G error related negativity syntactic positive shift

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- Theoretical label (e.g., Error Related Negativity, Syntactic Positive Shift).- ERP components may be difficult to dissociate -> Functional Significance may not be understood.Exogenous Components- Display passage of information along the sensory channels.- Very sensitive to physical nature of the signal.- In many cases not sensitive to “cognition.Endogenous Components- Appearance depends on processing of stimulus and not only on its physical nature.- Represents aspects of cognitive processing l Does not even require a stimulus to occur.Identification of ERP components- Polarity (+/-).- Latency (Onset & peak).- Scalp distribution (At which recording electrodes is the ERP maximal).- Task sensitivity (to which experimental manipulations is the ERP sensitive).Averaging and S/N Ratio: S/N ratio = (Signal Size) + (Noise Size)- 0.5 uV effect, 10 uV EEG, EEG noise -> 0.5:10 = 0.05:1.- Ideally want a value that is at least 1:1 ratio- Averaging increases S/N according to square root of Noise Size- Quadrupling N Doubles S/N (because square root (4)=2)- l If S/N = 0.05:1 on a single trial, 1024 trials gives us a S/N ratio of 1.6:.- Because square root (1024) =32 and .05 x 32 = 1.6Assumptions of Averaging1. All sources of Voltage are randomwith respect to time-locking event except the ERP.- True for a well-designed ERP experiment w/ no time-locked biological artifacts (ex: P3a). 2. The Amplitude of the ERPsignal is the same on each trail.- Violations of this assumption do not matter much.- We do not usually care if component varies in amplitude from trial to trial (But: patient work). 3. The Latency of the ERPsignal is the same on each trail.- Violations of this assumption matter a lot.
- Timing of the component may vary from trial to trial (example auditory presented words).- This is called Latency Jitter.- This average will contain a smeared-out version of a component w/ reduced amplitude.Summary of ERPs- They represent summed electrical responses of thousands of neurons, not single neurons.- Summed activity of a large population of pyramidal neurons with similar voltage fields that have an open field configuration.- They can only be measured from so called open field neuronal configuration.- Reflect post-synaptic potentials at the dendrites of the neuron.- Not the Action Potential because it is too fast & not synchronous.Some Methodological Issues- Especially longer latency ERPs contain not one but many effects.- Need to decompose waveformsMeaning of Scalp Distributions- Different scalp distributions suggest different neuronal sources but could also reflect relative contribution of different sources.Inverse Problem- Scalp recorded ERPs may not reflect the activity of brain regions underlying the recording electrode.- Spread of electrical current in the conducting media (head, skull scalp).- Orientation of the active brain tissue- Electrial Source Modeling????- Forward vs Inverse Problem???Lecture #4 (1/29/18) Sensation PerceptionSensation:Translation of external information (light, sound, touch, odor, time) into neural codes.Perception:Intimate links to memory recognition (or failure of recognition)*

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