cognitive neuroscience methods

neurotransmitters

• messages are passed via neurotransmitters

• excitatory - cause the neuron to fire

• action potential

what is cognitive neuroscience

the study of how brain structured and processes mediate cognitive behaviour

objectives of cognitive neuroscience

• understanding - how the mind is created by the brain, links between cognition and neuroscience

• build - models of how the brain works

• establish - which brain regions and brain circuits are involved in different tasks, what changes in the brain as a consequence of learning

• investigate - brain imaging, activity produced during cognition

challenges for cognitive neuroscience

• complex systems - around 1 mil neurons in 1 cubic mm of brain

• signal to noise ratio - needle in a haystack

• between individual differences - experience, individual variability

• within-individual differences - strategy, mood, task difficulty

techniques

brain structure

• anatomical dissection

• structural magnetic resonance imaging

brain function

• single electrode stimulation

• scalp-located electrodes - EEG, ERP

• functional MRI (fMRI)

• magneto-encephalography (MEG)

• positron emission tomography (PET)

• transcranial magnetic stimulation (TMS)

neuropsychology

• data from individuals with brain damage

MRI

• hydrogen atoms like constantly spinning magnets

• align when in a scanner’s magnetic field

• then radio waves sent in to make them face in a new direction

• when they relax and return to their previous alignment, they emit energy (i.e. ‘resonance’)

• this energy is what the scanner uses to create an image

MRI scans

• static structure of the brain

• hydrogen atoms in different tissues have different relaxation times and can be identified separately

• the lower the water content of an area, the fewer hydrogen atoms there will be emitting signals

• the weaker the signal, the darker the area appears on the scan

advantages of MRI

• excellent spatial resolution

• allows you to view the brain from multiple angles

• non-invasive

• excellent for looking at soft tissue

disadvantages of MRI

• no temporal information

• negative experience - noisy, have to stay very still, claustrophobic

• expensive

• older metal implants are not compatible

FMRI

• not just structure but function

• based on the same technique as MRI (magnets and radio waves)

• but interested in blood flow in the brain

• blood contains haemoglobin which contains iron which is magnetic

• deoxygenated blood is affected by a magnetic field differently than oxygenated blood

• when neurons are active, they burn energy - this is automatically replenished via oxygen carried by haemoglobin in the blood stream

• active parts of the brain contain more oxygen-rich blood

• by measuring the BOLD (blood oxygen level dependent) response in the MRI scanner, we can work out which parts of the brain were active recently

diamagnetic vs paramagnetic

haemoglobin is diamagnetic when oxygenated but paramagnetic when deoxygenated

• diamagnetic = has a weak magnetic field

• paramagnetic = has a strong magnetic field

this is called the BOLD response - Blood Oxygen Level Dependent

Piaget’s conservation of numbers task

• 2.5% of the 5-6 year olds could conserve numbers

• 99.7 of the 9-10 year olds could conserve numbers

but

our brains process a vast amount of information and perform multiple tasks at the same time so how can we tell if the brain activity is purely related to conservation?

conservation task - control experiment

• closely matches main experiment

• allows us to pinpoint what brain activity is related to extraneous processes e.g. visual information and environmental noises not related to the concept we’re interested in

• both groups were accurate on the colour task

advantages of fMRI

• excellent spatial resolution

• reasonable temporal resolution

• non-invasive

• tells us which parts of the brain are used in tasks

disadvantages of fMRI

• BOLD isn’t a direct measure of activity and care should be taken interpreting it

• experience - noisy, have to stay very still, claustrophobic

• expensive

• can’t have any metal-based equipment for stimulus presentation

positron emission tomography (PET)

• measures blood flow in the brain - more blood flow in active areas

• inject, inhale or swallow a mildly radioactive drug (tracer) which is absorbed into the blood stream

• measure how much of the tracer goes where in brain

• higher levels of activity show up as bright spots

• can also be used to evaluate certain brain disorders, such as tumours, Alzheimer’s disease and seizures

advantages of PET

• reasonable structural resolution

• direct reflection of activity

• no motion artifacts

• more comfortable experience than MRI/fMRI

disadvantages of PET

• no temporal resolution

• expensive

• injection of radioactive substance so ethical issues

• may need to do MR or CT as well

electroencephalography (EEG)

• measures electrical signals generated by the brain through electrodes placed at the scalp

• EEG signals are produced by partial synchronisation of cortical field activity and are measured as changes in voltage recorded at the scalp over time

how are EEG signals detected

• electrodes are placed on the scalp, connected with gel or conductive solution

• EEG signals are transported to an amplifier

• the amplifier measures the difference in voltage between the active electrode and a reference electrode

advantages of EEG

• very good temporal resolution

• direct reflection of activity

• not claustrophobic - can even be used by infants

disadvantages of EEG

• poor spatial resolution

• motion artifacts

• inverse problem - how can we be sure which brain regions generated the electrical activity?

magnetoencephalogram (MEG)

• the pryamidal cells of the cortex, when active, generate a significant magnetic field

• for both EEG and MEG, the synchronous firing of 10,000 neurons is required to produce a field which is large enough to measure

• magnetic fields are less distorted by the scalp than electrical fields

advantages of MEG

• excellent temporal resolution

• direct reflection of activity

• good spatial resolution especially if combined with MRI

disadvantages of MEG

• expensive

• inverse problem - how can we be sure which brain regions generated the electrical activity

brain legions - neuropsychology

lesion patients

• e.g. Phineas Gage

• injured 1840s

• no impairment of language, intelligence, movement or memory

• irresponsible, disregard for social conventions

transcranial magnetic stimulation (TMS)

• uses a changing magnetic field to induce weak electric currents in the cortex

• noni- - invasive method causing depolarisation or hyperpolarisation

advantages of TMS

• near portable

• can stimulate or lesion

disadvantages of TMS

• difficut to specify precise regions

• ony durfave reaion