The McGill Physiology Virtual Lab

Biomedical Signals Acquisition


EEG consists of all brain activity visible at the location of the electrodes at a particular point in time. Buried within the EEG, a more useful signal can be revealed in terms of understanding information processing in the brain.


This signal can be obtained through the technique of average processing when a repeated stimulus is delivered and many EEG traces - event related brain potentials (ERP) - are recorded. When the stimulus is a sound, auditory-related brain potentials (ARP) are recorded. The recorded EEG traces, which are time locked to multiple presentations of the same sound stimulus, are averaged. The averaging process tends to decrease the influence of random activity (non-event related EEG) and maintains the consistent event-related activity.

Recording ERP: Subject preparation and equipment setup

  • Negative (-) electrode positioned on Cz.

 Measure with the paper ruler the distance between de nasion and inion of the subject and the circumference of the head as indicated in the diagram above. Then affix the negative electrode in the midline just at the half way point of the distance between de nasion and inion (Cz).

  • Positive (+) electrode positioned on the ipsilateral mastoid process

  • Attach the earth (ground) electrode in Fp2 (above nasion, 10% of the distance between de nasion and inion).
  • Put the headphones on to the subject: a repetitive sound is going to be sent through the headphones.
  • The subject relaxes as much as possible, eyes closed, and concentrates on the sound delivered by the audio monitor.

A voltage delivered by the stimulator inside Powerlab is amplified by the audio monitor, and converted as an audible “click” through the head phones; by pressing start in the acquisition display, a segment of EEG which follows the audio stimulus is recorded.

Recording the ERP

Basic concepts of Event related potentials

 ERPs are classified according to the nature of the stimulus: visual, somato-sensory, and auditory; they can also be classified according to the latency at which their components occur after stimulus presentation: short latency (<100msec) and long latency (>100msec) potentials.

The shorter latency components are generated during the sensory stimulus processing stages (exogenous components). The longer latency components represent the cortical processing stages, which are less determined by the physical features of the stimulus (endogenous components).

Exogenous potentials

Endogenous potentials

- Depend on physical features of the sensory stimulus.

- Do not depend on physical features of sensory stimulus. They can be evoked, just with stimulus expectancy, even in the absence of stimulus.

- Do not depend on the subjects’ level of consciousness.

- Can change depending on the level of attention, its relevancy during the task and resources required for stimulus processing.

- Are not influenced by cognition processes.

- Related to cognition processing.

The classification into early or late components ERPs is useful in practical terms, however it is more theoretical than realistic since ERPs generation is a continuous process.  

In this session the stimulus used to evoke the responses is auditory, so the responses will be auditory related potentials. 

Early auditory related potentials include five positive waves that occur during the first 10 msec after stimulus presentation and are labeled from I to V according to their order of appearance. They are very stable in shape, amplitude and latency in subjects with no hearing impairment. It is well proven that these components are generated as a result of the activation of brain stem nuclei of the auditory pathway during auditory stimuli information processing. Due to their stereotyped behaviour, even during sleep and unconsciousness states, these potentials have been very helpful as an objective functional evaluation of auditory system in newborns and psychological deafness.

Long latency potentials are referred to those components that appear after 100 msec of stimulus presentation and are thought to represent cortical information processing. They are affected by level of attention, stimulus significance, task relevance and stimulus processing requirements.

We are going to record P100 (first positive [P] component appearing 100 ms after the stimulus) using auditory stimuli, although this component can also be evoked visually; the most important factor is that the stimulus must be unpredictable in time

This kind of potential has been used for psychophysical assessment in patients with cognitive and attention disorders such as Alzheimer’s dementia, schizophrenia, and speech disorders.

The aim of this lab is to record late latency auditory related potentials. An example of one of the 70 sweeps averaged by the software is displayed below.
The averaged data is shown below: at  100 ms following the stimulus, there is a positive peak (P100) followed by a negative deflection.

Click here to view some  EEG sweeps with the resulting averaged waveform
showing a late ERP component.


Variables analyzed from ERPs:

Absolute latency: is the time interval between stimulus presentation and the point of maximal value (peak) of a defined component. It is expressed in milliseconds and represents the time taken by the stimulus information to generate the component.

Relative latency (inter-peak latency): is the time interval between two components and measures the conduction of the impulse between two generators.

Amplitude: vertical distance measured from the trough to the maximal peak (negative or positive). It expresses information about the size of the neuron population and its activation synchrony during the component generation.

Duration: Time interval from the beginning of the voltage change to its return to the baseline. It is also a measurement of the synchronous activation of neurons involved in the component generation. Longer durations indicate less synchronous neuronal activation.

Control task

Ask the subject to close their eyes. Put the headphones on the subject. Set the volume button of the sound amplifier to zero. This way, the subject will not perceive any auditory stimulus delivery. Make sure that the subject does not hear anything!

What is the purpose of the control task? See the graph below showing the results obtained when 70 sweeps are averaged; the stimulus is also shown.

ERP following a random auditory stimulus
The figure above shows a "zoom" of the averaged data following the random auditory stimulus. Note the absolute latency and the peak to peak amplitude of the P100.
ERP following a random auditory stimulus, followed by another auditory stimulus delivered 300 ms later (double stimulus).
The figure above shows a "zoom" of the averaged data following a double stimulus. Note the absolute latency for each P100 and the peak to peak amplitude for each of the P100.
Problems encountered during recordings
Inspect the graph above: the trace shows an electromagnetic interference. You can determine  the period of cyclic waves and extrapolate the frequency which contaminates the trace. It is difficult to determine the variables of the ERP under study: how can you measure this trace?

How can the recording be improved? Refer to the artifacts section

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