15.2 Neuroimaging techniques and their applications

Neuroimaging techniques revolutionize our understanding of brain structure and function. From MRI's detailed anatomical images to fMRI's real-time activity tracking, these tools offer unprecedented insights into the neural basis of motivated behaviors.

PET imaging reveals neurotransmitter activity, while EEG captures rapid electrical signals. Each method has unique strengths, allowing researchers to explore different aspects of brain function and unravel the complex neural networks underlying motivation and emotion.

Neuroimaging Techniques in Physiological Psychology

Structural and Functional Imaging Methods

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• Neuroimaging techniques provide non-invasive methods to study brain structure and function in living subjects
• Structural imaging techniques provide detailed anatomical information about the brain
  • Computed tomography (CT) uses X-rays to create cross-sectional images
  • Magnetic resonance imaging (MRI) uses strong magnetic fields and radio waves to generate high-resolution 3D images
• Functional imaging techniques measure brain activity through changes in blood flow, metabolism, or neurotransmitter activity
  • Functional MRI (fMRI) detects changes in blood oxygenation
  • Positron emission tomography (PET) uses radioactive tracers to measure metabolic activity
  • Single-photon emission computed tomography (SPECT) tracks blood flow using gamma-emitting radioisotopes

Electrophysiological and Connectivity Imaging

• Electrophysiological techniques record electrical or magnetic signals generated by neuronal activity
  • Electroencephalography (EEG) measures electrical activity via scalp electrodes
  • Magnetoencephalography (MEG) detects magnetic fields produced by neuronal currents
• Diffusion tensor imaging (DTI) allows visualization of white matter tracts and structural connectivity in the brain
  • Uses MRI to measure water molecule diffusion along axon bundles
  • Produces tractography maps showing neural pathways

Strengths and Applications

• Each neuroimaging technique has specific strengths, limitations, and applications in physiological psychology research
• fMRI offers high spatial resolution (millimeters) but lower temporal resolution (seconds)
• EEG and MEG provide excellent temporal resolution (milliseconds) but lower spatial precision
• PET allows measurement of specific neurotransmitter activity but involves radiation exposure
• Multimodal imaging combines techniques to leverage their complementary strengths (PET-MRI)

fMRI for Motivated Behaviors

BOLD Signal and Neural Activity

• fMRI measures brain activity by detecting changes in blood oxygenation and flow, known as the BOLD (Blood Oxygen Level Dependent) signal
• The BOLD signal stems from active neurons requiring more oxygen, leading to increased blood flow in specific brain regions
• Hemodynamic response function (HRF) models the relationship between neural activity and BOLD signal changes
  • Typically peaks 4-6 seconds after stimulus onset
  • Allows researchers to infer timing of underlying neural events

Spatial and Temporal Characteristics

• fMRI provides high spatial resolution, allowing researchers to localize brain activity with millimeter precision
  • Can distinguish activity in adjacent brain structures (amygdala vs. hippocampus)
• Temporal resolution of fMRI lags behind neural activity by several seconds due to the hemodynamic response
  • Limits ability to study rapid cognitive processes

Experimental Designs and Analysis

• Event-related fMRI designs allow researchers to study brain responses to specific stimuli or behaviors related to motivation
  • Can present brief rewards or decision-making tasks and measure resulting brain activity
• Block designs compare brain activity between extended periods of different task conditions
  • Useful for studying sustained motivational states or mood induction
• Advanced fMRI techniques provide insights into functional connectivity and distributed neural representations of motivated behaviors
  • Resting-state fMRI examines intrinsic brain networks during task-free conditions
  • Multivariate pattern analysis (MVPA) detects subtle patterns of activity across multiple voxels

PET for Brain Function and Neurotransmitter Activity

Principles and Tracer Types

• PET imaging involves the injection of radioactive tracers that emit positrons, which are detected to create 3D images of brain activity or receptor distribution
• Different radioactive tracers measure various aspects of brain function
  • $^{18}$F-FDG (fluorodeoxyglucose) tracks glucose metabolism
  • $^{15}$O-water measures regional cerebral blood flow
  • $^{11}$C-raclopride binds to dopamine D2 receptors

Neurotransmitter Imaging

• PET can quantify neurotransmitter receptor density and occupancy, providing valuable information about the brain's chemical signaling systems
• Radiolabeled ligands specific to different neurotransmitter systems allow for the study of their roles in motivated behaviors and related disorders
  • Dopamine (reward and addiction): $^{11}$C-raclopride, $^{18}$F-fallypride
  • Serotonin (mood regulation): $^{11}$C-DASB, $^{18}$F-MPPF
  • Opioid (pain and pleasure): $^{11}$C-carfentanil, $^{11}$C-diprenorphine

Applications and Limitations

• PET imaging has been instrumental in understanding the neurochemical basis of addiction, reward processing, and mood disorders
  • Revealed decreased dopamine function in substance use disorders
  • Mapped serotonin receptor changes in depression
• Limitations of PET include lower spatial (3-6 mm) and temporal (minutes to hours) resolution compared to fMRI, as well as exposure to ionizing radiation
• Combined PET-MRI systems offer the potential for simultaneous acquisition of structural, functional, and molecular imaging data
  • Allows correlation of neurotransmitter activity with BOLD signal changes

EEG for Motivation and Emotion

Basic Principles and Frequency Bands

• EEG measures electrical activity generated by large populations of neurons using electrodes placed on the scalp
• EEG provides excellent temporal resolution, allowing for the study of rapid changes in brain activity on a millisecond scale
• Frequency bands in EEG are associated with different cognitive and emotional states
  • Delta (0.5-4 Hz): deep sleep, unconsciousness
  • Theta (4-8 Hz): drowsiness, meditation, memory consolidation
  • Alpha (8-13 Hz): relaxed wakefulness, inhibition
  • Beta (13-30 Hz): active thinking, focus, anxiety
  • Gamma (>30 Hz): cognitive processing, conscious perception

Event-Related Potentials and Emotional Processing

• Event-related potentials (ERPs) derived from EEG data provide information about the timing and sequence of neural processes in response to specific stimuli or events
  • P300 component reflects attention and context updating
  • N400 component associated with semantic processing
• EEG asymmetry, particularly in the frontal regions, has been linked to emotional valence and motivational direction (approach vs. withdrawal)
  • Greater left frontal activity associated with positive affect and approach motivation
  • Greater right frontal activity linked to negative affect and withdrawal motivation

Advanced Analysis Techniques

• Time-frequency analysis of EEG data can reveal oscillatory patterns associated with various aspects of motivated behavior and emotional processing
  • Wavelet analysis decomposes EEG signal into time-frequency representations
  • Allows study of event-related synchronization and desynchronization
• Advanced EEG techniques provide insights into the neural networks involved in motivation and emotion
  • Source localization estimates the neural generators of scalp-recorded activity
  • Connectivity analysis (coherence, phase synchrony) reveals functional interactions between brain regions