Abstract: Event-Related Potentials (ERPs) refers to the electrical events, specific to the cognitive activities of the brain that signal its task-related cognitive evaluation and decision-making processes following sensory input in various modalities. ERPs recording involve the use of a longer recording epoch following the stimulus, in order to record the subsequent cognitive activities. There are several types of cognitive ERPs including Contingent Negative Variation (CNV), Late Positive Complex (LPC or ‘P300’) and N400 (mismatch negativity), respectively. This article aims to demonstrate the auditory evoked potentials recording technique. Some examples of the cognitive event-related potentials testing done in the laboratory are also mentioned.
INTRODUCTION
The meaning of Event-Related Potentials (ERPs) can be understood by mean of the separated technical terminologies as (a) “Event-Related” refers to associated with, but not directly evoked by and (b) “Potentials” mean electrical events, in response to a call, as with sensory Evoked Potentials (EPs) recording. In other words, ERP means the electrical event which is functioned in cognitive activities of the human brain. This electrical event signals its task-related cognitive evaluation and decision-making processes following sensory input in various modalities (Remond and Torres, 1964; Petsche et al., 1970; Hallop, 1983; Hassett, 1978; Nuwer, 1985). ERPs differ from EPs mainly in the sense that they form a kind of follow-up or follow-through on the prior sensory EPs input, reflecting specifically longer-latency sensory-related cognitive information processing by the Central Nervous System (CNS), in addition to the earlier sensory EPs signal (Davis and Hiemoeller, 1968; Roth and Cannon, 1972; Buchsbaum et al., 1981, 1982a, b; Chiappa, 1983; Owen and Davis, 1985; Scherg and von Cramon, 1985; Spehlmann, 1985). Therefore, ERPs contain two signals of interest, rather than just one, as in conventional sensory EPs signal. Cortically, as EPs were centered on primary sensory cortex, ERPs are mainly focused on associative cortical and subcortical areas and therefore appear to arise from a much larger area of brain. This information is normally presented in the raw, ongoing electroencephalogram (EEG) all the time there is such task-related information processing going on. While this signal is generally higher-voltage than EPs, it is still: (1) usually too small to be seen and (2) still tends to get swallowed-up by the mass of order information processing going on in the brain at the same time (Jewett and Williston, 1971; Goff, 1974; Morstyn et al., 1983; Hood and Berlin, 1986). This article thus aims to demonstrate the auditory evoked potentials recording technique. Some examples of the cognitive event-related potentials testing done in the Clinical and Research Electro-Neurophysiological Laboratory, Mahidol University, Thailand, are also presented.
EVENT-RELATED POTENTIALS RECORDING
Recording: ERP recording is a method to tune-in to one specific part of the evaluation and decision-making processes that goes on in the brain most of the time, in order to try to get an indication of how the brain’s overall thinking system works. Recording ERPs mainly involves, beyond the standard EP recording technology, the use of a longer recording window or epoch following the stimulus, in order to record the subsequent cognitive activities (Lehmann and Skrandies, 1979, 1984). Based on its information content compared with a prior stimulus, most ERPs require a task performance involving either some sort of discrimination between two randomly alternating sensory inputs, or a conditional evaluation of a second stimulus. In addition, the provision must be made for the delivery of two stimuli, as well as either the separate or sequential recording of the brain’s responses to each (Pfurtscheller and Aranibar, 1977, 1979; Federico, 1984).
CHARACTERISTICS OF COGNITIVE EVENT-RELATED POTENTIALS
Cognitive ERPs include the following types of testing: (a) Contingent Negative Variation (CNV), (b) Late Positive Complex (LPC or P300, as applied in auditory, visual and missing stimulus testing modalities) and (c) N400 (mismatch negativity, especially used in various types of lexical studies), respectively (Jewett and Williston, 1971; Buchsbaum et al., 1981; Chiappa, 1983; Davis and Hiemoeller, 1968; Owen and Davis, 1985; Spehlmann, 1985). Figure 1 illustrates the diagrammatic representation of the typical Auditory P300 stimulus and recording set-up. In this case, a full multi-channel recording is being performed, with subsequent Topographic Brain Mapping (TBM) computerized display of the results (Duff, 1980; Duff et al., 1984; Kotchabhakdi, 2005).
| Fig. 1: | A diagrammatic representation of the ‘typical’ Auditory P300 stimulus and recording set-up (Kotchabhakdi, 2005) |
| Fig. 2: | Another name for P300 protocol is “Odd-Ball” paradigm (Kotchabhakdi, 2005) |
| Fig. 3: | P300 cognitive ERPs comparing between two elderly Thai people, a 61 year-old male of normal mental abilities and a 64 year-old female who was demented (Kotchabhakdi, 2005) |
One essential ingredient in any P300 protocol is for the stimulus to be unpredictable in occurrence. The stimulus can be auditory, visual or missing. Even if the stimulus is a missing stimulus protocol, stimuli must be predictable for the expected time of occurrence. This is usually achieved by randomizing the presentation of the so-called Target stimulus-the one to which the patient or subject must attend and on which he/she must then perform some kind of cognitive task, e.g., count them.
| Fig. 4: | The non-target responses to unattended stimuli of the P300 cognitive ERPs were quite comparable (and normal). On the other hand, their responses to the target or attended stimuli were quite different (Kotchabhakdi, 2005) |
This process of randomization is diagrammed in Fig. 2 which is a dual-stimulus type of protocol, with delivery of the Non-Target (unattended) stimulus being much more frequent than the Target. Another name for this type of protocol is “Odd-Ball” paradigm (Fig. 2).
One of the main areas of study where P300 Cognitive ERP testing has been used extensively is the Senile Dementia of the Alzheimer’s Type (SDAT).
SOME EXAMPLE DATA OF COGNITIVE EVENT-RELATED POTENTIALS TESTING
Figure 3 showed the results of a comparison study between two elderly Thai people, a 61 year-old male of normal mental abilities and a 64 year-old female who was demented. As can be seen from the Fig. 3, the non-target responses to unattended stimuli for both these individuals were quite comparable (and normal). However, their responses to the Target or attended stimuli were quite different, as can be clearly seen in Fig. 4. Additionally, the normal male’s Target P3b response component occurred at a latency of 316 msec, was of about 20 μV amplitude and was of <300 msec overall duration. On the other hand, the demented female’s Target P300 occurred at a prolonged latency of 476 msec, was only about 10 μV amplitude, while being also much more dispersed, with an overall duration of about 450 msec, respectively.
CONCLUSION
ERPs referred to the electrical events, specific to the cognitive activities of the brain that signal its task-related cognitive evaluation and decision-making processes following sensory input in various modalities. ERPs recording involved the use of a longer recording epoch following the stimulus, in order to record the subsequent cognitive activities.