Chapter 3 - Perception (Tulgren)

Sensation

The process of detecting physical stimuli (light, sound waves, chemicals, pressure) with sensory receptors and converting them into neural signals; the "outer experience"

Perception

The brain's process of organizing and interpreting sensory signals into meaningful experiences (objects, faces, sounds); the "inner experience"

Sensation vs. perception

Sensation = detecting raw input (photons hit your retina). Perception = interpreting it (recognizing a green light means go). The same sensation can be perceived differently depending on context and experience

Stimulus to perception: 4 steps

1) Stimulus (physical energy) 2) Sensation (receptors detect it) 3) Sensory coding/transduction (converted to neural signals) 4) Perception (brain interprets it)

Sensory coding

Translating a stimulus into neural signals the brain can use; includes both what the stimulus is (qualitative) and how much of it there is (quantitative)

Transduction

Converting physical energy (light, sound waves, pressure, chemicals) into electrical and chemical neural signals

Qualitative information

Tells the brain what the stimulus is (e.g. red vs. green); coded by WHICH neurons fire

Quantitative information

Tells the brain how much or how intense the stimulus is (e.g. dim vs. bright red); coded by HOW OFTEN neurons fire (firing frequency)

Bottom-up processing

Perception that starts with the raw sensory data and builds up to a whole: detect features, combine into forms, recognize the stimulus (data-driven)

Top-down processing

Perception guided by prior knowledge, expectations, and context: you form a hypothesis about the whole and then check features against it (concept-driven)

Contextual effect

Surrounding information and expectations change how we perceive a stimulus (e.g. the same gray block looks different against different backgrounds)

Retinex theory

Color perception is context driven: the brain compares light from a surface to light from surrounding areas, so a color looks the same under different lighting (color constancy)

Heuristics (perception)

Mental rules of thumb or shortcuts the brain uses to quickly organize and interpret input (note: unlike algorithms, they are fast but not guaranteed to be right)

Psychophysics

The study of the relationship between physical properties of stimuli and our psychological experience of them

Absolute threshold

The minimum stimulus intensity needed to detect a stimulus 50% of the time

Absolute threshold: why 50%?

There is no sharp line between detecting and not detecting; detection gradually increases with intensity, so the point where you detect it half the time is defined as the threshold

Difference threshold (just noticeable difference, JND)

The smallest difference between two stimuli that a person can detect 50% of the time

Weber's law

The JND is a constant proportion of the original stimulus, not a fixed amount (adding 1 penny to a bowl of 2 is noticeable, adding 1 to a bowl of 200 isn't)

Neural noise

Spontaneous firing of neurons even with no stimulus; makes it harder to decide whether a weak signal is really there

Signal detection theory

Theory that detection depends on both the sensory signal and the decision-maker's bias (expectations, motivation, costs of being wrong), not just sensitivity

Hit

Signal present and the person says "yes" (correct detection)

Miss

Signal present but the person says "no"

False alarm

Signal absent but the person says "yes"

Correct rejection

Signal absent and the person says "no" (correct)

Liberal responder

Says "yes" easily; gets many hits but also many false alarms (e.g. 89% hits, 41% false alarms)

Conservative responder

Says "yes" only when very sure; fewer false alarms but more misses (e.g. 45% hits, 26% false alarms)

Application: radiologist reading a scan

Signal detection: if the cost of missing a tumor is high, a liberal response criterion reduces misses but raises false alarms (extra biopsies)

Application: airport security screener

Signal detection: a liberal criterion catches more real threats (hits) but flags more harmless bags (false alarms); a conservative one misses more threats

Application: two people with different thresholds

Julie says she sees dim light at lower intensity than Regina; this could be real sensitivity differences or response bias (willingness to say yes)

Application: you think you felt your phone buzz but it didn't

A false alarm; expectation (top-down) and neural noise made you say "yes" when no signal was present

Application: a witness identifies the wrong person under pressure

Bias in judgment: a liberal criterion (eager to help police) raises false alarms in signal detection

Wavelength

Distance between wave peaks (lambda); determines the hue (color) of light; visible range about 400 nm (violet/blue) to 700 nm (red)

Amplitude

Height of a wave; for light it determines brightness (intensity), for sound it determines loudness

Frequency

How many waves occur per second; for sound it determines pitch (high frequency = high pitch)

Electromagnetic spectrum

Range of all energy waves (radio, microwave, infrared, visible light, ultraviolet, X-rays, gamma rays); humans only sense a narrow band (visible light)

Visible light wavelengths

Longer wavelengths look red, shorter look blue/violet

Cornea

Clear outer layer at the front of the eye; bends and focuses incoming light (does most of the focusing)

Iris and pupil

Iris is the colored muscle that adjusts the pupil, the opening that controls how much light enters the eye

Lens

Fine-tunes focus by changing shape to focus light on the retina

Accommodation

The lens changing shape to focus: it fattens (thickens) for near objects and flattens for distant objects

Application: eye strain after hours of reading

Accommodation: the muscles keep the lens fattened for near focus; looking into the distance lets the lens flatten and relax

Retina

Light-sensitive layer at the back of the eye containing photoreceptors (rods and cones), bipolar cells, and ganglion cells

Fovea

Center of the retina packed with cones; responsible for sharp central vision and detail

Parafovea

Region just around the fovea; contains both rods and cones, with detail vision lower than the fovea

Photoreceptors

Light-sensitive cells in the retina (rods and cones) that transduce light into neural signals

Rods

Photoreceptors for dim light and night vision; mostly in the periphery; no color; high sensitivity but low detail

Cones

Photoreceptors for bright light, color, and fine detail; concentrated in the fovea; three types (short, medium, long wavelength)

Application: seeing a faint star better out of the corner of your eye

Rods are in the periphery and are more sensitive in dim light; the fovea only has cones, which need more light

Application: why you can't read text in your peripheral vision

Fovea has dense cones for detail; periphery has mostly rods with low detail

Visual pathway in the retina

Light passes through ganglion and bipolar cells to photoreceptors; photoreceptors signal bipolar cells, which signal ganglion cells, whose axons form the optic nerve

Blind spot

Spot where the optic nerve leaves the eye; there are no photoreceptors there so no light is detected; the brain fills in the gap

Optic nerve

Bundle of ganglion cell axons that carries visual information from the eye to the brain

Optic chiasm

Where the optic nerves partially cross so that information from the left visual field goes to the right hemisphere and vice versa

Lateral geniculate nucleus (LGN)

Part of the thalamus that relays visual information from the optic nerve to the visual cortex

Primary visual cortex

Occipital lobe region where visual information first reaches the cortex; sensation becomes perception from here on

Pathway of vision

Cornea, pupil, lens, retina (rods/cones), bipolar cells, ganglion cells, optic nerve, optic chiasm, LGN of thalamus, primary visual cortex (occipital lobe)

Trichromatic theory (Young-Helmholtz)

Color vision results from three types of cones (short, medium, long wavelength; roughly blue, green, red) whose combined activity gives all colors

Opponent-process theory

Color is coded by opposing pairs of cells (red vs. green, blue vs. yellow, black vs. white); activating one member inhibits the other

Trichromatic vs. opponent-process

Trichromatic explains color detection at the cone level in the retina; opponent-process explains color coding after the cones (ganglion cells and the brain)

Negative afterimage

After staring at a color, you see its opposite (red then green); evidence for opponent-process theory

Application: color blindness

Usually results from missing or abnormal cone types (trichromatic theory); red-green color blindness affects the long- or medium-wavelength cones

Color constancy

Seeing an object as the same color under different lighting; explained by contextual processing (retinex theory)

Gestalt psychology

Perception organizes parts into meaningful wholes; the whole is different from the sum of its parts

Gestalt principles of organization

Rules the brain uses to group stimuli: similarity, closure, proximity, good continuation, figure/ground

Similarity

Similar items (shape, color) are grouped together

Closure

We fill in gaps to see complete shapes

Proximity

Items near each other are grouped together

Good continuation

We see smooth, continuous lines rather than abrupt changes (a line passing behind an object is seen as continuing)

Figure/ground

We separate a scene into the object of focus (figure) and the background (ground); can flip in ambiguous images (like Rubin's vase)

Subjective (illusory) contour

A contour we perceive even though there's no physical edge (like the Kanizsa triangle); result of closure

Application: reading text with missing letters

Closure and top-down processing: the brain fills in what's missing based on expectation

Application: seeing a team in matching jerseys as one group

Gestalt similarity; also relates to social categorization

Application: a clustered set of dots looks like two groups

Gestalt proximity

Application: a vase vs. two faces image

Figure/ground; your brain can only see one interpretation at a time

Application: you can read a blurry sign because you know what shop it is

Top-down processing: prior knowledge fills in missing details

Application: noticing a stranger's face in a crowd by detecting eyes, nose, mouth first

Bottom-up processing: starting with features and assembling a whole

Depth perception

Ability to see the world in three dimensions and judge how far away objects are, using binocular and monocular cues

Binocular cues

Depth cues that need both eyes: convergence and retinal disparity

Convergence

Eye muscles turn the eyes inward to focus on near objects; more strain means closer object

Retinal (binocular) disparity

Each eye gets a slightly different image; the brain uses the difference (greater for close objects) to judge depth

Monocular cues

Depth cues that work with one eye: occlusion, linear perspective, relative size, texture gradient, shading

Occlusion (interposition)

An object that blocks part of another is seen as closer

Linear perspective

Parallel lines appear to converge in the distance

Relative size

Of two similar objects, the one that makes a smaller image on the retina is seen as farther away

Texture gradient

Texture appears finer and denser with distance

Shading

Light and shadow patterns give the perception of three-dimensional shape and depth

Ponzo illusion

Two identical lines look different in length when placed between converging lines; the brain applies linear perspective and sees the "farther" line as larger

Application: watching a 3D movie with glasses

Retinal disparity: each eye receives a slightly different image, creating perceived depth

Application: a person on a distant hill looks tiny but you know they're normal size

Size constancy using depth cues (relative size and context), a top-down effect

Application: car in the distance looks smaller than the one nearby

Relative size depth cue (monocular)

Application: a painter makes a flat canvas look deep

Uses monocular cues such as linear perspective, occlusion, relative size, shading, and texture gradient

Application: closing one eye and still judging distance

Monocular cues, since binocular cues need both eyes

Sound waves: pitch and loudness

Frequency of sound waves is perceived as pitch; amplitude is perceived as loudness

Pitch

How high or low a sound is; determined by frequency (Hz)

Loudness

Perceived volume of a sound; determined by amplitude

Pathway of sound

Sound waves, outer ear/ear canal, eardrum, ossicles (malleus, incus, stapes), oval window, cochlea (basilar membrane, hair cells), auditory nerve, brain (auditory cortex in temporal lobe)

Eardrum (tympanic membrane)

Membrane that vibrates when sound waves hit it

Ossicles

Three tiny bones in the middle ear (malleus/hammer, incus/anvil, stapes/stirrup) that amplify and pass vibrations to the cochlea

Cochlea

Fluid-filled, snail-shaped structure in the inner ear where vibrations are converted to neural signals

Basilar membrane

Membrane inside the cochlea that vibrates in response to sound; different places respond to different frequencies