The EEG is poorly sensitive and specific to detect lesions compared to neuroimaging; its practical use is to determine the functional consequence of the lesion. Focal dysfunction (physiologic) may occur without an associated neuroimaging abnormality. Postictal states and hypoperfusion are examples of physiologic dysfunction; these are often reversible (disappear on repeat testing). Focal dysfunction causes disruption of the background architecture (wakefulness and sleep), asymmetric responses on activation procedures, and focal slowing. Severity of the focal dysfunction may be estimated based on the abundance of slowing, attenuation of amplitude, loss of reactivity, and increase of slower frequencies. Sporadic, intermittent, or fluctuating focal slowing that is reactive to external stimulation or endogenous state changes (such as arousal) may indicate physiological dysfunction. Focal intermittent rhythmic (monomorphic) delta activity such as Lateralized rhythmic delta activity (LRDA) specifically indicates epileptogenicity. It should be treated like an epileptic discharge despite the lack of a sharpness. Look for epileptic discharges that may accompany focal slowing. Focal slowing may occur in isolation, bilaterally, or in the setting of diffuse cerebral dysfunction.

Sporadic abnormalities are those that occur in singles, pairs or in abundance but are not repetitive. They indicate cortical dysfunction, but the central question is if they are epileptogenic. They key electrographic feature to classify the waveform is its sharpness Spike and sharp waves pointed in morphology and are typically epileptogenic, these terms are often used interchangeably. Slow waves are blunted in morphology and are typically not epileptogenic (except rhythmic focal slowing).