Klimesch (1995a, 1995b) and Verleger (1995) agree that the main power of the event-related potential (ERP) P300 is in the delta range. Klimesch, however, adds that the theta range is also of importance for the P300 response. This commentary summarizes results of ERP analysis in the frequency domain confirming the major importance of the delta response for the P300 wave. A role of the theta response for the P300 wave, however, is also demonstrable. Furthermore, our experimental data show that other frequency ranges also contribute to the P300 response. To our mind, this controversy is just an example of the recent trend towards a functional interpretation of event-related (or induced) brain rhythms.
2. MULTIPLE FREQUENCY RANGES PLAY A ROLE FOR THE P300 WAVE. ERP frequency analysis confirms Klimesch's view that a theta response component contributes to oddball P300 responses (Klimesch 1995a, 1995b; Klimesch et al., 1996; Basar-Eroglu et al., 1992): A comparison of responses to standard vs. oddball stimuli showed significant increases of both delta (1-3 Hz) and theta (3-6 Hz) responses (measurement window for theta responses: 250-500 ms after stimulus). Note that in another stimulation paradigm (with subjects attending the stimulus preceding a regularly omitted stimulus) theta increases without delta increases were observed (0-250 ms after stimulus; Demiralp & Basar, 1992). Klimesch's argument concerning the theta range may even be extended because theta is not the only frequency range contributing to the P300 response: P300-40 Hz compound responses (40 Hz responses with P300 latency) have been observed in intracranial recordings in cats where an animal model of P300-like responses was established (Basar-Eroglu & Basar, 1991) and in human scalp recordings (Basar et al., 1993).
3. FUNCTIONAL INTERPRETATION OF EVENT-RELATED BRAIN RHYTHMS - A NEW TREND. The frequency analysis of P300 responses is of special interest with respect to functional roles of brain rhythms. It is increasingly popular to view brain rhythms, not only in the delta and theta ranges (Klimesch 1995a), in relation to their psychophysiological correlates:
(a) The above-mentioned comparison of responses to oddball stimuli vs. responses to attended stimuli may permit a tentative differentiation of delta and theta responses with respect to their functional correlates (Basar-Eroglu et al., 1992).
(b) Experimental data show that alpha responses are more closely related to primary sensory processing than theta responses (Basar & Schurmann, 1996). Functional correlates of alpha rhythms in the brain have also been discussed at a conference in 1994 (Basar et al., 1996).
(c) The putative relationship between gamma (40 Hz) responses and "binding" (Gray et al., 1989, Eckhorn et al., 1988) is a further example of a functional interpretation. Taking into account the variety of gamma rhythms as classified by Galambos (1992), the range of functional correlates of gamma rhythms may be wider than supposed so far (Basar-Eroglu et al., 1996).
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