Niall Flashcards
What do we mean by the term stochastic (signals) when referring to the gross electrical activity
of the brain?
All brains generate spontaneous activity called the EEG. The technique of recording signals from the surface of the scalp which are specific to a particular stimulus, are called evoked potentials or event related potentials.
A stochastic signal has a element of chance associated with it & cannot be predicted exactly.
Describe the type of amplifier used when measuring Visual Evoked Potentials
The type of amplifier used is a differential amplifier. This amplifies the difference between two voltages but does not amplify particular voltages.
A differential amplifier receives two signals, one from an active electrode and one from a reference electrode and amplifies the difference between the two. Amplifiers always have band pass characteristics.
What are the typical frequency ranges to which amplifier are used?
The EEG on the surface of the scalp is quite small, around 100mV. This means that some kind of amplifier is needed and usually physiological signals are amplified using a differential amplifier. A differential amplifier receives 2 signals, one from an active and one from a reference electrode and amplifies only the difference between the two. If a 1Hz stimulation is used then
the amplifier is set at 0.3-30Hz. If a 30Hz stimulus is used then it’s set to 15-60Hz.
What are the two sources of noise when recording VEP’s?
Electrical and physiological noise. In order to minimise the bandwidth of noise, amplifiers have band pass characteristics – amplifiers that accepts certain band widths and rejects others.
Briefly describe signal averaging, include approximate voltages for the amplitude of signal and background noise.
Stimulus presentation triggers computer and then the stimulus is repeatedly presented. Computer samples EEG each time the stimulus is present so that the stimulus and response are time locked. VEP extracted by adding samples of EEG and dividing by the number of presentations. Noise = 100mV and signal =5mV Signal averaging is used as a way of extracting the signal from the neural noise.
Where on the human scalp are electrodes places in order to record a single channel of Visual Evoked Potentials.
Electrodes are fixed to the scalp with a gel ml/l,/lat is a good conductor. In the case of VEP’s, the active electrode is placed in the occipital regions (lower canthus/02) and the reference on the front of the head (temporal forehead/c2) so that this is visually silent (earth). 10-20 electrode system – active electrode placed at nasion and inion; reference placed away from where signal is.
Describe the stimulus conditions under which the frequency-domain or time-domain averaging are used to record visual evoked potentials?
For slow presentations use time domain, for fast presentation use frequency domain.
Sketch a diagram showing labelled x and y axis when VEP data are plotted in the time domain and frequency domain. ?
Electroencephalogram and its amp is approx. 100mV.
The spontaneous electrical activity of the brain is described as stochastic. What does this mean and why is it important?
The means the background activity of the brain is random. This activity is called the EEG. The technique of recording signals from the surface of the scalp which are specific to a particular stimulus, are called evoked potentials or event related potentials.
it is important because its random nature allows us to differentiate and highlight changes in the EEG due to a stimulus. E.g when doing time averaging, the stochastic part of the EEG goes away because its random, but the VEP stays because its not random (has the same voltage with each stimulus presentation)
The spontaneous signals of the brain are known as slow wave activity/EEG source. What is their origin?
V1 cortical cells. – apical dendrites of pyramidal neurons.
What is the main limitation of using flash, as opposed to pattern stimuli as a stimulus for the VEP? (2 marks)
Flash evoked responses can be recorded virtually anywhere on the scalp and it is this lack of localisation which tells us they are of limited value
Visual cortical neurons do not respond very well to this type of stimulation.
What is meant by the term evoked potentials?
A technique of recording the signal from the surfaces of the scalp which are specific to a particular stimulus.
An evoked potential is an electrical potential recorded from the nervous system of a human or other animal following presentation of a stimulus.
Name 3 locations on the scalp where electrodes are places in order to record evoked potentials from the visual system. A small sketch may help. LOOK UP.
Forehead – earth, ear – reference and inion.
3 electrodes: Occipital lobe (inion)- Active
Temporal forehead (top of head-mid frontal) - Reference
Forehead (visually silent) - Earth
All 3 electrodes equal one channel. If you want to record from multiple channels, you can have one common reference (mid frontal) And have an array of electrodes at the back of the head.
Describe the process of signal averaging.include approximate voltages for the
amplitude of signal and background noise (4 marks)
LOOK UP – NEED MORE DETAIL
Signal averaging can be by time or frequency. Signal averaging involves repeatedly sampling the EEG (with a computer) at the same time as the stimulus is presented. (locked in time)
Time averaging has a slow presentation so the system returns to its resting state between responses. The signal is sharply located in time- it has a sudden onset. The x axis is time. VEP extracted by adding samples of EEG and dividing by number of presentations.
Noise 100mV, signal 5mV.
Frequency averaging: stimulus presented at fast rates so the response is concentrated into a narrow bandwith and becomes sinusoidal. The x axis is frequency.
Describe the different methods for temporally modulating the stimulus when recording VEPs.
You can temporally modulate the stimulus when recording VEP by using a flash stimulus (on-off) and a contrast reversal (change in contrast stimulus). (phase reverses)
