W6: Methods and Limitations of Neuroscience

Discuss how neuroimaging techniques inform our understanding of brain function and behaviour

• To understand how the brain produces behaviour

• Identify the functions that are required for performing a behaviour and determine what circuits of neurons are responsible for each each of these functions

*Helps us understand how a healthy brain works, useful in psychiatry & treatment

Distinguish between temporal and spatial resolution

• Spatial resolution: “where something happens” the ability of an imaging technique to locate structures/activity

• Temporal resolution: “when something happens” the ability to detect changes in brain activity over given time period

Describe key research methods and neuroimaging techniques: lesion studies

*Understanding which parts of the brain are responsible for a behaviour

^circuits within the brain perform functions not behaviours (not 1 brain region is solely responsible for a behaviour)

^makes it hard to interpret results

• Naturally occurring lesions (human research ppts e.g. PG)

• Induced lesions (animal studies)

• Experimental ablation: brain tissue is deliberately destroyed, removed or inactivated and alterations in behaviour is observed

How are lesions created?

- Surgical lesions

- Radiofrequency (RF) lesions (electrical current destroys tissue)

🙂 can control size of lesion

X may destroy more than cell bodies- induce changes not intended

^control= sham lesions (don’t turn on lesion maker)

- Excitotoxic lesions (uses chemical substances, bind to glutamate receptors and causes an influx of Ca+- excites cell to death)

^more precise than RF

- Temporary inactivation (inject anaesthetic, blocks AP entering or leaving)

e.g. GABA receptors, reversible effects

Evaluation

🙂 moderate to high spatial resolution

X other structures might be damaged

X poor temporal resolution (doesn’t measure real-time activity)

X does not account for compensation

Describe key research methods and neuroimaging techniques: tract tracing

*Maps which neurons connect to which and in what direction

• Injecting tracers into specific brain regions (taken up by neurons)

• Efferent neurons via ANTEROgrade labelling

^labels the axons & terminal buttons of neurons whose cell bodies are located in a particular region

• Afferent neurons via RETROgrade labelling

^labels cell bodies that give rise to the terminal buttons that from synapses with cells in a particular region

How tract tracing works

• Use a stereotaxic apparatus to target a specific brain region

• Injected into a living brain, transported along the axons

• Animal is euthanised

• Tissue is sectioned & histological staining reveals the labelled pathways

Evaluation

🙂 high/very high spatial resolution

X poor temporal resolution

X invasive (not used in humans)

Describe key research methods and neuroimaging techniques: diffusion tensor imaging (DTI)

*Non-invasive tract tracing in humans

Shows white matter paths and how they joined together

Reveals bundles of myelinated axons (movement of water molecules in WM not random)

Evaluation

🙂 non-invasive, can be used in humans

X moderate resolution

X cannot show individual axons/connections

Imaging the structure and function of the brain

*Neuroimaging studies: the use of methods to visualise the structure and function of the brain

• Structural brain scans

--> tell us what the brain looks like and allow locating an area that has been affected by a condition like a stroke or a lesion- good for brain abnormalities

• Functional brain scans

--> tells us which part of the brain is actively doing something (i.e. which area of the brain activates under particular conditions)

Describe key research methods and neuroimaging techniques: computerised tomography (CT scan)

*Structural

• Uses X-rays to produce 2D cross-sectional images of the brain

• Reveals structural abnormalities in the brain and bones

• Shows tumours, bleeding, skull fractures

^contrast dye helps differentiate between normal and abnormal structures

Evaluation

🙂 good spatial resolution

🙂 widely available & fast

🙂 cheap & non-invasive

X radiation exposure

X poor temporal resolution (cannot track brain activity in real time)

Describe key research methods and neuroimaging techniques: MRI

• Strong magnetic field (aligns hydrogen protons)

• Radio wave knocks protons out of alignment

• Protons relax & emit signals as they realign

• Signals detected & creates image

*Measures hydrogen alignment

• Visualisation of brain soft tissue e.g. GM and WM

Evaluation

🙂 good spatial resolution

🙂 relatively accessible

🙂 non-invasive

X poor temporal resolution

X expensive & noisy

X cannot be used with metal implants

E.g. Maguire et al., 2000- structural change in hippocampi of taxi drivers

Describe key research methods and neuroimaging techniques: fMRI

* Detects changes in blood oxygenation levels

—> when neurons are active they use oxygen & blood flow increases

(indirectly measures metabolic change)

• Visualisation of brain activity associated with performing a cognitive task and/or behaviour e.g. facial recognition

Evaluation

🙂 best spatial resolution for brain-imaging

🙂 non-invasive & relatively accessible

X poor to moderate temporal resolution (seconds)

X loud environment

Describe key research methods and neuroimaging techniques: electroencephalogram (EEG)

*Study brain activity that occurs within milliseconds after a visual stimulus

• Measures voltage fluctuations produced by populations of neurons firing together

• Electrodes on scalp pick up brain’s electrical signals

• Applications…

- used in clinical settings to diagnose conditions such as epilepsy

- EEG-based research on functional networks in cognitive and affective processing

• Shows direct recording of underlying electrical brain activity associated with a cognitive task and/or behaviour

Evaluation

🙂 excellent temporal resolution (milliseconds)

🙂 tolerant of subject movement

🙂 non-invasive & silent

X low spatial resolution compared to fMRI

X analysis of acquired data can be very complex

X poorly measures neural activity that occurs below the upper layers of the brain

Describe key research methods and neuroimaging techniques: transcranial magnetic stimulation (TMS)

*Produces magnetic pulse which passes through the skull and can interfere with brain activity

• TMS machine sends a strong electric current to a coil

• Gives rise to fluctuating magnetic pulse which goes into the brain

• Pulses trigger electrical charges changing activity of nearby neurons

TMS for treating aphasia (excitatory)

- TMS over left dorsolateral cortex in people with progressive aphasia

- TMS vs controls

- Slower decline in brain metabolism and improvements in language with active TMS

TMS for motion perception (inhibitory)

- TMS over MT/V5 area in visual cortex

- "If MT/V5 is active and necessary for motion perception than disrupting it with TMS during motion viewing should impair motion perception"

- Hypothesis confirmed= region functionally necessary

• Can see if a brain region…

- becomes more or less active in a particular situation

- is or is not involved in a specific action

E.g. whether stimulating PFC improves working memory performance

Evaluation

🙂 good temporal resolution (milliseconds)

🙂 can be combined with other methods to record response to stimulation

🙂 non-invasive

X stimulates superficial cortical areas- cannot reach deep structures

X moderate spatial resolution (compared to fMRi)

X interindividual variability due to anatomy/NT levels

Describe key research methods and neuroimaging techniques: positron emission tomography (PET)

*Functional

*Visualises activity in different brain regions by tracking a radioactive tracer

• Radioactive tracer is injected into bloodstream

• Crosses blood brain barriers

• Binds to specific receptors

• PET scanner detects positron emissions

• Signal reflects binding potential

*Shows metabolic and NT activity

Evaluation

🙂 can measure specific molecules or NT systems

🙂 silent

X comparatively poor spatial resolution & temporal resolution (minutes) compared to fMRI

X invasive due to required use of radioactive tracers

X costly to operate

X short-lived events within the brain are likely to be missed