Klimesch (1995a), Verleger (1995), and Schurmann et al. (1996) agree that the main power of the event-related potential (ERP) P300 component is in the delta range. What is still debatable is the relationship between P300 and theta ERP activity. It is important to differentiate two aspects of the association between P300 and theta response: (1) Theta ERP activity may contribute directly to P300 expression, for example, P300 may contain theta power; and (2) the relationship between P300 and theta response may be functional and mediated by general processing in the theta frequency channel of the EEG rather than just being underlaid by the theta power within the P300 latency range. We present data in support of both aspects. The latter one may be an example of the interaction between delta and theta ERP components.
1. As pointed out by Klimesch (1995b), Stampfer and Basar (1985) have observed that prominent theta oscillations coincide with oddball P300, and Basar-Eroglu et al. (1992) have found that the average theta (3-6 Hz) responses within 250-500 ms after oddball stimuli were higher in amplitude than those to passive stimuli. Although these studies have not assessed quantitatively the relationship between P300 and the late theta response, they nonetheless indicate the presence of oscillatory theta activity during P300 development.
2. In a recent study (Yordanova & Kolev, 1996d), we evaluated theta responses to passive and oddball auditory stimuli by analyzing three single-sweep parameters: amplitude, phase-locking to stimulus, and amplitude enhancement relative to prestimulus theta activity (for methodology see Yordanova et al., 1996a; Yordanova & Kolev, 1996b). Single theta responses from the P300 latency range were significantly higher in amplitude, more tightly phase-locked, and more enhanced to oddball than to passive stimuli. Further, to assess quantitatively the association between P300 and theta response, correlational analyses across individuals were carried out for the parameters of single theta responses during P300 development and P300 parameters (amplitude and latency). High correlations were obtained for P300 amplitude and theta response magnitude (amplitude and enhancement), with no correlations found for P300 latency. These findings show that theta power in the P300 latency range may contribute to oddball P300 waveform and support the hypothesis of Basar (1992; Basar-Eroglu et al., 1992) and the unpublished data of Klimesch, Schimke, and Pienert (1995).
3. We have further approached this problem by testing the hypothesis that if theta power contributes to P300 expression, the rejection of theta activity from the ERPs should produce significant changes in P300 parameters (Kolev & Yordanova, 1993; Yordanova & Kolev, 1995). P300 was recorded in several conditions and P300 parameters measured before and after stop-band filtering in the theta (4-7 Hz) frequency range were compared. Both the amplitude and latency of P300 were significantly affected by the theta rejection. Also, the effect of theta rejection on P300 depended on the eliciting condition. Thus, an argument is provided in support of the view that upon specific task requirements the theta response may contribute to P300 waveform.
4. Intriligator and Polich (1994) have found an important association between spontaneous theta power and P300 amplitude. Demiralp and Basar (1992) have demonstrated a significant increase in theta response amplitudes within 0-250 ms after attended relative to passive stimuli. Klimesch et al. (1994) and Basar-Eroglu et al. (1992) have emphasized on the similar responsiveness of the event-related theta response and P300 to tasks engaging episodic memory or focused attention. These reports imply that the association of the event-related theta activity with P300 may be more complex and rather functional than just originating from a contribution of theta power to P300 waveform.
5. In our study mentioned above (Yordanova & Kolev, 1996d), we performed correlational analyses for the single theta responses preceding P300 appearance, for example, in the 0-300 ms epoch, and P300. P300 amplitude and latency were strongly associated with the phase-locking and enhancement of the early theta response. Also, there was a strong relationship between P300 amplitude and prestimulus theta rms amplitude. These results further demonstrate that P300 is strongly related with processing in the theta frequency channel of the EEG as reflected by the spontaneous (Intriligator & Polich, 1995), prestimulus, and early phase-locked ERP theta activity.
6. To additionally test the hypothesis of the functional relationship between P300 and event-related theta activity, we employed a developmental model (Yordanova & Kolev, 1996c; Kolev et al., 1994b). P300 latency has been found to decrease with increasing age in children (Courchesne, 1983; Polich et al., 1990), and the latency of the maximal theta response to auditory and visual stimulation has manifested a substantial delay in three year-old children compared to adults (Kolev et al., 1994a; Basar-Eroglu et al., 1994). We hypothesized that if P300 and theta response are interrelated, a speeding of theta response should accompany brain development, such that it correlates with the age-related P300 latency reduction. It is to be noted that the maximal amplitude in the averaged ERPs filtered in the theta (4-7 Hz) range is determined primarily by the phase-locked theta activity. Results from the multiple regression analysis demonstrated that theta response latency significantly decreased with age from 6 to 11 years, preceded P300 latency by approximately 200 ms, and entirely predicted the developmental reduction of P300 latency. Thus, by using a developmental model, the functional association between these two time and frequency domain ERP components was established.
7. Our data support the view of Klimesch and further indicate that event-related theta activity may not only contribute directly to P300 waveform expression, but may also modify P300 via other processes in the theta frequency channel of the EEG. If event-related delta activity contributes primarily to P300 (Verleger, 1995; Schurmann et al., 1995), the functional relationship between theta response and P300 implies that an interaction between delta and theta frequency ERP components may also exist.
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