Sensory systems

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Psy 111 Basic concepts in Biopsychology Lecture
6 Sensory Systems_ Vision
Website http//mentor.lscf.ucsb.edu/course/summer
/psyc111/
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Objectives

• Describe the general principles of sensory system
  organization.
• Define transduction and the various types of
  stimuli that the body detects.
• Explain the role of the thalamic relays in
  sensory systems.
• Illustrate parallel pathways.
• Describe topographical arrangement and
  hierarchical designs.
• Describe nature and properties of light.
• Identify the route of light into the eye to the
  retina.
• Describe the organization and circuitry of the
  retina
• Explain transduction of light in the two types of
  photoreceptors (rods and cones) how are they
  different?
• Describe the nature of color transduction.
• Explain the inputs to bipolar cells and ganglion
  cells.
• Describe the output pathways from the retina.
• Describe the lateral geniculate nucleus.
• Discuss the inputs and organization of primary
  visual cortex.
• Describe the secondary and association visual
  cortices use face cells example.
• Identify the two streams of higher visual
  processing.

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Sensory Systems

• Principles of sensory systems
• 2. Vision the photic sensory system

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What is sensation?

• Sensation the process of obtaining information
  about the environment and transmitting it to the
  brain for processing.
• Perception the process of interpreting sensory
  signals sent to the brain.

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Principles of Sensory Systems
Transduction Mechanism environmental energy -gt
biological energy Receptive fields each
transducing cell or neuron is receptive to info
from a specific subset of sensory
informationoften defined anatomically Parallel
Pathways quality of information is maintained
Relay Centers series of projection
neurons Topographically arranged shape of
information is maintained Hierarchical
organization convergence at relays to
cortex Cross Midline it just does
Note like all rules there are exceptions.
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Transduction Mechanisms

• Transduction conversion of stimulus energy into
  neural energy
• Law of specific nerve energy
• Each sensory modality is responsive to specific
  forms of environmental energies.

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Transduction Mechanisms

• Transduction change in membrane conductance.
• -produces graded receptor potential
• -summates to generate action potential
• -amount of stimulus determines frequency of
  action potentials
• Three types of Transduction
• Chemical
• Mechanical
• Photic

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Receptive fields
Entire receptive surface (e.g. light sensing
portion of retina, touch sensing portion of skin,
etc)
Area of input for a single cell receptive
field i.e. physical input to the receptive
portion of a single transducing cell or input to
(through the dendrites) of a single neuron.
For some types of sensory cells/neurons, there is
not an anatomically defined receptive field but
one defined on quality of information (e.g.
specific taste or odor molecules).
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Relay Centers
Sensory pathways involve multiple projection
neurons which relay info at specific sites
(nuclei) in the brain. Within each relay, there
is circuitry in which the sequential activation
of projection neurons is modulated by local
neurons
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Parallel pathways
Sensory information is moved from relay to relay
along parallel pathways
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Sensory System Relay Parallel Pathways

• Each sensory system is associated with a specific
  transduction mechanism localized in a
  transduction (sensory) organ.
• Transduction converts environmental info into
  biological signals that is transmitted along
  specific parallel pathways.
• These parallel neuronal pathways are relayed at
  specific points in the brain and is kept separate
  to maintain stimulus info.

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Pathways Thalamus
Thalamus is a major relay for all sensory systems
(for most prior to info arriving at
cortex). MDN-olfaction VPM-taste VPL-somatosensati
on MGN-auditory LGN-visual
Also, contains many motor nuclei and some
involved in complex processes (memory, emotional,
etc poorly understood).
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Pathways Primary Cortex

• Area of cortex that first receives sensory
  information of a given modality (i.e. vision,
  auditory, somatic, olfactory etc or motor) is
  referred to as primary sensory cortex

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Pathways Sensory, Motor, Association Cortex

• Cortical areas that are only associated with
  function of primary (sensory/motor) cortex are
  referred to as secondary sensory/motor cortex.
• Areas that are indirectly associated with primary
  cortices (involve multiple sensory modalities or
  sensory-motor information processing) are
  referred to as association cortex mediate
  higher cognitive functions.

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Topographical Arrangement
Shape of information is by anatomy maintained
in parallel sensory pathways
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Hierarchical Organization
Movement through sequential relays involves
multiple levels of processing such that the
once the info reaches the cortex has been highly
summarized or processed. -allows highly
complex integration and responses by nervous
system e.g. specialized cells to respond to
lines and specialized cell to respond to faces.
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Cross Midline
Info from one side of the body is projected to
the cortex on the contralateral side of the
brain. e.g. somatosensory (touch)
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Sensory Systems

• Principles of sensory systems
• 2. Vision the photic sensory system

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Electromagnetic Waves
Light is comprised of waves of photons Photons
are emitted with different energies resulting in
different wavelengths and different amounts
resulting in different amplitudes of waves.
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The visible spectrum
Short Wavelength
Long Wavelength
The visible spectrum is a small portion of the
total spectrum of wavelength of light. Keep in
mind we only perceive these as different colors
of light
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Movement of Light
Absorption is how eye detects energy ie transfer
of energy Transduction
Refraction is critical to pattern of light
shape detection
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Structure of the Eye
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But we really have two eyes

• Some perceptual processes in vision result from
  having two independent sites of visual input
• Convergence turning inward to see objects up
  close
• Binocular disparity difference in the position
  of the same image on 2 retinas ? enables you to
  construct a 3-D perception from two 2-D images on
  your retinas

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The visual field
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Passage of Light into the Eye
Where light is TRANSDUCED into biological energy
(i.e. detected).
Important for focus uses refraction to
appropriately bend light.
Where info is conducted to rest of brain.
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Focusing of light through the cornea
The lens refracts light to produce focus on the
the fovea
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Accommodation of the lens
The amount of refraction is controlled by ciliary
muscle determination of lens shape
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Organization of the Retina
Initial path of light
Primary visual neuron (projection)
Processing
Light detection
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Circuitry of the Retina
Ganglion Cells are primary visual neurons
. Input to ganglion cells is highly processed by
horizontal, bipolar, and amacrine cells.
Photoreceptors-detect light.
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Light decreases Na conductances
Light causes photoreceptor cell to hyperpolarize.
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Activation of Rod Photopigments
Photopigment opsin retinal sensitive to
light i.e. conformational change induced in
presence of photons.
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Transduction of light
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Transduction in Photoreceptors
Photopigment is (roughly) equivalent to a G
protein-coupled receptor.
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Types of Photoreceptors
Cones have small outer segment (less
photosensitive protein) -gt low sensitivity to
light levels
Rods have large outer segment (lots of
photosensitive protein) -gt high sensitivity to
light levels
Photopigments are contained in stacks in the
outer segment
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Responsiveness of Rods

• Rods account for most of vision under low light
  (scotopic) conditions. They are much more
  sensitive to light (activated with less photons)
  than cones.
• Broadly-tuned to light wavelength
• Only one type of rod (importance for color
  sensation/perception).

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Differential Responsiveness of Cones

• Cones account for human vision under bright light
  conditions.
• Low sensitivity (needs more photons to activate).
• Cones have different opsin proteins that are
  differentially activated by specific wavelengths
  of light allows color vision when info is
  maintained in separate parallel pathways.

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Regional Organization of Retina

• Distribution of photoreceptors
• center high cones low rods.
• periphery high rods low cones
• Level of convergence in circuitry
• center low convergence
• periphery high convergence

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Organization at the Fovea
Lateral displacement of non-photoreceptor cells
allows high resolution image formation (decreases
effect of light passing through other layers of
cells).
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Receptive fields
Entire retina receptive surface contains
100,000s cells.
Area of input for a single cell receptive
field i.e. physical point where photos come in
contact with the cell input to the receptive
portion of a photoreceptor cell
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Bipolar Cell Activation

• Bipolar cells have input from both photoreceptors
  and horizontal cells in a center-surround
  fashion.
• Direct photoreceptor to bipolar cell causes one
  response.
• Indirect photoreceptor to horizontal cell to
  bipolar cell causes opposite response.
• Bipolar cell response to photoreceptor
  transmitter can be depolarization or
  hyperpolarization
• -OFF-cells versus ON-cells
• .

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Ganglion Cell Output

• Ganglion cells primary visual neuron
• Innervated by Bipolar Cells and Amacrine cells
• Have Center-Surround of activation.
• Can be off (above) or on center but always
  have opposite center-surround.

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Ganglion Cell Output
OFF-center ganglion cell output
Retinal circuitry produces complex ganglion cell
responses with maximal firing produce by
light-dark borders which makes vision highly
contrast sensitive.
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Contrast Effects
Which is brighter? Center square on left or on
right. Contrast effects are created because
retinal output is sensitive to relative
stimulation.
Perception is not perfect reflection of reality.
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Types of Ganglion Cells
P Cells
M Cells
What are the benefits and pitfalls of large
receptive fields?
Trade-off between sensitivity and accuracy.
-small receptive field high spatial acuity but
only for small area low sensitivity - need high
level of stimulation over small area -large
receptive field low spatial acuity but for a
large area but high sensitivity need low level
of stimulation summed over large area
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Types of Ganglion Cells
M-type ganglion also undergo relative fast and
high level of habituation most sensitive to
dynamic changes in light levels.
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Color-opponent P Cell
Differential responses to color in P Ganglion
cells are due to connections with bipolar and
amacrine cells. (i.e. ganglion cells do not
detect light but chemical messengers). Note M
ganglion cells receive input from rods (not
sensitive to wavelength of light)
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Color Information Coding
Color information is coded by the relative
activation of three types of photopigments found
in cones.
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Illusions created by color-opponent processing
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Optic nerves and Chiasm
Formed by axons of ganglion cells.
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Non-thalamic targets of optic tract
Some axons from the optic nerve go out of main
pathway and relay into the hypothalamus
(important for sleep/wake cycle) and superior
colliculus (orientation reflex).
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Visual Pathways to the Cortex
Visual fields travel to contralateral cortex.
Note fields are not defined by the eyes
cortex receives info from ipsilateral and
contralateral eye.
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Topographical mapping in visual paths
Thalamic relay
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Lateral Geniculate Nucleus Visual relay in
thalamus
Color Vision
Again, we maintain important aspects of stimulus
quality by parallel pathways.
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Striate Cortex Primary visual cortex
Majority of LGN input goes to layer IV of cortex
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Cells respond to specific visual patterns.
Cell respond to a variety of line orientation but
maximal responding is induced only by a specific
orientation (i.e. pattern of input).
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A word about cortex layers in general

• The cortex consists of 6 layers of cells
• Each layer consists of different cell types
• Some layers receive information and process it
• Other layers send information following
  processing i.e. cortex contains multiple-layer
  circuits.
• In most cortex, the cells responding to a certain
  type of information are arranged in columns
  comprises a functional unit

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Column organization in Primary Visual Cortex.
Cells at same layers of cortex within different
columns respond to different types of input e.g.
maximal firing with line of different orientation.
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Column organization in Primary Visual Cortex.
Cells at different layers of cortex within a
column respond to same type of input e.g.
maximal firing with line of same
orientation. Column acts as a functional unit.
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Cells respond to specific visual patterns.
Cell responds to movement of a stimulus in either
direction but maximal responding is induced only
by motion in one direction.
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Higher cortical processing of vision

• Information is then passed on to visual
  association cortex?integration with other senses
  and generation of movement
• As you move up the visual hierarchy, the
  receptive fields become larger and the
  information processing is more complex and
  specialized e.g. Grandmother cells
• Information then passed on to subcortical
  structures?emotion, memory etc

Neurons in the occipital lobe form columns that
respond to basic shapes. e.g. line orientation
Neurons in the temporal lobe form columns that
respond to categories of shapes.
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Two streams of visual processing
Dorsal stream is specialized for motion and
believed to be involved in determining where
stimuli are for the control of behavior
Ventral stream is specialized for attributes and
believed to be involved in determing what
stimuli are for conscious perception.