Lambda waves are diphasic, or sometimes triphasic, occipital sharp waves that occur during eye movements for visual exploration (Evans, 1953). Therefore, they occur only during wakefulness and in a well-lit environment that includes visual details. Because most EEGs are recorded with the patient lying supine and looking at a blank ceiling that lacks visual details for exploration, lambda waves are not commonly present.

A sharply contoured, prominent phase within the lambda wave gives the wave a triangular or saw-tooth waveform. Its name is an apt description because the Greek capital lambda (Λ) is a triangle missing its base segment. Sometimes the prominent phase is the only one that is evident; thus, the wave appears monophasic. However, a second, smaller phase commonly follows the prominent phase, which produces the overall diphasic waveform. Sometimes another low amplitude phase precedes the higher amplitude prominent phase, which produces a triphasic wave that differs from typical triphasic waves by having two smaller wave components that are similar to each other surrounding the higher amplitude, prominent component. The prominent phase is positive and has an amplitude that typically is 20 μV and rarely greater than 50 μV. Its duration ranges from 100 to 200 milliseconds, and the wave as a whole has a duration that typically is 200 milliseconds and usually ranges from 150 to 300 milliseconds. Lambda waves usually are bilaterally synchronous with a field that is centered at the occipital pole (O1 and O2 electrodes) and possibly extending to include the parietal (P3 and P4 electrodes) and posterior temporal regions (T5 and T6 electrodes). Because of the broad distribution, they are best recorded with channels that include posterior electrodes and have long interelectrode distances. However, the positive polarity at the occiput is best observed by using a bipolar montage with a circumferential (hatband) chain. This montage includes both O1 and O2 in one electrode chain, which allows for a phase reversal between them. Rarely, lambda waves occur unilaterally. When they are unilateral, there is a shifting asymmetry in occurrence that does not indicate an overall lateralization. Lambda waves usually occur as seemingly random and isolated transients but may recur at intervals of 200 to 500 milliseconds.

Lambda waves do not occur before 1 year and are most common during the middle years of childhood. Between 1 and 3 years, lambda waves are larger and have longer durations, which may be up to 400 milliseconds. Through childhood, they progressively become lower amplitude and shorter, with stabilization of their waveform in early adulthood. The prevalence of lambda waves between 3 and 12 years has a maximum of about 80% (Niedermeyer, 1999). During the middle years of adulthood, the prevalence decreases to a maximum of about 40%.