Chapter 5: Sensation and Perception
Sensation to Perception (p.127)
Receive and translate environmental stimuli into nerve impulses
Making sense of what the senses tell us (processing and interpretation)
Giving meaning to the sensory information
1. Stimulus is received by sensory receptors.
2. Transduction: Receptors translate stimulus into nerve impulses.
3. Feature detectors analyze stimulus features.
4. Stimulus features are reconstructed into neural representation.
5. Neural representation is compared with previously stored information in brain.
6. Matching process results in recognition and interpretation of stimulus.
Sensory sensitivity (p.128):
Absolute Threshold: (limits of stimulus detectability)
lowest intensity at which a stimulus can be detected 50% of the time
The lower the threshold, the greater the sensitivity.
Variations among species (eg dogs)
Individual variation in detection:
Decision criterion: how certain one must be that a stimulus is present before they will say they detect it
Will change from time to time depending on hunger, expectation (horror movie), etc
Signal detection theory: factors that influence decisions about whether or not a stimulus is present
Typical signal detection experiments: see p.128 Hit/Miss/False alarm/Correct rejection
Personalities eg conservative people always say no
Manipulation with cost/reward
Subliminal Stimulus: stimulus so weak/brief that although it is received by the senses, it cannot be perceived
consciously (consumer experiment p.129)
Differences threshold: (ability to discriminate between stimuli)
the “just noticeable difference” that people can perceive 50% of the time.
Weber’s law: jnd is proportional to the intensity of original stimulus (eg weight is 1/50)
Smaller the fraction, greater the sensitivity to differences. Humans: better vision(1/60) than smell(1/4)
Sensory Adaptation: diminishing sensitivity to an unchanging stimulus
eg background sound disappearing, cold water only cold at first
Frees senses from constant stuff and allowing them to pick up informative changes that could help survival V ISION : VISUAL SYSTEM
Electromagnetic energy/Light waves – stimulus for vision, measured
in nanometers (nm)
The human eye (p.132)
Waves enter through cornea: transparent protective
structure at the front of eye
Behind is pupil: can dilate or constrict to control the
amount of light that enters
Pupil’s size is controlled by muscles in the colored iris
o Dark room – pupil dilate (bigger) so more light can
enter to improve optical clarity
o Bright room- pupil constrict (smaller)
Behind pupil is lens: elastic structure that becomes thinner(far)/ thicker (near) to focus on objects
Lens projects a image onto the retina: a multilayered light-sensitive tissue at the rear of the fluid-filled eyeball
The lens reverses the image but the brain reverses the visual input into the image we perceive
Ability to see clearly depends on len’s ability to focus.
o Myopia – nearsightedness (cant see far), lens focuses the image before the retina
o Hyperopia – farsightedness (cant see near) lens does not thicken enough and image is there before
focused on a point behind the retina
Photoreceptors (p.133): The retina contains two types of receptor cells (in retina):
black and white receptors
best in dim light
500 more times more sensitive to light than
cones, but don’t react to color sensations
Found mostly throughout retina
function best in bright illumination
found mostly in fovea: center of retina with no
rods. Cones decrease in concentration the
farther away they are from fovea.
How rods and cones send their message to the brain: Light > rods and cones have synaptic connections with
bipolar cells > synapse with ganglion cells > Optic nerve: bundle of ganglion cells axons > to brain.
Dark adaptation : gradual regeneration of photopigments that have been depleted by brighter
illumination. Cones adapt in 10 mins, rods continue to increase sensitivity for another 20 mins.
Color-Deficient Vision (p.137)
Trichromats: normal color vision, can see all 3 systems: blue-yellow, red-green, black-white
Dichromat: blind in one system (B-Y or R-G)
Monochormat: can only see B-W, total color-blind.
Analysis and Reconstruction of visual senses (p.138)
Feature detectors: cells fire selectively in response to visual stimuli that have specific characteristics
Eg celebrities, faces in arts, shapes, movements, etc. / + \ + — = A
Discovered by Hubel & Wiesel
Visual stimuli are analyzed by features detectors in the primary visual cortex, and the stimulus elements
are reconstructed and interpreted in visual association cortex. A UDITION : SOUND SYSTEM
Sound waves – stimuli for hearing.
Receptors are hair cells in organ of Corti of the inner ear.
Frequency: the number of sound waves (cycles) per second, how rapidly air vibrates
Measured in hertz (Hz) – 1Hz is 1 cycle per second
Related to pitch. Higher frequency/hz, the higher the pitch
Amplitude: the vertical size of sound waves – amount of compression and expansion
Measured in decibels (dB) – measure of physical pressures that occur at the
Related to loudness. Absolute threshold for hearing is 0db, increase of 10db
represents a tenfold increase in loudness. 140db+ can cause potential damage to
Quality of a sound
Complexity of a sound wave
The Ear (p.141)
Sound waves travel into auditory canal leading to the eardrum: a membrane that vibrates in response to sound
Beyond is middle ear (ossicles) with three tiny bones: hammer (malleus), anvil (incus) and stirrup (stapes)
First bone (hammer) is attached to eardrum, third (stirrup) is attached to another membrance.
The oval window is the boundary between middle and inner ear
Inner ear contains the cochlea: coiled snail-shaped tube about 3.5cm and is filled with fluids and contains the
basilar membrane: membrane that runs the length of the cochlea
Inside BM is organ of Corti: contains thousands of tiny hair cells that are sound receptors
Tip of hair cells are attached to another membrane, the tectorial membrane.
The hair cells synapse with neurons of the auditory nerve, send impulses to auditory cortex Auditory Transduction: from pressure waves to
When sound waves strike eardrum, pressure created
at oval window by 3bones of middle ear sets fluid
inside cochlea into motion. The fluid waves vibrate
the basilar membrane and the tectorial membrane,
causing a bending of hair cells in the organ of Corti.
The bending of hair cells triggers the release of
neurotransmitters to neurons of auditory nerve and
sent impulse to brain.
Within auditory cortex are feature-detector neurons
that respond to specific kinds of auditory input.
Hearing loss (p.143)
Conduction deafness: problem with structure of ear that transmit vibrations to the cochlea
Punctured eardrum or bones not functioning in middle ear -- Can be fixed with hearing aid.
Nerve deafness: damaged receptors within inner ear or damage to auditory nerve
Aging, disease or exposure to loud sounds
G USTATION : GUSTATORY SYSTEM – SENSE OF SMELL ( P .144)
Sensitive to chemical molecules rather than some form of energy.
Five basic taste: sweet, sour, salty, bitter, umami
Also have tactile (touch/texture) and temperature receptors
Taste buds: chemical receptors concentrated along the tip, edge and back of tongue
Each taste bud is most responsive to one or two of the basic taste qualities but responds weakly to the others as
Umami: increase the intensity of other taste
Humans have 500-10,000 buds, each consisting
several receptor cells arranged like segments of
When substance enters mouth, it interacts with
saliva to form a chemical solution that flows into
the taste pore and stimulates the receptor cells.
Taste results from complex patters of neural
activity produced by four types of taste receptors.
Genetic differences in ‘taste’.
Also culture and learning
Sense of taste can also discriminate between
nutrients and toxins.
Response to some taste qualities is innate.
o Eg newborn respond positively to sugar water and negatively to bitter substances
Most poisonous substances are bitter and this emotional response have hardwired into our physiology
High calorie foods are mostly sugar-rich
Most humans do not live in food-scarce environment anymore so preference for sweet substances evolved and
tend to over consume sweet foods
There are genetic differences in taste responses
Supertasters (25% of Americans):
o find broccoli, caffeine, and saccharin unpleasantly bitter, high sensitivity to sweet,salty, and spicy
o Have more taste buds! O LFACTION : OLFACTORY SYSTEM – SENSE OF SMELL (P .144)
Receptor are long cells in upper nasal cavity
Humans have 40 million olfactory receptors, dogs have 1 billion
Olfactory receptors resemble neurotransmitter binding sites on neurons (lock and key)
The receptors send input to the olfactory bulb: a forebrain structure above nasal cavity