Chapter 5: Sensation and Perception
How Do We Sense Our Worlds?
•way we experience the world is divided into two sections:
•sensation: the organs’ responses to external stimuli and the transmission of these
responses to the brain
•light, air vibrations, odor
•is an elementary experience such as colour or motion without the more
complex perceptual experience of what is being seen or what is moving
•perception: the processing, organization, and interpretation of sensory signals, it
results in an internal representation of stimulus
•often based on prior experiences which shape our expectations about new
sensory experiences
•unlikely to see a blue applep-shaped object as a real apple because you
know apples are not blue
•our perception of the world does not work like a camera/tape recorder, what
we sense is the result of brain processes that actively construct perceptual
experiences > allow us to adopt to our environments detail
Stimuli Must Be Coded to Be Understood by the Brain
•sensory coding: our sensory organs’ translation of stimuli’s physical properties
into neural impulses
•brain cannot process raw stimuli > must be translated into chemical and
electrical signals that the brain can interpret
•sensory coding begins with transduction: a process by which sensory
receptors produce neural impulses when they receive physical or chemical
stimulation
•to function properly, our brains need qualitative and quantitative information about
stimuli
•qualitative: knowing whether traffic light is green or yellow, difference
between salty and sweet
•quantitative: coded by the speed of a particular neuron’s firing
•a brighter light
Psychphysics Related to Stimulus Response
•psychophysics: examines our psychological experiences of physical stimuli
•how much change is required before we notice that change
Sensory Thresholds
•absolute threshold: the minimum intensity of stimulation that must occur before
you experience a sensation
•the absolute threshold for hearing is the faintest sound a person can detect
50% of the time
•difference threshold: the just noticeable difference between two stimuli > the
minimum amount of change required for a person to detect a difference
•the minimum change in volume required for you to detect a difference
•the difference threshold increases as stimulus becomes more intense
•Weber’s Law: states that the just noticeable difference between 2 stimuli is based
on a proportion of the original stimulus rather than on a fixed amount difference
•I / I ∆ 1/10 = 0.1 you can detect a single candle when you have 10
candles
Signal Detection Theory
•since people might convince themselves that a stimulus was presented due to
questioning
•signal detection theory (SDT): a theory perception based on the idea that the
detection of a faint stimulus requires a judgement - is is not an all-or-nothing
process
•any trial in which participants judge whether an event occurs can have 1 of 4
outcomes:
•hit: if signal is presented and observer detects it
•miss: if participant fails to detect a signal
•false alarm: if participant “detects” a signal that was not presented
•correct rejection: if signal is not presented and observer does not detect it
•response bias: refers to a participant’s tendency to report detecting the signal in
an ambiguous trial
•people’s expectations often influence the extent to which they are bias
Sensory Adaptations
•sensory systems are tuned to detect environmental changes
•sensory adaptations: a decrease in sensitivity to a constant level of stimulation
•researchers have noticed that if stimulation is presented continuously, the
responses of the sensory systems that detect it tend to diminish over time
•those who live near the airport become less aware of the airplane noises
over time
What Are the Basic Sensory Processes?
•only the neurons in the sensory organs respond directly to events in the world > the
neurons in the brain do not respond to events in the world, they only respond to input
from other neurons
In Taste, Taste Buds Detect Chemicals
•gustation: our sense of taste
•taste buds: sensory receptors that transduce taste information (mostly on the
tongue) - individual has from 500 to 10 000 taste buds
•every taste experience is composed of a mixture of 5 basic qualities: sweet,
sour, salty, bitter, umami (yummy)
•entire taste experience occurs in mouth and brain
•taste relies heavily on sense of smell, texture
•the same food can taste different to people because of the sensation
associated with that food
•cultural experiences influence taste preference
•what mothers ate during pregnancy effected children’s tastes
In Smell, the Nasal Cavity Gathers Odorants
•olfaction: sense of smell
•has the most direct route to the brain but may be the least understood
•olfactory epithelium: a thin layer of tissue embedded with smell receptors >
receptors transmit information to the olfactory bulb
•olfactory bulb: the brain centre for smell (below the frontal lobes)
•part of the brain (prefrontal cortex: involved with emotion and memory > smell can
evoke feelings and memories
•women have better sense of smell than men
•impaired sense of smell is associated with increased risk of mental decline and
Alzheimer’s
In Touch, Sensors in the Skin Detect Pressure, Temperature, and Pain
•haptic sense: sense of touch
•conveys sensations of temperature, pressure, pain, and a sense of where limbs are
in space
•anything that makes contact with our skin provides tactile stimulation which gives
rise to an integrated experience of touch
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•pain receptors are found throughout the body, not just the skin
•pain is part of a warning system that stops you from continuing activities that may
harm you
•most experiences of pain result when damage to the skin activated haptic
receptors
•nerve fibers that convey pain are thinner than those for temperature and pressure
> are found in all body tissues that sense pain: skin, muscles, membranes around
bones and joints, organs etc
•two types of pain (two kinds of nerve fibers):
•fast fibers: sharp immediate pain
•fast pain leads us to recoil from harmful objects > is protective
•slow fibers: chronic, dull, steady pain