UofR: Neural Basis of Cognition
Lecture 1
Neuroanatomy and Brain Function
• What is the link?
• Theory of Mass Action
– posited that each portion of the brain contributed
to all mental abilities
– initially supported by the fact that increasing
amounts of brain damage leads to increasing loss
of mental ability
– discredited only after more advanced tools and
methods lead to the ability to demonstrate
localization of function
Neuroanatomy and Brain Function
• Localization of Function
– Particular mental functions can be localized to specific
regions of brain tissue
– Localization of function must be supplemented by
communication between different parts of the brain in
order to be able to produce the sum total of mental
ability that is observed
– Damage to one area of the brain may disrupt
communication between other parts of the brain and
cause deficits not all of which are in functions
localized entirely to the site of damage.
Lesion Method
• Artificially induced lesions are extensively
used in animal experiments and have led to a
wealth of understanding.
• Such experiments cannot be performed on
humans.
• Major advances in understanding of the
human brain have been come from the study
of people who sustained brain damage in
some accident.
Lesion Method
• One may emphasize either knowledge about
neural substrates…
– Experimental group: a group of individuals whose
brain damages are as similar as possible in
location, extent, and, in some cases, cause
• … or knowledge about cognitive function.
– Experimental group: a group of individuals whose
behavioral symptoms are as similar as possible.
Double Dissociation
• The sets of cognitive deficits produced by
lesions in different areas are disjoint.
• Classic example:
– Broca’s area
• Lesion impairs speech production
• Speech comprehension is unimpaired
– Wernicke’s area
• Lesion impairs speech comprehension
• Speech production is unimpaired
Lesion Method
• Difficulties:
– The exact location and extent of a lesion are unlikely
to be consistent across a group of human subjects.
– the function of the lesioned area is not necessarily
known
– cognitive impairment may occur because a lesion
disrupts communication between two still functional
areas of the brain (disconnection syndrome)
– an impairment in cognitive function may be “silent" if
another part of the brain can perform the same tasks
as the one lesioned
Single, Group, Multiple-Case Studies
• Single-case studies:
– In-depth study of one patient
– Gives only one data point
• Group studies:
– Individual variability can be hidden by averaging
performance on a task
• Multiple case studies:
– Series of individuals with similar traumas are each
individually extensively studied and then additionally
analyzed as a group
Experimental Considerations
• Any experimental group must be compared to a
control group.
• The choice of a proper control group may be
difficult as each subject in an experimental group
must be matched with an individual of the
control group with a similar background.
• Consider: performance on a reasoning task is
measured in a neurologically intact high school
dropout as compared to a college-educated brain
damaged individual
Imaging Techniques
• Anatomical imaging:
– Computerized Axial Tomography (CAT)
– Magnetic Resonance Imaging (MRI)
• Functional imaging:
– Positron Emission Tomography (PET)
– Functional MRI (fMRI)
Computerized Axial Tomography (CAT)
• Technique:
– Uses x-rays to differentiate between structures of
different densities
– Cerebrospinal fluid (CSF) is less dense than brain
tissue, which is less dense than blood, which is
less dense than bone
– Denser matter appears lighter, less dense matter
appears darker
Computerized Axial Tomography (CAT)
• Advantages:
– Inexpensive
– Widespread
– Can be performed on all individuals
• Disadvantages:
– Relatively poor spatial resolution
Magnetic Resonance Imaging (MRI)
• Static field: aligns all particles’ magnetic moments
in the same direction
• Pulse sequence: frequency chosen so as to
resonate with certain particles, perturbing their
alignment (often adjusted for hydrogen atoms)
– A perturbed hydrogen atom will release a photon as it
falls back to an aligned state after an amount of time
(relaxation time) characteristic of the compound in
which it is located
– The relaxation time can be used to infer the location
of the type of compound to be localized
Magnetic Resonance Imaging (MRI)
• Advantages:
– Higher spatial resolution than CAT
– Avoids potentially harmful x-rays
• Disadvantages:
– Not every individual can be subjected to MRI
(pacemakers, metal in the body)
Positron Emission Tomography (PET)
• Blood flow is higher in areas of higher neural
activity
• Radioactively labeled compounds are
introduced into the bloodstream and thus the
brain
• Isotope decays by positron emission; positron
annihilates with electron, emitting two photos
in opposite directions
Positron Emission Tomography (PET)
• Advantages:
– Can localize specific substances in the brain
• H2O15: accumulates in brain in direct proportion to local
blood flow, marking neural activity
• Localizing dopamine receptors
• Disadvantages:
– Ionizing radiation – max 2-5 scans per year
– Low temporal resolution due to isotope half-life
• Advantages can much outweigh the
disadvantages for some purposes
Functional MRI (fMRI)
• Increased brain function is associated with increased
metabolism at the site of neural activity.
• Increased metabolism involves the deoxygenation of
blood; as blood contains iron, deoxygenation alters the
sensitivity of blood to a magnetic field.
• fMRI is an adaptation of MRI that is attuned to
measure the relative concentrations of oxygenated
versus deoxygenated blood on the scale of seconds and
is used to accurately determine sites of increased brain
function on a time scale that is shorter than that
allowed by PET.
Functional MRI (fMRI)
• Advantages:
– High spatial and temporal resolution
– No use of ionizing radiation
– Allows tracking of changes in a patient’s brain
function over time due to no usage limits
• Disadvantages:
– Same as MRI