4.3.1 Comparison with previous studies
To estimate the quality of the data, I compared the eects to those reported in the literature. Because I was not able to nd one study that measured the aSSR using a similar paradigm this is rather dicult. The only possibility is to compare results of studies dealing to the modulation of the steady-state in a wider range with those obtained in the present experiment.
It seems widely accepted that the amplitude of the steady-state increases with higher attention in the visual domain (Morgan 1996 [26], Chen et al.
2003 [5], Di Russo et al. 2002 [41]) and also in the auditory domain (Ross et al. 2004 [40]), although it is possible to obtain the opposite eect under certain conditions (Silberstein et al. 1990 [46]) or even no eect at all (Linden et al. 1987 [23]).
For the anterior cingulum source, I observed both an increase and a decrease of the amplitude with an increasing number of consecutive equal feedbacks, depending on the condition. This inconsistency may lead to doubts concern-ing the credibility of the data. Facts increasconcern-ing the condence in the data are that the dierence is observed consistently in all sources as well as the magnitude of the dierence compared to the standard-error. Although the standard-errors remained rather constant for all possible amounts of repeti-tions there is only a signicant dierence for a string-length of four. This is rather astounding as the number of trials averaged for each number of repe-titions decreased from 37 for a string-length of one to four for a string-length of four.
4.3.2 No eects in temporal sources
As stated in the section about the aims of this study I expected dierences in the amplitude of the aSSR in temporal sources. These eects are far from signicance (see tables 3.7 and 3.8) which is in agreement with Linden et al.
(1987) [23] but contrary to the ndings by Ross et al. (2004) [40] who found the eect of an amplitude-increase for attention. This observation is similar to the one made for the ERP amplitudes where no eect was reported, too.
An in-depth discussion on the absence of eects in the temporal sources will follow later in this chapter.
4.3.3 Eects in other areas
There was however a signicant main eect for "Condition" and a signicant interaction "Condition * Length" for the anterior cingulum as well as a trend for these eects in the left frontal source. Waberski et al. (2001) [48] also found an activation of the anterior cingulum using a mismatch-negativity design. This activation is reported to have a greater latency and can be dis-tinguished from temporal activations. As frontocentral areas like the anterior cingulum are consistently associated with higher functions like attention and expectancy the eect for the anterior cingulum taken together with the trend found in the left frontal source may reect changes in higher functions. The fact that the eects for the left frontal source did not reach signicance may be due to the low number of subjects. It remains to further research with more subjects to determine whether there is a contribution from this source.
Another question that needs to be discussed is why the amplitude does not dier for the rst three string-lengths. Comparing the results for the ampli-tude to the behavioral results reveals that both results share a distinct eect on string-length four. While in the behavioral data this is the rst point on which, for the absolute values, an eect was shown that was expected (re-peated presentation of the high tone leads to absolute higher expectancy for the low tone), it is the only point in the aSSR amplitude-data that showed a dierence at all.
One could argue that this eect is only due to the small number of tri-als that were analyzed for the string-length of four, compared to the other string-lengths. Although this seems plausible, I would have expected a larger standard-error for this group compared to the others, which clearly is not the case. Together with a p-value of less than 0.02, it is more likely that it is not some kind of phantom-eect although this is dicult to interpret.
One interpretation that seems possible is that in reference to the behavioral data it seems likely that after three times getting the same feedback there was a critical point for the expectancy as it is the point where it seems to have turned around. Assuming that this is the point where something has changed in the cognition of the subjects it seems logical to expect a change in the electrophysiological data on that point as well.
The question that remains is why the feedback for the high tone leads to a decrease in amplitude while it is increased by the feedback for the low tone.
One possible explanation may be the kind of feedback associated with each of the tones. While each "low tone" was followed by silence, each "high tone"
was followed by a high and loud noise. One possible speculation is that the subjects associated a negative emotional valence to the loud noise and thus to the "high tone" while the association with the "low tone" was rather positive as the silence that followed was not aversive. This would be in accordance with the study of Kemp et al. (2002) [22] who found an increase for visual stimuli with positive emotional valence in contrast to negative emotional valence.
There is also another possible explanation of the dierence of the amplitude.
The behavioral results (see gure 3.1) show that it is only after four times getting the same feedback that the subject's expectation for a change in the kind of tone was higher than for the tone to stay the same. Thus, after four times of getting the "low tone" feedback the expectancy for the next tone to be followed by an aversive noise increased. Vice versa after four times getting the "high tone" feedback, subjects expected the next tone to be followed by silence. According to studies like Moratti et al. (2006) [25] it is a common nding that the expectation of an aversive stimulus increases the amplitude of steady-state responses.
These two interpretations dier in one major aspect. The rst accounts
the dierence of the amplitude to conditional eects, which is a bottom-up process, while the second one explains it through changes in expectancy, clearly a top-down process. It remains to future research to design a study that is able to show whether one of these explanations is more credible than the other.
The rst interpretation may serve as a point of criticism for the study as associations are bottom-up inuences on cognition, something which I wanted to eliminate from the design. The amount of this confoundation should be subject for further research although I do not believe that it is the only source of the discussed eect.