QEEG/ERP OVERVIEW

 

Electroencephalograms (EEG) are recordings of the bio-electric activity of the brain measured along the scalp.  The EEG represents the combined synchronous and spontaneous activity of billions of neurons performing many physiologic and cognitive functions.

 

Certain characteristic frequencies of the EEG can be representative of specific cognitive states such as sleep, attentiveness, relaxation, etc.  The analysis of the spectral content of the EEG is called quantitative EEG (qEEG).  qEEG can be used to evaluate a range of brain functions.

 

 

Event-related Potentials (ERP) are part of the EEG generated by sensory and cognitive processing of external stimuli.  The stimuli can be auditory, visual, or tactile and are generally arranged in a long sequence of many repetitions.  These sequences can be designed to probe specific cognitive processes such as selective attention, memory encoding, or semantic processing.  The ERP provides a real-time electrophysiological biomarker of the underlying cognitive processes.

 

An auditory ERP test is performed while the test subject listens to a sequence of sounds through earphones or external speakers.  EEG activity is time-locked to each stimulus and is then recorded using electrodes positioned on the scalp.

 

The stimulus type, sequence, and timing, often called an ERP paradigm, is designed to cause the brain to enter a specific cognitive state or to initiate specific cognitive processes.  Many ERP paradigms have been extensively investigated in neuroscience and clinical research labs around the world.

 

One of the most common ERP paradigms in neuroscience research and clinical use is the auditory oddball.  In this paradigm, one or more infrequent (deviant) tones are randomly presented within a long stream of frequent (standard) tones.  The test subject is often instructed to press a button attached to the testing device when one of the deviant tones is heard.  This simple paradigm is often referred to as the P300 for the large deflection of the ERP which occurs at approximately 300ms.

 

Three  variations of the auditory oddball paradigm are commonly used:

 

 

The process of: 1) presenting a stimulus, 2) recording the brainwave activity, and 3) waiting for a short interstimulus delay, is called an ERP epoch.  Several hundred epochs are often needed to extract the desired ERP wave.  Each epoch is usually 1-3 seconds long, so most ERP tests require 10-60 minutes to complete.

 

Once an ERP test has been performed, the EEG data from the individual epochs are time-locked to the stimulus and averaged.  The background EEG activity (blue waves in the figure below) averages to zero and the resultant waves are the ERPs (red wave in the figure below).  The ERP represents only the bioelectric activity related to the cognitive functions elicited by the stimulus sequence.

 

 

A typical ERP brainwave profile will include several characteristic positive and negative peaks.  The height of the peak is the amplitude in microvolts (µV) and the time from stimulus onset is called the latency and is generally measured in milliseconds (ms).