Hoofdstuk 3 Flashcards
The electrophysiological method of single cell recordings
has been used to investigate questions related to neural representations
- single cell recordings measure the responsiveness of a neuron to a given stimulus (action potentials per second)
- single cell recordings can be obtained by placing a very small electrode either into the neuron itself (intercellular recording) or outside the membrane (extracellular recording). Extracellular Recording is the norm in mammalian brain
Electro Encephalography (EEG)
measurements of electrical signals generated by the brain through electrodes placed on different points on the scalp (TIMING of cognitive events and neural activity)
Event-related Potentials (ERP’s)
link the AMOUNT of change in voltage at the scalp with particular cognitive events (RATE of change becomes increasingly common)
Functional Imaging methods (fMRI)
have a better SPATIAL resolution than temporal resolution
Transcranial Magnetic Stimulation (TMS)
has both spatial + temporal resolution
Roll and Deco distinguish between 3 different types of representation that may be found at neural level:
1) local representation - all info about stimulus/event carried in one neuron (grandma cell)
2) fully distributed representation - all info carried in all neurons of a given population
3) sparse distributed representation - small portion of the neurons carry the info
- Quiroga - Halle Berry Neuron
Rate Coding
the informational content of a neuron may be related to the number of action potentials per second
The physiological basis of the EEG signal originates
in the postsynaptic dendritic currents, rather than the axonal currents associated with the action potential
Basic Requirements:
1) whole population of neurons must be active in synchrony to generate a large enough electrical field
2) this population must be aligned in a parallel orientation, so that they summate together, rather than cancel out
- neurons are arranged this way in the cerebral cortex
Temporal Coding
the synchorny of firing may be used by a population of neurons to code the same stimulus or event
* may be one mechanism for integrating information across spatially seprated populations of neurons
EEG
-compare voltage between 2 or more sights
- refrence often used is mastoid bone behind ears or a nasal reference
- electrodes - 10/20 system
- labeled according to location
F rontal
P arietal
O ccipital
T emperal
C entral
+ hemisphere involved (even for right, odd for left, Z for midline)
- EEG/ERP is not best equipped for detecting the location of neural activity
- signal-to-noise ratio is very low (the signal being the electrical response to the event and the noise being the background level of electrical activity)
Whether a peak is negative or positive (polarity)
has no real significance in cognitive terms, nor does it reflect excitation or inhibition. The polarity depends on the spatial arrangment of the neurons that are giving rise to the signal at that particular moment in time
Dipole
- positive ions flow into the dendrites when an excitatory neurotransmitter is released, leaving a net negative voltage in the extracellular space
- spearation of electrical charges that creates an electromagnetic field
- Dipoles from different neurons and different regions summate and conduct to the skull and these give rise to the characteristic peak and troughs of the ERP waveform
- timing + amplitude of those peaks links it to cognition
Frequency bands
- EEG Signal oscillates at different rates (named after greek letters
Alpha Waves: 7-14 Hz
Beta Waves 15-30Hz
Gamma Waves: 30+ - these oscillations arise because large groups of neurons tend to be in temporal synchorny with eachother with firing (action potentials) and in terms of their slower dendritic potentials (basis of EEG signal)
- Amount of Oscillations = power
Mental Chronometry
the study of the timecourse of information processing in the human nervous system
_ changes in the nature or efficiency of inromation processing will manifest themselves in the time it takes to complete a task
Sternberg (1969) developed a general method for dividing reaction times into different stages termed “the Additive Factors Method”
1) encoding the probe digit
2) comparing the probe digit to items held in memory
3) deciding which response to make
4) responding by executing the button press
Additive Factors Method assumes that if 2 variables affect different stages of processing, then they should have an additive effect on the overall time (parallel lines)
- if 2 variables affect the same stage of processing then the factors should have an interactive effect (non parallel lines)
The strength of this method is that one could take an unknown factor (sleep deprivation, parkinsons, reading ability) and determine whether this has an interactive effect on stimulus perceptibility (implying new factor affects perceptual encoding) or whether it has an interactive effect with the number of items in the array (implying new factor affects the comparison stage) or both (implying the new factor has effects at multiple levels)
Additive factors method assumes that the stages are strictly sequential, but this assumption is not always valid
N170
An ERP Component (negative potential at 170 ms), liked to perceiving facial structure
- stronges over right, posterior temporal electrode vibes
Mental Chronometry approach
applies to the analysis and interpretation of ERP data
- reaction time consists of a single measure that is assumed to reflect different stages/components
- ERP waveform consists of a series of peaks and troughs that vary continuously over time and are likely to have some degree of correspondence with different cognitive stages of processing (peaks + troughs = components or deflection)
Stages of Face Processing
1) perceptual coding of the facial image (location of eyes, mouth etc)
2) stage where facial identity is computed - mapping the perceptual code onto a store of known faces and representing the face irrespective of viewing conditions (lighting/viewing angle)
3) there may be a representation of the identity of the person that is not tied to any modality (e.g. responds to faces and names) and may enable retrieval of other types of knowledge (e.g. their occupation)
Associative Priming
refers to the fact that reaction times are faster to a stimulus if that stimulus is preceded by a stimulus that tends to co-occur with it in the environment
- associative priming interacts with stimulus degradation
- interacts with how perceptually distinctive a face is
- associative priming has a late effect (after 300ms) on the ERP waveform that is more consistent with a post-perceptual locus. Effects of stimulus degradation were found under 150ms
Traditionally 2 categories of ERP coponents
1) exogenous components - appear to depend on the physical properties of a stimulus (e.g. sensory modality, size, intensity). also called EVOKED potentials
2) endogenous components - appear to depend on the properties of the task (what the participant is required to do with the stimulus
- Exogenous tends to be earlier than endogenous
Inverse Problem
the difficulty of locating the sources of electrical activity from measurements taken at the scalp (ERP research)D
DIpole modelling
common attempt to solve inverse problem, that involves assuming how many dipoles (regions of electrical activity) contribute to the signal recorded at the scalp
Obtaining good spatial resolution
fMRI or Magneto encephalography (MEG)
MEG can be regarded parallel to EEG, however better spatial resolution + excellent temporal resolution
