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Chapter 6

PSYB65 Chapter 6 Detailed chapter notes


Department
Psychology
Course Code
PSYB65H3
Professor
Ted Petit
Chapter
6

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Chapter 6: Sensation and Perception: Vision
Module 6.1: Organization of Sensory Systems
Three main types of sensory cortex:
1. Primary Sensory Cortex – first part of the cortex that receives information relayed from the
senses through the thalamus
Of the visual cortical areas, has the most direct access to the information provided by
the sensory systems
2. Secondary Sensory Cortex
Receives much of its information from the primary sensory cortex
Is highly interconnected–receives information from other areas of the secondary sensory
cortex
Sends information to association cortex
3. Association Cortex
Any area of the cortex that receives information from more than one sense
Manner with which information is received and disbursed through the three cortical areas
suggests that sensory systems can be categorized into three organizing principles:
1. Sensory systems are characterized by hierarchical organization
2. Each level of the organization contains functionally distinct cortical areas
3. The processing of sensory information occurs in parallel throughout the cortex
Hierarchical Organization
- At the level of the eye, there are large number of specialized receptors that have one function
to transform light into signals that are meaningful in the nervous system
- At the level of the association cortex, job is more complex – to integrate information from a
number of sensory systems into a perception of the outside world
- As we move from the lowest to the highest level of sensory systems, the neurons changes
from having simple on/off sensory functions to those that respond optimally to stimuli of
greater complexity and specificity
Deficits that arise when the various levels are damaged are informative as to the
function of each of the levels
Segregation by Function
Within each of the three levels of the sensory cortex, there are specific areas that are involved in
processing specific aspects of the same sensory stimulus
Processing of Information is Parallel
- Information is transmitted throughout the sensory system in parallel – each level receives
some information from the level immediately below it and some information from levels
below that
- When information is processed in parallel through multiple pathways, information flows
through the levels of the sensory system rapidly and decreases the reliance of the system on
any one level of processing
Areas of sensory systems are very interconnected with each other (both within and
between levels)
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Module 6.2: The Visual System
Light: Stimulus for the Visual System
- Electromagnetic (EM) energy
Very short wavelengths – high-energy gamma rays, X-rays
Very long wavelengths – low-energy radio waves
Wavelengths of about 400-700nm = visible to human eye
- All EM energy is “light” but light is usually referred to the relatively narrow band of EM
energy that humans can sense with their eyes
- Two potential sources of light:
Light coming directly from something that is producing it – ie. Sun, lightbulb
Light that has travelled from a source to an object and is then reflected off the object or
multiple objects
- Different wavelengths of light are perceived as different colours
The Eye and Retina
- Light enters the eye through the pupil and is focused on the retina by the curvature of the
cornea and fine-tuned by the lens
Retina – a layered structure at the back of the eye that contains 5 different types of cells
(receptors, horizontal cells, bipolar cells, amacrine cells, and retinal ganglion cells),
each of which has different functions
-Receptors: converts light energy into neural responses, which are then transmitted to the
brain via the retinal ganglion cells
Receptors in retina (rods and cones) are specialized and are active under different types
of lighting conditions
Rods – typically active in low light levels; very sensitive to movement
Has low acuity; has multiple rods synapse on one bipolar cell, which in turn
converges on the same retinal ganglion cell with other bipolar cells
Cones – typically active in medium to bright light; responsible for high-acuity vision, or
vision that provides rich details and colors
Synapses 1:1 on bipolar cells, which then synapse individually on retinal ganglion
cells – quite sensitive to the activity of that cone
-Amacrine and Horizontal cells – lateral communication between the various cells (eg.
Communication between cones and other cones or retinal ganglion cells and other retinal
ganglion cells)
-Bipolar cells – synapse on the receptors, which in turn synapse on the retinal ganglion cells,
whose axons leave the retina via the optic nerve
- Light tends to travel in a straight line
- Sides of the Retina:
Nasal – closest to the nose
Temporal – closest to temples
- The retinal ganglion cells in the nasal hemiretina project contralaterally to the lateral
geniculate nucleus of the thalamus (LGN) and the retinal ganglion cells in the temporal
hemiretina project ipsilaterally to the LGN
Ensures that information about the right side of space from either eye stays together in
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the left LGN and that information about the left side of space from either eye stays
together in the right LGN
Left and right visual fields are segregated into different hemispheres in the brain
Retino-Geniculate-Striate System
- Beings in the eye and ends in the primary visual cortex
- Although the visual system conforms to the three principles of sensory organization, visual
information is perceived as an integrated whole
The Lateral Geniculate Nucleus of the Thalamus
- A 6-layered nucleus of the thalamus that receives information from the retina
Each layer receives different sensory information
Top 4 layers – Parvocellular / P layers – composed of neurons that have small cell
bodies
Bottom two layers – Magnocellular / M layers – composed of neurons that have large
cell bodies
-P pathway
Composed of parvocellular neurons in the LGN and the axons of the retinal ganglion
cells that synapse on them
Retinal ganglion axons belong to neurons that synapse on cones
Responsive to color, detail, and stationary or slowly moving objects
-M pathway
Composed of magnocellular neurons in LGN and axons of the retinal ganglion cells
that synapse on them
Retinal ganglion axons belong to neurons that synapse on rods
Responds to movement and orientation but does not respond to color and detail
- Information about the details and color of a moving object in the right visual field will be
sent through the P pathway to the top four layers of the left LGN, and the details about the
movement of the same object will be sent through the M pathway to the bottom two layers of
the left LGN
Striate Cortex
- Aka. Visual cortex – under microscope, has a striped appearance
- Information is transmitted ipsilaterally from the LGN to the primary visual cortex (V1)
Left LGN sends axons (via the optic radiations) to left V1
Right LGN sends axons to right V1
- Axons of LGN project and synapse on layer 4 of the cortex
Layer 4 send projections to the other 5 layers of V1, mostly with the layers either
immediately above or below them
Projections are forwarded to V2, V3, V4 complex, then middle temporal region (MT)
MT sends its projections dorsally to the parietal lobe
V4 sends projections ventrally to the temporal lobe
-V1
Receives projections from both the M and P pathways – is the gateway to the higher
cortical areas
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