Characterizing the effect of recent acoustic history properties on auditory fMRI responses

Olumide Ayodeji Olulade, Purdue University

Abstract

Limitations of the functional Magnetic Resonance Imaging (fMRI) technique include the non-linearity and limited dynamic range of the measured BOLD response. These confounds are exacerbated in auditory fMRI by interaction of responses to multiple (desired) stimuli with those driven by acoustic noise inherent in the imaging process. The inherent acoustic imaging noise can induce undesired neuronal responses that can mask responses to desired auditory stimuli. Similarly, activation accumulated over multiple desired stimuli has been demonstrated to elevate the baseline, thus reducing the dynamic range available for subsequent responses. To best evaluate responses to auditory stimuli, it is necessary to account for the presence of all recent acoustic stimulation, beginning with an understanding of the extent of memory in the composite neural/hemodynamic system, herein referred to as the relevant duration of acoustic history of an experiment. Additionally, characterization of the attenuating effects brought about by interaction between and among induced unwanted neuronal responses and responses to desired auditory stimuli is necessary. This dissertation focuses on the characterization of the duration of the afore-mentioned temporal memory and quantitative assessment of the associated response attenuation. This was achieved in two separate experiments involving the examination of different experimental parameters that affect the amount of acoustic information present in a typical auditory fMRI study. In the first experiment, two parameters—inter-stimulus interval (ISI) and repetition time (TR)—were varied during an fMRI session in which participants were asked to passively attend to an auditory stimulus. These two parameters directly influence the amount of acoustic noise (both desired and otherwise) present in an auditory fMRI experiment. Results of these experiments present evidence of a non-linear interaction between responses to acoustic imaging noise and desired auditory stimuli. As expected, the attenuating effects of these interactions become less significant as TR and ISI increase. In contrast to previous work, it has been found that attenuating effects persist up to 18 – 24s after a stimulus presentation. In the second study, the inter-stimulus interval was varied, along with other experimental parameters including the number of acquired slices (Nsl), the rate of acquisition (aRate), and the duration of the presented stimulus (sDur). The repetition time (TR) was kept fixed during these experiments. Results of this study suggest that the use of longer stimulus durations, as well as faster rates of acquisition may mitigate the attenuating effects of the inherent acoustic scanner noise.

Degree

Ph.D.

Advisors

Talavage, Purdue University.

Subject Area

Biomedical engineering|Medical imaging

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