Lecture 1 - Introduction to Brain Imaging Methods and Transcranial Magnetic Stimulation (TMS) Flashcards
Can you measure the activity of single neurons? If not, does this pose a problem to understanding cognitive processes?
No. There are no techniques that measure the activity of single neurons in humans. This does not pose as much of a problem as it may at first appear. This is because the brain is made up of a network of neurons that work together. Measuring a single neuron would not give us much information about how cognitive processes unfold.
What are some qualities of neurons and brain structure that allow the study of cognitive processes? And, what are examples of different brain imaging techniques that utilise these qualities to measure brain activity?
The brain is made up of an extremely large network of neurons that work together to allow for the function of the human mind and body. Neurons work by receiving signals from other neurons and sending this signal on or blocking it from going further. The way neurons send signals is through ionic gradients that allow for the sending of electrical signals/action potentials. The sending of action potentials as well as the effect these action potentials have at synapses, i.e. the release of neurotransmitters, all require energy in the form of ATP. ATP production from glucose requires oxygen.
These qualities of neuron function as well as the physiology of the brain are utilised in various brain imaging techniques. Electroencephalography (EEG) utilises voltage potentials, Magnetoencephalography (MEG) utilises magnetic field generated by neurons, Positron Emission Tomography (PET) utilise the energy consumption of neurons by radioactive labelling of glucose for example, Magnetic Resonance Imaging and Functional Magnetic Resonance utilise the large water content of the brain as well as the energy consumption of neuron activity, and lastly Transcranial Magnetic Stimulation uses the electrical qualities of neuron activity to generate virtual lesions or activate areas of the brain (a more structural approach).
Understanding cognition and the functions of the brain has for a long time been based on observing the effects brain injuries have had on the cognitive abilities and functions of survivors. What are four reasons we do not rely on just studying people with acquired brain injuries when it comes to studying cognition and brain function?
Firstly, in order for us to obtain a robust and significant understanding we need to see patterns across many people. Each person with an acquired brain injury will be unique in their injury and therefore we cannot draw robust conclusions from individual presentations.
Secondly, acquired brain injuries often do not occur in specific areas, and so it is difficult to determine which specific areas are responsible for or engaged in whatever cognitive task or function is impaired by the injury.
Thirdly, it can be unethical to use individuals with these injuries in experiments or as sources of scientific research.
Fourthly, the brain is very elastic. This means that oftentimes when the brain is injured it will develop new ways to perform cognitive and or physical tasks to compensate for the injury. This means that those with acquired brain injuries may not be good examples of the effects damage to that brain area actually are.
What are four qualities of TMS brain “lesions” that make them a good way of studying brain function?
Hint: there are four key qualities of these “lesions” that make them an appealing brain imaging/manipulation technique.
The virtual lesions created by the neural noise approach of TMS are short-lived, non-invasive, painless, and most importantly reversible.
How are virtual cortical lesions created using TMS?
Hint: neural noise approach and repetitive TMS approach .
The magnetic coil used in TMS creates an electrical field that can be applied to and transverse the scull and depolarise a small area of the cortex. Depolarisation of the cortical neurons makes them all fire action potenials at once. This means that coordinated communication between these neurons is lost and along with it the functions this communication generates, i.e. the same effect of a cortical lesion is created.
This can be done by applying a single pulse at a specific time to disrupt function (single pulse TMS) or by using repetitive firing of a TMS signal over a brain area, causing it to continually fire and eventually exhaust itself in term impairing its correct function.
What is the benefit of the figure-8 coil generally used for TMS?
The figure-8 coil allows for a more specific stimulation/depolarisation of a cortex area. The area has a radius of 1-4cm and penetrates about 5cm into the cortex.
