measuring brain activity and modification of brain

Brain “imaging”

Assess brain structure and function “non-invasively” without dissection or damage to the brain

Electroencephalography (EEG)

EEG refers to both electroencephalography (the equipment/method) and electroencephalogram (the data output - “writings of electricity from the head”)

1924 - first human EEG

1934 - first demonstration of epileptiform spikes with EEG

Electroencephalography strengths

• good temporal resolution (can discriminate vert brief events in time)

• Relatively cheap ($10,000-$100,000 per system). Many psychology departments owning multiple EEG systems

• Portable and possible to record EEG while people are moving around (important in the detection of epileptic seizures that can require people to wear an EEG cap for many days)

• Safe and well tolerated by participants - no real risks associated with placing recording electrodes on a person, beyond mild discomfort

Electroencephalography limitations

• poor spatial resolution. With more electrodes the spatial resolution may be improved, but it’s still difficult to determine precisely from which area of the underlying brain the signal has come

• Typically only detects activity on the surface of the cortex. It is very hard to detect activity from more central regions within the brain, as the electrodes are attached to the outside of the skull

Electrophysiology - single neurons

Hodgkin and Huxley recorded action potentials (electrical signals) in the giant axon of Atlantic squid in 1952 (won Nobel prize in 1963)

A subsequent explosion of research in the following decades was based on technical advancements

• development of microelectrodes

• “Multi-unit” recording electrodes

Much of the sensory and motor cortex was mapped using these methods in animal studies

Huber and Wiesel mapped the development and functional organisation of the visual system (won Novel prize 1981)

Electrophysiology - single neurons strengths

Records directly from individual neurons so is the best method to use if you want to know what the neurons are doing

Electrophysiology - single neurons limitations

High risks of infection as this technique is “invasive” penetrating the brain.

It is only possible to record from a few (up to ~100 neurons at at time which multielectrode arrays), so can only record individual neurons or small network activity.

Magnetic Resonance Imaging (MRI)

MRI exploits the magnetic properties of brain tissue

MRI coil generates a VERY strong magnetic field (earth’s magnetic field is about 1/1000 tesla, whereas the field created by MRI scanner is 1.5-7 tesla)

Magnetic field passes through the person’s head causing hydrogen atoms to align with the magnetic field

Radio frequency waves temporally disrupt this alignment causing a signal that can be detected by this machine

Because different areas of brain tissue contain different amounts of water, they emit different signals.

Analysis software converts detected signals into very detailed images of different structures in the brain.

Diffusion Tensor Imaging (DTI) - uses the same MRI equipment to detect the large axon tracts (“white matter”) that flow through the brain and connect different regions of cortex.

fMRI - functional imaging

Cognitive processes use energy

• The production of energy uses oxygen from hemoglobin (blood)

Oxygenated blood

• doesn’t distort surrounding magnetic field

Deoxygenated blood

• distorts surrounding magnetic field

= blood vessels became more visible as blood oxygen decreased

Blood-oxygen-level-dependent (BOLD) signal tracks the ratio fo oxygenated vs deoxygenated blood

BOLD dominates fMRI studies that map human brain function.

As the brain region uses energy there is an increase in blood flow to the region.

Because fMRI images reflect the chance in oxygen levels in the blood (not the neurons directly) there is a delay of a few seconds between time of neural activity and change in blood oxygen levels.

MRI - strengths

Very high spatial resolution, identifying exactly where the brain different structures are or different function is occurring

MRI is also very valuable tool because it can identify specific anatomical/structural and functional properties of different brain regions

MRI - limitations

MRI machines very expensive ~ $1 million for the latest equipment

Large equipment requires specialist facility with multiple rooms to enable staff to work - needs to be kept within a magnetically shielded room

Safety risks associated with large magnet, ensure no metal enters MRI room

Requires specialist staff with radiography training

Positron Emissions Tomography (PET)

Uses radioactive substances known as tracers to visualize glucose metabolism or the neurotransmitter/receptor function

Can also use radioactive tracers to bind selectively to proteins of interest

PET is currently used as a diagnostic tool for Alzheimer’s Disease (detecting buildup of Amaloid protein)

Positron Emissions Tomography (PET) - strengths

Can detect different chemical in the brain associated with either the metabolism (energy consumption), or specific neurotransmitter levels or receptors

Positron Emissions Tomography (PET) - limitations

Expensive and requires specialist facilities and staff

Relatively low spatial resolution compared to MRI

Signal requires radioactive tracers to be injected into the participant’s blood (considered very safe, but it is important that the risks are managed with the appropriate safety precautions)

Section summary of measuring brain activity

• different brain imaging methods provide an opportunity to investigate the structure and function of the brain

• Electrical recordings - electroencephalography (EEG) measures scalp activity single-unit recording of neurons

• Magnetic structural and functional brain imaging - correlates brain activity with behaviour (fMRI)

• Positron emission tomography (PET) - measures changes in brain chemistry (metabolism or neurotransmitter system function)

Brain modification

Brain regions can be permanently removed/destroyed or the brain activity can be temporarily decreased

OR

Brain areas can also be stimulated to enhance or increase brain activity in that region

Allows scientific research to be done. Unlike brain imaging techniques like fMRI which provide correaltional information, brain modifications provides information about causation.

Brain modifications - medical treatment

Many psychiatric and neurological conditions are associated with abnormal brain functions. Treatment of these disorders will often target the abnormal brain processes involved

In some cases, such as epilepsy or brain tumour, treatment might involve removal of sections of abnormal brain tissue

Drugs can also be used to selectively target abnormal function of specific neurotransmitter systems

Brain modifications - enhancement

Improvement of healthy function to above or better than normal. In contrast to medical use, which aims to improve impairments to achieve healthy function

Increasing claims around the use of both brain stimulation and drugs to improve brain function beyond typical normal levels

Ablation studies

Ablation - “to carry away”

Deliberate lesions allow a relatively high degree of precision

Eg. The limbic system contains a number of small structures including the hippocampus and amygdala - ablations in monkeys show only that the hippocampus plays a role in learning and memory

Portuguese physician Egan Moniz introduced prefrontal leucotomy for relief of psychiatric disorders - removing frontal lobes of chimpanzees made it calmer and more cooperative

Frontal leucotomy

Leucotome was inserted into one of several holes drilled in the skull. The wire was then extruded from the tip and the leucotome rotated to remove core of tissue.

Cutting implement was inserted above the eyelid, pushed through the base of the skull (thin and brittle just above eyes) and rocked from side to side to splice through frontal lobes, separating them from rest of the brain

Profound personality consequences

• apathy

• Emotional unresponsiveness

• Disinhibition

• Inability to plan

Electrical brain stimulation

Electrcial stimulation has also been used to reveal precise localization of cortical function.

Stimulate part of brain.

Non-invasive electrical brain stimulation - ECT

• electroconvulsive therapy (ETC) invented in Italy in the 1930s

• It was already known that seizures reduced psychiatric symptoms - as early as the 1500s, seizure inducing agents were used to treat psychiatric conditions

• ECT originally used to treat a range of mental illnesses - now used to treat severe depression

• Mechanism of action is unknown - electrical stimulation of the frontal lobes needs to be strong enough to cause a seizure

Non-invasive electrical brain stimulation - tDCS

A range of non-invasive methods for electrical brain stimulation now exist, however the efficacy and safety remains debated

The most common method is transcranial direct current stimulation (tDCS)

Non-invasive magnetic brain stimulation - TMS

In Transcranial Magnetic stimulation (TMS), a coil carrying an electrical current generates a brief, focal magnetic pulse which activates a small region of cortex (approx 10-15 mm) underlying the coil

Section summary of modifications of brain

• introduced historical and current methods that inform our understanding of the functional role of different cortical areas (treatment, enhancement, science0

• Removal/surgical ablation of cortex

• Invasive and non-invasive electrical stimulation with ECT and tDCS

• Non-invasive magnetic stimulation with TMS increasing or decreasing activity (“virtual lesions”)