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Cognitive Neuroscience:Knowledge From Cognitive Deficits

Chapter Two

McBride & Cutting

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Review of the Basics

• The Neuron

– Structure

– Action Potential

– synapse

– Neurotransmitters, hormones and neuroimmunology

• Ways to in intervene at the synapse

• Localization of function

• Neural Circuits

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Neurons!

Basic building blocks and transmission lines of the nervous system.

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Structure of a Neuron

• Cell Body - metabolism

• Dendrites – receive signals

• Axons (nerve fibers) – transmit signals.

• Axon Terminals – branches to transmit signal to multiple areas if the nervous system.

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Communication Within a Neuron

Receptors on Dendrites

- Sensory (chemical reactions)

- Chemicals from other neurons

- Chemicals from endocrine glands

- Chemicals from outside sources (e.g., Drugs)

Chemical Messages can be Excitatory or Inhibitory.

Effect on dendrite is a summed effect.

- threshold

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Action Potential

- all or none law

- digital message (yes/no)

- one strength

- one speed

- one way (dendrite to Axon)

Neuron firing

If input at the dendrite reaches threshold, the neuron will fire. If it does not, the neuron will fail to fire. Each neuron can produce only one of two responses at a given moment, fire or fail to fire. The pattern of firing and failing to fire is like a digital code. It does not increase or decrease its rate, strength or change its direction.

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Inside and outside of neuron are fluid environments.

Fluid chemicals have ionic charges.

- sodium, potassium and chloride(electrolites)

Resting Potential

- inside negative relative to outside.

- sodium-potassium pump

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If threshold is reached at dendrite, then an action potential

begins.

Gates in membrane open allowing sodium to flow in.

Inside becomes more positive.

Momentary change in charge.

Passed along the membrane, carrying message to Axon

Terminal.

The Positive Charge Can Be Measured Using Electrodes

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Synapse

• Fluid filled space between the axon terminal of a presynaptic neuron and the dendrites postsynaptic neuron.

Neurotransmitters(chemicals) are released into Synaptic space.

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Neurotransmitters

Neurons communicate with one another by releasing chemical messengers, called neurotransmitters, into the synapse, where they are then taken up by specific receptors on neighboring cells. There are many types of neurotransmitters in the brain—what they have in common:• they are produced inside a neuron, • released into the synapse, • and then cause an excitatory or inhibitory effect

on receptor cells, helping to propagate or downgrade action potentials.

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Neurotransmitters are cleared from the synaptic

space by

1) re-uptake into the terminal button

2) destroyed by enzymes

Drugs Effects on Synapse

• Mimic Natural Neurotransmitters

• Block receptor with another molecule so that neurotransmitter cannot bind to its receptor on the postsynaptic neuron

• Block reuptake – e.g., Selective Serotonin Reuptake Inhibitors (SSRIs, e.g., Prozac).

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Neural Circuits

Neurons are interconnected with one another to form circuits, much as electronic components are wired together to form a functional circuit. Many neural circuits together form a neural network, just as many electronic circuits together form a device such as a computer or a TV.

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Hormones

Hormones - similar effects as neurotransmitters

- effect the firing of neurons via the dendrites.

Differences:

-Released into blood stream from endocrine glands

-Have wider ranging, longer lasting effects

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Psychoneuroimmunology (PNI): interactions among psychological processes and the immune and nervous systems.

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Although the interrelationships are complex, research indicates that psychological factors (e.g., stress, happiness, beliefs), biological factors (e.g., hormonal and immune responses) affect and are directly affected by the nervous system. It also indicates that understanding psychological health (e.g., depression, Alzheimer's) will require a better understanding of factors related to stress, wellbeing and immune responses.

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The Cerebral Cortex

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Localization of Function

Specific areas of the brain have specific functions (processes).

Damage to specific areas produces specific function deficits.

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Cortex has 2 halves (hemispheres)Each controls opposite side of the body.

Lateralization of brain function - some neural

functions, or cognitive processes (e.g., language)

tend to be more dominant in one hemisphere than

the other. The two hemispheres are highly

connected by the corpus callosum.

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Each hemisphere has 4 lobes1) Frontal Lobe

- Primary motor cortex (controls movement)

- prefrontal cortex(planning of movements and working memory)

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2) Parietal Lobe- primary somatosensory cortex

Sensations and location of body partsDamage - Unilateral Neglect- can sense the neglected side but fail to

to attend to it.

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3) Occipital lobe- primary visual cortex

4) Temporal Lobe- primary auditory (hearing) area.- complex aspects or vision

- face blindness (right hemisphere)- language (left hemisphere)

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Distributed Processing

Newer Technology has allowed us to identify the neural circuits that underlie many processes.

Functions (e.g., daydreaming, reading, understand a scene in a movie) involve processes that are distributed across multiple areas.

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Cognitive Neuropsychology

Study of Brain Damaged Patients

Assumption of Modularity

Cognitive system is divided into a set of functional units (named modules) that can be composed into a larger application.

E.g., several modules (color, motion, form)

Contribute to visual processing.

27 Intro and History

Dissociation – A patient performs at the same level as normals on one task but is impaired on other a second task.

E.g., Retrograde Amnesia – impaired LTM with normal STM.

28 Intro and History

Double Dissociations

Patients exist who are impaired on X but not Y.

Others are impaired on Y but not X.

Evidence that X and Y are independent processes.

29 Intro and History

Measures in Cognitive Neuroscience

• Measuring Brain Activity- Single-Cell Recording- EEGs

• Brain Imaging- MRIs- PETs- FMRIs- MEGs

• Transcranial magnetic stimulation (TMS)

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Methods of Measuring Brain Activity Patterns

Single-Cell Recording

• Animal studies

- feature detectors (chp. 3)

- Mirror Neurons

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Electroencephalography (EEG)

Event-related potentials (ERPs) are very small voltages generated in the brain structures in response to specific events or stimuli. Time locked to sensory, motor or cognitive events.

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Same Stimuli presented several times and lined up according to stimulus onset.

Averaged ERPs allow genuine effects to be distinguished from background neural activity.

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Recordings can be taken from several locations on the scalp

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Poor spatial resolution –but excellent temporal resolution.

Brain Imaging Techniques

Magnetic Resonance Imaging (MRI)

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Positron Emission Tomography (PET)

Small amount of radioactive material (tracer) given through a vein (IV). Radioactivity is detected at active areas of the brain.

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MRIs and PETS have Poor Temporal resolution (activity is result of activation over 30 to 60 secs) but reasonably good spatial resolution.

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Functional Magnetic Resonance Imaging (fMRI)

Imaging test that uses powerful magnets and radio waves to create pictures of the body.

60 Minutes Video (to 4.23) – Just and Mitchell study

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fMRI

fMRI signals reflect brain activity indirectly, by measuring the oxygenation of blood (brain fuel) flowing near active neurons.

Spatial resolution very good.

Temporal Resolution is not great for learning about many cognitive processes

2 to 3 secs. Video

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fMRI

The fMRI signals show relative neuronal activity, meaning that the fMRI signal analysis is always compared to reference neuronal activity.

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Brain Imaging in Practice: What does love look like?

Zeki And Romaya (2010)

fMRI studies of the neural correlates of romantic love in 24 subjects, half of whom were female (6 heterosexual and 6 homosexual) and half male (6 heterosexual and 6 homosexual). Compared the pattern of activity produced in their brains when they viewed the faces of their loved partners with that produced when they viewed the faces of friends of the same sex as their partner to whom they were romantically indifferent.

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The pattern of activation and de-activation was very similar in the brains of males and females, and heterosexuals and homosexuals. We could therefore detect no difference in activation patterns between these groups. ( from Abstract)

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Magnetoencephalography (MEG)Uses Superconducting Quantum Interference Device (SQUID). Like an fMRI but more sensitive. Magnetic fields pass through the skull and scalp and measured outside of the head using the MEG’s very sensitive scanner.

Excellent Temporal Resolution and Good Spatial Resolution.

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MEG advantages over fMRI

- MEG signals are obtained directly from neuronal electrical activity, fMRI is indirect (O2 usage).

- MEG signals are able to show absoluteneuronal activity whereas the fMRI signals show relative neuronal activity, meaning that the fMRI signal analysis is always compared to reference (control) neuronal activity.

Disadvantages of MEG

• Very Expen$ive

• Limited availability

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Transcranial magnetic stimulation (TMS)

Uses magnetic fields to excite or

inhibit neural activity.

Transcranial direct current stimulation (tDCS)Excites neural activity using electric current.

Effects of both are temporary.

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rTMS as Treatment for Depression

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Can all Mental Processes Be Explained in Terms of Brain Activity?

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Famous Neuroscientists

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A. R. Luria

• Mind of a mnemonist (Synesthesia)

• The man with a shattered world (Plasticity & Pure Alexia)

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Oliver Sacks

• Awakenings

• The man who mistook his wife for a hat (object agnosia)

• Musicophilia: Tales of Music and the Brain

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V. S. Ramachandran

Referred to as the “House” of neuroscience!

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