Transcript Chapter 4

Chapter 4:
Cortical Organization
An Exploration of Spatial Organization
• Electronic map on V1
– Retinotopic map is an electron map of the
retina on the cortex
– Cortical magnification – a small area of the
fovea is represented by a large area on the
visual cortex
Figure 4-1 p78
Figure 4-2 p78
Brain Imaging Techniques
• Positron emission tomography (PET)
– Person is injected with a harmless
radioactive tracer
– Tracer moves through bloodstream
– Monitoring the radioactivity measures
blood flow
– Changes in blood flow show changes in
brain activity
Brain Imaging Techniques - continued
• PET - subtraction method
– Brain activity is determined by:
• Measuring activity in a control state
• Measuring activity in a stimulation state
• Subtracting the control activity from the
stimulation activity
Brain Imaging Techniques - continued
• 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 4-3 p79
Figure 4-4 p79
The Cortex is Organized in Columns
• Cortical magnification factor
– Fovea has more cortical space than
expected
Figure 4-5 p80
The Cortex is Organized in Columns
• Visual cortex shows:
– Location columns
• Receptive fields at the same location on
the retina are within a column
– Orientation columns
• Neurons within columns fire maximally
to the same orientation of stimuli
• Adjacent columns change preference in
an orderly fashion
• 1 millimeter across the cortex represents
entire range of orientation
Figure 4-6 p80
Figure 4-7 p81
Figure 4-8 p81
Figure 4-9 p81
The Cortex is Organized in Columns continued
• Visual cortex shows
– Ocular dominance columns
• Neurons in the cortex respond
preferentially to one eye.
How Do Feature Detectors
Respond to a Scene?
• Tiling – columns working together to cover the
entire visual field.
Figure 4-10 p82
Figure 4-11 p82
Figure 4-12 p82
Streams for Information About What
and Where
• Lesioning or Ablation Experiments
– First, an animal is trained to indicate
perceptual capacities.
– Second, a specific part of the brain is
removed or destroyed.
– Third, the animal is retrained to determine
which perceptual abilities remain.
– The results reveal which portions of the
brain are responsible for specific
behaviors.
Streams for Information About What
and Where - continued
• Ungerleider and Mishkin experiment
– Object discrimination problem
• Monkey is shown an object
• Then presented with two choice task
• Reward given for detecting the target
object
– Landmark discrimination problem
• Monkey is trained to pick the food well
next to a cylinder
Streams for Information About What
and Where - continued
• Ungerleider and Mishkin - Using ablation, part
of the parietal lobe was removed from half the
monkeys and part of the temporal lobe was
removed from the other half.
– Retesting the monkeys showed that:
• Removal of temporal lobe tissue
resulted in problems with the object
discrimination task - Where pathway
• Removal of parietal lobe tissue resulted
in problems with the landmark
discrimination task - What pathway
Figure 4-13 p83
Figure 4-14 p84
Streams for Information About What
and Where - continued
• What pathway also called ventral pathway
• Where pathway also called dorsal pathway
• Both pathways:
– originate in retina and continue through two
types of ganglion cells in the LGN.
– have some interconnections.
– receive feedback from higher brain areas.
Streams for Information About What
and How
• Where pathway may actually be “How”
pathway or action pathway
– Dorsal stream shows function for both
location and for action.
– Evidence from neuropsychology
• Double dissociations: two functions
involve different mechanisms and
operate independently
Table 4-1 p85
Streams for Information About What
and How - continued
• Behavior of patient D.F.
– Damage to ventral pathway due to gas
leak
– Not able to match orientation of card with
slot
– But was able to match orientation if she
was placing card in a slot
– Other patients show opposite effects
– Evidence shows double dissociation
between ventral and dorsal pathways
Figure 4-16 p85
Behavior of People Without Brain Damage
• Ganel experiment was designed to
demonstrate a separation of perception and
action in non-brain-damage subjects.
Figure 4-17 p86
Modularity: Structures for Faces, Places,
and Bodies
• Module - a brain structure that processes
information about specific stimuli
– Rolls measured the response neurons in
the 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 4-18 p87
Figure 4-19 p87
Figure 4-20 p88
Areas for Faces, Places, and Bodies in the
Human Brain
• Evidence from humans using fMRI and the
subtraction technique show:
– Fusiform face area (FFA) responds best to
faces.
– Parahippocampal place area (PPA)
responds best to spatial layout.
– Extrastriate body area (EBA) responds
best to pictures of full bodies and body
parts.
Figure 4-21 p88
Figure 4-22 p89
Where Vision Meets Memory
• MTL structures are extremely important in
memory
– H.M.
– hippocampus
Figure 4-23 p90
Figure 4-24 p90
Experience and Neural Responding
– Experience-dependent plasticity in humans
• Brain imaging experiments show areas
that respond best to letters and words.
• fMRI experiments show that training
results in areas of the FFA responding
best to:
–Greeble stimuli
–Cars and birds for experts in these
areas
Figure 4-25 p91