MEGSIM Auditory Neuromag //
Empirical Auditory Data from Elekta Neuromag Vectorview (306 channels)
Three pure tones of different frequencies (500 Hz, 2000 Hz, and 4000 Hz) were presented to obtain a tonotopic map. In addition, white noise was also presented intermixed with the tones (focal vs. extended source conditions). Noise spreads spectral energy over a wide frequency range in contrast to pure tones, and thus increases the size of the activated cortical patch. Based on the tonotopic map characterizing the cochlea through primary auditory cortex, low frequencies are represented rostrally and high frequencies caudally. White noise should stimulate extended tissue covering a range of frequencies. The tones and white noise (200 trials for each stimulus) were randomly presented at an average inter-stimulus interval of 1000 ms. The auditory stimuli were generated on the Presentation computer using a Creative Labs Soundblaster audio card. The sound was delivered to the subject's ear canal using sound transducers connected with plastic tubing to ergonomically designed earplugs. A dB attenuator was used to adjust the intensity of the tones. Hearing thresholds were determined individually for each subject.