Setting the „signal suppression hypothesis“ to a stress test. Experiments to breakup the circular argumentation using behavioral findings and the lateral distractor positivity (Pd)

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A central prediction of the „signal suppression hypothesis“ is the proactive suppression/inhibition of salient distractor stimuli in visual search. Under the condition that features of the distractor are known in advance it was shown with behavioral data in the „capture probe“ design that recognition of letters superimposed upon stimuli of the search display was below the recognition of letters at non-target stimuli. Together with the occurance of the distractor positivity (Pd), a positive going component of the visual evoked potential contralateral to the respective distractor that is presented together with a target stimulus, having a latency of about 200 ms, this was seen as experimental evidence for the model. However, for both dependent variables alternative interpretations are possible. The fact that non-target stimuli always had the identical color as the target, global feature-based facilitation of non-target stimuli might be possible. If that were the case, higher letter recognition here, would be a consequence of facilitation and not suppression/inhibition. Further, the interpretation of the Pd as a neural signature of stimulus suppression/inhibition is far from beeing settled or unambigously clear. A number of studies provided results that were at odds with that interpretation. In the current project we intend to set the Pd to a stress test to check/scrutinize its commonly accepted functional significance as neural signature of distractor suppression/inhibition. The proposed project is planned as a dual center project together with the Center for Mind/Brain Sciences (CIMeC) of the University of Trento to document the reliability of our findings, because results might question some of the crucial predictions of the active suppression account of the Pd. At the end of the project we expect that our results will force the need to reformulate a number of studies and might question the idea of proactive distractor suppression/inhibition in general.