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an interaction of "states" of both observers enters the state description of the experiment. An instruction for this purpose or a thorough explanation about its implications was absent in our experiment.

- Finally, and possibly most importantly, there is a possible conceptual difference between the two experiments. The observation by the preobserver in the second experiment was incomplete in the sense that this observer was unaware if (s)he observed a quantum event or a classical event. One could argue qualitatively that this lack of knowledge corresponds to only a partial collapse and hence a situation where preobservation does not really make a difference or makes a smaller difference for the final observer. Interestingly, under this assumption one would expect a smaller or no difference between the preobserved and non-preobserved condition but one still would expect a difference between the classical and the quantum events. This is exactly what was found. Some consistent differences were found when comparing the AEP of quantum events with those of classic events (see Fig. 2.11 and Table 2.4).

No differences were expected on the basis of identical beep frequency, duration or simulated decay times between events. Only the latter could possibly differ between the two conditions. The table of classical latencies was generated by recording the radioactive decay in the same experiment. Radioactive decay, however, has such a large variance that it is difficult to ensure a perfect simulation. When we later compared the average of the table with the average of radioactive events that occurred during the recording of the subjects, we noticed considerable differences. The real quantum events had average latencies that sometimes fell below a tenth of those of the table. The selection was therefore unlucky. It is not, however, straightforward to attribute the differences to this difference in latency. When one expects a stimulus at a certain time, a contingent negative variation (CNV) will precede the moment of stimulation after which the AEP will start at a more negative baseline. In the AEP of the frontal and frontocentral pooling this CNV is clearly seen (Figs. 2.9 and 2.10 between 250ms and 0ms before stimulus), in which there does not seem to be a differential effect between conditions. A CNV will only have an effect on the amplitude, not on the latency of early endogenous components. Also, the difference wave (Fig. 2.11) shows a consistent difference in the positive direction. This cannot be explained by a difference of latency for that would result in a difference wave that crosses the baseline. It must be explained in terms of a difference in surface. Interpreting this difference, however, is inappropriate at this stage of investigation and needs further investigation.

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