Chapter 2: the Neural Basis for Cognition

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the human brain is divided into 3 main structures…

  1. the hindbrain

  2. the midbrain

  3. the forebrain

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The Hindbrain

  • located at the very top of the spinal cord

  • contained structures crucial for controlling key life functions like heart rate

  • plays an essential role in maintaining posture and balance

  • helps control level of alertness

  • includes…

    • cerebellum

    • pons

    • medulla

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Cerebellum

  • largest area of the hindbrain

  • coordination of bodily movements

  • various other roles

  • damage: cause problems in spatial reasoning, discriminating sounds, and integrating input received from various sensory systems

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Pons

  • main connection between cerebellum and the rest of the brain

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Medulla Oblongata

  • breathing

  • heart rate

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The Midbrain

  • important part in coordinating the precise movements of the eyes as they explore the visual world

  • contain circuits that relay auditory information from the ears to areas in the forebrain where information is processed and interpreted

  • help to regulate the experience of pain

  • relays information between the brain and the spinal cord

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Cerebral Cortex

  • thin layer covering cerebrum but includes lots of cortical tissue

  • makes up 80% of the human brain

  • deepest groove in the surface of the cortex is longitudinal fissure

    • runs from the front to the back and separates the left and right hemispheres

  • other fissures divide the cortex into 4 lobes

    • frontal lobes (central fissure separates them from parietal lobes)

    • parietal lobes (lateral fissure separates them from temporal lobes)

    • temporal lobes

    • occipital lobes

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Subcortical Structures

  • thalamus

  • hypothalamus

  • limbic system

    • amygdala

    • hippocampus

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Thalamus

acts as a relay station for most sensory information going to the cortex

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Hypothalamus

crucial role in behaviours that serve biological needs (eating, drinking, sex)

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Limbic System

  • essential for learning and memory

    • includes amygdala and hippocampus (HM had large portions of these removed)

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Amygdala

  • key role in emotional processing

  • greater activation of amygdala while witnessing an event will lead to better, longer-lasting memories

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Commissures

thick bundles of fibers that carry information back and forth between the two hemispheres

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Corpus Callosum

  • largest commissure

  • sometimes there are medical reasons to sever the corpus callosum and some other commissures (like for epilepsy)

    • patient then becomes a split brain patient

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Split-Brain Patient

  • communication between the two halves is severely limited

  • research with these patients has taught us a lot about the specialized function of the brain’s two hemispheres

    • confirmed that many aspects of language are in the left hemisphere

    • confirmed that right hemisphere is crucial for a number of tasks involving spatial judgement

  • important not to overstate the contrast between the two halves

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Neuropsychology

  • the study of the brain’s structures and how they relate to brain function

  • study of cases about brain damage through accident, disease or birth defect falls within the domain of neuropsychology

  • consequences of brain lesions depend on which hemisphere is damaged

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Clinical Neuropsychology

  • specialty of neuropsychology

  • seeks (among other things) to understand the functioning of intact, undamaged brains by means of careful scrutiny of cases involving brain damage

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Neuroimaging techniques

  • produce precise, 3D pictures of a living brain

    • structural and functional imaging

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Structural Imaging

  • generating a detailed portrait of the shapes, sizes, and portions of the brain’s components

    • CT Scan

    • MRI

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Functional Imaging

  • tells us about activity levels throughout the brain

    • PET Scan

    • fMRI

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fMRI

measures oxygen content in blood flowing through each region of the brain to provide an index of the level of activity in that region (can offer a precise picture of the brain’s moment-by-moment activities)

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neurons function

  • main function is to communicate with one another using chemical signals called neurotransmitters

    • also communication within a neuron

    • neurons get signals from one end of the cell to another with electrical pulses

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Electroencephalography

  • recording of voltage changes occurring at the scalp that reflect activity in the brain underneath

    • creates an EEG: recording of the brain’s electrical activity

    • often used to study broad rhythms in the brain’s activity

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Power of Combining Techniques

  • each technique has its strengths and weaknesses, we deal with limitations by combining techniques

    • seek data from multiple sources to make a whole image

    • combine EEG with fMRI so we can see when and where activity took place in the brain

    • combine fMRI with CT so that findings about brain activation can be linked to a person’s brain anatomy

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How to get around only correlational data from neuroimaging techniques

  • ex; Fusiform Face Area is especially active whenever a face is being perceived so there is a correlation between a mental activity and a pattern of brain activity

    • does this mean FFA is needed for face perception?

  • we can get additional data from lesions - if a brain area is damaged and disrupts a particular function it’s an indication that the site does play some role in supporting that function

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Transcranial Magnetic Stimulation

  • technique that creates a series of strong magnetic pulses at a specific location on the scalp and these pulses activate the neurons directly under this scalp area

    • can be used as a way to see what happens when we stimulate certain neurons

    • because it disrupts the ordinary function of these neurons it creates a temporary lesion which allows us to identify what functions are compromised when a particular bit of brain tissue is briefly turned off

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Localization of Function

  • finding out where functions originate in the brain

  • reveals enormous overlap between the brain structures needed for two activities

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Primary projection areas

  • primary motor projection areas

  • primary sensory projection areas

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Primary Motor Projection Areas

  • “departure points” for signals leaving the cortex and controlling muscle movement

    • evidence comes from studies where investigators apply mild electrical current to this area in anesthetized animals

    • stimulation produces specific movement depending on the site it’s applied to

      • show a pattern of contralateral control (stimulation to left hemisphere leads to movement on the right side of the body and vice versa)

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Primary Sensory Projection Areas

“arrival points” for information coming from the eyes, ears and other sense organs

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Why are they called projection areas?

  • because they seem to form “maps” of the external world with particular positions on the cortex responding to particular parts of the body or locations in space

    • located on a strip of tissue towards the back of the frontal lobe

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Sensory Areas

information arriving from the skin senses is projected to a region in the parietal lobe just behind the motor projection area (somatosensory area)

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Sensory Projection Areas

  • differ from each other in important ways but also have features in common and they parallel the attributes of the motor projection area

    • provides a “map” of sensory environment

  • includes somatosensory area, visual area, auditory area

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Somatosensory Area

  • each part of the body’s surface is represented by it’s own region on the cortex

    • areas that are near to one another are represented by similarly nearby areas in the brain

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Visual Area

  • each region of visual space has its own cortical representation

    • adjacent areas of visual space are usually represented by adjacent brain sites

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Auditory Projection Area

  • different frequencies of sound have their own cortical site

    • adjacent brain sites are responsive to adjacent frequencies

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Assignment of Cortical Space

  • governed by function, not by anatomical proportions

    • in the parietal lobes: parts of the body that aren’t very discriminating to touch get relatively little cortical area (even is they’re physically large) and more sensitive areas (lips, tongue, fingers) get more space

    • in the occipital lobes: more cortical surface is devoted to the fovea (part of the eye most sensitive to detail)

    • in auditory areas: some frequencies of sound get more cerebral coverage than others

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Association Areas - brain space

motor and sensory areas make up only about 25% of the human cerebral cortex, the rest is association cortex

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Apraxia cause

caused by lesions in the frontal lobe

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Agnosia cause

  • caused by lesions in the occipital or rearmost parietal lobe

    • usually affect one modality only

      • visual agnosia: can recognize a fork by touching it but not by looking at it

      • auditory agnosia: unable to identify familiar voices but can still recognize the face of the person talking

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Unilateral Neglect Syndrome

  • produce lesions (usually in parietal lobe) where individuals seem to ignore half of the visual world

    • shave only half their face

    • eat food on half the plate

    • read only half of words

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Aphasia

lesions in areas near the deep groove that separates the frontal and temporal lobes can result in language disruptions

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The Brain’s Major Divisions

  • Hindbrain

  • Midbrain

  • Forebrain

  • Subcortical Structures

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Forebrain

  • higher order processing

  • most interesting brain region

  • structure that surrounds the midbrain and most of the hindbrain

  • cerebral cortex

    • outer surface (grey matter)

    • folds increase volume

  • two hemispheres (separated by the corpus callosum

    • 4 lobes in each

    • some specialization

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Scanning Methods

  • Event Related Potentials (ERP)

  • Computerized Axial Tomography (CAT/CT Scans)

  • Positron Emission Tomography (PET Scans)

  • Magnetic Resonance Imaging (MRI & fMRI Scans)

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Event Related Potentials (ERPs)

  • electrical activity on the scalp

    • measure changes in the EEG in the brief periods just before, during and after the event

  • patterns of negative and positive voltage

  • excellent temporal resolution (the “when”)

    • can tell exactly when part of the brain is activated

  • poor spatial resolution (the where”)

    • not great for showing exactly where

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Computerized Axial Tomography (CAT/CT Scan)

  • series of x-rays taken at various angles then reconstructed in a computer to create an image

  • allows for the study of anatomy and structure, but not function

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Positron Emission Tomography (PET Scan)

  • measures blood flow, often via radioactive glucose tracers

  • measures brain activity over time (not instantaneous)'

  • fair spatial resolution (where)

    • hard to tell when structures are small and close together

  • Poor temporal resolution (when)

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Magnetic Resonance Imaging (MRI and fMRI)

  • use a giant magnet to measure fluctuations in the magnetic fields

  • fMRI measures rapid changes in blood and oxygen flow in the brain

  • helps determine localization of function (excellent spatial resolution)

  • weak temporal resolution

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Broca’s and Wernicke’s Areas

  • located in the left hemisphere

  • both involve language

    • Broca - language production

    • Wernicke- language comprehension

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Broca’s Aphasia

  • expressive aphasia

  • disrupted production a-grammatical speech

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Case Study - Mike

  • broca’s aphasia

  • struggles to produce his speech, it takes a lot of effort to get the words out

  • the speech he does produce is very basic, sparse and is often single words

  • his ability to understand language is not impaired

  • he listens to himself talk

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Wernicke’s Aphasia

  • receptive aphasia

  • difficulty understanding

  • speech is often a “word salad”

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Case Study - Bryon

  • good prosody - speech sounds fluent and flow at a normal pace

  • his sentences are relatively well-structured, but don’t convey much information

    • uses nouns and verbs in grammatically correct spots

  • occasionally makes up words (neologisms)

  • has some difficulty understanding what is being said to him

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