Transcript Document

Chapter 4: The Organized
Brain
Overview of Questions
• How can brain damage affect a person’s
perception?
• Are there separate brain areas that determine
our perception of different qualities?
• How has the operation of our visual system
been shaped by evolution and by our day-today experiences?
Lateral Geniculate Nucleus of the Thalamus
Maps: Representing Spatial Layout
• Retinotopic map - each place on the retina
corresponds to a place on the LGN
• Determining retinotopic maps - record from
neurons with an electrode that penetrates the
LGN obliquely
– LGN has 6 layers
– Stimulating receptive fields on the retina
shows the location of the corresponding
neuron in the LGN
Lateral Geniculate Nucleus
Retinotopic mapping of neurons in the LGN.
The Map on the Cortex
• Cortex shows retinotopic map too
– Electrodes recording from a cat’s visual
cortex shows:
• Receptive fields on the retina that
overlap also overlap in the cortex
• This pattern is seen using an oblique
penetration of the cortex
Retinotopic mapping of neurons in the cortex.
The Map on the Cortex - continued
• 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
• How do we know this stuff?
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
The subtraction technique
Brain Imaging Techniques - continued
• Functional magnetic resonance imaging (fMRI)
measures blood flow by:
– 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
Purple and teal areas show
the extent of stimuli that
were presented while a
person was in an fMRI
scanner. (b) Purple and
teal indicates areas of the
brain activated by the
stimulation in (a).
Organization in Columns
• LGN receives signals for right and left eyes
– Layers 2, 3, and 5 receive input from the
ipsilateral eye
– Layers 1, 4, and 6 receive input from the
contralateral eye
• Electrodes inserted perpendicular to the
surface show that receptive fields along the
track are in the same location in the retina
Cross section of the LGN showing layers.
Organization in Columns - continued
• 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 4.9 When an electrode penetrates the cortex perpendicularly, the receptive fields of the neurons
encountered along this track overlap. The receptive field recorded at each numbered position along the
electrode track is indicated by a correspondingly numbered square.
All of the cortical neurons encountered along track
A respond best to horizontal bars (indicated by the
red lines cutting across the electrode track.) All of
the neurons along track B respond best to bars
oriented at 45 degrees.
Organization in Columns - continued
• Visual cortex shows (cont.)
– Ocular dominance columns
• Neurons in the cortex respond
preferentially to one eye
• Neurons with the same preference are
organized into columns
• The columns alternate in a left-right
pattern every .25 to .50 mm across the
cortex
Figure 4.11 (a) How a peppermint stick creates an image on the retina and a pattern of activation on the
cortex. (b) How a long peppermint stick would activate a number of different orientation columns in the
cortex.
Lesioning or Ablation Experiments
1. An animal is trained to indicate perceptual
capacities
2. A specific part of the brain is removed or
destroyed
3. The animal is retrained to determine which
perceptual abilities remain
•
The results reveal which portions of the
brain are responsible for specific behaviors
What and Where Pathways
• Ungerleider and Mishkin (1983)
– Object discrimination
problem
• Monkey is shown an
object
• Then presented with two
choice task
• Reward given for
detecting the target
– Landmark discrimination
problem
• Monkey is trained to pick
the food well next to a
cylinder
What and Where Pathways - continued
• Ungerleider and Mishkin (cont.)
– 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 landmark discrimination task - What pathway
(I see it, but don’t know where it is)
• Removal of parietal lobe tissue resulted in problems
with the object discrimination task - Where pathway
(I know where it is, but I don’t know what it is)
The monkey cortex, showing the what, or ventral
pathway from the occipital lobe to the temporal
lobe, and the where, or dorsal pathway from the
occipital lobe to the parietal lobe.
What and Where Pathways - continued
• What pathway also called doral pathway
• Where pathway also called ventral pathway
• Both pathways originate in retina
– Ventral pathway begins in small or medium
ganglion cells
• Called P-cells
• Axons synapse in layers 3, 4, 5, & 6 of
LGN
• Called parvocellular layers
What and Where Pathways - continued
– Dorsal pathway begins in large ganglion
cells
• Called M-cells
• Axons synapse in layers 1 & 2 of LGN
• Called magnocellular layers
• Ablation research with monkeys shows:
– Parvo channels send color, texture, shape
and depth information
– Magno channels send motion information
The dorsal and ventral streams in the cortex originate with the magno
and parvo ganglion cells and the magno and parvo layers of the LGN.
The red arrow represents connections between the streams. The
dashed blue arrows represent feedback - signals that flow “backward.”
Retinal ganglion cells and their functions.
What and Where Pathways - continued
• 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 Double dissociations in TV sets and people.
What and How Pathways Neuropsycholgical Evidence
• 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 Performance of D.F. and a person without brain damage for two tasks: (a) judging the
orientation of a slot; and (b) placing a card through the slot. See text for details. (From the Visual Brain in
Action by A. D. Milner and M. A. Goodale. Copyright ©1995 by Oxford University Press. Reprinted by
permission.)
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 4.17 (a) Rod and frame illusion. Both small lines are oriented vertically. (b) Matching task and
results. (c) Grasping task and results. See text for details.
Modularity: Structures for Faces, Places,
and Bodies
• Module - a brain structure that processes
information about specific stimuli
– Inferotemporal (IT) cortex in monkeys
• One part responds best to faces while
another responds best to heads
• Results have led to proposal that IT
cortex is a form perception module
– Temporal lobe damage in humans results
in prosopagnosia
Figure 4.18 (a) Monkey brain showing location of the inferotemporal cortex (IT) in the lower part of the
temporal lobe. (b) Human brain showing location of the fusiform face area (FFA) in the fusiform gyrus,
which is located under the temporal lobe.
Figure 4.20 Response of a neuron in the IT cortex for which the person’s head is an important part of the
stimulus because firing stops when the head is covered. (From “Recognition of Objects and Their
Components Parts: Responses of Single Units in the Temporal Cortex of the Macaque,” by E. Washmuth,
M. W. Oram, and D. I. Perrett, 1994, Cerebral Cortex, 4, Copyright © 1994 by Oxford University Press.)
Modularity: Structures for Faces, Places,
and Bodies - continued
• Evidence from humans using fMRI and the
subtraction technique show:
– Fusiform face area (FFA) responds best to
faces as well as when context implies a
face
– 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 fMRI response of the human fusiform face area. Activation occurs when a face is present (E) or
is implied (D) but is lower when other stimuli are presented (A,B,C,F). (Reprinted with permission from Cox,
D., Meyers, E., Sinha, P. (2004). Contextually evoked object-specific responses in human visual cortex,
Science, 304, 115-117.
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
Evolution and Plasticity: Neural
Specialization - continued
• Plasticity of neurons also shapes sensory
responses
– Experience-dependent plasticity in animals
• Monkeys trained to recognize specific
view of unfamiliar object
• Other views of object showed decline in
recognition as object rotated from
trained view
• Neurons in the IT cortex showed
maximal response to the trained
orientation
Figure 4.23 (a) Stimuli like those used by Logothetis & Pauls (1995). (b) Monkey’s ability to recognize the
training shape and rotated views of the shape that were not seen during training. (c) Response of neurons
in the IT cortex of the trained monkey to the training shape and the rotated shape.
Evolution and Plasticity: Neural
Specialization - continued
– 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 4.24 (a) Greeble stimuli used by Gauthier. Participants were trained to name each different Greeble.
(b) Brain responses to Greebles and faces before and after Greeble training. (a: From Figure 1a, p. 569,
from Gauthier, I., Tarr, M. J., Anderson, A. W., Skudlarski, P. L., & Gore, J. C. (1999). Activation of the
middle fusiform “face area” increases with experience in recognizing novel objects. Nature Neuroscience, 2,
568-573.)
Figure 4.25 Ways that the brain is organized.
Sensory Code: Representation of
Environment
• Sensory code - representation of perceived
objects through neural firing
– Specificity coding - specific neurons
responding to specific stimuli
• Leads to the “grandmother cell”
hypothesis
• Recent research shows cells in the
hippocampus that respond to concepts
such as Halle Berry
Figure 4.29 (a) Location of the hippocampus and some of the other structures that were studied by Quiroga
and coworkers (2005)
Sensory Code: Representation of
Environment - continued
– Problems with specificity coding:
• Too many different stimuli to assign
specific neurons
• Most neurons respond to a number of
different stimuli
• Distributed coding - pattern of firing across
many neurons codes specific objects
– Large number of stimuli can be coded by a
few neurons
Sensory Code: Representation of
Environment - continued
– Coding can be distributed across many
brain areas
• Monkeys’ IT cortex shows overlap of
activation caused by different stimuli
• fMRI experiments with humans show the
same type of effect
• Thus, although there is specific
response within modules, there is also
activation across modules for specific
stimuli
Figure 4.26 How faces could be coded according to (a) specificity coding and (b) distributed coding. The
height of the bars indicates the response of neurons 1, 2, and 3 to each stimulus face. See text for
explanation.