Event-Related Potentials (ERPs)

Event-Related Potentials (ERPs)

Brain ActivitiesEvent-related potential (ERP) is a very small voltage brain activity that is the direct result of a specific sensory, cognitive or motor event. It is any stereotyped electrophysiological response to a stimulus. The transient electric potential shifts (the ERP components) are time-locked to the stimulus onset. Each ERP component reflects the brain’s activity which is associated with one or more mental operations. The modern neuroscience considers ERPs the reflections of complex neurophysiological processes overlapped in time and space. They are in turn associated with distinct psychological operations.

The assessment of ERPs provides a non-invasive means of evaluating brain functioning. ERPs are measured with electroencephalography (EEG), which requires placing electrodes on the scalp, and amplifying and recording the neural activity generated in the brain. The EEG signal at any time is the representation of many simultaneously ongoing brain processes. Therefore, the brain response to a single stimulus or event of interest is too small to be visible on the EGG trace. In order to see the time-locked event’s potential, multiple repetitions of the event is conducted and the EEG traces are averaged. During this process other processes not related to the stimulus cancel each other, but the time-locked event builds up and the potential related to that event becomes observable.

The ERP waveforms consist of a series of positive and negative voltage deflections, which are related to a set of underlying neural events. These events are labelled with a letter (N or P) indicating the negative or positive polarity, followed by a number indicating either the latency in milliseconds or the component’s ordinal position in the waveform. For example, a negative-going peak, which occurs about 100 milliseconds after a stimulus is presented, is called the N100 (its latency is 100 ms) or N1 (it is the first peak and is negative).

There are 2 categories of ERPs in humans. The early components occurring within the first 100 milliseconds after stimulus, are termed ‘sensory’ or ‘exogenous’, as they depend largely on the physical parameters of the stimulus. In contrast, ERPs generated in later parts reflect the way in which the subject evaluates the stimulus and are termed ‘cognitive’ or ‘endogenous’ ERPs. It is during these moments that the incoming information is processed.

Compared with behavioural assessment procedures, ERPs provide a continuous measure of information processing between a stimulus and a cortical response, making it possible to determine which stage(s) of information processing are being affected by the designed test protocol. Furthermore, ERPs can provide a measure of processing of stimuli even when there is no behavioral change. It is relatively difficult to localise the exact source of an ERP. However, a mathematical computation process called independent component analysis (ICA) may be able to extract multiple, functionally distinct sources of an ERP generated by separate regions of cortex. This method greatly increases the benefit of ERPs by providing a cleaner and less ambiguous measure of source activity. ICA is applied to a collection of ERPs recorded from a large number of subjects. The methodology was described in several papers (Kropotov, Ponomarev, 2009; Kropotov et al., 2011, 2012) and in two monographs published by Academic Press, Elsevier  (Kropotov, 2009) and by Springer (Mueller, Cadrian, Kropotov, 2011).  This new analysis method opened a new approach for discriminating groups of patients (ADHD, schizophrenia, OCD, TBI, PTSD), as well as aiding for diagnosis of brain dysfunction in a single patient.

In our clinic ERPs are recorded and analysed as part of the brain function profile (BFP) assessment process.