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An essential step in EEG interpretation is the identification of features that are statistically deviant compared to normative data, a process that defines abnormalities and establishes their significance in appropriate clinical context. EEG patterns evolve with maturation; neonates reveal distinct features that regress within 4–6 weeks after birth to be replaced by patterns characteristic of infancy and early childhood. The patterns also vary with the child's state at time of recording, the level of alertness during wakefulness, and cycling through stages of sleep. Recognizing the considerable variability of normative EEG features is thus a prerequisite to interpretation.
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Once an abnormality is identified, the next goal of interpretation is to define its location and extent. While localization of abnormality is useful in most clinical settings, it gains particular relevance for patients with intractable epilepsy undergoing evaluation for resective surgery. Guidelines for localization were initially described by Adrian and Matthews1 and have been subsequently addressed in several comprehensive reviews.2,3,4,5 Principles of electric field theory were addressed by Nunez,6 the physiological substrates of EEG sources became better appreciated after the introduction of intracranial recordings.7,8 The pitfalls and caveats underlying many of the physical and physiological assumptions used in localization of epileptic sources were reviewed by Jayakar et al.9
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The growing success of resective surgery over the past few decades increased the need for localizing accuracy. This led to the development of automated computational systems for 3-D source localization of spikes recorded on EEG. More recently, magnetoencephalography (MEG) and a hybrid technique where EEG spikes trigger activation of functional MRI signal have also been utilized. While these new methods are gaining popularity in many epilepsy surgery centers, the set of underlying assumptions are not generally well understood. The main focus of this review article is to discuss the principles of localizing seizure foci; the intent is not to reiterate the standard teachings of localization but rather to emphasize the limitations and pitfalls underlying many of the assumptions used in visual or automated analyses and the potential for misinterpretation.
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THE EPILEPTIC GENERATOR
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Localization is facilitated by modeling the spike generator, the choice of a model is dictated by the distribution of the excitatory (negative) and inhibitory (positive) postsynaptic potentials within the generator. Since the majority of cortical neurons are arranged in parallel and activate synchronously by virtue of their interconnections, the respective potentials summate to generally produce layer of negativity on the superficial aspect of the cortex, and positivity on its deeper aspect, that is, a dipole layer.
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The orientation of a generator greatly affects its field distribution on the scalp.4 If a generator is located over a superficial gyrus, the dipole is oriented vertically to the scalp, that is, it is a "vertical dipole" and is the commonest orientation observed on the scalp EEG. ...