Smell, Taste, Pain, Hearing, and Psychophysics

2/10/2012

Chapter 7

Sensation

• What we sense is based upon what it was important for us to respond to in the distant past • What would you expect was important to a dog, based on its most important senses? To a bird? • What do you think is the most important sense for humans? What does that tell us about our past?

Sensation vs. Perception

• Sensation - what we take from our environment and translate into neural signals – Example: the sweetness of a wine • Perception - organization and meaningful interpretations of sensory input – Example: that the wine will remind us of another favored wine, or of one that made us sick (!)

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Sensory Experience

Physical stimulus

Physiological response

Sensory experience

How We Study Senses

• Psychophysics: The physical characteristics of stimuli plus our sensory experience of the stimuli • Important concepts – Absolute threshold – Difference threshold or just-noticeable difference (jnd)

Senses and Sensory Receptors

• Transduction - The sensory action potential, activated when a physical stimulation causes an electrical change in receptor cells – Smell = olfactory receptor cells – Taste = taste receptor cells on taste buds – Pain = A-delta fibers, C fibers – Hearing = cilia (hair cells) in basilar membrane, inner ear

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Sensory Adaptation

• When we acclimate to a stimulus • Happens because we are designed to sense change better than stagnation – Examples • Your friend’s perfume • Bright lights • Loud music

Senses We’ll Review Today:

• Smell • Taste • Pain • Hearing

Review!

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Functions of Smell

• Important environmental changes – Example: fire • Recognition – Examples: siblings, offspring • Infant bonding • Mate selection – Example: major histocompatibility complex)

How Smell Works

• Different types of stimuli trigger different glomeruli (intermediary between olfactory receptors and olfactory bud) • Signals are sent to the limbic system and hypothalamus then to the temporal lobe (piriform cortex) and frontal lobe (orbitofrontal cortex)

Function of Taste

• Informs us what to eat or what not to eat – Naturally occurring bitter tastes in general are toxic – We have cravings for sweet and umami tastes because high calorie foods were rare in past environments

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Taste

• We taste via taste receptors on taste buds • More taste buds = more taste sensitivity, especially for bitter tastes • 5 tastes; each taste has distinct receptor cells and brain areas for sensing and perceiving – Sweet – Salty – Bitter – Sour – Umami

Brain Areas Related to Taste

• Temporal and parietal lobes – Separate areas interpret each taste • Orbitofrontal cortex – Where taste and smell mingle

Intermission: Sensory Interaction • Senses do not operate independently of each other • True of all senses, but we experience the interaction most acutely with taste and smell

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Pain

• In what way is pain different from the other senses?

• Somatosense – Pain receptors all over body – Feel pain as bodily event, not as coming from outside of us – Emotional tones - recognizable facial expression, consumes conscious mind – Motivation to stop source of pain

Pain Receptors

• A-Delta Fibers - thicker, myelinated, faster – Some respond to strong pressure or extreme hot and cold – Stop what you’re doing! (first pain) • C Fibers - thin, unmyelinated, slower – Some respond to strong pressure, hot and cold, or chemical burns on skin – Don’t do it again! (second pain)

The Neuroscience of Pain

• Pain produces a reflexive response – When confronted with a pain-inducing stimulus, pain nerves connected to the brainstem cause withdrawal from stimulus • Pain is sometimes mind over matter – Nerves also send messages to the somatosensory cortex, limbic system, and prefrontal cortex – Phantom limb patients experience the sensation of their missing limb even if the nerves and spinal cord portion are damaged

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And Now, The Ear!

Hearing

• Vibration of air moving outward from the stimulus as a wave

• Which sound has a lower amplitude, your library voice or your outside voice? • Which sound has a lower frequency, a baby crying or a dog growling?

How We Hear Different Frequencies (Pitch) • Variation based on peak amplitude • Traveling wave theory of pitch – If cilia fires from a proximal portion of the basilar membrane, we hear high pitch; if they fire from the distal portion, we hear low pitch – Auditory masking of high pitch by low pitch • Loss of high pitch with age because proximal cilia are activated for all pitches – Tonotopic organization of auditory cortex

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Traveling Wave Theory of Pitch (cont.) • High pitch = firing from cilia at proximal portion of basilar membrane; low pitch = firing from distal • Auditory masking of high pitch by low pitch - low pitch travels down basilar membrane, high pitch stays proximal • Loss of high pitch because those cilia are activated for all pitch, low not • Also, tonotopic organization of auditory cortex (certain areas for certain pitches)

Interpreting What We Hear

• We attend to: – Direction from which the sound originates (sound localization) – Gender and other identifying information of the speaker • Cortical neurons respond differentially based on pitch, length, speech pattern

Assessment

• How do we get information from the external world inside? • What is the difference between sensation and perception? • What is the function of smell? • What are the five basic tastes? • How do we hear different pitches?