Discussion

Experiment 1 was designed in order to test the oddball protocol and to collect data from healthy participants. The results showed attention capture as revealed by less accuracy and more time necessary to categorize S2 after the presentation of a deviant S1 than after the presentation of a standard S1. In brief, a deviant stimulus captured attention in the oddball protocol used. The decrease of categorization performance due to the preceding deviant stimuli involved involuntary attention, since there was no need to discriminate between standard and deviant stimuli in the irrelevant to-be-ignored channel. It seems likely that the S1 stimuli were not completely ignored by the subjects since they provided a warning signal for the forthcoming S2 stimuli. Moreover, participants attested in the debriefing that their responses were partly based on the melody created by the sequence of the two sounds S1 and S2. However, the frequency of S1 was irrelevant, and, therefore, the effects may be regarded as a result of an involuntary attention switch triggered by a task-irrelevant deviant event, i.e., a frequency change. The attention capture was greater when the interval between the to-be-ignored stimulus S1 and the target S2 was smaller, that is, less detrimental effects of the preceding task-irrelevant deviant S1 on the processing of relevant S2 occurred with the longest ISI (150 and 200 ms). This suggests that the detrimental effects of deviants are very short-lived, which was already reported in a previous study [Schröger, 1996].

This effect of attention capture has been interpreted as a capacity of our central nervous system to detect inconsistency among a homogeneous set of stimuli and to automatically direct informational processing toward the deviant stimulus [Schröger, 1997]. According to Schröger, this deviance detection is a special case of discrimination between an infrequently occurring irregular event (the deviant stimulus) and a frequently occurring regular event (the standard stimulus). His model, mainly based on electrophysiological data (mismatch negativity wave – MMN), suggests that conscious deviance detection may be based in part on the output of an obligatorily operating deviance detection system that does not rely on explicit intention to detect deviants and that is active even in the absence of attention. However, processes involved in other stimulus processing, even attentive processing may modulate processes taking place in the preattentive stages of processing. Thus, processing of the tinnitus signal might affect such an automatic attention switching, particularly in the case of chronic disabling tinnitus where tinnitus could attract and keep attention on its processing. Experiment 2 was designed to test this hypothesis in tinnitus sufferers.