Task 3

Question 1

EEG

Answer 1

high temporal resolution
-quite sensitive to noise
- EEG can detect tangential and radial dipoles
EEG yields a continuous recording of overall brain activity.

EEG may originate from both cortical sulci, which would produce tangential dipoles, and cortical gyri, which would produce radial dipoles, as long as these are large enough or near enough to the skull to be measurable. Deeper sources of electrical stimulation are more “noisy”.

Question 2

MEG

Answer 2

temporal & spatial resolution

• rather rarely because expensive
• and easily distorted, because it measures weak magnetic fields
• Meg cannot detect radial (standing up like I)dipoles

Question 3

EEg signal

Answer 3

detect little Curren produced by action potentials within the brain. detects extracellular charge( doesn’t detect wat happens in the cell but the difference in multiple cell)

Question 4

external noise

Answer 4

easy to control

Question 5

2 ways with egg can be interpreted

Answer 5

by using compute simulations (simpler, pro and con because of confounding)

epileptic patients ( more realistic, but since epileptic not sad eto generalise)

Question 6

Single-cell recording

Answer 6

The primary goal of single-cell recording experiments is to determine what experimental manipulations produce a consistent change in the response rate of an isolated cell (i.e. moving arm). These measurements of changes are made against a baseline activity, which varies widely between brain areas.
It soon became clear that the behaviour of cells might be more than just the sum of its parts. Th e function of an area might be better understood by identifying the correlations in the firing patterns of groups of neurons rather than identifying the response properties of each individual neuron. This led to the development of multiunit recording

Question 7

Electroencephalography (EEG)

Answer 7

when populations of neurons are active together, they produce electrical potentials large enough to be measured by non-invasive electrodes that have been placed on the scalp. The electrodes (usually 20 – 256), embedded in a cap, are bigger than those used for single-cell recordings.
The electrical potential can be recorded at the scalp because the tissues of the brain, skull, and scalp passively conduct the electrical currents produced by synaptic activity . The fluctuating voltage at each electrode is compared to the voltage at a reference electrode, which is usually located on the mastoid bone at the base of the skull. The recording from each electrode reflects the electrical activity of the underlying brain region. The record of the signals is referred to as an electroencephalogram. EEG yields a continuous recording of overall brain activity.

Question 8

Erpgraphs

Answer 8

While EEG focuses on the brain’s global activity, one approach focuses on how brain activity is modulated in response to a particular task, which requires extracting an evoked response from the global EEG signal. EEG traces recorded from a series of trials are averaged together by aligning them relative to an external event, such as the onset of a stimulus or response. This alignment eliminates variations in the brain’s electrical activity that are unrelated to the events of interest. ERPs can be important for detecting tumours or other illnesses.

N is negative, p positive

N100 is negative that occured 100 milliseconds

waves are 100ms are modulated by attention

Question 9

difference time frequency & Erp

Answer 9

Just as with ERP, activity is linked to an event and measured over time; but the strength of the activity in different EEG frequencies is measured, rather than summing the signal of all of the activity

Question 10

MEG

Answer 10

 Same temporal resolution but better spatial resolution

As with EEG, MEG traces can be recorded and averaged over a series of trials to obtain event-related fi elds (ERFs

Thanks to the good spatial resolution, MEG is used in neurosurgery and helps to identify seizures and locate tumours.

Question 11

MEG has 2 cons:

Answer 11

(1) It can only detect surrent flow if that flow is oriented parallel to the surface of the skull. For this reason, the neurons that can be recorded with MEG tend to be located within sulci, where the long axis of each apical dendrite tends to be oriented parallel to the skull surface.
(2) Magnetic fields generated by the brain are extremely weak. To be effective, the MEG device requires a room that is magnetically shielded from all external magnetic fields, including the Earth’s magnetic field. To detect the brain’s weak magnetic fi elds, the sensors, known as superconducting quantum interference devices (SQUIDS), are encased in large, liquid-helium-containing cylinders that kee p them colder than 4 degree s Kelvin.

Question 12

Electrocorticogram (ECoG)

Answer 12

Electrocorticogram (ECoG)
ECoG is similar to EEG, excpect that electrodes here are placed directly on the surface of the brain (either outside dura or beneath it). Thus, ECoG is appropriate only for people who are undergoing neurosurgical treatment. The implants are left there for a week.
ECoG electrodes measure electrical signals before they pass through the scalp and skull. Thus, there is far less signal distortion compared with EEG. Th is much cleaner signal results in excellent spatial and temporal resolution. The electrodes can also be used to stimulate the brain and to map and localize cortical and subcortical neurologic functions, such as motor or language function. In ECoG studies, neural repsonses were found shortly (100ms) after the stimulus was presented.

Question 13

dipole

Answer 13

 Dipole: a region of positive charge separated from a region of negative charge by some distance.
 Source: The region of positive charge
 Sink: The region of negative charge

Question 14

The neural source of EEG

Answer 14

EEG arises from synchronized synaptic activity in populations of cortical neurons (pyramidal cells organized along cortical columns). Excitation of the postsynaptic neurons creates an extracellular voltage near the neural dendrites that is more negative than elsewhere along the neuron.
In the case where an electrode is equidistant from both source and sink of a dipole, the electrode will measure a net neutral; so, an electrode can only detect dipoles when the electrode is closer to either the positive or negative end of the dipole.

Question 15

2 major types of dipoles can be measured:

Answer 15

2 major types of dipoles can be measured:

1) Radial dipoles: oriented perpendicular to the surface
2) Tangential dipoles: oriented parallel to the scalp surface

Question 16

In EEG, the (positive and negative) dipoles from multiple neurons in a region will sum together and to measure a nonzero (measurable) signal, neurons must be

Answer 16

(a) Arranged in a parallel fashion
 If the neurons are all arrayed in the same orientation, then their signals can sum to form a larger signal. In any other configuration, the individual dipoles’ positive and negative ends will sum and cancel each other out.
(b) Synchronously active
 Yields a net charge on the scalp-facing side of the dipole sheet, rather than charges cancelling each other out and
 Gives us a signal large enough to be measured

Question 17

How EEG travels from the brain to the recording device

Within the brain

Answer 17

Volume conduction is responsible for the propagation of the EEG signal within the brain
Volume conduction = process by which a pool of ions repels nearby ions of the same charge
In the brain, different tissue densities may impede or permit ion flow, based on their inherent electrical properties as well as physical arrangement. Signals from large dipoles travel further than those of small dipoles.

Question 18

Capacitor =

Answer 18

Capacitor = two pools of charges separated by an insulating layer (a dielectric).

Question 19

settling

Answer 19

Electrons flow between the gel and electrode creating half-cell (DC) potentials. This is why EEG system users are instructed to allow the electrodes to “settle” for a few minutes before beginning to record: the settling corresponds to the electrochemical interaction between the electrode and gel reaching a steady state.

Question 20

Spatial smearing

Answer 20

Spatial smearing = a dipole will influence the charge in a range of scalp locations, not just on the scalp immediately above the dipole

Question 21

eeg limitation

Answer 21

The limitation of EEG is therefore not the alignment of dipoles but the size of the electric field that has to be large enough to be measurable at the surface
.
EEG measures ALL electrical activity (that coming from the brain but also that coming from other sources). This voltage from other sources is noise. Signal to noise ratio=high

Question 22

Why is electrode gel necessary?

Answer 22

Why is electrode gel necessary?
The EEG signal within the brain is carried via volume conduction, but once the signal reaches the skull, it can no longer be carried by ions because ions cannot pass through the skull. The electrode gel saturatew the space beneath an electrode, filling in the air pockets between hairs and thus providing a conductive path from scalp to electrode. This makes another capacitor.

Question 23

internalvs externalnoise

Answer 23

External noise
The electrical power supply in buildings (External Noise) is the loudest kind of noise. Electric field strength decreases exponentially with the distance from the source to the measurement. This problem of external noise may be solved by passive shielding (piece of conductive metal that surrounds the region to be shielded and cancels out or shunts away electromagnetic radiation) and active electrodes (place an amplifier as close to the electrode as possible).
Internal noise
Internal noise is problematic because it cannot be excluded or eliminated. These noise sources can be minimized to some degree by controlling environmental factors and participant behaviour, but, generally, internal sources of noise must be dealt with through various methods of filtering and artifact detection in postprocessing.

Question 24

Event-related potentials (ERPs)

Answer 24

electrical potentials generated by the brain that are related to specific internal or external events (e.g., stimuli, responses, decisions).

ERPs are commonly used to determine which specific mental process is influenced by a given manipulation. ERPs can also be used to assess the anticipatory processes that occur before a stimulus and the performance monitoring processes that occur during and after a behavioural response.

Another key attribute of ERPs is that they can be used to “covertly” monitor mental activity in the absence of a behavioural response

In order to measure ERPs, the action being studied must be time-locked to a discrete, observable event. We must also be able to repeat the condition we are examining for many times, in order to average it later on (impossible with for example deception). ERP studies usually focus on specific ERP components (i.e. related to language, motor preparation, executive functioning, …)

Question 25

rep component

Answer 25

ERP component = voltage deflection that is produced when a specific neural process occurs in a specific brain region.
 Many components will be elicited by a stimulus in a given task, and the different components sum together to produce the observed ERP waveform (+ and – peaks)

Question 26

Naming components

Answer 26

Naming most commonly begins with N or P (+ or -), followed by a number indicating the peak latency (e.g. 170 ms) or the ordinal position of the peak within the wave (e.g. P2  this is preferable because sometimes the exact latency fluctuates). Using the polarity to label the component is also problematic because any given component will produce a positive potential on one side of the head and a negative potential on the other side of the head. Moreover, some components vary in polarity depending on the experimental conditions

Another problem is that a given label may refer to a completely different component when different sensory modalities are considered (i.e. auditory P1 is different from visual P1). .

Question 27

ERP components can be divided into three main categories:

Answer 27

(a) exogenous sensory components that are obligatorily triggered by the presence of a stimulus (but may be modulated to some degree by top-down processes)
(b) endogenous components that reflect neural processes that are entirely task dependent, and
(c) motor components that necessarily accompany the preparation and execution of a given motor responses

Question 28

ERP Eigenschaften

Answer 28

p3a-novelty-is elicited by highly distinctive improbable stimuli, even when the task does not require discrimination of these stimuli

p3b sensitive to task probability- That is, it is larger for improbable stimuli only when the task requires sorting the stimuli in a way that makes a given stimulus category improbable

In infants and children, ERPs can be used to investigate the mental processes of their poor behavioural control. In elderly, we can investigate whether only specific mental processes slow down. In mental health disorders, we can determine exactly which processes are impaired

Question 29

Emotion related ERP components

Answer 29

Emotion inducing pictures influence many ERP components and might increased them.

• Early posterior negativity (=negative potential over the visual cortex in the N2) is enhanced by positive emotions and reflects the e recruitment of additional perceptual processing for emotion-inducing stimuli.
• Late positive potential (=positive voltage in the P3 wave in the parietal lobe) is extended and distributed through subjective emotional experiences.

Question 30

ERP localization

Answer 30

The spatial resolution of ERP is already limited and it loses even more precision when 2 dipoles are localized together (especially when they are close together – like in (D) which looks similar to (A), and when there is a lot of noise).
When we don’t know how many dipoles were involved in a signal, we add external constraints. Although these constraints produce a unique solution, they do not necessarily produce the correct solution. There is a very small margin if error in localizing ERPs (it is very hard to see whether an error was made). But even though it is very hard, localizations are sometimes possible.
ERPs can be localized using the general hypothesis testing approach: a hypothesis leads to some predictions which are then tested like the prediction of the scalp distribution, the prediction that damage to the location eliminates the ERP component, …
Difference waves = strategy in which the ERP waveform elicited by one trial type is subtracted from the ERP waveform elicited by another trial type
 vulnerable (they isolate processes that are differentially active for 2 trial types)
 with this it was found for example that schizophrenics have slower response selection processes (60ms slower) and that this difference is due to delayed LRP (see figure) and therefore not due to slower perception (because the early P3 waves are very similar).

Question 31

Recording the Electroencephalogram

Answer 31

The EEG is quite small, so the signal from each electrode is usually amplified by 1,000 to 100,000 times. Filters and cut-offs make sure that no noise or unusual values are included in the analysis.
Each electrode name begins with one to two letters denoting a general brain region (Fp for frontal pole, F for frontal lobe, C for central sulcus, P for parietal lobe, O for occipital lobe, T for temporal lobe). The letters are followed by a number that reflects the distance from the midline (1 is close the midline; 5 is far from the midline). Odd numbers are used for the left hemisphere and even numbers are used for the right, with z for zero when the electrode is on the midline.
Against all odds, the more electrodes does not mean the better. It might lead to poorer signal quality and lower statistical power. An intermediate number of electrodes is therefore usually best.

Voltage is the potential for electrical charges to move between two locations, and the EEG is therefore measured as the voltage between two electrodes. One is called the active electrode and the other is called the reference electrode. Since there is no electrically neutral location, it is important to realize that the voltage attributed to a given site is really the potential between two sites, and brain activity at both the active and reference sites contribute to the recorded signal.

Question 32

Artefact rejection and Correction

Answer 32

Artefacts (i.e. eye movements) are picked up by the EEG and require special treatment because they might distort and confound the experiment. This is why usually the participant is asked to maintain his gaze on a fixation cross. Thus might lead to the problem that a large number of trials might be rejected and that there might be ERP activity that is due to the mental effort to fixate and supress blinks.

Question 33

The number of trials that must be performed depends on several factors:

Answer 33

• Size of the ERP response
• Amplitude of unrelated EEG activity
• Amplitude of nonneural activity
• Number of participants
• Magnitude of differences between conditions

Question 34

Extracting averaged ERPs from the EEG

Answer 34

ERPs are usually isolated from the ongoing EEG by a simple averaging procedure. To make this possible, it is necessary to include event codes in the EEG recordings that mark the events that happened at specific times. These event codes are then used as a time-locking point to extract segments of the EEG surrounding each event.
To extract stimulus related activity, the EEG segments following each infrequent stimuli (X) are averaged into one waveform and those following frequent stimuli (O) into another (in the oddball paradigm).

Depending on the clarity of the wave, the number of trials can go as high as 100 – 500 (P1 wave). The averaging procedure is based on an assumptio4n that does not always hold: the timing of the neural response must be the same across trials. Variability in timing can lead to a smaller amplitude or even to a complete cancellation of the averaged response.

Question 35

Attention’s components

Answer 35

• Voluntary attention = our ability to intentionally attend to something
  o Goal-driven
• Reflexive attention
  o Bottom-up, stimulus-driven
• Overt attention
  o When you turn your head to orient towards a stimulus
• Covert attention
  o Not obviously turning your head to a stimulus (something many people seem incapable of

Question 36

The Cocktail Party Effect

Answer 36

The Cocktail Party Effect
= selective auditory attention allows you to follow and participate in a conversation even when you’re in a noisy surrounding (i.e. at a cocktail party)

Question 37

Broadbent’s model

Answer 37

Broadbent’s model = information processing system has a limited-capacity stage or stages through which only a certain amount of information can pass

 Early selection: the idea that a stimulus can be selected for further processing, or it can be tossed out as irrelevant before perceptual analysis of the stimulus is complete
 Late selection: all inputs are processed equally by the perceptual system

o Implies that attentional processes cannot affect our perceptual analysis of stimuli
o Selection takes place at higher stages of information processing that involve internal decisions about whether the stimuli should gain complete access to awareness, be encoded in memory, or initiate a response

Question 38

early vs late selection

Answer 38

 Early selection: the idea that a stimulus can be selected for further processing, or it can be tossed out as irrelevant before perceptual analysis of the stimulus is complete
 Late selection: all inputs are processed equally by the perceptual system

Question 39

• Problem with early selection models:

Answer 39

o Cocktail party effect - sometimes salient information from the unattended ear was consciously perceived, for example, when the listener’s own name or something very interesting was included in a nearby conversation

Question 40

early vs late selection

Answer 40

 Early selection: the idea that a stimulus can be selected for further processing, or it can be tossed out as irrelevant before perceptual analysis of the stimulus is complete

 Late selection: all inputs are processed equally by the perceptual system

Question 41

Anne treisannn

Answer 41

• Anne Tresiman: unattended channel info was not completely blocked from higher analysis but was degraded or attenuated instead
  o Thus, information in the unattended channel could reach higher stages of analysis, but with greatly reduced signal strength

Question 42

Limited capacity processes

Answer 42

• Limited capacity: human performance suffers when overloaded by multiple inputs →assumed by both early-and late selection models
• Capacity limitations result in processing bottlenecks (this overcomes the problem of limited capacity)
• The location of the bottleneck is the central difference between both models

Question 43

Neurophysiology of voluntary visual attention

Answer 43

• EXPERIMENT: subjects are told to covertly attend to stimuli presented at one location and ignore those in another while fixating a central cross
• Modulations in the visual ERPs begin between 70-90ms after stimulus onset (occipital P1; first major positive wave)
• P1 is bigger for stimuli appearing in an attended location (P1 attention effects)  only visuospatial attention
• Attentional affects for more complex tasks (e.g. stimulus colour, orientation, what object is) are observed later in the ERPs
• Earliest spatial attention effects are generated in the extra-striate cortex (early visual processing)

Question 44

Reflexive attention

Answer 44

• Reflective spatial attention: when things in the environment attract our attention without our cooperation  is activated by stimuli that are conspicuous in some way
• The effects of reflective attention can be demonstrated by examining how a task-irrelevant flash of light affects RT to subsequent task-relevant target stimuli  reflexive cueing or exogenous cueing
• Flashes do not predict target location, but responses are still faster in trials where location of light and target is the same (only when the target is presented shortly after the light)
• When time between light and target increases, responses are slower for same location trials (inhibitory aftereffect/inhibition of return)  this prevents being “stuck” at one location for too long which might be dangerous (voluntary attention however could still focus on such locations)

Question 45

Neurophysiology of reflexive visual attention

Answer 45

• Neural processing in the visual cortex is heightened when reflexively attending a stimulus, too (when target quickly follows the “cue” in the same location)
• ERPs (occipital P1) are enlarged for targets that quickly follow a sensory cue at the same location and diminished for targets that appear long after the cue

Question 46

late election

Answer 46

All inputs are processed equally by the perceptual system, but selection follows to determine what will undergo additional processing and be represented in awareness
- Attentional processes cannot affect our perceptual analysis of stimuli

Question 47

early selection

Answer 47

Broadbent’s model: sensory input that can enter higher levels of the brain for processing are screened so that only the most important or attended events pass through.
Problem: cocktail party effect – information that is actually unattended (hearing ones name) is still processed and consciously perceived
Solution: unattended channel information was not completely blocked from higher analysis but was degraded or attenuated instead