Psychology Sensation and Perception: Chemical Senses, Psychophysics, Touch, and Vision

Chemical senses: similarities and differences

Both detect chemicals with receptors and combine into flavor. Smell detects airborne molecules (olfactory epithelium, hundreds of receptor types); taste detects dissolved molecules (taste buds, five basic tastes)

Orthonasal vs. retronasal olfaction

Orthonasal: odors sniffed in through the nose. Retronasal: odors reach the nose from the mouth during eating; key to flavor

Evolutionary functions of smell

Finding food and judging spoilage, detecting danger, identifying mates and kin, social communication (pheromones), and navigation

Olfactory receptors and binding

G-protein-coupled receptors on ORN cilia (about 350 to 400 types in humans, one type per neuron); binding depends on odorant shape and size; one odorant activates many receptors and one receptor responds to many odorants (combinatorial coding)

Olfactory receptor neurons (ORNs)

Receptor cells in the olfactory epithelium at the top of the nasal cavity; regenerate about every 30 to 60 days

Olfactory map

ORNs with the same receptor type converge on the same glomeruli in the olfactory bulb; each odor produces a distinct pattern of glomeruli activity

Olfactory pathway

ORNs, olfactory bulb (glomeruli, mitral cells), piriform cortex, amygdala, entorhinal cortex, then orbitofrontal cortex; reaches cortex without going through the thalamus first

Roles of amygdala, entorhinal cortex, and OFC in smell

Amygdala and entorhinal cortex link odors to emotion and memory; OFC supports conscious odor perception and combines smell with taste

Odor detection and discrimination

Detection thresholds vary by odorant and person; humans can discriminate a very large number of odors, but naming or identifying them is poor without cues or context

Five basic tastes and functions

Sweet (energy), salty (sodium), sour (acids, possible spoilage), bitter (toxins), umami (amino acids, protein)

Taste buds

Clusters of 50 to 100 receptor cells with a taste pore, found in papillae (fungiform, foliate, circumvallate; filiform have none); replaced about every 1 to 2 weeks

Taste pathway

Cranial nerves VII, IX, X, then nucleus of the solitary tract, thalamus, insula and frontal operculum, then orbitofrontal cortex

Tongue map myth

False; all five tastes can be detected wherever taste buds are found

Nontaster, taster, supertaster

Based on PROP bitterness and number of fungiform papillae. Nontaster (about 25%): weak or no bitterness. Taster (about 50%): moderate. Supertaster (about 25%): most taste buds, extremely bitter, finds bitter and spicy foods intense

Flavor as multisensory

Combines taste, retronasal smell, touch and texture, temperature, trigeminal sensations, vision, and sound (e.g., pinching the nose reduces flavor, food color changes flavor judgments, crunch sounds change perceived freshness)

Trigeminal sensations

Chemical irritation and touch sensations in the mouth and nose (e.g., spiciness from capsaicin, coolness from menthol)

Psychophysics and psychological scale

Psychophysics studies the relationship between physical stimuli and perception. A psychological scale relates stimulus intensity to perceived magnitude

Magnitude estimation

Observers assign numbers to stimuli to match perceived magnitude; used to build psychological scales

Stevens' power law

P = K x S^n. Exponent n < 1: compression (brightness); n > 1: expansion (electric shock); n about 1: linear (line length)

Fechner's law

Perceived magnitude is proportional to the log of stimulus intensity

Method of limits (procedure)

Experimenter presents ascending or descending series in steps; observer says yes or no; threshold is the average crossover point

Method of constant stimuli (procedure)

Fixed set of intensities presented many times in random order; threshold is the intensity detected 50% of the time

Method of adjustment (procedure)

Observer adjusts the intensity until the stimulus is just barely detectable; threshold is the average setting

Method of limits: pros and cons

Pros: quick and simple. Cons: errors of habituation and anticipation; less precise

Method of constant stimuli: pros and cons

Pros: most accurate, random order avoids anticipation, gives psychometric function. Cons: time consuming, many trials, fatigue

Method of adjustment: pros and cons

Pros: fast, easy, observer is active. Cons: less precise and controlled, biased by observer's control and starting point

Psychometric function

S-shaped graph of stimulus intensity vs. proportion detected; threshold is the intensity at 50% detection

Absolute vs. difference threshold

Absolute threshold: minimum intensity detected 50% of the time. Difference threshold (JND): smallest detectable difference between two stimuli

Weber's law

The JND is a constant fraction of the standard stimulus: DI/I = K (Weber fraction)

Signal detection theory outcomes

Hit (yes, signal present), miss (no, signal present), false alarm (yes, signal absent), correct rejection (no, signal absent)

Sensitivity (d prime) in SDT

How well the observer separates signal from noise; not affected by criterion; increases with stronger signals and better observers

Criterion in SDT

Internal cutoff for saying yes; liberal criterion gives more hits and more false alarms, conservative gives fewer of both; affected by expectations and payoffs, not sensitivity

ROC curve

Plots hit rate vs. false alarm rate across criteria; a curve farther from the diagonal means higher sensitivity

Merkel receptor and Ruffini cylinder

Both slowly adapting (SA). Merkel: small receptive field, shallow, fine detail and texture. Ruffini: large receptive field, deep, skin stretch and finger position

Meissner corpuscle and Pacinian corpuscle

Both rapidly adapting (RA). Meissner: small receptive field, shallow, light touch and grip control. Pacinian: large receptive field, deep, high frequency vibration and fine texture

Tactile acuity

Ability to detect fine spatial detail on the skin; highest on fingertips and lips (small receptive fields, many receptors)

Proprioception

Sense of body and limb position and movement. Receptors: muscle spindles, Golgi tendon organs, joint receptors. Pathway: dorsal column medial lemniscal

Nociception

Sense of pain. Receptors: nociceptors (free nerve endings). A-delta fibers carry fast sharp pain; C fibers carry slow dull pain. Pathway: spinothalamic. Function: warns of tissue damage

Thermoception

Sense of temperature. Receptors: warm and cold thermoreceptors (free nerve endings, TRP channels). Pathway: spinothalamic. Function: detects temperature changes and helps regulate body temperature

Dorsal column medial lemniscal vs. spinothalamic pathway

Dorsal column: fast, touch and proprioception, crosses in the medulla. Spinothalamic: pain and temperature, crosses in the spinal cord. Both go through the thalamus to the cortex

Primary somatosensory cortex (S1)

Parietal lobe, postcentral gyrus (behind the central sulcus); receives input from the thalamus; contralateral body representation

S1 features

Somatotopic map (homunculus); cortical magnification for sensitive areas (hands, lips); plastic (maps change with experience or injury)

Light

Visible part of the electromagnetic spectrum (about 400 to 700 nm); wave and particle (photon); wavelength determines hue

Cornea

Transparent front of the eye; most of the focusing power (about 80%); fixed focus

Lens

Behind the pupil; adjustable focusing power (about 20%); changes shape for accommodation

Retina

Back of the eye; contains photoreceptors; image is focused on it upside down

Accommodation

Lens changes shape to focus: for near objects ciliary muscles contract and lens thickens; for far objects they relax and lens flattens

Myopia (nearsightedness)

Far objects blurry; eyeball too long or refractive power too strong; focus falls in front of retina; corrected with concave (diverging) lenses

Hyperopia (farsightedness)

Near objects blurry; eyeball too short or refractive power too weak; focus falls behind retina; corrected with convex (converging) lenses

Presbyopia

Age-related loss of lens flexibility; near point moves farther away; trouble reading up close; reading glasses or bifocals

Astigmatism

Uneven curvature of cornea or lens; blurry or distorted vision at all distances

Cataract

Clouding of the lens; hazy vision, glare, faded colors; treated with lens replacement surgery

Macular degeneration

Damage to the macula and fovea; loss of central vision, peripheral vision preserved; trouble reading and recognizing faces

Retinitis pigmentosa

Photoreceptor degeneration starting with rods; night blindness and loss of peripheral vision (tunnel vision)

Rods

About 120 million; peripheral retina, none in fovea; very sensitive; dim light; black and white; low acuity; high convergence; one pigment

Cones

About 6 million; concentrated in fovea; bright light; color vision (three types: S, M, L); high acuity; low convergence

Rods vs. cones: similarities

Both are photoreceptors with outer segments containing visual pigment, transduce light into electrical signals, and synapse onto bipolar cells

Fovea and blind spot

Fovea: center of retina with only cones and highest acuity. Blind spot: where the optic nerve exits, no photoreceptors

Dark adaptation

Sensitivity increases in the dark; cones adapt first, then rods take over at the rod-cone break

Visual pathway

Photoreceptors, bipolar cells, ganglion cells, optic nerve, optic chiasm (nasal fibers cross), LGN of thalamus, primary visual cortex (V1, occipital lobe)

Superior colliculus

Midbrain structure that receives retinal input and controls eye movements and orienting

Dual pathways of vision

Ventral (what) pathway to the temporal lobe for object identification; dorsal (where or how) pathway to the parietal lobe for location and action

Ungerleider and Mishkin experiment

Monkeys with temporal lesions failed object discrimination (what); monkeys with parietal lesions failed landmark discrimination (where)

Milner and Goodale experiment (D.F.)

D.F. (ventral damage) could not report slot orientation but could post a card into it, showing separate perception (what) and action (how) pathways

Receptive field

Area of the retina or visual field where stimulation changes a neuron's firing

Ganglion cell receptive fields

Circular center-surround; on-center and off-center types; center and surround are antagonistic, so cells prefer small spots or edges over diffuse light

Simple cells

Elongated receptive field with separate excitatory and inhibitory areas; respond best to a bar or edge of a specific orientation at a specific location

Complex cells

Respond best to a bar of a specific orientation anywhere in the receptive field; many prefer a specific direction of movement

Hubel and Wiesel

Discovered orientation-selective simple and complex cells in cat visual cortex

Magnocellular pathway

Large cells; fast, transient; large receptive fields; motion, location, low contrast; not color sensitive; low detail; dorsal stream

Parvocellular pathway

Small cells; slower, sustained; small receptive fields; color, fine detail, form, texture; high detail; ventral stream

Pictorial depth cues

Monocular cues in a flat image: occlusion, relative height, relative size, familiar size, linear perspective, texture gradient, atmospheric perspective, shading and shadows

Occlusion

Closer objects block the view of farther objects

Linear perspective

Parallel lines appear to converge toward a vanishing point in the distance

Atmospheric perspective

Distant objects look hazier and bluer because of particles in the air

Reading graphs: axes and type

Independent variable on x-axis, dependent variable on y-axis; bar graphs compare categories, line graphs show trends; check labels, units, and whether the axis is truncated

Reading graphs: error bars

Show variability or uncertainty (SEM, SD, or confidence interval); non-overlapping error bars often suggest a reliable difference

Reading graphs: interactions

Non-parallel lines in a line graph suggest an interaction (the effect of one variable depends on another); a main effect is a difference averaged across the other variable

Reading graphs: psychophysics and vision curves

Psychometric function: threshold at 50% detection. ROC curve: farther from diagonal means higher sensitivity. Dark adaptation curve: rod-cone break is the second drop in threshold