2 Neuroanatomy and Brain Function What is the link?Theory of Mass Actionposited that each portion of the brain contributed to all mental abilitiesinitially supported by the fact that increasing amounts of brain damage leads to increasing loss of mental abilitydiscredited only after more advanced tools and methods lead to the ability to demonstrate localization of function
3 Neuroanatomy and Brain Function Localization of FunctionParticular mental functions can be localized to specific regions of brain tissueLocalization 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 observedDamage 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.
4 Lesion MethodArtificially 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.
5 Lesion MethodOne 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.
6 Double DissociationThe sets of cognitive deficits produced by lesions in different areas are disjoint.Classic example:Broca’s areaLesion impairs speech productionSpeech comprehension is unimpairedWernicke’s areaLesion impairs speech comprehensionSpeech production is unimpaired
7 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 knowncognitive 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
8 Single, Group, Multiple-Case Studies Single-case studies:In-depth study of one patientGives only one data pointGroup studies:Individual variability can be hidden by averaging performance on a taskMultiple case studies:Series of individuals with similar traumas are each individually extensively studied and then additionally analyzed as a group
9 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
11 Computerized Axial Tomography (CAT) Technique:Uses x-rays to differentiate between structures of different densitiesCerebrospinal fluid (CSF) is less dense than brain tissue, which is less dense than blood, which is less dense than boneDenser matter appears lighter, less dense matter appears darker
13 Computerized Axial Tomography (CAT) Advantages:InexpensiveWidespreadCan be performed on all individualsDisadvantages:Relatively poor spatial resolution
14 Magnetic Resonance Imaging (MRI) Static field: aligns all particles’ magnetic moments in the same directionPulse 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 locatedThe relaxation time can be used to infer the location of the type of compound to be localized
16 Magnetic Resonance Imaging (MRI) Advantages:Higher spatial resolution than CATAvoids potentially harmful x-raysDisadvantages:Not every individual can be subjected to MRI (pacemakers, metal in the body)
17 Positron Emission Tomography (PET) Blood flow is higher in areas of higher neural activityRadioactively labeled compounds are introduced into the bloodstream and thus the brainIsotope decays by positron emission; positron annihilates with electron, emitting two photos in opposite directions
19 Positron Emission Tomography (PET) Advantages:Can localize specific substances in the brainH2O15: accumulates in brain in direct proportion to local blood flow, marking neural activityLocalizing dopamine receptorsDisadvantages:Ionizing radiation – max 2-5 scans per yearLow temporal resolution due to isotope half-lifeAdvantages can much outweigh the disadvantages for some purposes
20 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.
22 Functional MRI (fMRI) Advantages: Disadvantages: High spatial and temporal resolutionNo use of ionizing radiationAllows tracking of changes in a patient’s brain function over time due to no usage limitsDisadvantages:Same as MRI