Figure 1.
Schematic representation of neural mechanisms as evoked by the masking stimuli (MS).
A. The lower part of the figure shows the frequency spectrum of the MS. The upper part is a highly simplified representation of neural activity in the auditory cortex as evoked by the MS. Neurons (excitatory = red, inhibitory = blue) are represented tonotopically. Transparent colors indicate reduced activation of inhibitory and excitatory neurons. Enlarged circles represent enhanced activation of neurons. B. Neurons with characteristic frequencies (CF) corresponding to the outer frequencies of the masking stimulus (OUT CF), the edge frequency bands (EFB) around the notch (EFB CF) and the frequencies within the notch (NOTCH CF) are further defined. A, B, C, D and E show the hypothesized modulation of neural mechanisms by MS with different spectral contrasts. The amount of lateral inhibition of neurons with NOTCH CF is hypothesized to be modulated in the following order (beginning with the greatest lateral inhibition of NOTCH CF): 3/8 oct +30 dB >7/8 oct +30 dB>NFN >3/8 oct −30 dB >7/8 oct −30 dB.
Figure 2.
Schematic representation of a trial sequence.
A masking stimulus (MS) of 3 s was followed by a test stimulus (TS) of 1 s with an inter-stimulus interval of 0.5 s.
Figure 3.
Auditory evoked magnetic field and contour map for the N1m response – experiment 1.
Example of the auditory evoked magnetic field and the corresponding contour map of a representative subject for N1m responses.
Figure 4.
Grand averaged source waveforms, normalized source localizations and normalized N1m responses – experiment 1.
A. Grand averaged source waveforms for the N1m time window in the first experiment. N1m source strength is smallest in both amplified edge frequency band (EFB) conditions and greatest in both attenuated EFB conditions. The left panel shows the normalized source locations of both equivalent current dipoles (ECD) transformed to a standardized magnetic resonance imaging (MRI) brain. B. Mean normalized N1m values demonstrating the steady decrement of N1m responses with the smallest N1m source strength in the 3/8 octave amplified condition and the greatest in the 7/8 octave attenuated condition.
Figure 5.
Auditory evoked magnetic field, contour map and normalized source localization for ASSR – experiment 1.
A. Example of the auditory evoked magnetic field and the corresponding contour map of a representative subject for auditory steady state responses (ASSR) in the first experiment. B. Normalized source locations of both equivalent current dipoles (ECD) transformed to a standardized magnetic resonance imaging (MRI) brain.
Figure 6.
Auditory evoked magnetic field and contour map for the N1m response – experiment 2.
Example of the auditory evoked magnetic field and the corresponding contour map of a representative subject for N1m responses.
Figure 7.
Grand averaged source waveforms and normalized N1m responses – experiment 2.
A. Grand averaged source waveforms for the N1m time window in the second experiment. N1m source strength is lower in both amplified edge frequency bands (EFB) conditions than in the notch-filtered noise (NFN) condition. N1m source strength in the narrower EFB amplification condition (1/24 oct +30 dB) seems to be greater than in the wider EFB amplification condition (1/8 oct +30 dB). The left panel shows the normalized source locations of both equivalent current dipoles (ECD) transformed to a standardized magnetic resonance imaging (MRI) brain. B. Mean normalized N1m values confirming the observed results in the grand averaged source waveforms.
Figure 8.
Auditory magnetic evoked field, contour map, normalized source localization and interaction for ASSR – experiment 2.
A. Auditory evoked magnetic fields and the corresponding contour map of a representative subject for auditory steady state responses (ASSR) in the second experiment. B. Normalized source locations of both equivalent current dipoles (ECD) transformed to a normalized magnetic resonance imaging (MRI) brain. C. Interaction effect between the factors NFN-type and hemisphere for normalized ASSR source strengths. Error bars denote +/−1 standard error. Normalized ASSR source strengths are greater in both amplified conditions in the left hemisphere. NFN condition does not seem to differ between hemispheres.