Sensation and Perception: Vision, hearing and the other senses

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Sensation and Perception Vision, hearing and the
other senses

• Week 12
• HP502 Introductory Psychology Learning, Memory
  and Cognition
• Mr Simon Morris
• BA(Soc.Sc.) Monash, PostGrad Dip. Psych. Monash
• smorris_at_students.ballarat.edu.au

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Lecture outline

• Vision
• Hearing (audition)
• Smell
• Other senses

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Vision

• Vision is the best understood of all the senses.
• The function of the eye is to detect
  electromagnetic radiation (light).
• Vision is functional in that it allows for
  detection of
• Movement (Is that moving object a predator or a
  friend?)
• Colour (Is that fruit ripe or is it spoiled?)

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The nature of light

• Light detection is particularly useful because
• Light travels rapidly (in contrast to sound waves
  in hearing) allowing for rapid detection of
  events in the environment.
• Light travels in straight lines (no distortion)
  the geometry of objects is preserved in visual
  perception.
• Light interacts with the surfaces of objects in
  the environment (is reflected or absorbed).
• Electromagnetic energy travels in waves
  characterised by patterned movement, or
  oscillation.
• Different forms of radiation have waves of
  different lengths, or wavelengths.
• For example gamma rays are as short or shorter
  than the diameter of an atom, while radio waves
  may oscillate once in a kilometre.

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Electromagnetic Spectrum
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The electromagnetic spectrum

• Electromagnetic (EM) energy travels in waves or
  oscillations.
• The waves vary in frequency with gamma waves at
  the low end (10-5 nM) and radio waves at the
  upper end (1013 nM).
• Humans are sensitive to only a small range of the
  EM scale (400-700 nM).
• The physical dimension of wavelength translates
  in the psychological dimension of colour, just as
  the physical intensity of light is related to the
  subjective sensation of brightness.

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The human eye and retina
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Elements of the eye

• Cornea The tough transparent tissue covering the
  front of the eyeball.
• PupilThe opening of the centre of the iris that
  constricts or dilates to regulate the amount of
  light entering the eye.
• Lens The disc shaped elastic structure of the
  eye that focuses light.
• Retina The light sensitive layer of tissue at
  the back of the eye that transduces light into
  neural impulses.

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Structure of the retina
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Elements of the retina

• The retina is a multi-layered structure about as
  thick as a sheet of paper.
• ConesOne of two types of photoreceptors
  specialised for colour vision which allow
  perception of fine detail.
• Rods Are more sensitive to light than cones, and
  allows vision in dim light. Produce black, white
  and grey.
• Bipolar cells Neurons in the retina that combine
  information from many receptors and excite
  ganglion cells.
• Ganglion cells Nerve cells in the retina that
  integrate information from multiple bipolar
  cells, the axons of which bundle together to form
  the optic nerve.

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Transduction of Light

• Light travels through the retina and impinges on
  photoreceptors at the back of the eye.
• Both rods and cones contain photosensitive
  pigments that change chemical structure in
  response to light.
• This process is called bleaching because the
  pigment breaks down when exposed to light,
  leading to photoreceptors to lose their
  characteristic colour.
• Bleaching must be reversed before a photoreceptor
  is restored to full sensitivity.

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Receptive fields

• Once the rods and cones have responded to
  patterns of light, the nervous system must
  somehow convert these patterns into a neural code
  to allow the brain to reconstruct the scene.
• Each ganglion cell has a receptive field.
• A receptive field is a region within which a
  neuron responds to appropriate stimulation.
• Neurons at higher levels of the visual system
  (the brain) also have receptive fields, which
  means at the higher and higher levels of
  processing, the visual system keeps creating maps
  of the scene the eye has observed.

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Receptive fields .
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Receptive fields .

• Receptive field That aspect of the external
  world that produces a change in firing rate of a
  given sensory cell.
• Sensory neurons show a baseline rate of firing
  rate of firing can increase or decrease in
  response to external stimuli.
• Centre-surround shape A spot of light placed on
  the centre of the field produces an on response
  (increase in firing rate), the same spot of light
  placed on the outside of the field reduces the
  firing rate.

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Visual Pathways in Brain
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From the eye to the brain

• Impulses first pass through the optic chiasm,
  where the optic nerve splits.
• Information from the left half of each retina
  (which comes from the right visual field) goes to
  the left hemisphere, and vice versa.
• The information then travels to the brain via the
  optic tracts, which are a continuation of the
  ganglion cells.
• From there, the visual information flows along
  two separate pathways within each hemisphere.

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From the eye to the brain

• The first pathway projects to the lateral
  geniculate nucleus of the thalamus and then to
  the primary visual cortex of the occipital lobes.
• Neurons on the lateral geniculate nucleus
  preserve the map of the visual space in the
  retina. i.e., neighbouring ganglion cells
  transmit information to thalamic neurons next to
  each other, which in turn transmit this retinal
  map to the visual cortex.

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From the eye to the brain

• The second short pathway projects to a clump of
  neurons in the midbrain known as the superior
  colliculus, which is involved in controlling eye
  movements.
• Its neurons respond to the presence of absence of
  visual stimulation in parts of the visual field
  but cannot identify specific objects.
• Visual processing in the superior colliculus, and
  perhaps at the level of the lateral geniculate
  nucleus, leads to visual responses that can guide
  behaviour outside of awareness.

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Visual cortex

• From the lateral geniculate nucleus, visual
  information travels to the primary visual cortex
  in the occipital lobes. This is sometimes also
  known as the striate cortex.
• The striate cortex is the first stop in the
  cortex for all visual information. Neurons begin
  to make sense of visual information through the
  use of feature detectors.
• Feature detectors are neurons that only fire when
  stimulation in their receptive field matches a
  very specific pattern. E.g., particular
  direction, specific size or length, colour,
  contrast, texture

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The what pathway

• From the primary visual cortex, visual
  information flows along two pathways.
• The what pathway runs from the striate cortex
  in the occipital lobes through the lower part of
  the temporal lobes. This involved in determining
  what an object is.
• Primitive features such as lines are integrated
  into more combinations such as shapes.
• At other locations along the pathway, the brain
  processes features of the object such as colour
  and texture.

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The where pathway

• The where pathway is involved in locating an
  object in space, following its movement, and
  guiding movement towards it.
• This pathway runs from the striate cortex through
  the middle and upper regions of the temporal
  lobes and up into the parietal lobes.
• Their locations make sense, with the what
  pathway located directly below those involved in
  language, particularly among naming objects. The
  where pathway is adjacent to circuits in the
  parietal lobes that processes information about
  the position of the body in space.

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Perception of Colour

• Colour is a psychological property. E.g., grass
  is not green to a cow because they lack colour
  receptors.
• Three dimensions of colour
• Hue is the apparent colour of an object (red,
  blue, etc)
• Saturation is the purity of the colour (extent to
  which diluted with black or white)
• Lightness is the extent to which a colour is
  light or dark.
• Three different types of cones are found in the
  eye.
• Cones are sensitive to different wavelengths of
  light
• S-cones code for blue light.
• M-cones code for green light.
• L-cones code for red light.

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Cone response curves
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Theories of colour vision

• Young-Helmholtz (Trichromatic Theory) Colour is
  explained by differential activation of three
  colour receptors in the eye.
• Opponent-Process Theory Colours are derived from
  activity of three antagonistic systems
  (Black-white, Red-green, Blue-yellow).

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Negative colour afterimages
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Hearing

• Hearing, or audition, allows sensation at a
  distance and is therefore of tremendous adaptive
  value.
• The stimulus energy underlying hearing is sound.
• The ear transduces sound, and sends it along the
  the neural pathways for auditory processing.

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The nature of sound

• All sound is produced by vibrations produced in
  the air. These rhythmic pulsations of acoustic
  energy (sound) spread outward from the vibrating
  object as sound waves.
• Sound waves grow weaker with distance, but they
  travel at a constant speed, roughly 340 metres
  per second.

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The nature of sound

• Acoustic energy has three important properties
• Frequency Each round of expansion and
  contraction between air molecules is known as a
  cycle. The number of cycles per second determines
  the sound waves frequency. Frequency is a
  measure of how often a wave cycles. Frequency is
  expressed in hertz, or Hz
• Frequency corresponds to the psychological
  property of pitch, which is the quality of a tone
  from low to high.
• Complexity Most sounds are a combination of
  sound waves, each with a different frequency.
  Complexity refers to the extent to which a sound
  is composed of multiple frequencies.
• This corresponds to the psychological property of
  timbre, or texture of the sound.

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The nature of sound

• Amplitude Amplitude refers to the height or
  depth of a sound wave, that is, the difference
  between its maximum and minimum pressure level.
• The amplitude of a sound wave corresponds to the
  psychological property of loudness the greater
  the amplitude, the louder the sound.
• Amplitude is measured in decibels (dB). Zero
  decibels is the absolute threshold at which most
  people can hear a 1000 Hz tone.

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Frequency and amplitude
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The ear

• Transduction of sound occurs in the ear, which
  consists of an outer, middle and inner ear.
• The outer ear collects and magnifies sounds in
  the air the middle ear converts waves of air
  pressure into movements of tiny bones and the
  inner ear transforms these movements into sound
  waves in fluid that generate neural signals.

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The Ear
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Transduction

• The hearing process begins in the outer ear,
  which consists of the pinna and the auditory
  canal.
• The pinna is not essential for hearing, but its
  irregular shape helps locate sounds in space,
  which bounces off its folds differently when they
  come from various locations.
• At the end of the auditory canal is a thin,
  flexible membrane known as the eardrum or
  tympanic membrane.
• The eardrum reproduces the cyclical vibration of
  the object that a created the noise on a
  microcosmic scale.
• When the eardrum vibrates it sets in motion three
  tiny bones in the middle ear, called ossicles.

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Transduction

• These bones named for the distinctive shapes, are
  called the malleus (hammer), incus (anvil) and
  stapes (stirrup).
• The ossicles further amplify the sound by two or
  three times before transmitting vibrations to the
  inner ear. The stirrup vibrates against a
  membrane called the oval window, which forms the
  beginning of the inner ear.

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Transduction of Sound
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The inner ear

• The inner ear consists of two sets of
  fluid-filled cavities the semicircular canals
  (involved in balance) and the cochlea (involved
  in hearing).
• The cochlea is a three-chambered tube in the
  inner ear. When the stirrup vibrates against the
  oval window, the oval window vibrates, causing
  pressure waves in the cochlear fluid. These waves
  disturb the basal membrane, which separates two
  of the cochlea chambers.
• Attached to the basilar membrane are the ears
  15,000 receptors for sound called the hair cells.
  
• The movement of the hair cells triggers action
  potentials in sensory neurons forming the
  auditory nerve, which transmits auditory
  information to the brain.

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Neural pathways
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Neural pathways

• Sensory information transmitted along the
  auditory nerves ultimately finds its way to the
  auditory cortex in the temporal lobes.
• The auditory nerves from each ear projects to the
  medulla, where the majority of its fibres cross
  over to the other hemisphere.
• From the medulla, bundles of axons project to the
  midbrain (inferior colliculus), then on to the
  thalamus.
• The thalamus transmits information to the
  auditory cortex in the temporal lobes, which has
  sections devoted to different frequencies.
• In humans and other animals, some cortical
  neurons on the left temporal lobe respond
  exclusively to particular sounds characteristic
  of language of the particular animal.

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Sound localisation

• Humans use two main cues for sound localisation
  differences between the two ears in loudness and
  timing of the sound.
• High-frequency sounds particularly rely upon the
  relative loudness in the ear closer to the source
  as the head blocks some of the sound from hitting
  the other side.
• Low-frequency sounds relies more on the
  split-second difference in the arrival time of
  sound at the two ears.

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Smell

• Smell (olfaction) serves a number of functions.
  It enables us to detect danger, discriminate
  palatable from spoiled foods and recognise
  familiar others.
• Many other species communicate through
  pheromones, scent messages detected through an
  auxiliary olfactory system that regulates the
  sexual behaviour of many animals and directs a
  variety of behaviours in insects.

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Olfaction
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Transduction

• The environmental stimuli for olfaction are
  invisible molecules of gas emitted by substances
  and suspended in the air.
• The thresholds for recognising most odours are
  remarkably low as low as one molecule per 50
  trillion molecules of air for some molecules.
• Although the nose of the sense organ for smell,
  the vapours can enter threw the nose or mouth.
• When food is chewed, vapours travel up the back
  of the mouth into the nasal cavity this process
  accounts for much of the flavour.

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Transduction

• Transduction of smell occurs in the olfactory
  epithelium, a small pair of thin structures less
  than 2.5cm2 in diameter at the top of the nasal
  cavities. Chemical molecules in the the air
  become trapped in the mucus of the epithelium,
  where they make contact with the olfactory
  receptor cells that transduce the stimulus into
  olfactory sensations.

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Neural pathways

• The axons of olfactory receptor cells form the
  olfactory nerve, which transmits information to
  the olfactory bulbs, multi-layered structures
  that combine information from receptor cells.
• This information then travels to the primary
  olfactory cortex, a primitive region of the
  cortex deep in the frontal lobes.
• Unlike other senses, smell is not relayed through
  the thalamus on the way to the cortex.
• However the olfactory cortex has projections to
  both the thalamus and limbic system, so that
  smell is connected to both taste and emotion.

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Other senses

• Skin senses
• Proprioceptive senses
• Vestibular sense provides information about the
  position of the body in space by sensing gravity
  and movement.
• Kinesthesia provides information about the
  movement and position of the limbs and other
  parts of the body relative to one another.

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Conclusions