Auditory Steady State Response, an MEG study
I studied Auditory Steady State Responses (ASSR) to Amplitude Modulated Sinusoids with Dr. Jonathan Z. Simon, reverse engineering the auditory system in an attempt to model it through a transfer function that can extract simplified part of speech from the analysis of Magnetoencephalography (MEG) brain waves. The high temporal resolution of MEG is ideal for investigations of the neural processing of oscillatory sounds and their temporal dynamics. We built the signal processing tools necessary to extract clear and reliable time series resulting in 3 IEEE EMBS articles (removing noise, detecting significant signals, and fitting dipoles). Research followed by testing various hypotheses and publishing: ‘Sensitivity to Temporal Modulation Rate and Spectral Bandwidth in the Human Auditory System: MEG Evidence’ in the Journal of Neurophysiology 2012. Major findings were that global synchronized auditory oscillations are phase locked to slow temporal modulations and amplitude modulation is inversely correlated with neural sensitivity, but not bandwidth.
Abstract: Slow acoustic modulations below 20 Hz, of varying bandwidths, are dominant components of speech and many other natural sounds. The dynamic neural representations of these modulations are difficult to study through noninvasive neural-recording methods, however, because of the omnipresent background of slow neural oscillations throughout the brain. We recorded the auditory steady-state responses (aSSR) to slow amplitude modulations (AM) from 14 human subjects using magnetoencephalography. The responses to five AM rates (1.5, 3.5, 7.5, 15.5, and 31.5 Hz) and four types of carrier (pure tone and 1/3-, 2-, and 5-octave pink noise) were investigated. The phase-locked aSSR was detected reliably in all conditions. The response power generally decreases with increasing modulation rate, and the response latency is between 100 and 150 ms for all but the highest rates. Response properties depend only weakly on the bandwidth. Analysis of the complex-valued aSSR magnetic fields in the Fourier domain reveals several neural sources with different response phases. These neural sources of the aSSR, when approximated by a single equivalent current dipole (ECD), are distinct from and medial to the ECD location of the N1m response. These results demonstrate that the globally synchronized activity in the human auditory cortex is phase locked to slow temporal modulations below 30 Hz, and the neural sensitivity decreases with an increasing AM rate, with relative insensitivity to bandwidth.
Wang, Y., Ding, N., Ahmar, N., Xiang, J., Poeppel, D., & Simon, J. Z. (2011). Sensitivity to temporal modulation rate and spectral bandwidth in the human auditory system: MEG evidence. Journal of neurophysiology, 107(8), 2033-2041.
Ahmar, N. E., & Simon, J. Z. (2005, March). MEG adaptive noise suppression using fast LMS. In Neural Engineering, 2005. Conference Proceedings. 2nd International IEEE EMBS Conference on (pp. 29-32). IEEE.
Ahmar, N. E., Wang, Y., & Simon, J. Z. (2005, March). Significance tests for MEG response detection. In Neural Engineering, 2005. Conference Proceedings. 2nd International IEEE EMBS Conference on (pp. 21-24). IEEE.