Psychophysiological Methods in Neuroscience -Summary Notes
Psychophysiological Methods in Neuroscience
Psychophysiological methods are noninvasive neuroscience techniques used across disciplines to answer questions about psychology, mental events, and behavior.
Many techniques are classified as psychophysiological, each with strengths and weaknesses.
Understanding these techniques is important for researchers and consumers to evaluate the meaning of results.
Learning Objectives
Define psychophysiology within neuroscience.
Review and compare examples of psychophysiological methods.
Understand the advantages and disadvantages of different methods.
History
Phineas Gage case study (mid-19th century):
Railroad worker who experienced a severe brain injury when a tamping iron went through his head.
Lost part of his left frontal lobe but survived.
Personality changes: became impulsive, had trouble carrying out plans, and used vulgar profanity.
This case suggests specific brain areas are associated with psychological phenomena.
Studying the brain is important in psychology.
Methods have been developed to safely measure nervous system activity to understand psychology and its relationship with biology.
Introduction
Psychophysiology: research where the dependent variable (what is measured) is a physiological measure, and the independent variable (what is manipulated) is behavioral or mental.
Typically noninvasive with awake human participants.
Physiological measures include blood flow, neural activity, heart rate variability, and eye movements.
These measures provide information about emotion, cognition, and their interactions.
Offers flexible tools for researchers to answer questions about behavior, cognition, and health.
Psychophysiological methods are a subset of neuroscience methods.
Brain injuries have widespread consequences for other areas.
Psychophysiology examines the relationship between physiology and behavior/mental events, but does not replace the latter with the former.
Example: Happiness is a state of pleasurable contentment associated with physiological measures, but the measures are not happiness itself.
Inferences can be made about cognitive/emotional state based on self-report, physiology, or overt behavior.
Psychophysiology addresses inferences about internal events and the physiology itself.
Central Nervous System (CNS)
Overview of popular psychophysiological methods, each with a range of data-analysis strategies.
Methods discussed focus on the central nervous system.
Structural Magnetic Resonance Imaging (sMRI)
Noninvasive technique to view anatomical structures.
Participant is placed in a strong magnetic field, causing atoms in the body to align.
Low-energy radio frequencies are pulsed, absorbed by the atoms causing them to tip over.
Atoms return to their aligned state, giving off electromagnetic radiation, which is measured and transformed into a 3D picture.
In research, sMRI is used to: compare the size of structures in different groups or increase the accuracy of spatial locations measured with functional magnetic resonance imaging (fMRI).
Functional Magnetic Resonance Imaging (fMRI)
Measures changes in tissue activity, such as neural activity during thought.
Builds on sMRI principles.
When neurons fire, they use energy, which is replenished by glucose and oxygen from the blood.
Oxygen is transported by hemoglobin (with binding sites for oxygen).
Oxygenated hemoglobin: hemoglobin with oxygen.
Deoxygenated hemoglobin: hemoglobin without oxygen.
When neurons fire, oxygen is consumed, reducing oxygenated hemoglobin.
The body compensates by providing an abundance of oxygenated hemoglobin.
When neural activity declines, oxygenated hemoglobin returns to its original level (takes seconds).
fMRI measures the change in the concentration of oxygenated hemoglobin (blood-oxygen-level-dependent (BOLD) signal).
Important facts about fMRI:
Measures blood volume and blood flow to infer neural activity (does not measure neural activity directly).
Poor temporal resolution (precision with respect to time), but excellent spatial resolution (ability to distinguish objects in space) when combined with sMRI.
Temporal resolution is on the order of seconds, and spatial resolution is on the order of millimeters.
Inverse relationship between temporal and spatial resolution.
Valuable for identifying specific brain areas associated with tasks.
Clinically, fMRI is used prior to neurosurgery to identify language areas.
fMRI allows researchers to identify differential or convergent patterns of activation associated with tasks.
Excellent tool for comparing brain activation in different tasks and/or populations.
Figure 1: Example of fMRI analyses overlaid on an sMRI image.
Blue and orange shapes represent areas with significant changes in the BOLD signal, indicating changes in neural activation.
Electroencephalography (EEG)
Technique for studying brain activation.
Uses electrodes (2-256) to measure the difference in electrical charge (voltage) between points on the head.
Electrodes are fastened to a flexible cap.
Measures naturally occurring electrical activity in the brain (does not introduce new electrical activity).
Measures neural activity directly (unlike fMRI which measures a correlate of that activity).
Electrocorticography (ECoG): electrodes placed within the skull, directly on the brain, typically used prior to medical procedures for localizing activity (e.g., epileptic seizures).
Invasive procedure allowing for more precise localization.
EEG measures from the scalp are noninvasive.
Localization is less precise when measuring from the scalp because electrical activity must travel through the skull and scalp.
Advantage: excellent temporal resolution (data recorded thousands of times per second).
EEG analyses investigate the change in amplitude or frequency components of the recorded EEG on an ongoing basis or averaged over trials.
Figure 2: Example of EEG analysis output.
Panel A: Changes in the relative strength of different frequencies in the EEG data over time.
Panel B: Changes in the amplitude in the instantaneous EEG voltage over time.
Magnetoencephalography (MEG)
Noninvasive technique for measuring neural activity.
The flow of electrical charge (current) associated with neural activity produces very weak magnetic fields.
Magnetic fields are detected by sensors placed near the scalp.
The number of sensors used varies from a few to several hundred.
Special rooms shielded from magnetic fields are needed to avoid contamination of the signal.
Has the same excellent temporal resolution as EEG.
Not as susceptible to distortions from the skull and scalp.
Magnetic fields pass through tissue relatively unchanged, providing better spatial resolution than EEG.
MEG analytic strategies are nearly identical to those used in EEG.
MEG recording apparatus is more expensive than EEG.
EEG and MEG are excellent for elucidating the temporal dynamics of neural processes.
Example: How long after reading an unexpected word in a sentence does someone recognize it as unexpected?
EEG and MEG methods allow researchers to investigate the degree to which different parts of the brain communicate with each other.
Allows for a better understanding of brain networks and their role in different tasks and psychopathology.
Positron Emission Tomography (PET)
Medical imaging technique to measure processes in the body, including the brain.
Relies on a positron-emitting tracer atom introduced into the bloodstream in a biologically active molecule (e.g., glucose, water, or ammonia).
Positron: a particle much like an electron but with a positive charge.
Example: fludeoxyglucose (similar to glucose) concentrates in areas with higher metabolic needs.
Tracer molecule emits positrons, which are detected by a sensor.
The spatial location of the tracer molecule can be determined based on the emitted positrons.
Allows researchers to construct a 3D image of brain areas with the highest metabolic needs (most active).
Images usually represent neural activity over tens of minutes (poor temporal resolution).
PET images are often combined with computed tomography (CT) images to improve spatial resolution.
Tracers can be incorporated into molecules that bind to neurotransmitter receptors.
Allows researchers to answer unique questions about the action of neurotransmitters.
Few research centers have the equipment required to obtain images or create positron-emitting tracer molecules.
Transcranial Magnetic Stimulation (TMS)
Noninvasive method that causes depolarization or hyperpolarization in neurons near the scalp.
Not considered psychophysiological because the independent variable is physiological.
Qualifies as a neuroscience method because it deals with the function of the nervous system.
Can be combined with psychophysiological methods.
A coil of wire is placed above the participant’s scalp.
Electricity flowing through the coil produces a magnetic field.
The magnetic field travels through the skull and scalp, affecting neurons near the surface of the brain.
When the magnetic field is rapidly turned on and off, a current is induced in the neurons, leading to depolarization or hyperpolarization.
Single- or paired-pulse TMS depolarizes site-specific neurons in the cortex, causing them to fire.
Over primary motor cortex: produces or blocks muscle activity.
Over primary visual cortex: produces sensations of flashes of light or impairs visual processes.
Valuable tool for studying the function and timing of specific processes.
Repetitive TMS produces effects that last longer than initial stimulation.
TMS is able to explore neural plasticity (the ability of connections between neurons to change).
Has implications for treating psychological disorders and understanding long-term changes in neuronal excitability.
Peripheral Nervous System
Psychophysiological methods have focused on the central nervous system, but considerable research has also focused on the peripheral nervous system.
Methods include skin conductance, cardiovascular responses, muscle activity, pupil diameter, eye blinks, and eye movements.
Skin Conductance
Measures the electrical conductance (inverse of resistance) between two points on the skin, which varies with the level of moisture.
Sweat glands (controlled by the sympathetic nervous system (SNS)) are responsible for this moisture.
Increases in skin conductance can be associated with changes in psychological activity.
Example: studying whether psychopaths react to fearful pictures in a normal way.
Provides relatively poor temporal resolution.
Easy way to measure SNS response to a variety of stimuli.
Cardiovascular Measures
Include heart rate, heart rate variability, and blood pressure.
The heart is innervated by the parasympathetic nervous system (PNS) and SNS.
Input from the PNS decreases heart rate and contractile strength.
Input from the SNS increases heart rate and contractile strength.
Heart rate can be monitored using a minimum of two electrodes.
Psychological activity can prompt increases and decreases in heart rate, making it a sensitive measure of cognition.
Measures of heart rate variability are concerned with consistency in the time interval between heartbeats.
Changes in heart rate variability are associated with stress and psychiatric conditions.
Figure 3: Example of an electrocardiogram (used to measure heart rate and heart rate variability).
Cardiovascular measures allow researchers to monitor SNS and PNS reactivity to various stimuli or situations.
Example: Does an arachnophobe's heart rate increase more than someone not afraid of spiders when viewing pictures of spiders?
Electromyography (EMG)
Measures electrical activity produced by skeletal muscles.
Measures the voltage between two points (similar to EEG).
Used to determine when a participant first initiates muscle activity to engage in a motor response or the degree to which a participant begins to engage in an incorrect response.
Used in emotion research to identify activity in muscles used to produce smiles and frowns.
Possible to detect very small facial movements that are not observable from looking at the face.
The temporal resolution of EMG is similar to that of EEG and MEG.
Eye Blinks, Eye Movements, and Pupil Diameter
Eye blinks are assessed using EMG electrodes placed below the eyelid.
Electrical activity associated with eye blinks or eye movements can be measured with electrodes placed on the face near the eyes.
A camera can be used to record video of an eye, which is valuable when determining the absolute direction of gaze.
With a calibration period, eye position is extracted from each video frame and compared with data from the calibration phase.
Allows researchers to identify the sequence, direction, and duration of gaze fixations.
Example: When viewing pleasant or unpleasant images, people spend different amounts of time looking at the most arousing parts, which can vary as a function of psychopathology.
Pupil diameter can be measured and recorded over time from the video record.
Pupil diameter is controlled by SNS and PNS inputs.
Commonly used as an index of mental effort when performing a task.
When to Use What
There are no definitive answers for which tool is right for a given question.
Guidelines to consider:
Interested in brain structures associated with cognitive control? → fMRI or PET.
Interested in how cognitive control unfolds over time? → EEG or MEG.
Interested in studying the bodily response to fear? → Peripheral nervous system measures.
Key: Define the question that is needed to be answered.
Think about the strengths and weaknesses of different psychophysiological measures.
Pick one, or several, whose attributes work best for the question at hand.
It is common to record several at once.
Conclusion
Overview of psychophysiological methods shows different techniques available to researchers studying a range of topics.
Some studies use several methods in sleep assessments or multimodal neuroimaging.
Psychophysiological methods have applications outside of mainstream psychology in areas where psychological phenomena are central.
Examples of applications for each method are provided.
The field is continually evolving, with new methods and applications being developed.
The variety of methods and applications provide limitless possibilities for researchers.
Vocabulary
Blood-oxygen-level-dependent (BOLD): The signal typically measured in fMRI that results from changes in the ratio of oxygenated hemoglobin to deoxygenated hemoglobin in the blood.
Central nervous system: The part of the nervous system that consists of the brain and spinal cord.
Deoxygenated hemoglobin: Hemoglobin not carrying oxygen.
Depolarization A change in a cell’s membrane potential, making the inside of the cell more positive and increasing the chance of an action potential.
Hemoglobin: The oxygen-carrying portion of a red blood cell.
Hyperpolarization A change in a cell’s membrane potential, making the inside of the cell more negative and decreasing the chance of an action potential.
Invasive Procedure: A procedure that involves the skin being broken or an instrument or chemical being introduced into a body cavity.
Lesions: Abnormalities in the tissue of an organism usually caused by disease or trauma.
Neural plasticity: The ability of synapses and neural pathways to change over time and adapt to changes in neural process, behavior, or environment.
Neuroscience methods: A research method that deals with the structure or function of the nervous system and brain.
Noninvasive procedure A procedure that does not require the insertion of an instrument or chemical through the skin or into a body cavity.
Oxygenated hemoglobin: Hemoglobin carrying oxygen.
Parasympathetic nervous system (PNS): One of the two major divisions of the autonomic nervous system, responsible for stimulation of “rest and digest” activities.
Peripheral nervous system: The part of the nervous system that is outside the brain and spinal cord.
Positron: A particle having the same mass and numerically equal but positive charge as an electron.
Psychophysiological methods: Any research method in which the dependent variable is a physiological measure and the independent variable is behavioral or mental (such as memory).
Spatial resolution: The degree to which one can separate a single object in space from another.
Sympathetic nervous system (SNS): One of the two major divisions of the autonomic nervous system, responsible for stimulation of “fight or flight” activities.
Temporal resolution: The degree to which one can separate a single point in time from another.
Voltage: The difference in electric charge between two points.