Top-down modulation of the auditory Steady State Response in a task-switch paradigm

Abstract

Our perceptual system is constantly exposed to a huge amount of sensory information. Two main concepts exist which describe how our brain selects the relevant information for further processing: Top-down theories refer to selection processes by higher-order brain areas whereas bottom-up theories characterise data-driven processing. The present study is concerned with the investigation of auditory selective attention, a central top-down mechanism for selection of auditory information. Amplitude-modulated tones (modulated by 20 Hz and 45 Hz respectively) that elicit auditory steady-state responses were chosen as stimuli.Until now, the impact of selective attention on the auditory steady state response is not well understood and remains to be clarified. In aid of this, a dichotic listening experiment was designed which allowed the study of selective attention within the auditory modality. Fourteen subjects were exposed to binaural stimulation while switching between left and right ear focus. As they had to detect target tones in the prior designated ear they were forced to re-adjust their focus of attention from trial to trial. Thus, the impact of selective attention on the auditory steady-state response could be investigated within the auditory system.The MEG data of 14 normal-hearing subjects were collected and analysed. Data sets were projected to 8 regional sources, including the two left and the two right temporal sources I was interested in. To disclose attention-mediated effects on the auditory steady-state response, the amplitude of the respective modulation frequencies was identified using a Fast Fourier Transformation. Significant results, revealing an attention-mediated impact on the auditory steady-state response, were found: Selective attention emerged to inhibit the auditory steady-state responses of attended stimuli in the ipsilateral hemisphere. In contrast, no modification of neuronal activity was observed in the contralateral hemisphere. These findings contradict the existing literature which favours an enhancement of neuronal activity due to attention. Task switching within the auditory modality was found to be one of the crucial factors for these surprising results.