Chapter 4: Sensation and Perception

Sensation and Perception: The Distinction

• Sensation: stimulation of sense organs: for example absorbing energy from light by the eyes.

• Perception: selection, organization, and interpretation of sensory input

• Psychophysics = the study of how physical stimuli are translated into psychological experience

Figure 4.1 The distinction between sensation and perception

Psychophysics: Basic Concepts

• Sensation begins with a detectable stimulus• Fechner: the concept of the threshold, what

is the weakest detectable stimulus– Absolute threshold: absolute threshold is

the minimal amount of stimulation that an organism can detect stimulus 50% of the time

– Just noticeable difference (JND): smallest difference detectable• Weber’s law: size of JND proportional

to size of initial stimulus

Figure 4.2 The absolute threshold

Psychophysics: Concepts and Issues

• Signal-Detection Theory: Signal detection theory holds that the detection of sensory information is influenced by two things…1) noise in the system (irrelevant stimuli in the environment that elicit neural activity), and 2) decision making processes

• Subliminal Perception: Many researchers, using very different methods, have demonstrated that perception can occur without awareness

• Sensory Adaptation: Decline in sensitivity

Figure 4.3 Signal-detection theory

Vision: The Stimulus

• Light = electromagnetic radiation, that travels as a wave…the wave travels quickly…the speed of light.– Amplitude: perception of brightness– Wavelength: perception of color– Purity: mix of wavelengths

• perception of saturation, or richness of colors.

Figure 4.5 Light, the physical stimulus for vision

The Eye:Converting Light into Neural Impulses

• The eye:The eye has two main purposes, providing a “house” for the neural tissue that receives light, the retina, and channeling light toward the retina.

• Components: – Cornea: where light enters the eye – Lens: focuses the light rays on the retina– Iris: colored ring of muscle, constricts or

dilates via amount of light– Pupil: regulates amount of light

Figure 4.7 The human eye

The Retina: An Extension of the CNS

• Retina: piece of neural tissue that lines the back of the eye, absorbs light, processes images

• Optic disk: Axons from the retina to the brain converge at the optic disk, a hole in the retina where the optic nerve leaves the eye optic nerve connection/blind spot

• Receptor cells:– Rods: black and white/low light vision– Cones: color and daylight vision

• Adaptation: becoming more or less sensitive to light as needed

The Retina: An Extension of the CNS

• Information processing:– Receptive fields: the collection of rod and

cone receptors that funnel signals to a particular visual cell in the retina

– Lateral antagonism: or lateral inhibition, occurs when neural activity in a cell opposes activity in surrounding cells.

Figure 4.8 Nearsightedness and farsightedness

Figure 4.9 The retina

Figure 4.10 The process of dark adaptation

The Retina and the Brain:Visual Information Processing

• Optic Chiasm: the point where the axons cross to different hemispheres

• Light rods and cones neural signals bipolar cells ganglion cells optic nerve optic chiasm opposite half brain

• Main pathway: lateral geniculate nucleus (thalamus) primary visual cortex (occipital lobe) (90% of axons go to LGN)– magnocellular: where (ex brightness)– parvocellular: what (ex color)

The Retina and the Brain:Visual Information Processing

• Second pathway: superior colliculus thalamus primary visual cortex– Helps with the perception of motion and

coordination of visual input to other senses

• These 2 channels use parallel processing– They both extract different info from the

same stimulus

Figure 4.13 Visual pathways through the brain

Figure 4.15 The what and where pathways from the primary visual cortex

Hubel and Wiesel:Feature Detectors and the Nobel Prize

• Early 1960’s: Hubel and Wiesel– Microelectrode recording of axons in

primary visual cortex of animals– Discovered feature detectors: neurons that

respond selectively to lines, edges, etc.– Groundbreaking research: Nobel Prize in

1981• Later research: cells specific to faces in the

temporal lobes of monkeys and humans

Basics of Color Vision

• Wavelength determines color – Longer = red / shorter = violet

• Amplitude determines brightness• Purity determines saturation

Figure 4.16 The color solid

Figure 4.17 Additive versus subtractive color mixing

Theories of Color Vision

• Trichromatic theory - Young and Helmholtz (mid 1800’s)– This theory holds that the human eye has

three types of receptors with differing sensitivities to different light wavelengths…one for red, one for green, and one for blue

• Opponent Process theory – Hering (1878)– 3 pairs of antagonistic colors– red/green, blue/yellow, black/white

• Current perspective: both theories necessary

Figure 4.18 The color circle and complementary colors

Perceiving Forms, Patterns, and Objects

• Reversible figures: drawing that is compatible with two interpretations that can shift back and forth

• Perceptual sets: a readiness to perceive a stimulus in a particular way

• Inattentional blindness: failure to see fully visible objects or events in a visual display

• Feature detection theory - people detect specific elements in stimuli and build them up into recognizable forms, bottom-up processing

Perceiving Forms, Patterns, and Objects

• Form perception - involves top-down processing…clearly emphasized by the Gestalt psychologists, who demonstrated that the whole is more than the sum of its parts

• Subjective contours: phenomenon whereby contours are perceived where none actually exist

• Gestalt psychologists: the whole is more than the sum of its parts– Reversible figures and perceptual sets

demonstrate that the same visual stimulus can result in very different perceptions

Figure 4.25 The principle of figure and ground

Figure 4.28 A famous reversible figure

Figure 4.22 Feature analysis in form perception

Figure 4.23 Bottom-up versus top-down processing

Figure 4.24 Subjective contours

Principles of Perception

• Gestalt principles of form perception:– figure-ground, proximity, similarity,

continuity, closure, and simplicity– The Gestalt emphasis is still felt in the stud

y of perception, as they had useful insights that have stood the test of time, raised important issues

Principles of Perception

• Recent research:– Distal (stimuli outside the body) vs.

proximal (stimulus energies impinging on sensory receptors) stimuli

– Perceptual hypotheses• Context, your brain makes a decision as

to what an object is• Objects that are thought to be “out of

place” are more easily remembered

Principles of Perception

• Phi Phenomenon– The illusion of movement presented by

visual stimuli in rapid succession• Figure and ground:

– Figure is the object being looked at– Ground is the background on which it

stands• Proximity: we group things that are close

together

Principles of Perception

• Closure: your brain fills in the gaps to complete objects forms

• Similarity: we group objects of similar size or shape

• Simplicity: we group elements together to form a simple figure

• Continuity: we follow objects in the direction we are led

Figure 4.26 Gestalt principles of perceptual organization

Figure 4.27 Distal and proximal stimuli

Figure 4.29 The Necker cube

Figure 4.30 Context effects

Depth and Distance Perception

• Depth perception involves interpretation of visual cues that indicate how near or far away something is

• Binocular cues – clues from both eyes together – retinal disparity: objects within 25 feet

project images to slightly different locations on the left and right retinas

– Convergence: feeling the eyes converge toward each other as they focus on a target.

Depth and Distance Perception

• Monocular cues – clues from a single eye – motion parallax : having images of

objects at different distances moving across the retina at different rates

– Accommodation: change in the shape of the lens as the eye focuses

– pictorial depth cues: cues about distance that can be given in a flat picture

Depth and Distance Perception

• Monocular cues (cont.)– pictorial depth cues: cues about distance that

can be given in a flat picture• Linear perspective: lines converge in the

distance• Texture gradients: small details can not be

seen from far away• Interposition: objects that come b/w you and

another are closer• Relative size: closer objects are bigger• Height in plane: distant objects are higher in the

picture• Light and shadow: gives direction and distance

Stability in the Perceptual World:Perceptual Constancies

• Perceptual constancies – stable perceptions amid changing stimuli – when a person walks toward you, they get

larger perceptually…do you think they are growing? No

– Size– Shape– Brightness– Hue– Location in space

Optical Illusions:The Power of Misleading Cues

• Optical Illusions - discrepancy between visual appearance and physical reality

• Famous optical illusions: Muller-Lyer Illusion, Ponzo Illusion, Poggendorf Illusion, Upside-Down T Illusion, Zollner Illusion, the Ames Room, and Impossible Figures

• Cultural differences: Perceptual hypotheses at work, Muller-Lyer and Poggendorf demonstrate the importance of perceptual hypotheses.

Figure 4.41 The Ames room

Windows Mac OS X

The Ames Room

Figure 4.37 The Muller-Lyer illusion

Figure 4.38 Explaining the Muller-Lyer Illusion

Figure 4.39 Four geometric illusions

Figure 4.42 Three classic impossible figures

Hearing: The Auditory System

• Stimulus = sound waves (vibrations of molecules traveling in air)– Amplitude (loudness)– Wavelength (pitch)– Purity (timbre)

• Wavelength described in terms of frequency: measured in cycles per second (Hz)

– Frequency increase = pitch increase • Amplitude is a description of sound pressure and is

measured in decibels (db). Perceived loudness is higher with increasing decibel level.

Figure 4.44 Sound, the physical stimulus for hearing

The Ear: Three Divisions

• External ear (pinna): collects sound• Middle ear: the ossicles (hammer, anvil,

stirrup)• Inner ear: the cochlea

– a fluid-filled, coiled tunnel – contains the hair cells, the auditory

receptors– lined up on the basilar membrane

Figure 4.46 The human ear

Figure 4.47 The basilar membrane

The Auditory Pathway

• Sound waves vibrate bones of the middle ear • Stirrup hits against the oval window of

cochlea• Sets the fluid inside in motion• Hair cells are stimulated with the movement

of the basilar membrane• Physical stimulation converted into neural

impulses • Sent through the thalamus to the auditory

cortex (temporal lobes)

Theories of Hearing: Place or Frequency?

• Hermann von Helmholtz (1863) – Place theory: proposed that perception of

pitch corresponds to the vibration of different portions, or places, along the basilar membrane

• Other researchers (Rutherford, 1886)– Frequency theory: perception of pitch

corresponds to the rate, or frequency, at which the entire basilar membrane vibrates, causing the auditory nerve to fire at different rates for different frequencies

Theories of Hearing: Place or Frequency?

• Like with research in theories of color vision, researchers argued about these two competing theories for almost a century. It turns out that both are valid - in part

• Georg von Bekesy (1947) – Traveling wave theory: the whole basilar

membrane does move, but the waves peak at particular places, depending on frequency

Auditory Localization:Where Did that Sound Come From?

• Two cues critical:– Intensity (loudness) – Timing of sounds arriving at each ear

• Head as “shadow” or partial sound barrier

• Timing differences as small as 1/100,000 of a second

Figure 4.48 Cues in auditory localization

The Chemical Senses: Taste

• Taste (gustation)• Physical stimulus: soluble chemical

substances – Receptor cells found in taste buds

• Pathway: taste buds -> neural impulse -> thalamus -> cortex – Four primary tastes: sweet, sour, bitter,

and salty– Taste: learned and social processes

The Chemical Senses: Taste

• The ability to taste may depend on the density of taste buds (genetics)– 25% Non Tasters– 25% Supertasters (much more sensitive to

certain tastes: sweet and bitter)– 50% medium tasters

Figure 4.49 The tongue and taste

The Chemical Senses: Smell

• Smell (Olfaction)• Physical stimuli: substances carried in the air

– dissolved in fluid, the mucus in the nose– Olfactory receptors = olfactory cilia

• Pathway: Olfactory cilia -> neural impulse -> olfactory nerve -> olfactory bulb (brain)

The Chemical Senses: Smell

• The olfactory receptors synapse directly with cells in the olfactory bulb at the base of the brain. Olfaction is the only sense, therefore, that is not routed through the thalamus

• Odors are not easily classified, and primary odors have not really been delineated.

• Humans can distinguish among about 10,000 odors, but for some reason have a hard time attaching names to odors quite frequently

Figure 4.51 The olfactory system

Skin Senses: Touch

• Physical stimuli = mechanical, thermal, and chemical energy impinging on the skin.

• Pathway: Sensory receptors -> the spinal column -> brainstem -> cross to opposite side of brain -> thalamus -> somatosensory (parietal lobe)

• Temperature: free nerve endings in the skin; specific for cold and warmth

Skin Senses: Touch

• Pain receptors: also free nerve endings – Two pain pathways:

• the fast pathway that registers localized pain and relays it to the brain in a fraction of a second

• the slow pathway that lags a second or two behind and carries less localized, longer-lasting aching or burning pain.

Skin Senses: Touch

• Pain– Can be influenced by expectations and

belief, mood, personality, and other factors– You can mentally block pain – Gate-control theory (Mezlack and Wall

1965): incoming pain signals must pass through a gate in the spinal cord that can be closed (to block pain signals)• No scientific data to support

Skin Senses: Touch

• Pain (cont.)– Endorphins are our body’s natural pain

killer– The periaqueductal path (PAG) in the brain

seems to help suppress pain– Certain types of neural glia cells also help

suppress pain

Figure 4.53 Pathways for pain signals

Other Senses: Kinesthetic and Vestibular

• Kinesthesis - knowing the position of the various parts of the body– Receptors in joints/muscles

• Vestibular - equilibrium/balance– Semicircular canals: the largest part of the

vestibular system; these are fluid filled canals that contain hair cells similar to those in the basilar membrane

– When your head moves, the fluid moves, moving the hair cells, and initiating neural signals that travel to the brain.