Effects of visual load on auditory processing

In daily life, people need to be able to focus on a task while ignoring any task-irrelevant background noise. For example, people who work in an open-space office may have to work on a report while ignoring the background talk among co-workers. Theories of attention argue that processing of task-irrelevant auditory information should become attenuated when attentional capacity is exhausted by task-relevant stimuli, for example a visual task. According to early-filter theory, top-down attenuation of auditory responses is possible at various stages of the auditory pathway through multiple recurrent loops. Furthermore, the adaptive filtering model of selective attention suggests that filtering occurs early when concurrent visual tasks are demanding (e.g., high load) and late when tasks are easy (e.g., low load). The main aim of this thesis was to investigate whether auditory processing is attenuated during concurrent visual load manipulation, and if so, at which stage of the auditory pathway. Recurrent loops that are present throughout the entire auditory pathway should allow top-down modulation of even earliest responses, especially when accompanied by a highly demanding task. Recording these auditory responses during concurrent visual tasks with different levels of load could potentially show at which stages of auditory processing the filtering happens when the task demands are high or low, or there is no task at all. This thesis tested the effects of visual load manipulation on the responses originating from different stages of the auditory pathway: mismatch negativity (MMN) to duration in Study I, MMN to frequency in Study III, auditory steady-state responses (ASSRs) to a 40-Hz modulation frequency in Study IV, and ASSRs to 20-, 40-, and 80-Hz modulation frequencies in Study V. Additionally, Study II compared different control conditions (cascade vs. no-repetition control) for evoking MMN to find a design that reduces confounds in the MMN. Results of Study II showed that cascade and no-repetition control are comparable control conditions for evoking MMN. For the effects of visual load manipulation on auditory MMN (low vs. high load), results showed moderate evidence for the effect of load on duration MMN and moderate evidence for no effect of load on frequency MMN. However, results for the duration MMN might be confounded by physical differences between the low load and high load conditions in the visual task, and by a biased oddball paradigm used to evoke the MMN. Thus, it is most likely that auditory MMN is not affected by visual load (low vs. high). For the effects of visual load on ASSRs (no, low, and high load), results showed moderate to strong evidence for no effects of load manipulation on ASSRs, but some comparisons were inconclusive. The convincing evidence obtained in Studies IV and V points to the robustness of ASSRs against visual load manipulation. Because it is most likely that neither MMN nor ASSRs are affected by visual load manipulation, results presented in this thesis support the idea that attentional resources are modality specific. Because it is possible that filtering occurs at one stage of auditory processing and does not change with the concurrent task demands, the present results do not rule out the early-filter theory. However, results are inconsistent with the adaptive filtering model because filtering of task-irrelevant responses does not vary with the demands of the concurrent task