Transcript Lecture 9 Powerpoint file

Exam 1 week from today
• in class
• assortment of question types including written
answers
Read this article!
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Final Project
• Research grant proposal
• start thinking about a hypothesis and research
question
• start thinking about the techniques you would use to
answer the question
How does the visual system represent
visual information?
• Brainstorm this: what are the different ways the visual
system might encode a feature?
How does the visual system represent
visual information?
• Brainstorm this: what are the different ways the visual
system might encode a feature?
How does the visual system represent
visual information?
• Brainstorm this: what are the different ways the visual
system might encode a feature?
– “labeled lines”
• many different subnetworks of neurons - activity in a network
indicates presence/nature of a feature
– spike timing
• absolute rate or # of spikes per second might indicate
presence/nature of a feature
• “multiplexed”
– Hybrid of these two
Visual Pathways
• Image is focused on the
retina
• Fovea is the centre of visual
field
– highest acuity
• Peripheral retina receives
periphery of visual field
– lower acuity
– sensitive under low light
Visual Pathways
• Retina has distinct layers
Visual Pathways
• Retina has distinct layers
• Photoreceptors
– Rods and cones respond to
different wavelengths
Visual Pathways
• Retina has distinct layers
• Amacrine and bipolar cells
perform “early” processing
– converging / diverging input
from receptors
– lateral inhibition leads to
centre/surround receptive
fields - first step in shaping
“tuning properties” of higherlevel neurons
Visual Pathways
• Retina has distinct layers
– signals converge onto
ganglion cells which send
action potentials to the
Lateral Geniculate Nucleus
(LGN)
– two kinds of ganglion cells:
Magnocellular and
Parvocellular
• visual information is
already being shunted
through functionally distinct
pathways as it is sent by
ganglion cells
Visual Pathways
• visual hemifields project
contralaterally
– exception: bilaterally
representation of fovea!
• Optic nerve splits at optic
chiasm
• about 90 % of fibers project
to cortex via LGN
• about 10 % project through
supperior colliculus and
pulvinar
– but that’s still a lot of fibers!
Note: this will be important
when we talk about
visuospatial attention
Visual Pathways
• Lateral Geniculate Nucleus
maintains segregation:
– of M and P cells
– of left and right eyes
P cells project to layers 3 - 6
M cells project to layers 1 and 2
Visual Pathways
• Primary visual cortex
receives input from LGN
– also known as “striate”
because it appears striped
on some micrographs
– also known as V1
– also known as Brodmann
Area 17
Visual Pathways
• Primary cortex maintains
distinct pathways
• M and P pathways synapse
in different layers
W. W. Norton
How does the visual system represent
visual information?
How does the visual system represent features of
scenes?
• Vision is analytical - the system breaks down the
scene into distinct kinds of features and represents
them in functionally segregated pathways
• but…
• the spike timing matters too!
Visual Neuron Responses
• Unit recordings in LGN
reveal a centre/surround
receptive field
• many arrangements exist,
but the typical RF has an
excitatory centre and an
inhibitory surround
• these receptive fields tend to
be circular - they are not
orientation specific
How could the outputs of such cells be transformed into a cell with
orientation specificity?
Visual Neuron Responses
• LGN cells converge on “simple” cells in V1 imparting orientation
specificity
Visual Neuron Responses
• V1 maintains a map of
orientations across the retina
because each small area on
the retina has a
corresponding cortical
module that contains cells
with the entire range of
orientation tunings