Functional magnetic resonance imaging (fMRI) of the auditory cortex : an event-related study using pure tone stimulation

Introduction. The use of Functional Magnetic Resonance Imaging (fMRI) in studying central auditory pathways expanded our knowledge of the neurophysiology of hearing. In various studies, an array of diverse auditory stimuli has been used, including pure tones and words, in a "Block" design. Few studies, thus far, have been performed using an event-related design. None of these has used pure tone stimulation.Subjects and methods. In eight out of 13 subjects enrolled and scanned in this project, data have been acquired and processed. Binaural pure tone stimulation at 2kHz and one higher frequency at 6, 8, or 12 kHz, at 90 dB SPL was presented to all subjects, using a stroboscopic design in an attempt to eliminate cortical response to scanning noise. In addition, an unusually long time of repetition (TR) interval of 10 seconds was utilized. The aim in data processing was to generate: (a) a Peak Height Map (PHM) with selection of areas of stimulation based on the maximal response; (b) a statistical map from the raw data of the images.Results. No consistent morphology was detected in the BOLD event-related curves, corresponding to the percentage change from baseline over time. Such curves revealed significant oscillation with marked dips below baseline, highly suggestive of an irregular noise pattern. Comparing the curves to each other, no discernible shape or similarities were noted.Conclusion. Attempting to extract both the temporal and spatial characteristics of the cortical response to pure tone stimuli resulted in a dispersal of data over many parameters, therefore "diluting" it. This, in turn, yielded a negative result where the responses were "drowned" revealing but a noise pattern