Download Mind, Brain & Behavior

Mind, Brain & Behavior
Friday
February 7, 2003
From Nerve Cells to
Cognition (Cont.)
Chapter 18
Mapping the Sensory Cortex

Single cell measurements show specific
responses when part of the body is touched.



Penfield mapped sensory cortex.
Different sizes of representation correspond to
amount of innervation in that body region.
Different species of animals rely on different
parts of the body for information and thus
have different sensory maps.
Maps Can Be Modified


Maps depend on experience (use).
Inputs to the sensory cortex are formed based
on Hebbian correlated firing:


Cells that fire together, wire together.
Syndactyly (webbed fingers) – fingers are not
represented independently.

When surgery separates the fingers, they become
independently represented in cortex.
Phantom Limb Syndrome



Patients with amputated limbs continue to
sense the missing limb.
Originally thought to be caused by signals
coming from the spinal cord from scar tissue.
Now thought to originate from representation
areas as they are remapped (other functions
expand into the area for the lost limb).
Receptive Fields

Mountcastle – identified receptive fields of
sensory neurons in skin.


Receptive fields overlap, so each area of skin is
monitored by multiple neurons.
Size of the receptive field varies in different
parts of the body.
Organized in Columns

Sensory cortex contains separate columns for
each modality (touch, pressure, temperature,
pain).


Within each column, all neurons respond to the
same type of sensory receptor.
Different types of sensory receptors are dominant
in different areas of the sensory cortex.
Integration of Modalities

Integration is accomplished through layered
processing:




The submodalities converge on common cells.
Response properties of neurons at higher levels
become more complex.
The size of the receptive field increases at each
level of processing.
Complex stimulus properties emerge from
elementary properties.
Parietal Association Areas


Areas 1 & 2 merge inputs from areas 3a, 3b.
Posterior parietal cortex (areas 5 & 7)
integrate sensory information with visual and
auditory information.



Lesions affect spatial perception, visuomotor
integration, directed attention.
Astereognosia – inability to recognize objects by
touch.
Neglect syndrome – ignore half of body.
Attention

Both posterior parietal cortex and frontal
cortex are active when attention is shifted
from one object to another.



Posterior when sensory input shifts attention.
Frontal when a motor response is made.
Extinction – inability to shift attention to an
object on opposite side from lesion.
Cognition and the Cortex
Chapter 19
Association Areas



Once thought to be “silent” areas.
Modern evidence suggests that association
areas are higher order processing centers for
sensory or motor information.
Two sources of information:


Those with brain damage due to accident or
disease.
Those with surgery – well-defined lesions.
Three Association Areas




See Figure 19-2
Prefrontal (1) – weigh consequences of future
actions and plan motor responses.
Limbic (2) – allows emotions to affect motor
planning.
Parietal-temporal-occipital (3) – processes
sensory information for perception and
language.
Hemispheric Specialization



The left and right hemispheres have different
cognitive capabilities.
The corpus callosum and anterior commissure
permit hemispheres to coordinate activity.
Epileptic patients may have a commisurotomy
in which pathways between hemispheres are
severed.

Called “split-brain” patients.
Split-Brain Functioning

Normally, split-brain patients show little
impairment.




Hemispheres function independently.
Information must be presented to just the left
or right hemisphere, to see differences.
Left hemisphere – aware and verbal.
Right hemisphere – automatic.
Normal Brains

Both hemispheres work together in the normal
brain – it makes little sense to talk about
“right brain” or “left brain.”


Interaction of hemispheres needed to identify
objects by touch.
Wada test – used to demonstrate which
hemisphere speech is lateralized to.

Patient unable to continue counting with sodium
amytal.
Lateralization of Language



Nearly all right-handed people (96%) have
language in the left hemisphere.
Most left-handed people also have language
in left hemisphere, 15% have language in the
right hemisphere, a small percentage have it
in both.
The hemispheres are anatomically
asymmetrical in the areas associated with
language.