W1L2 - Electrical signals in the brain Flashcards
What are the methods for intracanial, extracranial (a) electrical recordings and (b) electrical stimulations
Intracranial Recording
- Single cell animal studies
- ECoG
Extracranial Recording
- EEG
- ERP
Intracranial Stimulation
- DCES
Extracranial Stimulation
- tDCS
Extracellular Recordings of Single Neurons: What did anesthetised and awake behaving studies on anmmal tell us? Can we study multiple neurons?
Anaethsized Studies
- Sensory and Motor
Awake behaviour Studies:
- Higher level functions like attention
Mulitple neurons can be studied with electrode arrays
What is Local Field Potential. What is it? What are the cons?
LFPs:
- Not related to individual neurons
- Measures neural activity up to 3mm from electrode
- Use same electrocode as single unit recording
Cons:
- LFPs likely represents summed activity of large numbers of neurones with synchronous input
- More likely to reflect type cells with dendrites facing in the same direction away from cell body
- e.g., pyramidal cells
- Same type of cells
ECoG overview. What is it used to clinically
- Uses 2-256 electrocedes in an array placed directly on exposed surface
- Records LFP (Probably pyramid cells)
- Used to treat epilepsy by identifying region generating seizures
- DCES uses the same electrode
ECoG Pros as a Research Tol
- Understanding neural function
- High spatial and temporal resolution
- Both single and multi-unit recording
- Confirms electrophysiological recordings from animal models
- Understanding how indirect methods relate to direct neural responses
- BOLD poor temporal
- EEG poor spatial
ECoG + BOLD Finger Flexion Results. What is the implication?
7T fMRI prior to ECoG in finger flexion
“to what degree does localisation of neural activity from BOLD correspond to ECoG”
Results
- High frequency ECoG (65-95Hz) matches localised BOLD
- BOLD co-localises rapid neural changes at fine spatial scale (mm scale)
Implications
- Showed that 7T fMRI reliably captures important aspects of neural activity
EEG Overview. What are the cons?
- Electrical activity measured from large number of synchronous, aligned neurons
- Usually recording pyramial neurons (same as LFP)
- Best for Gyri, not sulci
EEG Pros and Cons
Pros
- Cheap
- Good Temporal Resolution
Cons
- Poor Spatial Resolution
- Not good for deep structures
- Voltage drops off rapidly with distance, so activity from deep sources is difficult to detect
How does EEG move to ERP
x1000 trials + signal averaging
DCES Overview and Cons
Overview
- Stimulation of Single Neurons
- Mostly on awake behaving non-human primates
- Using ECoG electrodes to stimulate
Cons
- Clinical patients limit the basic research
- Must have epilepsy
- No choice in electrode location
- Gyri; Biased to seizures
- Surgery
- Expensive
tDCS Overview and Aim
Overview
- Passing a weak DC current between electrodes placed on the scalp
- Extra-cranial
Aim
- Primarily to improve mental function
tDCS vs other techniques
- Does not require medical intervention (Non-invasive)
- Uses DC to influence brain activity
- Uses weak current to influence brain activity
How does tDCS work
- Small current passed between two electrodes on the scalp
- Assume that current flows though the brain
- Neurons under the anode more easily activated than they otherwise would be
- Excitation: Anode
- Inhibition: Cathode
- Not generating action potentials, but changing response of neurons
- Neurons under the anode more easily activated than they otherwise would be
tDCS pros and cons
Pros
- Non invasive
- Cheap to purchase and use
- Easy to use
- Safe when using established protocols
- Straight forward ethics
Cons
- Precise mechanisms elusive
- Difficult to precisely and selectively stimulate a target brain region
Does scientific evidence suggest tDCS is effective? What is the criticism (of the scientific evidence)?
Meta-analysis found no reliable effect.
Criticism of meta-analysis
- Not enough studies
- Hetereogneity of poor designs (gold-rush)
What are the difficulties in establishing whether tDCS is effective?
- High prevalence of “adverse” events = strong placebo
- No active sham control
- Participants can tell whether they’re in sham or active
Has the rapid increase in studies contributed to the tDCS confusion?
- High rates of “new” findings biases against verification
- Gold rush mentality (citations, funding, no replication)
Has the way we do science contributed to the confusion tDCS. What are the phenomenas?
1.) File drawer phenomenon
- Publish positive results
- Ignore negative or non confirmatory results
2.) Forking path phenomenon
- Lack of specific predictions in the absence of a good understanding of how tDCS works
3.) Increase in importance of science communication
- Expectation > Truth with single result
- Single result can define field if widely promoted
