PSYC 324: Lecture 11 - Measuring Neural Activity

MT 2 Content

4 Steps in Studying Brain Function

1. Observe and describe:

• Examine brain structure and activity.

• Correlations between structure/activity and behaviour.

2. Build theory:

• Develop explanations of brain function based on observations.

• Develop testable predictions of brain function.

3. Test theory:

• Manipulate brain function or behaviour to test predictions.

• Helps establish causal relationships rather than simply correlations.

4. Refine:

• Revise theories based on findings.

Testing Neural Hypotheses with Behavioural Manipulation

• Neurons in the primate prefrontal cortex (PFC) appear to maintain information about cues during the delay phase of a working memory task.

• These neurons continue firing while information is being held in working memory.

• Increasing the delay increases how long these neurons fire.

Prefrontal Cortex (PFC) & Working Memory

• Individual PFC neurons fire throughout the delay period when an animal holds information online.

• Once the animal makes its response, neuronal activity decreases.

• Demonstrates how neural activity can be studied by manipulating behavioural task demands.

4 Dimensions of Brain Observation

• Spatial resolution.

• Spatial scope.

• Temporal resolution.

• Temporal scope.

Spatial Resolution vs Spatial Scope

• Spatial resolution: How precisely a method can identify the location of brain activity or structures.

• Spatial scope: How much of the brain can be observed.

**Generally, greater spatial resolution comes at the expense of spatial scope, and vice versa.

Temporal Resolution vs Temporal Scope

• Temporal resolution: How precisely a method can distinguish changes over time.

• Temporal scope: How long activity can be monitored.

**Generally, greater temporal resolution can come at the expense of temporal scope, and vice versa.

7 Key Considerations When Choosing a Brain Measurement Method

• Direct vs indirect measurement.

• Freely moving/naturalistic behaviour.

• Invasiveness.

• Can it be done pre-mortem?

• Signal-to-noise ratio.

• Subjects.

• Cost.

Direct vs Indirect Measurement

• Direct: Measures neural activity itself (e.g., electrical activity using electrophysiology).

• Indirect: Measures a byproduct or correlate of neural activity.

• Example: fMRI measures changes in blood oxygenation as a proxy for neural activity.

Freely Moving / Naturalistic Behaviour

• Whether subjects can move and behave naturally during measurement.

• Some methods restrict movement.

• Example: fMRI requires subjects to lie inside a scanner, limiting naturalistic behaviour.

Invasiveness & Pre-Mortem Measurement

• Invasiveness: Whether a method requires entering or altering the body/brain (e.g., implanting electrodes).

• Pre-mortem: Whether measurements can be obtained while the subject is alive.

• Post-mortem tissue has limitations because the circumstances surrounding death can substantially affect brain function.

Signal-to-Noise Ratio (SNR)

• Measures how robust a signal is relative to background noise.

• Higher SNR = clearer, more reliable signal.

• Lower SNR = more noise relative to the signal being measured.

Human vs Non-Human Animal Brain Research Methods

• Humans:

  • Behaviour.

  • Blood/CSF/tissue samples.

  • Imaging.

  • Electrophysiology.

• Non-human animals:

  • All of the above, plus cell-type-specific monitoring and neurochemical monitoring.

Behaviour as a Measure of Brain Function

• All behaviour is fundamentally derived from brain activity.

• Behaviour can therefore be used to infer brain function.

• Used in clinical assessment and neuroscience research.

3 Applications of Behavioural Assessment

• Psychiatric diagnoses.

• Neurocognitive testing.

• Neurological examinations.

Psychiatric Diagnoses Through Behaviour

• Psychiatric conditions are diagnosed based on behavioural symptoms.

• Examples:

  • Depression.

  • Addiction.

  • Anxiety.

  • Schizophrenia.

Neurocognitive Testing

• Assesses specific cognitive processes and abilities.

• Can reveal which cognitive functions are impaired.

• Example: Identifying cognitive deficits associated with dementia.

Neurological Examinations

• Used to assess neurological function.

• Can help identify the origin of a stroke.

• Can examine dermatomes and myotomes.

Behaviour in Neuroscience Research

• Behaviour is often used to assess the causal impact of experimental manipulations.

• Researchers manipulate brain function and examine resulting behavioural changes.

Blood / CSF / Saliva Samples

• Biological samples used to measure chemicals such as hormones.

• Can be collected multiple times.

• Allows researchers to examine changes in response to events (e.g., a stressor).

Cerebrospinal Fluid (CSF) Collection

• CSF can be collected through a lumbar puncture.

• A needle is inserted between vertebrae to access CSF surrounding the spinal cord.

• The fluid is withdrawn through the needle for analysis.

Tissue Samples

Localized brain tissue samples can be collected and analyzed for:

• Gene transcription.

• Protein translation.

• Other chemicals.

2 Sources of Brain Tissue Samples

• Post-mortem tissue.

• Surgical tissue.

Post-Mortem vs Surgical Tissue

• Post-mortem: Brain tissue collected after death.

• Surgical: Brain tissue collected during a surgical procedure.

• Collection is limited by availability and ethical/practical considerations.

• Post-mortem tissue has caveats because the circumstances of death can affect brain function and tissue measurements.

Non-Dynamic Imaging

• Provides a snapshot of brain structure or molecular characteristics.

• Does not continuously measure changes in brain activity over time.

• Includes:

  • Histology/microscopy.

  • CT/CAT.

  • MRI.

  • PET.

Histology / Microscopy

• Examines brain tissue using microscopic techniques.

• Can identify specific proteins, mRNA sequences, and cellular structures.

• Requires tissue sections.

• Includes immunohistochemistry and in situ hybridization.

Immunohistochemistry

• Uses fluorescently labelled antibodies that bind to specific proteins.

• Allows researchers to identify the location of particular proteins in brain tissue.

• Fluorescent signals are visualized using a microscope.

In Situ Hybridization

• Uses fluorescently labelled nucleotide probes that bind to specific mRNA sequences.

• Allows researchers to identify where particular mRNA molecules are expressed.

• Detects gene expression rather than directly labelling proteins.

Immunohistochemistry vs In Situ Hybridization

• Immunohistochemistry: Fluorescent antibodies bind to specific proteins.

• In situ hybridization: Fluorescent nucleotide probes bind to specific mRNA sequences.

• Both require tissue sections and allow microscopic visualization.

Computerized Tomography (CT / CAT)

• Essentially a 3D X-ray.

• Detects differences in tissue density.

• Has relatively poor spatial resolution.

• Fast and inexpensive.

• Particularly useful for detecting brain bleeds.

• Can be combined with contrast agents.

CT Angiogram

• CT scan combined with a contrast agent.

• Improves visualization/labeling of blood vessels.

• Helps examine vascular structures.

Magnetic Resonance Imaging (MRI)

• Uses an extremely strong magnet and radiofrequency pulses from an electrical coil.

• Causes hydrogen protons in water to emit signals.

• Signals are processed to produce highly detailed brain images.

• Stronger magnets generally produce better image resolution.

MRI – T1 vs T2

Different magnetic field and radiofrequency applications highlight different tissue characteristics.

T1 MRI

T1:

• White matter = lighter.

• Grey matter = darker.

• Ventricles = darker.

T2 MRI

• T2:

  • White matter = darker.

  • Grey matter = lighter.

  • Ventricles = white/brighter.

Diffusion Tensor Imaging (DTI)

• MRI technique used to visualize white matter tracts.

• Examines the diffusion of water molecules through brain tissue.

• Helps identify the organization and pathways of white matter connections.

Positron Emission Tomography (PET)

• Detects radioactive isotopes.

• An isotope is often attached to a drug or tracer that binds to a specific brain target (protein or other molecule).

• The signal indicates where and how much of the target molecule is present.

• Also frequently used in cancer detection.

PET – Radiotracers

• Radioactive isotopes are attached to molecules that bind to specific targets.

• The tracer travels through the body and binds to its target.

• PET detects the radioactive signal to determine the location and amount of the target molecule.

CT vs MRI vs PET

• CT: Detects tissue density; useful for identifying bleeds.

• MRI: Produces highly detailed structural images of brain tissue.

• PET: Detects radioactive tracers to examine the distribution of specific target molecules.

Dynamic Imaging

• Examines changes in brain activity over time.

• Includes:

  • Functional MRI (fMRI).

  • Electrophysiological techniques.

• Allows researchers to monitor activity rather than simply obtain a structural snapshot.

Functional MRI (fMRI)

• MRI specialized to detect changes in blood oxygenation.

• More active brain regions require increased blood supply.

• Measures changes in oxygenated blood as an indirect indicator of neural activity.

• Can examine changes in brain activity over time.

fMRI – Limitations

• Measures a proxy for neuronal activity, not neuronal firing directly.

• Cannot distinguish the activity of individual neurons.

• Does not identify specific cell types.

• Requires subjects to remain in a scanner, limiting naturalistic behaviour.

fMRI & Fear Generalization

• Researchers can pair one stimulus with an aversive event and present another, different stimulus.

• Examine how much fear generalizes from one stimulus to another.

• Individuals with PTSD may show greater fear generalization.

• This can be associated with particular patterns of brain activity.

Electrophysiology

• Measures electrical potentials in the brain.

• Can measure the activity of individual neurons or populations of neurons.

• Includes intracellular, juxtacellular, local field potential (LFP), and EEG recordings.

Intracellular vs Juxtacellular Recordings

• Intracellular: Measures electrical activity from inside a neuron; typically performed ex vivo.

• Juxtacellular: Measures activity from very close to an individual neuron; typically performed in vivo.

Local Field Potentials (LFPs)

• Measure collective electrical activity from populations of nearby neurons.

• Recorded using electrodes inside the brain.

• Reflect the combined activity of multiple neurons rather than one individual neuron.

Electroencephalography (EEG)

• Measures collective electrical activity from outside the brain.

• Uses multiple electrodes attached to the scalp.

• Records electrical signals generated by populations of neurons.

• Can measure oscillatory brain states.

EEG – Temporal vs Spatial Precision

• Very high temporal precision (millisecond timescale).

• Poor spatial precision.

• Much more flexible than MRI.

• Can monitor rapid changes in electrical brain activity.

3 Applications of EEG

1. Seizure diagnosis and monitoring.

2. Sleep staging.

3. Measuring oscillatory brain states.

Single-Unit Recordings

• Measure electrical activity from individual neurons.

• Electrodes are implanted in specific brain regions.

• Can be used to examine the activity of individual neurons.

• Often performed in clinical contexts involving neurological disease.

Single-Unit vs LFP Recordings

• Single-unit: Measures electrical activity of individual neurons.

• LFP: Measures collective electrical activity of nearby neuronal populations.

• Both can be recorded using electrodes implanted in the brain.

• Often used in seizure monitoring.

Clinical Electrophysiology

• Electrodes may be implanted in different brain regions to monitor neural activity.

• Commonly performed in the context of neurological disease, particularly seizure monitoring.

• Often involves monitoring activity in the hippocampus.

• Neural activity can be monitored and, in some cases, targeted with electrical stimulation or current.

Pros vs Cons of Non-Human Animal Research

PROS:

• Simpler brains make them easier to study.

• High anatomical, genetic, and behavioural homology with humans.

• Greater control over environment and past experiences.

• Can record genetically defined cells.

• Can manipulate genes, molecules, and neural circuits.

CONS:

• Findings are not always generalizable to humans.

• Certain psychological phenomena may not be present (e.g., gender).

• Simpler brains do not fully represent human brain complexity.

Anatomical, Genetic & Behavioural Homology

• Homology refers to shared characteristics due to common evolutionary ancestry.

• Non-human animals share many anatomical, genetic, and behavioural similarities with humans.

• Makes animal models useful for investigating brain function.

Genetically Defined Cells

• Animal models allow researchers to identify and monitor specific populations of neurons based on their genetic characteristics.

• Allows more precise investigation of particular cell types.

• Researchers can also manipulate specific genes, molecules, and neural circuits.

GCaMP (Genetically Encoded Calcium Indicator)

• Genetically encoded indicator used to monitor neural activity.

• Fusion of modified green fluorescent protein (GFP) and calmodulin.

• Fluorescence increases when calcium binds to GCaMP.

• Allows researchers to monitor activity in genetically defined neural populations.

GCaMP – Bound vs Unbound States

• Unbound state (low calcium): Little/no fluorescence.

• Bound state (calcium present): Calcium binds to GCaMP, producing fluorescence.

• Neuronal activity/action potentials lead to calcium influx.

• Changes in fluorescence can be used as an indicator of neural activity.

Miniature Microscopes (Miniscopes)

• Small microscopes used to record GCaMP fluorescence in the brain.

• Allow researchers to monitor neural activity in behaving animals.

• Can record activity from many neurons and multiple neural cell types.

• Provide a way to study brain activity during more naturalistic behaviour.