Cognitive Neuroscience

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Cognitive Neuroscience

Cognitive neurosicence links the fields of cognitive psychology and neuroscience; it is the study of how the brain structures and processes mediate cognitive behaviour. Cognitive psychologists are interested in the study of cognitive processes such as attention, perception, thinking, imagining, planning and speaking. Neuroscience is the scientfic study of the structure and function of the nervous system and biological processes.

Cognitive neuroscience has made more progress in the last 20 years than most disciplines have ever made.

The main objective of cognitive neuroscience research is to understand how the brain creates the mind and the link between thoughts/processes (cognition) and the biological (neuroscience). The field of cognitive neuroscience aims to build theoretical models of how the brain works which are advanced through further research. Cognitive neuroscientists aim to discover which brain regions are involved in particular tasks and what changes in the brain as a result of learning.Researchers investigate this with the use of brain imaging techniques and by monitoring the activity produced during cognition.

The are many technquies for achieving the aims of cognitive neuroscience, some of which involve trying to understand the structure of the brain and others that try and uncover brain function.

Brain structure:

Anatomical dissection, Magnetic Resonance Imaging, Comparative neurophysiological techniques, Diffusion tensor imaging.

Brain function:

Electroencephalography and Enterprise resonance imaging, Functional MRI (fMRI), Magnetoenephalogram, Transcranial Magnetic Stimulation.

fMRI

Functional magnetic resonance imaging (fMRI) can be used to obtain maps of functional activation in the brain. When neurons are are active they use oxygen. BOLD fMRI uses the oxygen level in the blood to create an image. Blood contains haemoglobin. When oxygen is bound to haemoglobin it is called oxyhaemoglobin; when oxygen has dissociated from haemoglobin it is called deoxyhaemoglobin. The magnetic propoerties of haemoglobin differ depending on whether it carries oxygen - deoxyhaemoglobin is paramagnetic (magnetic) while oxyhamoglobin is diamagnetic (not really magnetic). Hence, it is possible to distinguish between oxygen-rich and oxygen-depleted blood. So when neurons fire, and use up oxygen, oxyhaemoglobin is converted to deoxhaemoglobin. This increase in deoxyhaemoglobion and its magnetic properties is what causes a increase in the signal, causing the image to become brighter. Oxyhaemoglobin decreases the signal and makes the image darker. The important point to remember about fMRI is that it is measuring the amount of oxygen and not the level of activity and so care must be taken during interpretation.

Advantages: Doesn't use harmful radiation unlike other methods such as CT and PET scans; arguably the safest method of neuroimaging as it is noninvasive; equipment is easy to use; produces high resolution images; gives reasonable temporal resolution; and is a way of getting sturcural information with the same session it is used.

Disadvantages: Expensive; only produces clear images when the patient lies completely still; only monitors blood flow in the brain and cannot assess the activity of individual neurons; results can be difficult to interpret; metal based euqipment cannot be used for stimulus presentation; and it is claustrophobic


EEG (Electroencephalography)

EEG measures electrical activity generated by the brain through electrodes placed on the scalp. After electrodes are placed on the scalp, EEG signals are transported to an amplifier. The electrical signal from the EEG is then compared to a reference electrode. Jasper (1958) proposed a labelled system for electrodes according to their location. Letters were used to describe lobe and letters use to describe hemisphere (odd numbers for left, even numbers for right). For example F2 = right frontal lobe. Recordings are gathered by measuring the voltage difference from an active electrode and a reference electrode (somewhere that will not be influenced by the investigation, e.g. the mastoid bone behind the ears)

Advantages: Very good temporal resolution, avoids motion artefacts and can be used by anyone including infants.

Disadvantges: Weak spatial resolution, there is an inverse problem of knowing which brain regions cause the activity that is measured, blinking can disrupt EEG signal (but can monitor this or blink between tasks), and you can not use tasks with vocal responses as jaw movements can disrupt the EEG signal

Data recorded via EEG can be task independent or task dependent:

In a task independent recording the participant is not asked to do a task. An example of when this method may be useful is when a doctor is looking for evidence of epilepsy. In this case, a patient would be asked to sit and do nothing. The recording would be watched by a neurologist who would look for evidence of seizures at any point during the recording.

In a task dependent recording the participant is asked to do a task. This is the method that is most commonly used in cognitive psychology/cognitive neuroscience and is useful for finding out how the brain processes information. In this case a participant is asked to complete a cognitive task and the researcher will look at the EEG signals to find out how the brain responds to different parts of the task e.g. detecting visual stimuli on a computer screen and pressing buttons on a keypad when a participant detects certain stimuli. It is likely that this will involve the participant completing many trials of the same task over and over, so that after the EEG session the researcher is able obtain an average signal by combining the data from all of the trials. Basically, in this type of design the bits of the EEG signal that the researcher will analyse depends on the part of the task that was being completed at the time.


'Transcranial magnetic stimulation TMS'

This was invented by Tony Barker and colleagues (1985), and it uses electromagnetic induction to cause either hyperpolarisation or depolarisation in the cortex. Electromagnetic induction is used to induce weak electric current in the cortex. It can cause motor evoked potential, for example limbs may twitch, and it can also be used to produce a simulated 'lesion' of a brain region by preventing normal functioning of that region.

Advantages: It is portable; can simulate or cause lesions; it is reversible; and lesions can be moved within same subject

Disadvantages: It is difficult to specify the precise regions that are showing activity (hereas patients with lesions can be precisely detected using MRI); low spatial resolution; and only surface regions are shown.


PET

Positron Emission Tomography is the process of injecting a radioactive glucose substance into the body which can be traced. A common one used is oxygen-15. When this radioactive tracer is in the blood stream, it decays, emitting a positron. This positron, then collides with an electrode which releases two photons. A machine around the head then detects the photons and a spatial image can be constructed. The participant is then given a task that is thought to require the use of a particular brain region. The experimenter will then measure where in the brain the substance can be found and then compare this with the resting state.

Advantages: There is a direct reflection of activity and the patient is able to move around; relatively pleasant; and can be used to investigate particular neuronal pathways and study the effects of drugs (for example Volkow studied cocaine abuse by using a radioactive tracer that has a similar profile to that drug).

Disadvantages: The injection of a radio active substance could be seen as unethical; there is no temporal resolution; no stuctural information is given and so a MRI or CT scan is necessary as well; and it is expensive.


MEG

Magnetoencephalogram is the process of recording magnetic fields that are generated by the activation of the pyramidal cells of the cortex. Like the EEG, MEG requires the synchronised firing of 10 000s neurons to create a field that is large enough to measure.

Advantages: excellent temporal resolution and good spatial resolution especially when it is combined with MRI.

Disadvantages : expensive and there is an inverse problem for finding what stimulus causes which brain region to become active.

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