesthesia

Proprioception is the sense of body position and kinesthesia the sense of body movement, both from receptors in muscles and joints. Two words, because static position and motion are separate reports.

Three entries, and the first draws a distinction that changes how the whole topic reads. The detection of tissue damage and the experience of pain are two different things — and once they are separated, the two entries after it make sense.

Two things.

Psych 0447 Pain and Nociception

Nociception is the detection of tissue-damaging stimuli; pain is the experience the brain constructs from that signal and its context. Separating them explains why identical damage produces different pain.

The gate.

Psych 0448 Gate Control Theory

A spinal mechanism that can amplify or block pain signals before they reach the brain. The signal is not a simple pipe from injury to awareness.

And who controls it.

Psych 0449 The Gate Is Controlled From Above

The gate is influenced by signals descending from the brain as well as by input arriving from the body. Which is the mechanism behind attention and expectation changing how much something hurts.

Five entries, and all of them happen before you know what you are looking at. Perception separates an object from its background and then groups the pieces — and none of this is a decision anybody makes.

The separation.

Psych 0451 Figure-Ground

Perceptual separation of an object from the background it sits against. The first thing perception does, and everything else depends on it having happened.

The first grouping.

Psych 0453 Proximity

Elements near one another perceived as belonging to one group. Distance alone, with no other property involved.

The second.

Psych 0455 Similarity

Elements resembling one another perceived as belonging to one group. Resemblance alone, and it can override distance.

The third.

Psych 0457 Continuity

Elements arranged on a line or curve perceived as continuing along it. The path is inferred and then it is seen.

The fourth.

Psych 0459 Closure

Incomplete figures perceived as whole, with gaps filled in. The filling happens without any report that something was missing.

Two entries on what perception adds. The image on the retina changes constantly as you move, and the object does not appear to change — and separately, what you expect to see shapes what you actually perceive.

Stability.

Psych 0461 Perceptual Constancy

Perceiving an object's size, shape and brightness as stable despite the sensory input changing. The stability is constructed — the input genuinely does change.

Readiness.

Psych 0463 Perceptual Set

A readiness to perceive stimuli in a particular way, produced by expectation, context or motivation. Part One's top-down processing, named as a standing state rather than an event.

Four entries, and the first is a method with a test built in. Name the principle assigning an element to its group — and then say what change would break that assignment. If nothing would break it, the principle was not the one doing the work.

The method.

Psych 0465 Identifying Gestalt Principles in an Image

Naming which principle assigns an element to its group, and what change would break that assignment. The second half is the check on the first.

The first image.

Psych 0466 Grouping in a Logo

In a logo, similarity and proximity are the principles most often used to make separate marks read as one object. Deliberate design, using the same mechanisms.

The second.

Psych 0468 Grouping in a Crowd Photograph

In a crowd photograph, figure-ground and proximity determine which people read as a group and which as background. The photographer did not decide this; the viewer's perception did.

The third.

Psych 0470 Grouping in a Page of Text

On a page of text, proximity determines paragraph boundaries and continuity determines that a line reads as one line. Typography is an applied use of this topic, whether or not anyone says so.

One capstone, and it takes a question with an obvious short answer and refuses the short answer. Why do two eyes beat one? Not only for the binocular geometry — the full account has receptor layout in it, monocular cues, and top-down interpretation.

The capstone. One scene, fully explained.

Psych 0472 Why Two Eyes Beat One

Combining receptor layout, binocular geometry, monocular cues and top-down interpretation into one account of a single scene. Every layer of both parts of this unit, on one image.

Five things you can now do, and Unit Four is complete. You can trace a sound from the outer ear through the middle ear's bones, into the cochlea, to the hair cells that transduce it, and on to the temporal cortex. You can say what separates the two kinds of hearing loss: one blocks sound before the cochlea with the receptors intact, the other damages the hair cells or the nerve itself. You can separate nociception from pain — detection of damage against an experience the brain constructs — and say that the spinal gate is influenced from the brain as well as from the body. You can name the five grouping principles: figure-ground, proximity, similarity, continuity, and closure. And you can say which principle assigns an element to its group, with the test being what change would break the assignment.