FFRs to Mandarin tones in macaques and guinea pigs

The raw data were high-pass filtered using a second-order zero-phase shift FIR filter. Time-locked epochs were extracted for all vowels in both polarities. The epochs that exceeded amplitudes of 250 μV were rejected. The FFRs in both the polarities for each tone were averaged to obtain a total of four FFR waveforms (one for each tone).

Custom MATLAB and R routines were used to filter and average signals appropriately to obtain LFPs and MUA from the macaque and GP recordings. The current source density (CSD) in the macaque was computed from the LFP signals derived from the electrodes with 150 μm spacing, spatially smoothing the LFP signal using a Gaussian filter (SD = 250 μm) and obtaining the second spatial derivative method using the finite difference approximation. In the GP, CSD was computed from the LFP signals derived from alternate electrode contacts (60 μm spacing), spatially smoothing the LFP signal using a Gaussian filter (SD = 125 μm) and obtaining the second spatial method using the finite difference approximation. The sink with the earliest latency in the CSD post-stimulus onset was used to identify the thalamorecipient layers.