Modulation masking in monaural and binaural conditions

The shape of hypothetical modulation filters was measured in monaural and binaural signal presentation. A sinusoidal 4-kHz carrier was amplitude modulated by low-low-noise (LNN) [A. Kohlrausch et al., Acta Acust. 83, 659–669 (1997)] and a probe tone of a frequency varying from 2 to 256 Hz. Using a three-interval forced-choice (3IFC) procedure the threshold of modulation by the probe tone was determined for the LNN of 16-Hz center frequency and 8-Hz bandwidth, and 64-Hz center frequency and 8-, 16-, or 32-Hz bandwidth. In the monaural condition, signals were presented to the right ear. In the binaural condition, modulation by the LNN was presented to the right ear, and modulation by the probe tone to the left ear. Results showed that masking in the modulation rate domain is characterized by a broad tuning with the highest values of about 17 dB for the monaural condition, and 4 to 9 dB in the binaural condition occurring for probe-tone frequency equal the center frequency of the LNN. The large effect in binaural condition confirms the idea that modulation detection by hypothetical modulation filter bank occurs on a higher level in the auditory system. [Work supported by the Polish Ministry of Education and Science, Grant No. 4T07B03230.]