Problem 3 Flashcards
Single cell recording
An electrode is inserted into an animals brain
–> this way changes in electrical activity can be measured
but: done extracellularly to avoid damage
What is the primary aim of single cell recording ?
To determine what experimental manipulations produce a consistent change in the response rate of an isolated cell
e.g. does the firing rate increase when the p. moves his arm
Baseline activity
Refers to the fact that neurons are constantly firing even in the absence of stimulation
–> varies widely from brain area to area
Receptive field
Refers to a region of space a single cell responds to
–> preferred area
Multiunit recording
Technique of single cell recording but where hundreds of cells can be recorded at the same time
Electroencephalography
EEG
Measures electrical potentials of neuron populations through electrodes that are placed on the scalp
–> the recording from each electrode reflects the electrical activity of the underlying brain region
In which ways are EEGs helpful ?
One may be able to detect abnormalities in brain function
–> as it shows overall brain activity
Event related potential
ERP
Refers to a change in electrical activity that is shown in form of a tiny signal embedded in the ongoing EEG that was triggered by a stimulus
–> this evoked signal can be extracted from the global EEG signal
THUS: potential that is related to a specific motor or sensory event
Electrocortogram
ECoG
Similiar to EEG except that the electrodes are placed directly on the surface of the brain
–> only appropriate for people undergoing neurosurgical treatment
Why is ECoG far less signal distortive than EEG?
The electrodes measure electrical signals before they pass through the scalp and skull
–> thus one has much cleaner signal results in excellent spatial + temporal resolution
Positron emmission tomography
PET
Refers to a functional imaging technique that uses radiotracers (radioactive) to visualize and measure changes in metabolic processes and blood flow while the participant is engaged in cognitive tasks
–> this way brain regions that are activated are identified
Where is the primary source of the EEG ?
Synchronized synaptic activity in populations of cortical neurons
Dipole
Is composed of two equal and opposite charges that are separated by a small distance
Sink
Dipole
Region of negative charge
Source
Dipole
Region of positive charge
How does an EEG measure ERPs with the aid of dipoles ?
- Electrodes detect the sum of positive and negative charges in their vicinity
- Electrode can only detect dipoles that are closer to either the positive or negative end of the dipole
- The dipoles from multiple neurons in a region will sum together
- -> sum is measurable as a single dipole - To be able to sum these together the neurons must be
a) arranged in a parallel fashion
- -> if not the signals will cancel each other out
b) synchonously active
Tangential dipole
Are parallel oriented to the surface
Volume conduction
Is the process by which a pool if ions repels nearby ions of the same charge
–> they in turn do the same with other ions of the same charge and on and on ..
THUS: a “wave” of charge travels through the extracellular space, which is how signals propagate through a wire
Does the volume conduction also occur in the brain in the same manner ?
No,
because that would mean a cascade of firing APs thus constant seizures
also: dipoles are not all the same size so signals from a large dipole travels much farther
Capacitor
Refer to 2 pools of charges that are separated by an insulated layer
Dielectric
Refers to an insulated layer that separates the 2 pools of the capacitor
–> this way the charges stay separated and don’t mix and become neutral
Can MEG detect dipoles anywhere in the cortex ?
No,
as it can only measure magnetic fields that leave the skull
Can EEG detect dipoles anywhere in the cortex ?
It can measure any dipole whose source and sink are not equidistant to the electrode
–> thus it is sensitive to both tangential and radial dipoles
Limitations in the detection/measurement of dipoles of the EEG
The size of the electric field produced by a set of neurons has to be large enough to be measurable at the surface
–> the larger the pool of neurons involved in synchronous activity, the stronger the electric field, and the further they can be measured
Why is electrode gel necessary ?
Whilst EEG signal within the brain is carried via volume conduction, once the signal reaches the skull it can no longer be carried by ions because they can’t pass through the skull
–> thus the gel is used
How does EEG solve the problem of external noise
- Passive shielding
2. Active electrodes
Passive shield
Conductive metal that surrounds the region to be shielded and cancels out electromagnetic radiation
N170 component
Refers to a negative-going wave over visual cortex
–> typically peaks around 170 ms after stimulus onset
Why is the N170 notable ?
Because it is larger when the eliciting stimulus is a face compared with when the stimulus is a nonface object
–> thus the brain is able to distinguish between faces and other objects
What did studies testing N170 find?
- Face processing is largely automatic but can be modulated by attention under some conditions
e. g. faces hard to percive - It is modulated by expertise
e. g. bird experts exhibit an enhanced N170 in response to birds - It is abnormal in ASD children
- Face specific processing is present early in infancy and becomes better over time
What are ERPs especially useful for ?
- Answering question about the timing of mental processes
- Determining which cognitive processes are influenced by experimental manipulation
- Assessing the anticipatory processes that occur before a stimulus onset
- Monitoring mental activity in the absence of a behavioral response
When are ERPs not really helpful ?
- When averaging of many trials is not possible
- Processes slower than 2 seconds are difficult to see in ERP
- For answering neuroanatomical question
ERP components can be divided into 3 main categories.
Name them
ERP components are defined by their polarity (positive or negative going voltage), timing, scalp distribution, and sensitivity to task manipulations
- -> there are
1. Exogenous
- Endogenous
- Motor
Exogenous sensory component
Are triggered by the presence of a stimulus
BUT: may be modulated to some degree by top down processes
Endogenous component
Refelct neural processes that are entirely task dependent
Motor component
Accompany the preparation + execution of a given motor response
Why do the components usually begin with either a P or an N followed by a number?
P = Positive going
N = Negative going
Number = Indicates the peak latency waveform or the ordinal position of it
P3/ P300 Wave
Is elicited in the process of decision making, thus categorizing and evaluating a stimulus
–> most common endogenous ERP component as it is linked to a person’s reaction to a stimulus
Bereitschaftspotential/
Readiness potential
Refers to the negative voltage/activity than can be observed over a period of several hundred ms before voluntary muscle movement
–> if one creates averaged ERPs time locked to motor response rather than stimulus one can see the ERP components reflecting the processes that lead up to the response
Lateralized readiness potential
Increase in electrical activity at the surface of the brain reflecting the preparation of motor activity on a certain side of the body
–> thus happens when a person gets ready to move one arm, leg, or foot.
Contra vs ipsilateral
A portion of the RP is larger over the hemisphere contralateral to the response than over the ipsilateral hemisphere
–> the difference in voltage between the two hemispheres can be used to isolate the response-specific activity.
The process of orienting our selective attention can be divided into 2 categories.
Name them.
- Voluntary attention
2. Reflexive attention
Overt attention
When you turn your head to orient toward a stimulus
Covert attention
Paying attention to something w/o orienting your head or eyes to it
Bottleneck theory
Suggests that individuals have a limited amount of attentional resources that they can use at one time
–> info and stimuli are ‘filtered’ somehow so that only the most salient and important information is perceived
e.g. involves early vs late selection
Early selection
Stimulus can be selected for further processing
or
it can be deemed irrelevant before perceptual analysis of the stimulus is complete
Late selection
- All inputs are processed equally by the perceptual system
- Attentional selection determines what will undergo additional processing and be represented in awareness
–> thus attentional processes cannot affect perceptual analysis of stimuli
Cueing studies
Are used to measure the effect of attention
- Presented a cue that directs attention to one location on a video screen
- Must press button as fast as they can following presentation of target stimulus
What do findings of cueing studies show ?
- Participants respond faster when the cue correctly predicts the target’s location
- RTs are slower when the stimulus appears at an unexpected location
Reflexive (exogenous) attention
Refers to things in the environment that attract our attention w/o our cooperation
e.g. extremely salient stimuli
Inhibitory aftereffect/
Inhibition of return (IOR)
When more than 300 ms pass btw the task irrelevant stimulus and the target
–> then participant will respond more slowly to stimuli that appear in vicinity of the flash
Reflexive cueing task
Hopefinger
Recorded ERPs in response to target stimuli and found that
- Short cue to target inter stimulus interval
–> P1 wave is larger for targets that quickly follow a sensory cue at the SAME location + vice versa
- Long cue to target inter stimulus interval
–> P1 wave is smaller for targets that follow a sensory cue at the SAME location but larger where both occur at DIFFERENT ones
