Although most odorants we encounter in daily life are mixtures of several chemical substances, we still lack significant information on how we perceive and how the brain processes mixtures of odorants. We aimed to investigate the processing of odor mixtures using behavioral measures and functional magnetic resonance imaging (fMRI). The odor mixture contained a target odor (ambroxan) in a concentration at which it could be perceived by half of the subjects (sensitive group); the other half could not perceive the odor (insensitive group). In line with previous findings on multi-component odor mixtures, both groups of subjects were not able to distinguish a complex odor mixture containing or not containing the target odor. However, sensitive subjects had stronger activations than insensitive subjects in chemosensory processing areas such as the insula when exposed to the mixture containing the target odor. Furthermore, the sensitive group exhibited larger brain activations when presented with the odor mixture containing the target odor compared to the odor mixture without the target odor; this difference was smaller, though present for the insensitive group. In conclusion, we show that a target odor presented within a mixture of odors can influence brain activations although on a psychophysical level subjects are not able to distinguish the mixture with and without the target. On the practical side these results suggest that the addition of a certain compound to a mixture of odors may not be detected on a cognitive level; however, this additional odor may significantly change the cerebral processing of this mixture. In this context, FMRI offers unique possibilities to look at the subliminal effects of odors.

Kognitive Kontrolle wird besonders in solchen Momenten deutlich, wenn eine geplante Handlung gestört wird. Weil zwei widerstreitende Verhaltenstendenzen gleichzeitig bestehen oder anlaufen, entsteht ein Konflikt. Experimentell können kognitive Konflikte beispielsweise mit dem Stroop-Paradigma hergestellt und untersucht werden (Stroop, 1935). Es ist dabei eine aktuelle Frage, wie Konflikte zeitlich verarbeitet werden und wo im Gehirn diese Verarbeitung geschieht. Zeitlich können Konflikte beispielsweise dann entstehen, wenn die Informationen des Stimulus abgerufen werden oder auch erst dann, wenn die intendierte Antwort tatsächlich für die Artikulation ausgewählt werden muss. Eine weiterführende Frage ist, ob sich die entsprechenden Ergebnisse für verschiedene Stroop-Varianten unterscheiden. In der vorliegenden Arbeit wurden diese Fragen systematisch für die Verarbeitung von Objekten und Zahlen mit zwei Varianten des Stroop-Paradigmas untersucht. In der vorliegenden Dissertation präsentiere ich Ergebnisse von Reaktionszeitstudien und fMRT-Experimenten zum zeitlichen Ablauf und zu neuronalen Substraten kognitiver Konflikte während der Verarbeitung von Objekten und Zahlen. Um die Konflikte zeitlich und räumlich lokalisieren zu können, wird die Abrufphase und die Antwortphase separat modelliert. Die Ergebnisse deuten darauf hin, dass die Konflikte eher während des Abrufs als bei der Antwortauswahl stattfinden. Außerdem wird geschlussfolgert, dass die Konflikte für Zahl- und Objektrepräsentationen nicht auf gemeinsamen neuronalen Substraten basieren. Die Ergebnisse meiner Reaktionszeitstudien und der MRT-Studien deuten also darauf hin, dass Konflikte bei der Verarbeitung von Objekten und Zahlen zwar einem ähnlichen zeitlichen Verlauf folgen, aber offenbar in unterschiedlichen neuronalen Netzwerken verarbeitet werden. / In daily life, we constantly have to adjust our goals and plans to changing task demands and internal needs. Our ability to balance the initiation and inhibition of our actions, and to solve resulting conflicts between them, is referred to as cognitive control. To study the processes of cognitive control, the Stroop Paradigm has become a popular tool (Stroop,1935). The Stroop Paradigm is frequently used to address central questions of cognitive control. It is, for instance, an open issue, where and when in the processing stream cognitive conflicts arise. Do they arise early, for example, during the retrieval of target and distractor? Or do they occur late, when the response is prepared for execution? Another debate is concerned with the question whether the findings agree for different Stroop variants (Van Maanen et al., 2009). In this dissertation I present research on the temporal characteristics and the neural substrates of cognitive conflicts during the processing of objects and numbers. To better understand the locus of the conflict, the retrieval phase and the response phase are modelled separately. The results from several reaction time studies and from two fMRI experiments speak to the issue that processing costs occur during retrieval, i.e., early in the processing stream, for both, object and number representations. The results further indicate that the processing of the conflict between target and distractor for number and object representations do not rely on common neural substrates. I will thus present the results from behavioural and functional imaging experiments, showing similar temporal patterns for the conflicts in both systems, but distinct underlying neural networks.

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Although most odorants we encounter in daily life are mixtures of several chemical substances, we still lack significant information on how we perceive and how the brain processes mixtures of odorants. We aimed to investigate the processing of odor mixtures using behavioral measures and functional magnetic resonance imaging (fMRI). The odor mixture contained a target odor (ambroxan) in a concentration at which it could be perceived by half of the subjects (sensitive group); the other half could not perceive the odor (insensitive group). In line with previous findings on multi-component odor mixtures, both groups of subjects were not able to distinguish a complex odor mixture containing or not containing the target odor. However, sensitive subjects had stronger activations than insensitive subjects in chemosensory processing areas such as the insula when exposed to the mixture containing the target odor. Furthermore, the sensitive group exhibited larger brain activations when presented with the odor mixture containing the target odor compared to the odor mixture without the target odor; this difference was smaller, though present for the insensitive group. In conclusion, we show that a target odor presented within a mixture of odors can influence brain activations although on a psychophysical level subjects are not able to distinguish the mixture with and without the target. On the practical side these results suggest that the addition of a certain compound to a mixture of odors may not be detected on a cognitive level; however, this additional odor may significantly change the cerebral processing of this mixture. In this context, FMRI offers unique possibilities to look at the subliminal effects of odors.