Transcript Visual Brain

Chapter 4:
The Visual Cortex and Beyond
Overview of Questions
• How can brain damage affect a person’s
perception?
• Are there separate brain areas that determine
our perception of different qualities?
Figure 4.1 (a) Side view of the visual system, showing the three major
sites: the eye, the lateral geniculate nucleus, and the visual cortex.
(b) Visual system showing how some of the nerve fibers from the
retina cross over to the opposite side of the brain at the optic chiasm.
Pathway from Retina to Cortex
• Signals from the retina travel through the
optic nerve to the
– Lateral geniculate nucleus (LGN)
– Primary visual receiving area in the
occipital lobe (the striate cortex)
– And then through two pathways to the
temporal lobe and the parietal lobe
Visual Areas
Areas V1 – V5
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Visual
Cortex
How do we study visual cortex?
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Hubel and Wiesel
Single cell recording in visual cortex.
Implant one cell in visual cortex.
Shine light on retina.
See is we can get that cell to respond.
What does the single cell like to “see”.
Figure 2.17 Recording electrical signals from a fiber in the optic nerve of an anesthetized cat. Each point
on the screen corresponds to a point on the cat’s retina.
Receptive Fields
• Area of receptors that affects firing rate of a
given neuron in the circuit
• Receptive fields are determined by
monitoring single cell responses.
• Research example for vision
– Stimulus is presented to retina and
response of cell is measured by an
electrode.
Neural Activity
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Overlap in Receptive Fields
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The Map on the Striate Cortex
• Cortex shows retinotopic map.
– Electrodes that recorded activation from a
cat’s visual cortex show:
• Receptive fields on the retina that
overlap also overlap in the cortex.
Neurons in Striate Cortex
• Simple cortical cells
– Side-by-side receptive fields
– Respond to spots of light
– Respond best to bar of light oriented along
the length of the receptive field
• Orientation tuning curves
– Shows response of simple cortical cell for
orientations of stimuli
Figure 4.6 (a) The receptive field of a simple cortical cell.
(b) This cell responds best to a vertical bar of light that covers the
excitatory area of the receptive field. The response decreases as the
bar is tilted so that it also covers the inhibitory area.
Orientation tuning curve of a simple cortical cell for a neuron that
responds best to a vertical bar (orientation = 0). (From Hubel &
Wiesel, 1959.)
Neurons in Striate Cortex - continued
• Complex cells
– Like simple cells
• Respond to bars of light of a particular
orientation
– Unlike simple cells
• Respond to movement of bars of light in
specific direction
Figure 4.8 (a) Response of a complex cell recorded from the visual
cortex of a cat. The stimulus bar is moved back and forth across the
receptive field. The cell fires best when the bar is positioned with a
specific orientation and is moved in a specific direction
Response of an end-stopped cell recorded from the visual cortex of
the cat. The stimulus is indicated by the light area on the left. This cell
responds best to a medium-sized corner that is moving up (*).
Neurons in Striate Cortex – Edge detector
• End-stopped cells
– Respond to:
• Moving lines of specific length
• Moving corners or angles
– No response to:
• Stimuli that are too long
Feature Detectors
• Neurons that fire to specific features of a
stimulus
• Pathway away from retina shows neurons
that fire to more complex stimuli
• Cells that are feature detectors:
– Simple cortical cell
– Complex cortical cell
– End-stopped cortical cell
Table 4.1 Properties of cortical neurons
Vision
Visualized
With
FMRI
Fovea
Periphery
Figure 4.17 (a) Red and blue areas show the extent of stimuli that were presented while a person was in an
fMRI scanner. (b) Red and blue indicates areas of the brain activated by the stimulation in (a). (From
Dougherty et al., 2003.)
Brain Imaging Techniques - fMRI
• Functional magnetic resonance imaging (fMRI)
– Hemoglobin carries oxygen and contains a ferrous
molecule that is magnetic
– Brain activity takes up oxygen, which makes the
hemoglobin more magnetic
– fMRI determines activity of areas of the brain by
detecting changes in magnetic response of
hemoglobin
• Subtraction technique is used like in PET
Figure 4.14 The magnification factor in the visual system: The small area of the fovea is represented by a
large area on the visual cortex.
Maps and Columns in the Striate Cortex
• Cortical magnification factor
– Fovea has more cortical space than
expected
• Fovea accounts for .01% of retina
• Signals from fovea account for 8% to
10% of the visual cortex
• This provides extra processing for highacuity tasks.
Figure 4.24 How a tree creates an image on the retina and a pattern
of activation on the cortex.
Other Cortical Areas
• Vision begins to processed by V1-V5
• Then goes to other lobes of the brain for
further processing.
• What we have seen. Object identification.
• Where we have it. Locating object in world.
Figure 4.27 The monkey cortex, showing the what and the where
pathways. The where pathway is also called the how pathway. (From
Mishkin, Ungerleider, & Macko, 1983.)
What and Where (How) Pathways
• Where pathway may actually be “How”
pathway
– Dorsal stream shows function for both
location and for action.
– Evidence from neuropsychology
• Single dissociations: two functions
involve different mechanisms
• Double dissociations: two functions
involve different mechanisms and
operate independently
Table 4.2 A double dissociation
What and How Pathways - Further Evidence
• Rod and frame illusion
– Observers perform two tasks: matching
and grasping
• Matching task involves ventral (what)
pathway
• Grasping task involves dorsal (how)
pathway
– Results show that the frame orientation
affects the matching task but not the
grasping task.
Figure 4.30 (a) Rod
and frame illusion.
Both small lines are
oriented vertically.
(b) Matching task
and results. (c)
Grasping task and
results.
Modularity: Structures for Faces, Places,
and Bodies
• Module - a brain structure that processes
information about specific stimuli
– Inferotemporal (IT) cortex in monkeys
• Responds best to faces with little
response to non-face stimuli
– Temporal lobe damage in humans results
in prosopagnosia.
Figure 4.32 (a) Monkey brain showing location of the
inferotemporal (IT) cortex. (b) Human brain showing
location of the fusiform face area (FFA), which is located
under the temporal lobe.
Figure 4.33 Size of response of a neuron in the monkey’s IT cortex that responds to face stimuli but not to
nonface stimuli. (Based on data from Rolls & Tovee, 1995.)
Monkey
Face
Cells
Evolution and Plasticity: Neural
Specialization
• Evolution is partially responsible for shaping
sensory responses:
– Newborn monkeys respond to direction of
movement and depth of objects
– Babies prefer looking at pictures of
assembled parts of faces
– Thus “hardwiring” of neurons plays a part
in sensory systems
Margaret Thatcher Illusion