Validity of ERPs

When assessing the validity of ERP measures, the goal of the experimenter is to evaluate if the ERP measure is actually quantifying what it is supposed to quantify. In practice, however, there are a number of strategies that can be adopted when assessing the validity of ERP effects. One strategy consists of finding correlations with other variables that have already been subjected to a validation process. For example, correlations with behavioural variables often serve this purpose. In that case validity is

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assessed by comparing different levels of analysis. Another possibility is validating an ERP effect by comparing it with other established ERP effects whose validity has already been assessed. Similarities in the time-course, waveform shape and topographical distributions may point towards similarities in the processes involved, especially if in both cases there is also a correlation with behavioural variables.

Validation in neuroscientific research proceeds by successive approximation: often what an effect is deduced by excluding what an effect is not, given the previous literature. New experiments are often designed to limit the interpretative scope with regards to an effect: by excluding some interpretations the interpretations that are still standing will be stronger. For example, to answer the questions: “does the Left-Parietal effect measure recollection?”, or “is the amplitude of the P3b a measure of stimulus categorization?”, it is therefore necessary to gradually exclude alternative hypothesis and refine how the effects measure what they measure.

1.5.3.1 Example: Validating the Mismatch Negativity

An interesting example of validation of an ERP effect is represented by the Mismatch Negativity (MMN). The mismatch negativity, considered to be an automatic form of discrimination occurring in the auditory cortices, has been deeply investigated to understand exactly what kind of discrimination it is correlated with. Does it measure a relatively simple form of memory present in the auditory cortices and, if so, is this

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form of memory dependent on the physical characteristics of the stimuli or on their relative probability? As MMN experiments typically involve the presentation of standard stimuli (high probability) intermixed with deviant stimuli (low probability), experimenters have tried to understand if the differential activation of auditory cortices occurred as a function of the different probabilities, or of the physical dissimilarities between standard and deviant stimuli, or was independent from those factors.

A key experiment investigating the MMN allowed excluding two of the possible interpretations. Tervaniemi and colleagues (1994) showed how the MMN did not depend on probability or physical features of the stimuli. The experiment used regularly descending pitch sequences intermixed with an ascending tone. The MMN negativity was present in the contrast between ERPs to descending pitch stimuli with ERPs to the deviant ascending tone, independent of the probability or physical features of the stimuli. Those results suggest that the MMN is sensitive to deviations to preceding stimulation, even when the preceding stimulation constitutes an “abstract rule”, such as a descending sequence of tones. The validation process used to examine the MMN led to the conclusion that the effect is the product of a discrimination process where the deviant stimulus is incongruent with the memory representation of the preceding stimulation (Naatanen et al., 2007), regardless of the probability or the physical characteristics of the deviant. More generally this example illustrates the importance of a gradual exclusion of alternative hypothesis when investigating what an experimental effect is actually measuring. Similarly in the current work we

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investigated what the N400 effect is and is not a measure of, by showing the effect‟s sensitivity to association relationships when semantic relationships are kept constant (Chapter 8) and excluding the possibility that the N400 effect and the Bilateral Frontal old/new effect reflect the same underlying phenomenon (Chapter 9).

1.5.3.2 From validation to Application

When the validity of an ERP effect has been assessed, the effect can then be used in applied and clinical settings, for example, as a diagnostic tool. Assessing the validity of ERP measurement is therefore a necessary prerequisite before the measure of interest reaches clinical and other applied domains. Importantly, however, the validity of an ERP measure is necessary but not sufficient in assessing its utility in applied settings (similarly to reliability being a necessary but not sufficient prerequisite for validity). The degree of importance of an ERP measure within the applied domain will be a function of many factors, including for example how the ERP measure discriminates between clinical and non-clinical populations, how reliable the ERP measure is at an individual level.

Importantly, reliability of ERPs at the individual level is a fundamental criterion in assessing if an ERP measure can be used as a diagnostic tool. If an ERP effect is not reliably seen at the individual level, but is seen only at the group level, it cannot be included by clinicians among their diagnostic tools. Results presented in Chapter 5 and

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further discussed in Chapter 10 suggest that amongst the three effects investigated in the current thesis, the N400 effect and the P3b effect are reliably seen at the individual level and can have potential applied and clinical applications, while the Left Parietal old/new effect is measured reliably only at the group level.