How do we perceive our environment?
• Complex stimuli are broken into individual features, relayed to the CNS, then reassembled as our perception
Sensation and Perception Terminology
• Stimulus: physical agent that interacts with a receptor
• Receptor: structure that transduces a physical stimulus
• Transduction: process of converting a physical stimulus into a neural signal (sensation)
• Sensations: neural representation of physical stimuli
• Perceptions: cognitive interpretations of sensations
Modes of Perception
• Bottom-up processing: our perceptions are the result of incoming sensations produced by physical stimuli
• Top-down processing: our perceptions are the result of experience, knowledge, bias, and contextual influences on incoming sensations
Vision: Summary
• Stimulus: 400-700 nm electromagnetic radiation
• Receptors: rods & cones (know the difference)
• Transduction: Light activates sends action potential through optic nerve to the brain
• Pathway: ½ of eye crosses to form contralateral visual field perceptions in each occipital lobe
• CNS Areas: occipital lobe - feature detectors (simple/complex cells) reassemble perception
• Perception: Influenced by top-down “shortcuts”
Air pressure changes that unfold over time
Three dimensions for the stimulus
• Frequency of the sound – high versus a low sound
• Amplitude – loudness of a sound
• Complexity – how many frequencies are mixed together
6
Auditory stimulus
400 Hz
1500 Hz
Auditory: Transduction
• The tectorial membrane “bounces” on the hair cells in rhythmic fashion to physically open ion channels and produce action potentials in the cochlear nerve
Auditory: Afferent Signals
• Receptors in the cochlea are “tuned” to send action potential only for certain frequencies from high (outermost) to low (innermost).
• More receptors/ afferent signals for 500-5,000 Hz
Auditory: Pathways
• Each auditory cortex receives input from both ears BUT primarily from the contralateral ear.
• Neural signal goes to the primary auditory cortex in the temporal lobe.
Auditory: Perception (Left Brain)
• There are specialized brain regions for complex sounds and language. (secondary auditory cortex = language & music)
Green = primary auditory cortex
Brown = language comprehension area
Gold = language production area
Auditory: Perception (Right Brain)
• The same areas of the right hemisphere appear to be specialized to detect all environmental sounds other than language.
Auditory: Summary
• Stimulus: 20-20,000 Hz sound waves
• Receptors: Hair cells in the cochlea
• Transduction: Physical opening of ion channels in the cochlea by the tectorial membrane
• Afferent Signals: unevenly distributed to allow most signals for range of human speech
• Pathway: contralateral to primary auditory cortex
• CNS Areas: Temporal lobe; Wernickes, Brocas, and right hemisphere specializations
• Perception: Complex processing of language in the left hemisphere and music in the right hemisphere.
The chemical senses . . .
• Taste and olfaction are grouped together as “chemical senses” because chemicals are the stimulus for both senses.
PHYSICAL STIMULUS:
• Taste (gustatory system) stimuli are dissolved chemicals (in solution or saliva)
• Smell (olfactory system) stimuli are typically chemicals suspended in the air
Olfactory Receptors
• Olfactory receptors are found in the back of the nasal cavity. (regenerate)
• Protected by mucous layer
• Olfactory receptors are modified neurons with cilia
Olfactory Transduction
• Olfactory receptors are similar to the neurotransmitter receptors (both stimuli are chemicals, right?)
• Odorants fit only in certain receptors like a lock & key
LOCK & KEY
Olfactory: Signals
• Chemicals bind to the receptor causing a reaction in the neuron to send a neural signal to the olfactory bulb
• Each odor has its own pattern of activity
LOCK & KEY
Olfactory: Pathways & CNS Areas
• What is your perception of odors?
• Olfactory bulb to:
• Amygdala – emotional value of stimuli
• Frontal Lobe – memories, moods
Olfactory Perception
• Projections to the limbic system and frontal lobe closely link smell with memories and emotions
• Evolutionary pressure to remember harmful stimuli – strong long-term memory mechanism
• Olfaction is a powerful component to flavor linked with gustation (taste).
Olfactory: Summary • Stimulus: Chemicals in air from nostrils or mouth
• Receptors: Olfactory receptor neurons with lock & key shape binding
• Transduction: Binding actives sending signal
• Afferent Signals: Odors produce unique patterns
• Pathway: Olfactory bulb Amygdala & Frontal Lobe
• CNS Areas & Perceptions: Amygdala (emotional value), Frontal Lobe (memory association), Orbitofrontal cortex (flavor perception)
Gustatory: Transduction
Chemicals dissolve in saliva
and enter the taste bud pits
Chemicals are interact
with receptors on the
taste receptor cells
Taste cells release
neurotransmitters
on afferent taste
neurons
Gustatory: Pathways
• 3 sensory nerve relay taste sensations to the brain
• Insular cortex = primary taste quality (between the temporal & parietal lobes)
Gustatory: CNS Areas
• Many secondary areas
Limbic system
& Frontal Lobe
Hypothalamus – hunger & satiety
Flavor Perception
• Flavor - the overall perception of an oral stimulant
• Includes taste, smell, texture, temperature, even color & appearance
% subjects correctly identify
flavored solutions
Black bars = normal
Gray bars = pinched nose
Gustatory: Summary
• Stimulus: Chemicals in saliva or solutions
• Receptors: Taste receptor cells in taste buds
• Transduction: Different for each taste category
• Afferent Pathway: 3 nerves from oral cavity to the NST of the brainstem
• CNS Areas & Perceptions: Insular cortex (primary taste categories), Amygdala (cravings and aversions), Frontal Lobe (memory association), Hypothalamus (hunger & satiety) Orbitofrontal cortex (flavor perception)
Somatosensory: Transduction
• Mechanoreceptors (Meisners, Merkel, Ruffini, Pacinian, and Hair cells): physical movement opens ion channels to depolarize the neuron and send action potentials to the brain
• Bare or Free Nerve endings: detect increases or decreases in temperature and chemicals released in response to tissue damage (like histamines) (pain detection)
• Pain following injury is often 2 sensations:
• Sharp immediate pain
• Dull throbbing secondary pain
• Speed difference due to myelin on axons!
Pain Perception
- unmyelinated neurons
- myelinated neurons
Immediate Sensation Delayed Sensation Overall Perception
Controlling Pain Sensations & Perceptions
• Pain perception can be modified!
• Reduction of the sensation through the :
Gate control theory
Natural Pain Management System
• Efferent signal
from the brain to
the spinal cord at
pain sensation
level.
Pain input
to brain
Brain output
to block pain
• Localized release of endorphins to block the sensation at a precise site
Somatosensory: Summary • Stimulus: mechanical, thermal, and chemical
• Receptors: Mechanoreceptors & Free nerve endings
• Transduction: Physical movement, change in temp, or chemicals released by tissue damage
• Afferent Pathway: Dorsal column pathway for touch, anterolateral pathway for temp and pain
• CNS Areas & Perceptions: Postcentral gyrus is the primary somatosensory cortex (touch organized by body part); anterior cingulate gyrus (pain)
• Gate control theory is natural pain management
Psychophysics
• Psychophysics is the study of the our perceptions (behavior) to physical stimuli
• The first field of experimental psychology!
• Two types of thresholds:
– Absolute: can you detect a stimulus or not?
– Difference: can you detect a change in a stimulus?
Signal Detection Theory
• The sensory system must deal with interference and noise to detect appropriate stimuli
False
alarm
Correct
decision
Correct
decision
Miss
Actual situation
NO sense Sense
Sensation
No
sensation
Person’s
Decision
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